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Volcanic aerosols change the atmospheric composition and thereby affect weather and climate. Aerosol dynamic processes such as nucleation, condensation, and coagulation modify the shape, size, and mass of aerosol particles, which influence their atmospheric lifetime and radiative properties. Nevertheless, most models omit these processes for ash particles. In this work, we explore the ash aerosol aging and sulfate production during the first 4 days following the 2021 La Soufrière (St. Vincent) eruption with the ICON‐ART model (ICOsahedral Nonhydrostatic model with Aerosol and Reactive Trace gases). Online coupling of ICON‐ART with a one‐dimensional volcanic plume model calculates volcanic emission, which makes it possible to resolve the different eruption phases of the noncontinuous La Soufrière eruption. We compared our simulated aerosol distribution and composition with observations from the Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument, the Multiangle Imaging SpectroRadiometer (MISR) Research Aerosol (RA) Algorithm, and the Barbados Cloud Observatory (BCO). We show that online coupling is essential to adequately model the emissions and plume development close to the volcano. The modeled aerosol aging is in very good agreement with observations from MISR near the emission source and with CALIOP at larger distances. Furthermore, particle aging occurs faster in the troposphere than in the stratosphere due to the availability of water vapor and OH, but a layer of coated ash appears at the plume top due to faster oxidation of SO2 and lofting by aerosol‐radiation interaction. This paper gives the first direct comparison of aerosol aging in volcanic eruption plumes between simulations and observations. Plain Language Summary: Large volcanic eruptions can influence weather and climate, and endanger aviation and public health. To constrain these effects and risks, it is critical to reliably predict the volcanic plume dispersion. However, the atmospheric lifetime of the ash released during an eruption is influenced by many factors, such as emission height, meteorology, and aerosol dynamical processes. Aerosol dynamical processes lead to growth and aging of volcanic plume particles. They include the formation of new particles from precursor gases, condensation on existing particles, and coagulation of particles. This paper investigates the formation of aged ash particles in model simulations and observations following the 2021 La Soufrière eruption. We consider ash aging both close to the volcano and during further transport. We found that ash aging takes place already close to the volcano and the fraction of aged particles increases with distance from the source. During further transport, a layer of aged ash particles forms at the plume top due to interaction of these particles with radiation and subsequent warming of the plume. Key Points: Resolving individual eruption phases is essential for modeling the 2021 La Soufrière eruptionCombination of modeling and satellite data confirm, for the first time, the aging of volcanic ashAsh aging and sulfate production rates depend on distance from the source and altitude [ABSTRACT FROM AUTHOR]

Black carbon (BC) absorbs solar radiation, increasing the buoyancy and vertical ascent of absorbing aerosol in the atmosphere. This self‐lofting process has been observed for individual plumes in the troposphere and lower stratosphere but here we show it occurring at broader scales through enhanced large‐scale ascent over BC‐rich regions. This is demonstrated in a pair of simulation using the UKESM1 Earth‐System model where BC aerosols were modeled either with or without the ability to absorb radiation. With absorption included the annual global mean concentration of BC in the upper troposphere and lower stratosphere (8–22 km) rose by up to 50% and the column loading over some remote oceanic regions more than doubled. The increase in aerosol height was particularly notable over the southeast Atlantic where biomass burning aerosol from Africa was elevated up to 1 km higher when their absorption was included. Similar effects were seen over the Arctic where the absorbing haze was transported in at higher levels and surface concentrations were halved. The absorption by BC also increased ascent over southern Asia, which tended to thicken the Asian brown cloud during the dry season but in the wet season enhancing ascent promoted deep convection and had the tendency to deplete the aerosol through wash‐out. We conclude that representing aerosol absorption accurately is important in simulating the vertical distribution, transport and abundance of aerosol in the Earth‐system that will affect their interactions with climate. Plain Language Summary: Black carbon is an important component of atmospheric aerosols as it absorbs solar radiation thereby heating the atmosphere and potentially warming climate. The localized heating can also affect clouds and atmospheric motions making the regional and climate effects more complex and uncertain. The vertical distribution and geographic spread of the aerosol is also key to how it interacts with the climate system. In this paper we highlight the fact that the absorption taking place in BC aerosols can affect its vertical ascent in the atmosphere and therefore how it becomes distributed across the globe. This self‐lofting process was demonstrated in a climate model and was particularly important in aiding the elevation and downwind transport of absorbing smoke layers from regions such as central Africa, as well as increasing the amount of BC reaching the upper troposphere and lower stratosphere. Key Points: Absorption of solar radiation helps to raise the altitude of black carbon aerosol by increasing buoyancy and vertical ascentThis self‐lofting mechanism has been captured in a climate model where radiative heating affects the large‐scale ascent and circulationSelf‐lofting increased the altitude and long‐range transport of black carbon to remote regions and the upper troposphere [ABSTRACT FROM AUTHOR]

Low‐level jets (LLJs) drive frequent emission of mineral dust in the central and western Sahara in boreal summer. A major hotspot for this process is central Algeria, northern Mali and Mauritania, through which blow the dry near‐surface northeasterly Harmattan winds, with a peak in dust emission around the low‐lying Tidihelt region. North African orography is thought to contribute to the strength of the LLJ over the Bodele dust source in Chad, but its influence on erosivity over summertime source regions remains unquantified. In this paper, the contribution of central Saharan orography to the strength of Harmattan LLJs and associated dust emission frequency is tested. An idealized simulation with flattened Hoggar mountains is compared with a control using the Met Office Unified Model at 12 km horizontal resolution. In the absence of the Hoggar mountains, dust emission frequency estimated using an empirical relationship with surface wind speeds is found to decline across the entire northeasterly "LLJ alley," including by 31% in the Tidihelt where composited jet surface winds drop from 9.0 to 7.3 m s−1 under a more easterly regime. The mountains are linked to a low‐level leeward geopotential height perturbation, with a northern limb reinforcing northeasterlies through the Tidihelt. Dome‐shaped elevated heating situated over the Hoggar mountains explains the difference between the simulated wind fields in the two experiments. These findings suggest that central Saharan orography plays an important role in sustaining erosive dusty conditions during boreal summer. Plain Language Summary: A major driver of dust storms in the central and western Sahara is a strong wind (referred to as a jet) which forms overnight at elevation and then hits the surface after sunrise. This process is important in the remote central Algerian Tidihelt region, home to a highly active dust source. In this paper we estimate the effect that the nearby Hoggar mountains have upon strong wind events in the Tidihelt during summer. We do this by comparing two computer model simulations of the atmosphere over North Africa. One has a realistic representation of the Hoggar mountains, whereas in the other the mountains are flattened to a uniform level. Our results show that the mountains strengthen these winds by heating the atmosphere relative to the surrounding desert, producing a local region of low pressure. Using satellite observations of dust plumes we estimate that without the mountains the frequency of dust emission events from the Tidihelt would be reduced by about a third due to weaker winds. The findings show how mountains can play an important role in the meteorology responsible for dust storms. Key Points: A model experiment is used to test the impact of the Hoggar mountains on dust emission frequency in the SaharaThe mountains generate a leeward low due to elevated heatingRemoving the mountains reduces emission frequency in the highly active Tidihelt by 31% [ABSTRACT FROM AUTHOR]

The Deep Blue (DB) aerosol retrieval algorithm has recently been applied to Advanced Very High Resolution Radiometer (AVHRR) data to produce a first version (V001) of a global aerosol optical thickness (AOT) data set. In this paper, we critically evaluate these AVHRR AOT data over China by comparison with ground‐based reference data from China Aerosol Remote Sensing Network for the period 2006–2011. The evaluation considers the impact of the surface (type and reflectance) and the aerosol properties (aerosol loading, aerosol absorption) on the quality of the retrieved AOT. We also compare the AVHRR‐retrieved AOT with that from Moderate Resolution Imaging Spectroradiometer over major aerosol source regions in China. We further consider seasonal variations and find, in general, a good agreement between AVHRR AOT and the reference data sets. The AVHRR retrieval algorithm performs well over dark vegetated surfaces, but over bright surfaces (e.g., desert regions) the results are less good. The AVHRR algorithm underestimates the AOT, with 32.1% of the values lower than the estimated error envelope of ±0.05 ± 0.25τ. In particular over the desert, the AVHRR‐retrieved AOT is frequently underestimated and for AOT ≤ 0.6 the values are on average 0.05 too low due to the pixel filtering, and dust storms are missed. The comparison of the AVHRR AOT with MODIS collection 6 and CARSNET data indicates that improvements are needed for, for example, AVHRR calibration and cloud/aerosol flagging. The analysis presented in this paper contributes to a better understanding of the AVHRR AOT product over China. Key Points: The 53.5% of the AOT values are within the estimated error envelop of ±0.05 ± 0.25τThe AVHRR‐retrieved AOT is frequently underestimated over the desert regionsAOT biases at 550 and 660 nm are −0.057 and −0.093, respectively [ABSTRACT FROM AUTHOR]

The physicochemical properties and origins of atmospheric aerosols in the Tibetan Plateau (TP) region are a research topic of great interest, but an in‐depth understanding of this topic is challenging, partially due to a lack of intensive in situ observations. Thus, a field campaign was conducted over Yadong, a remote area on the south slope of the TP from June 11 to 31 August 2021. The aerosol loading was low, with a black carbon mass concentration of 147.4 ± 98.4 ng·m−3. Aerosol single‐scattering albedo was low (0.73 ± 0.11 at 550 nm) and increased from 450 to 700 nm wavelength. Organic matter (OM) accounting for 69.6% of the total aerosol mass and relatively high secondary organic carbon ratios, highlighting the importance of secondary formation. An interesting phenomenon observed was that the evolution of aerosols was mainly characterized by diurnal variation, which could not be explained by large‐scale atmospheric processes such as Indian summer monsoon. Instead, it was found that regional mountain‐valley winds between the Himalayas and South Asia transported polluted air masses toward the TP, especially in the afternoon when regional valley wind are expected to be the strongest and the boundary layer in South Asia is deepest. Additionally, daytime local valley wind further elevated these aerosols to higher altitudes on the TP. This paper provides insights into the transport mechanisms of aerosols from South Asia to the TP. These findings are of great importance since aerosols exhibit significant diurnal variations in the TP region. Plain Language Summary: Previous studies focused on the analysis of the physical or chemical properties of aerosols on the Tibetan Plateau, but this study provides a comprehensive examination of both. The findings reveal that aerosols on the southern slope of the Tibetan Plateau exhibit strong absorption efficiency. Aerosol single‐scattering albedo was low (0.73 ± 0.11 at 550 nm), which may be attributed to aerosol secondary generation and coating. Finally, the mechanism of pollutant transport from South Asia to the Tibetan Plateau was analyzed relies on site observations, satellite, and reanalysis data to highlight the link between diurnal variations of pollutants and transport mechanism. The specific transport mechanism be understood uniformly across different scales, including Indian summer monsoon, regional mountain‐valley winds between the Himalayas and South Asia, and local mountain‐valley winds circulation. Key Points: Aerosol single‐scattering albedo was low (0.73 ± 0.11 at 550 nm) and secondary organic matter was the major aerosol componentThe evolution of aerosols was mainly characterized by diurnal variation that was related to transport mechanism over YadongThe Himalayas‐South Asia regional mountain‐valley winds combined with local mountain‐valley winds transport aerosols to the Tibetan Plateau [ABSTRACT FROM AUTHOR]

The SMART‐s (Spectral Measurements for Atmospheric Radiative Transfer—spectroradiometer) acquires Sun/sky observations for retrieving optimal information on trace gases and aerosols with minimal assumptions. Overall, the algorithm of SMART‐s incorporates a series of retrievals, from fundamental quantities (i.e., column abundance of trace gases and aerosol loading) to higher‐order geophysical parameters (e.g., aerosol physicochemical properties and vertical profiles), utilizing Sun/sky spectral radiance measurements. This paper describes the theoretical basis for column retrievals of trace gases and aerosols. Associated profile retrievals will be presented in follow‐up papers. The current algorithm retrieves the fine/coarse mode of the particle size distribution and spectral complex index of refraction and, thereby, the spectral aerosol single‐scattering albedo ω0. SMART‐s retrieval is unique particularly in its high spectral resolution of the complex index of refraction and ω0 from near‐ultraviolet to near‐infrared wavelengths, which is pivotal information for atmospheric chemistry, climate and other inversions. We theoretically assessed information content and retrieval accuracy of the algorithm and compared different type of measurements including the Aerosol Robotic Network (AERONET) and standard Pandora. For the same levels of radiometric accuracy, SMART‐s measurements provide the most informative aerosol retrievals based on theoretical error analyses. Higher spectral resolution measurements are particularly beneficial for particle size distribution and fine‐mode refractive index retrievals. We applied this algorithm to the AERONET Sun/sky measurements at Kanpur, India, in 2016 to assess algorithm consistency. Even with different assumptions and numerical methods for the inversion, SMART‐s retrieved aerosol parameters agreed well with the AERONET operational products (e.g., absolute mean bias errors less than 0.01 for ω0). Key Points: The SMART‐s algorithm retrieves aerosol particle size distributions and spectral complex index of refraction from about 310 to 800 nmWe applied the SMART‐s algorithm to the AERONET Sun/sky measurements and the retrievals were consistent with the operational products of AERONETHigh spectral resolution of the SMART‐s retrievals can provide unique information for atmospheric chemistry, air quality, and climate studies [ABSTRACT FROM AUTHOR]

The retrieval of the asymmetry parameter from nephelometer measurements can be challenging due to the inability to detect the whole angular range. Here, we present a new method for retrieving the asymmetry parameter of ice crystals with relatively large size parameters (>50) from polar nephelometer measurements. We propose to fit the angular scattering measurement with a series of Legendre polynomials and the best fitted coefficients give the asymmetry parameter. The accuracy of the retrieval is analyzed by accessing the smoothness of the phase function, which is closely linked to the complexity of ice particle. It is found that the uncertainty of retrieval could be smaller than 0.01, provided the measured intensity profile is smooth enough. As an application, we report an case study on Arctic cirrus, which shows a mean value for the asymmetry parameter of 0.72. Plain Language Summary: The asymmetry parameter of ice crystals is a parameter that can largely determine cirrus cloud's interaction with solar radiation energy, and therefore its magnitude is important for climate and weather prediction models. In‐situ measurements using nephelometers is a direct way to measure partial angular scattering functions, and the accuracy of these measurements is of upmost importance. In this paper, we report a novel method for retrieving the asymmetry parameter from polar nephelometer measurements. Depending on the smoothness of the measured angular scattering function, the accuracy of the retrieval could be very high. We report a case study over the Arctic region, showing a low asymmetry parameter around 0.72. Key Points: A new algorithm is developed for retrieving the asymmetry parameter of large and complex ice crystals from polar nephelometer measurementsAccuracy of retrieval could be better than 0.01, provided sufficient smoothness of the phase functionA case study of Arctic cirrus measurement during the CIRRUS‐HL campaign is reported [ABSTRACT FROM AUTHOR]

The physical and optical properties of aerosols and the change of planetary boundary layer height (PBLH) are of great scientific significance for the study of atmospheric environment and air quality. This paper intends to present integrated observations of aerosol physical‐optical characteristics and PBLH properties under different air pollution levels in Shouxian (National Climatology Observatory (32°26' N, 116°47' E)), China. The combined measurements of ground‐based Lidar and Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) are analyzed from December 2016 to November 2017 in this study. Results show that the extinction coefficients decrease significantly with height below 2 km. The extinction coefficients below 1 km, the aerosol optical depth (AOD), and the PBLH increase with the aggravation of pollution. The daily average aerosol extinction coefficient near the surface is about 0.78 km−1, the AOD is 0.87 ± 0.33 m (mean ± std, the same below), and the PBLH is 827 ± 132 m in polluted conditions (PM2.5 > 115 μg m−3). With the increase in pollution and the decrease in PBLH, more aerosols are accumulated at a lower height. The daily average ratios of AOD below PBLH (AODPBLH) to total AOD (AODtot) are about 64% in polluted conditions. According to the CALIPSO‐observed aerosol vertical profile, aerosols are identified for about 20.54% of the vertical space below 6 km altitude under polluted conditions. The spherical fine particles with lower particulate depolarization ratio (PDR, 79% of the PDR is <0.2), lower color ratio (CR, 64% of the CR is <0.75), and stronger scattering ability are dominant in 0∼2 km. Key Points: With the increase in pollution and the decrease in planetary boundary layer height, more aerosols are accumulated at the lower heightThe aerosol extinction coefficients decrease significantly with height below 2 km, and aerosol scattering ability is strong in 0∼2 kmThe regular fine particles with lower particulate depolarization ratio (PDR) (79% of the PDR is <0.2), lower lower color ratio (CR) (64% of the CR is <0.75) are dominant in 0∼2 km [ABSTRACT FROM AUTHOR]

Long‐range transport and vertical distribution of aerosols are important factors for assessing the uncertainty in aerosol radiative forcing. This paper reveals the vertical distribution and trends of aerosol optical properties in China using 15 years of Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) data. The Hybrid Single‐Particle Lagrangian integrated trajectory model was used to analyze the transport and trends of aerosols in the layer with the highest occurrence frequencies of dust, polluted dust, polluted continental and elevated smoke aerosols. The results indicated that (a) there were significant regional and seasonal differences in the vertical distribution of aerosols. The aerosol optical depth (AOD) trend in a given region depends on the changes in the aerosol type with the highest frequency and the layer corresponding to the largest AOD in the vertical profile. The frequency of polluted dust aerosols was the highest in Beijing‐Tianjin‐Hebei (BTH) and Central China. The considerable decrease in the 0–2 km AOD led to a significant trend of the column AOD. (b) The changes of AOD and main aerosol types in a region are also affected by the changes of aerosol sources and long‐range transport pathways. In the BTH, dust aerosols originated from the Mongolian Plateau, accounting for 57.88% of the total trajectories. The Pearl River Delta was dominated by elevated smoke aerosols, with trajectories mainly originating from the Myanmar and Vietnam, accounting for 27.38% and 29.59%, respectively. The trend of 15‐year backward trajectories of dust aerosols on the Tibetan Plateau indicated that the trajectory from India is increasing. Plain Language Summary: Aerosols are one of the main factors affecting global climate change, influencing global radiation through direct and indirect effects. Long‐range transport aerosols and their vertical distribution are important factors for assessing the uncertainty in aerosol radiative forcing. At present, there is limited research on the vertical distribution and transport pathways of different types of aerosols in multiple regions over China. Considering that aerosols in different regions of China exhibit large vertical distribution and transport differences, we revealed the vertical distribution and trends of aerosol optical properties in China using 15 years of Cloud‐Aerosol Lidar with Orthogonal Polarization data. The Hybrid Single‐Particle Lagrangian Integrated Trajectory model was used to analyze the transport and trends of different aerosols. We found significant regional and seasonal differences in the vertical distributions of aerosols. The changes in the aerosol optical depth and main aerosol types in a given region were also significantly affected by the changes in aerosol transport sources and transport pathways. This study provides comprehensive insight into the vertical distribution and transport characteristics of different types of atmospheric aerosols in the Chinese region over the last 15 years, which is very important to further evaluation of the impact of different aerosol types on the radiative effects and the environment. Key Points: The vertical distribution of aerosol optical properties exhibits notable seasonal and regional differences using 15 years of Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) dataThe trend of total aerosol optical depth (AOD) mainly depends on the joint changes of the highest frequency aerosol and the maximum AOD layer in a vertical profileThe changes of AOD and main aerosol types in a region are also affected by the changes of aerosol transport sources and transport pathways [ABSTRACT FROM AUTHOR]

The method to derive aerosol size distributions from in situ stratospheric measurements from the University of Wyoming is modified to include an explicit counting efficiency function (CEF) to describe the channel‐dependent instrument counting efficiency. This is motivated by Kovilakam and Deshler's (2015, https://doi.org/10.1002/2015JD023303) discovery of an error in the calibration method applied to the optical particle counter (OPC40) developed in the late 1980s and used from 1991 to 2012. The method can be applied to other optical aerosol instruments for which counting efficiencies have been measured. The CEF employed is the integral of the Gaussian distribution representing the instrument response at any one aerosol channel, the aerosol counting efficiency. Results using the CEF are compared to previous derivations of aerosol size distributions (Deshler et al., 2003, https://doi.org/10.1029/2002JD002514) applied to the measurements before and after Kovilakam and Deshler's correction of number concentration for the OPC40 calibration error. The CEF method is found, without any tuning parameter, to reproduce or improve upon the Kovilakam and Deshler's results, thus accounting for the calibration error without any external comparisons other than the laboratory determined counting efficiency at each aerosol channel. Moments of the new aerosol size distributions compare well with aerosol extinctions measured by Stratospheric Aerosol and Gas Experiment II and Halogen Occultation Experiment in the volcanic period 1991–1996, generally within ±40%, the precision of OPC40 moments, and in the nonvolcanic period after 1996, generally within ±20%. Stratospheric Aerosol and Gas Experiment II and Halogen Occultation Experiment estimates of aerosol surface area are generally in agreement with those derived using the new CEF method. Plain Language Summary: Since 1971, balloon‐borne instruments have been used to measure the size distribution of sub‐micrometer‐sized particles in the stratosphere. These particles, or aerosol, are important for two reasons: (1) They cool the planet by scattering sunlight back to space and (2) they facilitate chemical reactions, which destroy ozone. Ozone shields the Earth from harmful ultraviolet rays from the Sun. Stratospheric aerosol have also been measured from satellites since the 1980s using instruments which measure aerosol extinction, the extent to which sunlight is scattered or absorbed by stratospheric particles. The two measurement methods, satellite‐ and balloon‐borne instruments, have had a long‐standing discrepancy between satellite extinction measurements and balloon‐borne measurements used to calculate extinction. This paper resolves that discrepancy through laboratory investigations of the counting efficiency of the balloon‐borne instruments. When counting efficiency is included in the method to derive aerosol size distribution functions from the balloon‐borne measurements, from which extinction and aerosol surface area can be calculated, the two measurement methods are in reasonable agreement. Resolving this discrepancy between the two instruments allows the local balloon‐borne measurements to be used for testing the retrieval of aerosol surface area from the global satellite measurements. Stratospheric aerosol surface area is required for global climate models. Thus, this paper provides a basis to characterize the confidence, which can be placed in the global satellite estimates of aerosol surface area for the modeling community. Key Points: Aerosol size distribution retrievals from Wyoming in situ data are modified to account for laboratory counting efficiency measurementsAerosol extinction estimates from the new size distribution retrievals are in good agreement with both SAGE II and HALOE measurementsAerosol surface area densities from SAGE II v7.0 agree with the new in situ estimates, which have increased over previous estimates [ABSTRACT FROM AUTHOR]

Biomass burning releases a vast amount of aerosols into the atmosphere, often leading to severe air quality and health problems. Prediction of the air quality effects from biomass burning emissions is challenging due to uncertainties in fire emission, plume rise calculation, and other model inputs/processes. Ensemble forecasting is increasingly used to represent model uncertainties. In this paper, an ensemble forecast was conducted to predict surface PM2.5 during the 2018 California Camp Fire event using the National Oceanic and Atmospheric Administration (NOAA) HYSPLIT dispersion model at 0.1° horizontal resolution. Different combinations of four satellite‐based fire emission data sets (FEER, FLAMBE, GBBEPx, and GFAS), two plume rise schemes (Briggs and Sofiev), various meteorology inputs, and model setup options were used to create the forecast ensemble, for a total of 112 experiments. The performance of each ensemble member and the ensemble mean were evaluated using ground‐based observations, with four statistical metrics and an overall rank. The ensemble spread of the 112 members reached 1,000 μg/m3, highlighting the large uncertainty in wildfire forecast. The ensemble mean displayed the best performance. Each fire emission product contributed to one or more members among the top 10 performers, revealing the forecasting dependence on both the quality of fire emissions data and model representation of emission, transport, and removal processes. In addition, an ensemble size reduction technique was introduced. With the help of this technique, the ensemble size was reduced from 112 to 28 members and still produced an ensemble mean that yielded comparable or even better performance to that of the full ensemble. Key Points: The difference between satellite fire emission data sets is large, which reaches a factor of 10Ensemble forecast with different emission data sets, meteorology, and plume rise schemes can reduce forecast uncertaintyThe new ensemble size reduction technique can save calculation time while improving the performance of the ensemble forecast [ABSTRACT FROM AUTHOR]

Aerosol‐cloud interactions remain a major source of uncertainty in our understanding and modeling of the Earth's hydrological cycle. Based upon a diagnostic and modeling analysis utilizing the latest field measurements from the Integrative Monsoon Frontal Rainfall Experiment (IMFRE) 2018, this paper reports the effects of aerosols on the cloud properties along the Mei‐yu front over the Middle Reaches of Yangtze River in China. Numerical experiments with the Weather Research and Forecasting (WRF) model suggest that increasing aerosol number concentration reduces surface precipitation by ~8.8% and delays the onset of rainfall by ~30 min. Furthermore, warm clouds are suppressed but the convective cores are slightly intensified. This corresponds to an overall aerosol effect of "localization and invigoration" of the Mei‐yu rainfall and thus an elevated probability of short‐term heavy rainfall. The signals of "convective invigoration" with a bulk scheme in this study are relatively weak compared to those simulated by bin microphysics. The increased aerosol concentration strengthens Mei‐yu front and changes local morphology of the front, consistent with earlier studies demonstrating positive effects of convective heating on the genesis and maintenance of Mei‐yu front via conditional instability of the second kind (CISK) and diabatic generation of potential vorticity. Also discussed are the uncertainties of bulk microphysics in simulating aerosol‐cloud interactions, which may shed light on the design of future field campaigns to further understand the impact of aerosol‐cloud interactions on weather and climate over China in boreal summer. Key Points: Increased aerosol concentration suppresses warm clouds and slightly strengthens convection,leading to an overall effect of "localization and invigoration" of the Mei‐yu rainfallEnhanced latent heating in polluted conditions intensifies updrafts of convective coresIncreased aerosol concentration strengthens the Mei‐yu front and changes the local morphology of the front [ABSTRACT FROM AUTHOR]

The Atlantic multidecadal variability (AMV) acts as a key source of multidecadal variability and plays an important role in climate change in many regions of the Northern Hemisphere, resulting in great social and economic impacts. This paper shows that, since the late 19th century, the AMV has experienced a significant increasing trend. The AMV intensified at a rate of 0.0040°C per decade from 1870 to 2014, which is statistically significant at the 95% confidence level. The AMV teleconnects the amplified signal from the North Atlantic across the Eurasian continent to North America through a circumglobal teleconnection pattern, which further intensifies the regional climate variability and climate extremes in North Africa, Europe, East Asia, and North America. Results from the single‐forcing and all‐but‐one‐forcing experiments of the Community Earth System Model, version 1, indicate that anthropogenic aerosol emissions have played a more important role in the AMV intensification compared to other external forcings, especially during the period from the 1900s to 1990s. Key Points: The AMV has experienced a significant increasing trendThe intensified AMV has amplified the regional climate variability and climate extremes in the Northern HemisphereAnthropogenic aerosol emissions have played an important role in the AMV intensification [ABSTRACT FROM AUTHOR]

Decadal‐scale trends in aerosol and cloud properties provide important ways for understanding aerosol‐cloud interactions. In this paper, by using MODIS products (2003–2017), we analyze synergetic trends in aerosol properties and warm cloud properties over global ocean. Cloud droplet number concentration (CDNC) and aerosol parameters (aerosol optical depth, angstrom exponent, and aerosol index) show consistent decreasing trend over East Coast of the United States (EUS), west coast of Europe (WEU), and east coast of China (EC), and no significant trend in liquid water path is found over these regions during the period 2003–2017. Over regions with significant long‐term trends of aerosol loading and CDNC (e.g., EUS and WEU), the sensitivity of CDNC to aerosol loading based on the long‐term trend is closer to those derived from ground and aircraft observations and larger than those derived from instantaneous satellite observations, providing an alternative way for quantifying aerosol‐cloud interactions. A clear shift in the normalized probability density function of CDNC between the first 5 years (2003–2007) and the last 5 years (2013–2017) is found, with a decrease of around 50% in the occurrence frequency of high CDNC (>400 cm−3) over EUS and WEU. The relative variances of cloud droplet effective radius generally decrease with decreasing aerosol loading, providing large‐scale evidence for the effects of anthropogenic aerosols on the dispersion of cloud droplet size distribution. The long‐term satellite data sets provide great opportunities for quantifying aerosol‐cloud interactions and further confronting these interactions in climate models in the future. Key Points: Three major pollutant outflow regions show consistent decrease of CDNC, AOD, angstrom exponent, and aerosol indexThe sensitivity of CDNC to aerosol loading based on the long‐term trend is closer to those derived from ground and aircraft observationsThe relative variances of cloud droplet effective radius generally decrease with decreasing aerosol loading [ABSTRACT FROM AUTHOR]

Decades of atmospheric research have focused on the Western North Atlantic Ocean (WNAO) region because of its unique location that offers accessibility for airborne and ship measurements, gradients in important atmospheric parameters, and a range of meteorological regimes leading to diverse conditions that are poorly understood. This work reviews these scientific investigations for the WNAO region, including the East Coast of North America and the island of Bermuda. Over 50 field campaigns and long‐term monitoring programs, in addition to 715 peer‐reviewed publications between 1946 and 2019, have provided a firm foundation of knowledge for these areas. Of particular importance in this region has been extensive work at the island of Bermuda that is host to important time series records of oceanic and atmospheric variables. Our review categorizes WNAO atmospheric research into eight major categories, with some studies fitting into multiple categories (relative %): aerosols (25%); gases (24%); development/validation of techniques, models, and retrievals (18%); meteorology and transport (9%); air‐sea interactions (8%); clouds/storms (8%); atmospheric deposition (7%); and aerosol‐cloud interactions (2%). Recommendations for future research are provided in the categories highlighted above. Key Points: A total of 50+ field studies and 700+ papers illustrate the complexity of atmospheric phenomena over the West North Atlantic and North American East CoastThe widest body of work has been devoted to atmospheric chemistry and has characterized urban outflow and marine emissionsMultidisciplinary topics such as aerosol‐cloud and air‐sea interactions have not been sufficiently addressed and warrant high priority [ABSTRACT FROM AUTHOR]

This paper reports an exceptional biomass burning (BB) advection event from Alaska registered at Ny‐Ålesund from 10 to 17 July 2015 with particular interest on the influence of the airborne particle characteristics on the optical properties of the aerosol during the event. To this purpose we considered two DEKATI 12‐stage aerosol samples spanning the entire advection and analyzed them by scanning electron microscopy techniques. Aerosol chemical data and microphysical properties were also evaluated in order to correlate any change of individual particle characteristics with the bulk properties of the aerosol. The results of individual particle analysis depict a complex event characterized by a first phase (P1) of massive input of BB carbonaceous particles (i.e., tar balls, popcorn refractory particles, and organic particles), and by a second phase (P2) dominated by inorganic salts. The peculiar feature of this BB event is the exceptionally large grain size of the subspherical organic particles at the beginning of the event with respect to the background. At these conditions a significant increase of the scattering efficiency may occur even for a small increase of the size parameter. Results of the simulation of the complex refractive indices (n‐ik) confirm this evaluation. Aerosol evolution during the event resulted from the combination of three distinct occurrences: (a) progressive rotation of air mass circulation toward non‐BB source areas, (b) development of a thick fog layer in the planetary boundary layer, and (c) sea salt spray direct advection of local/regional provenance. Key Points: The case study is an exceptional biomass burning advection event from Alaska registered in the Arctic in July 2015Results of analyses point at two distinct phases of the event dominated by organic carbon particles and inorganic salts, respectivelyDistinct aging effects involving organic carbon and inorganic salt particles at different relative humidity conditions are hypothesized [ABSTRACT FROM AUTHOR]

The Deep Blue (DB) algorithm has been used to retrieve aerosol optical depth (AOD) and Ångström exponent (AE) over land from multiple satellite instruments, including the Moderate Resolution Imaging Spectroradiometers (MODIS) aboard the Terra and Aqua platforms and the Visible Infrared Imaging Radiometer Suite (VIIRS). This study first validates the latest MODIS (Collection 6.1) and VIIRS (Version 1) DB data products against Aerosol Robotic Network observations. On global average, the typical level of uncertainty in AOD is slightly better than ±(0.05 + 20%) relative to Aerosol Robotic Network. AE is quantitatively more uncertain but qualitatively shows skill at distinguishing between fine‐mode and coarse‐mode dominated aerosol columns. Results are also compared with the previous MODIS Collection 6. The stability of the three DB data sets ranges from 0.005–0.01 AOD per decade. Second, spatial and temporal patterns in AOD and AE are compared between the three data sets. It is found that they all show similar patterns of spatial coverage, which is predominantly linked to cloud cover, snow, and polar night. Regional time series of AOD also show highly consistent seasonal and interannual variations and are strongly correlated, although have offsets in some regions due to a combination of algorithmic and sensor‐related differences. Plain Language Summary: Aerosols are small particles in the atmosphere like desert dust, volcanic ash, smoke, industrial haze, and sea spray. Understanding them is important for applications such as hazard avoidance, air quality and human health, and climate studies. Satellite instruments provide an important tool to study aerosol loading over the world. However, individual satellites do not last forever, and newer satellites often have improved capabilities compared to older ones. This paper evaluates the latest version of the Deep Blue algorithm for monitoring aerosols as applied to the Moderate Resolution Imaging Spectroradiometers (MODIS) and Visible Infrared Imaging Radiometer Suite (VIIRS) satellite instruments. The two MODIS sensors provide data from 2000 and 2002 onward, while the first VIIRS was launched in late 2011, and VIIRS will carry on the MODIS data records into the future. The evaluation is performed by comparing to ground‐truth data which are part of (National Aeronautics and Space Administration) NASA's global Aerosol Robotic Network. The stability in time and consistency between the MODIS and VIIRS data sets are also examined. Key Points: VIIRS and MODIS Deep Blue show very similar validation results against AERONETDecadal stability in retrieved AOD is about 0.01 per decade or betterThe data sets show consistent seasonal and interannual variations in regional AOD [ABSTRACT FROM AUTHOR]

This paper presents the results of long‐term observations (1964–2010) of direct solar flux at two stations in the Tatra Mountains in Poland (Central Europe). The measurements obtained from a Linke‐Feussner actinometers are used to estimate broadband (0.29–2.9 μm) and wideband (0.53–0.63 μm) aerosol optical depth (BAOD and WAOD, respectively). Both BAOD and WAOD at Mount Kasprowy Wierch (1,991 m a.s.l.) and in Zakopane (851 m a.s.l.) show similar annual cycles with peaks during spring and summer. Coincidence of actinometer observation at the top of the Tatra Mountains and in Zakopane valley allows determination of AOD in the column between 851 and 1,991 m a.s.l. Annual variabilities of both BAOD and WAOD for this layer are similar to PM10 concentration measurements in Zakopane. Conversely, BAOD and WAOD obtained at both stations show opposing annual variability, which is consistent with the Aerosol Robotic Network and model results in Central Europe. Long‐term data show significant reduction of both BAOD and WAOD in Zakopane (−0.018 ± 0.003 and −0.023 ± 0.003, respectively, per decade) and lower reductions at Mt. Kasprowy Wierch (−0.005 ± 0.002 and −0.009 ± 0.002, respectively, per decade). Trends for BAOD and WAOD defined between 851 and 1,991 m a.s.l. are the same (−0.013 ± 0.004 per decade), which corresponds to the reduction of WAOD and BAOD by about 60% between 1964 and 2010. Conversely, emission data for the Zakopane region obtained from Emissions Database for Global Atmospheric Research inventories shows reduction of SO2 by a factor of 3 and of NOx and PM10 by about 40% between 1988 and 2010. Key Points: Annual cycle AOD as well as long‐range trend varies with altitude in the Tatra MountainsAOD long‐range negative trend is significantly higher in the lower station than at the mountain top indicating reduction of human emissionAOD defined between valley and mountain top shows positive correlation with monthly means PM10 concentration [ABSTRACT FROM AUTHOR]

This is a reply to the comment of Peng and Bi (2019) on the paper of Li et al. (2018, https://doi.org/10.1029/2018JD028956), which introduced a method to estimate aerosol effective density (ρe) based on a joint use of measurements of the scanning mobility particle sizer (SMPS) and the aerodynamic particle sizer (APS). In this reply, we describe the extension of the Cunningham slip correction factor (Cc) to the expression of the ρe and analyzed the useful but incomplete expression of ρe equation on Cc in Peng and Bi (2019) resulting from neglecting aerodynamic diameter. Meanwhile, in order to estimate Cc‐related error on ρe, Peng and Bi (2019) used an unreasonable scheme and an overestimated shape factor, which leads to a large overestimation (55.5%) of the error. We modify the error budget scheme and perform a theoretical error analysis on neglecting Cc during the ρe measurement in practice. The results suggest an average bias of 17.68% on ρe resulting from Cc. Together with other error sources presented in Li et al. (2018, https://doi.org/10.1029/2018JD028956), the typical uncertainty on ρe based on this "SMPS‐APS" joint‐use method is 21.06%, which is still reasonable in practice. Key Points: Reply to the comment of Peng & Bi (2019) on the paper of Li et al. (2018) published in JGR‐Atmospheres. [ABSTRACT FROM AUTHOR]

The impacts of aerosol and thermodynamics on electrification and lightning activities have been investigated in detail using the Weather Research and Forecasting Model coupled with a double‐moment microphysics parameterization and an explicit electrification lightning scheme. To obtain a varied combination of convective available potential energy (CAPE) values and aerosol concentrations, a sounding was modified consistently and initiated with five sets of aerosol concentrations that served as cloud condensation nuclei. The simulated electric processes respond to the varying dynamical and microphysical characteristics associated with the different CAPE and aerosol conditions. Under high CAPE circumstances, the augmentation of ice‐phase particle leads to the enhancement of non‐inductive charging primarily through the dynamic processes. Increased aerosol content further invigorates the electrification through microphysical processes. Elevated aerosol loading under low CAPE conditions increases cloud droplet and ice crystal numbers. Larger graupel particle size further leads to the enhanced electric intensity and lightning discharges. Plain Language Summary: A weather forecast model with an embedded lightning scheme was used to investigate the effects of aerosol and atmospheric conditions on electrification and lightning activity. Varied combinations of moisture and temperature profiles and aerosol concentration were considered in the simulations. The results showed that the electrification and lightning activity vary under different meteorological and aerosol conditions. Under high convective available potential energy (CAPE) conditions, the increased ice‐phase particle content leads to the enhancement of rebounding ice collisions mainly through the dynamic processes. Elevated aerosol loading under lower CAPE conditions invigorates lightning activity due to larger graupel particles participating in the thunderstorm electrification. Key Points: High convective available potential energy enhances non‐inductive charging due to augmentation of ice‐phase particle through dynamic processElevated aerosol under low atmospheric instability conditions enhances lightning due to larger graupel participating in electrificationAn anomalous charge structure appears under polluted conditions, owing to positively charged graupel [ABSTRACT FROM AUTHOR]

Desert dust accounts for a substantial fraction of the total atmospheric aerosol loading. It produces important impacts on the Earth system due to its nutrient content and interactions with radiation and clouds. However, current climate models greatly underestimate its airborne lifetime and transport. For instance, super coarse Saharan dust particles (with diameters greater than 10 µm) have repeatedly been detected in the Americas, but models fail to reproduce their transatlantic transport. In this study, we investigated the extent to which vertical turbulent mixing in the Saharan Air Layer (SAL) is capable of delaying particle deposition. We developed a theory based on the solution to a one‐dimensional dust mass balance and validated our results using large‐eddy simulation (LES) of a turbulent shear layer. We found that eddy motion can increase the lifetime of suspended particles by up to a factor of 2 when compared with laminar flows. Moreover, we found that the increase in a lifetime can be reliably estimated solely as a function of the particle Peclet number (the ratio of the mixing timescale to the settling timescale). By considering both the effects of turbulent mixing and dust asphericity, we explained to a large extent the presence of super coarse Saharan dust in the Caribbean observed during the Saharan Aerosol Long‐Range Transport and Aerosol‐Cloud‐Interaction Experiment (SALTRACE) field campaign. The theory for the lifetime of coarse particles in turbulent flows developed in this study is also expected to be applicable in other similar geophysical problems, such as phytoplankton sinking in the ocean mixed layer. Plain Language Summary: Dust emitted from desert surfaces on Earth impacts climate and weather by affecting cloud formation and interacting with solar and terrestrial radiation. It also carries important nutrients that feed land and ocean ecosystems. Therefore, it is important to accurately predict for how long these particles remain suspended in the atmosphere. However, current models underestimate this time and the associated horizontal transport. For example, it is still not well understood how super coarse dust particles (which deposit faster than smaller particles) are able to move from the Sahara desert to the Americas. Our study tested the hypothesis that vertical mixing caused by turbulence in the upper atmosphere (above about 2 km from the ground) is one of the mechanisms responsible for that phenomenon. Using the principle of mass conservation, as well as computer simulations, we found that turbulence can even double the period of time that dust remains suspended in the atmosphere. When also factoring in the irregular shape of dust particles, which decreases their deposition speeds, our theory was able to explain to a great extent observations of super coarse Saharan particles in the Caribbean. This suggests that turbulence is a key mechanism by which dust gets transported over long distances. Key Points: Turbulent mixing is capable of at most doubling the atmospheric lifetime of coarse aerosols in elevated layersAirborne lifetime of particles depends only on the relative strength of gravitational settling and turbulent mixingDust asphericity and free tropospheric mixing help explain the transport of super coarse Saharan dust to the Caribbean [ABSTRACT FROM AUTHOR]

Pyrocumulonimbus (PyroCb) clouds have a complex origin dependent on fire dynamics and meteorological conditions. When a pyrocumulonimbus cloud develops and is maintained over a period of time, it can inject significant aerosol into the troposphere and lower stratosphere, resulting in a longer‐term (months to years) occurrence of aerosol in the stratosphere. In this work, we investigate the British Columbia wildfires on 12–13 August 2017 using a multi‐scale simulation framework. We use the output of a physics‐based wildfire model (FIRETEC) with parameterized energy, particle, and gas emissions to drive the upper atmospheric aerosol mass injection within a regional cloud resolving model (HIGRAD). We demonstrate that vertical motions produced by latent heat release of the condensation of ice and cloud particles within the PyroCbs induce another 5 km of lifting of the simulated aerosol plume. Primary black carbon and organic aerosols (OAs) alone may not be enough to explain the observed aerosol burden, thus we show that secondary OA produced via condensation of gases by the fires, ash, and possibly dust can enhance lofted aerosol mass. A simulation with all emission mechanisms active, driven by the observed fuel load and environmental conditions, reasonably reproduces an aerosol profile inferred from observational data. Plain Language Summary: The British Columbia 2017 summer fires injected significant amounts of aerosol into the upper troposphere and lower stratosphere. This modeling study is an attempt to represent and understand the complex combustion, aerosol, and cloud physics behind this injection. This event was partly characterized by a standard fuel type, forest, and a non‐standard fuel type, slash piles, associated with bark beetle kill and subsequent forest management practices. The burning of these two fuel types were undertaken in detailed combustion simulations that then provided aerosol and heat sources for large‐scale pyrocumulonimbus simulations. The large‐scale simulations illustrate the complex relationship between the burning fuel types and latent heat associated with cloud formation with this energy release mechanism lifting the aerosol plume to observed heights. Additionally, to represent observed aerosol amounts other aerosols such as secondary organic aerosol from condensation of gases produced by the fires, ash, and dust were included in the large‐scale simulations. Key Points: Formation of PyroCb clouds are multi‐faceted and complexPyroCbs have the potential to eject large amounts of aerosol to various layers of the atmosphereSimulating PyroCb formations requires high‐fidelity tools covering multi‐scale physics [ABSTRACT FROM AUTHOR]

The third generation of the radiative flux profile data product, called ISCCP‐FH, is described. The revisions over the previous generation (called ISCCP‐FD) include improvements in the radiative model representation of gaseous and aerosol effects, as well as a refined statistical model of cloud vertical layer variations with cloud types, and increased spatial resolution. The new product benefits from the changes in the new H‐version of the ISCCP cloud products (called ISCCP‐H): higher spatial resolution, revised radiance calibration and treatment of ice clouds, treatment of aerosol effects, and revision of all the ancillary atmosphere and surface property products. The ISCCP‐FH product is evaluated against more direct measurements from the Clouds and the Earth's Radiant Energy System and the Baseline Surface Radiation Network products, showing some small, overall reductions in average flux uncertainties; but the main results are similar to ISCCP‐FD: the ISCCP‐FH uncertainties remain ≲10 Wm−2 at the top‐of‐atmosphere (TOA) and ≲15 Wm−2 at surface for monthly, regional averages. The long‐term variations of TOA, surface and in‐atmosphere net fluxes are documented and the possible transient cloud feedback implications of a long‐term change of clouds are investigated. The cloud and flux variations from 1998 to 2012 suggest a positive cloud‐radiative feedback on the oceanic circulation and a negative feedback on the atmospheric circulation. This example demonstrates that the ISCCP‐FH product can provide useful diagnostic information about weather‐to‐interannual scale variations of radiation induced by changes in cloudiness as well as atmospheric and surface properties. Plain Language Summary: The article describes the updated version of the International Satellite Cloud Climatology Project (ISCCP) radiative profile flux product, ISCCP‐FH. This version has several important improvements over its previous two versions in its radiation model and input data sets of clouds, aerosol and other atmospheric and surface physical properties as well as ancillary data sets. Its spatial resolution is increased to 110 km. It now has uncertainties ≲10 Wm−2 at the top‐of‐atmosphere (TOA) and ≲15 Wm−2 at surface for monthly, regional averages based on validations against the direct observations at TOA and surface, slightly improved over the previous versions. We also describe long‐term variations of the radiative energy intensity based on the product and give an example to study cloud‐radiation feedback using this product. We expect the new product to be used in climate studies like its previous versions. Key Points: The radiative flux profile data product (called ISCCP‐FH) is described. It benefits from the new ISCCP cloud products (called ISCCP‐H)The product is evaluated against the Clouds and the Earth's Radiant Energy System and the Baseline Surface Radiation Network measurementsThe long‐term variations of top‐of‐atmosphere, surface and in‐atmosphere net fluxes are documented and a possible cloud feedback is investigated [ABSTRACT FROM AUTHOR]

Predicting cirrus cloud properties with confidence requires a sound understanding of the relative roles of homogeneous and heterogeneous ice formation. This study explores the effect of mineral dust and contrail‐processed aviation soot particles as ice‐nucleating particles (INPs) competing with liquid solution droplets in cirrus formation. We study aerosol‐cirrus interactions by accounting for atmospheric variability in updraft speeds and INP number concentrations. Our results confirm the dominant role of mineral dust in ice nucleation events in cirrus clouds. In addition, we show that pre‐existing thin cirrus may suppress ice formation when updraft speeds are low. We find that homogeneous freezing of liquid solution droplets dominates clear‐sky aerosol‐cirrus interaction above a threshold updraft speed determined by total number concentrations and ice nucleation abilities of INPs. When mineral dust particles exceed number concentrations of 10 L−1, they reduce homogeneously nucleated ice crystal numbers significantly and even prevent homogeneous freezing for frequently observed local updraft speeds between 10 and 20 cm s−1. When both mineral dust and aviation soot particles coexist with solution droplets, dust typically prevents ice nucleation by aviation soot. Aviation soot exerts a notable impact on cirrus ice numbers only if updrafts are weak, large soot particles are present in number concentrations that are considerably higher than typically observed in emission measurements, and/or number concentrations of mineral dust and other INPs are low. Overall, our results elucidate the role of aviation soot‐cirrus interactions in the presence of other INP types. Plain Language Summary: Understanding ice formation from atmospheric aerosol particles in cirrus clouds is important to correctly predict the impact of cirrus on climate. Here, we study for the first time on the process level cirrus ice formation from freezing of liquid solution droplets in competition with ice nucleation on mineral dust and aviation‐produced soot particles. Our results show that in the majority of cloud‐forming updrafts, mineral dust particles dominate cirrus formation over droplet freezing and that they outcompete aviation soot in producing cloud ice. Moreover, already existing cirrus clouds often prevent ice formation from these particle types altogether. Our results help evaluate cirrus parameterizations within global models and constrain model‐based estimates of the global climate impact of aviation. Key Points: Mineral dust outcompetes contrail‐processed aviation soot in aerosol‐cirrus interactionsMineral dust reduces homogeneously nucleated ice crystal numbers more effectively than aviation sootStrong potential of pre‐existing cirrus clouds to suppress new cloud ice formation [ABSTRACT FROM AUTHOR]

Humic‐like substances (HULIS) are unresolved compounds isolated from organic matter in atmospheric aerosols. Understanding the chemical structures and sources that control the hygroscopicity of HULIS is important for predicting its environmental impacts and possibly useful for parameterization of the hygroscopicity of organic aerosol (OA). Here, the hygroscopic growth of HULIS from total suspended particles samples in Beijing was analyzed using a hygroscopicity tandem differential mobility analyzer. The hygroscopicity parameter of HULIS (κHULIS) was determined to be 0.06 ± 0.03, and was found to correlate positively with O:C and f44 from offline aerosol mass spectrometry. Additionally, κHULIS was correlated positively with oxygenated organic aerosol (OOA) and correlated negatively with cooking OA and biomass burning OA, for which the source‐related OA types were from mass‐spectral‐based positive matrix factorization analyses. Further analysis resolved substantial κ values of 0.09 and 0.22, respectively, for less‐oxidized and more‐oxidized OOA, in contrast to the low κ values for cooking OA and biomass burning OA. Extensions of O:C‐ and source‐based parameterizations reasonably estimated the hygroscopicity of total OA with a moderate contribution from HULIS. This result and the fact that the obtained OOA contributions to κ of OA were similar to those from different environments suggest the applicability of parameterizing the hygroscopicity for a wide range of OA in particular based on source‐related structural characteristics. Plain Language Summary: Humic‐like substances (HULIS) are ubiquitous components of atmospheric organic aerosol (OA). Understanding and parameterizing the hygroscopicity of HULIS is expected to facilitate predictions of aerosol budgets and related environmental impacts. In this study, the hygroscopicity parameters of HULIS were determined and found to be positively correlated with the oxygen‐to‐carbon elemental ratio (O:C). Furthermore, contributions from the sources of HULIS to their hygroscopicity were resolved, and a significant influence of oxygenated OA mass on the hygroscopicity of HULIS was revealed. The parameterization of hygroscopicity based on O:C or source information reconstructed the hygroscopicity of HULIS well, particularly for source‐based approach. These two parameterization approaches reasonably estimated the hygroscopicity of total OA in the studied aerosol, and the contributions from oxygenated OA masses were consistent with those in other environments, both of which suggest wide applicability of this parameterization method for OA fractions in addition to total OA. Key Points: The hygroscopicity parameter (κ) of Humic‐like substances (HULIS) was 0.06 ± 0.03, and it correlated positively with O:C and f44Organic mass assigned to the oxygenated organic aerosol (OA) controls the hygroscopicity of HULISSource‐based parameterization of the κ of OAs is applicable for different ranges of OA components [ABSTRACT FROM AUTHOR]

Ångström exponents (α) allow reconstruction of aerosol optical spectra over a broad range of wavelengths from measurements at two or more wavelengths. Hyperspectral measurements of atmospheric aerosols provide opportunities to probe measured spectra for information inaccessible from only a few wavelengths. Four sets of hyperspectral in situ aerosol optical coefficients (aerosol‐phase total extinction, σext, and absorption, σabs; liquid‐phase soluble absorption from methanol, σMeOH‐abs, and water, σDI‐abs, extracts) were measured from biomass burning aerosols (BBAs). Hyperspectral single scattering albedo (ω), calculated from σext and σabs, provide spectral resolution over a wide spectral range rare for this optical parameter. Observed spectral shifts between σabs and σMeOH‐abs/σDI‐abs argue in favor of measuring σabs rather than reconstructing it from liquid extracts. Logarithmically transformed spectra exhibited curvature better fit by second‐order polynomials than linear α. Mapping second order fit coefficients (a1, a2) revealed samples from a given fire tended to cluster together, that is, aerosol spectra from a given fire were similar to each other and somewhat distinct from others. Separation in (a1, a2) space for spectra with the same α suggest additional information in second‐order parameterization absent from the linear fit. Spectral features found in the fit residuals indicate more information in the measured spectra than captured by the fits. Above‐detection σMeOH‐abs at 0.7 μm suggests assuming all absorption at long visible wavelengths is BC to partition absorption between BC and brown carbon (BrC) overestimates BC and underestimates BrC across the spectral range. Hyperspectral measurements may eventually discriminate BBA among fires in different ecosystems under variable conditions. Plain Language Summary: New measurement techniques for atmospheric aerosols are becoming available to obtain high spectral resolution optical information over a broad range of ultraviolet (UV)–visible–infrared wavelengths. Here, we measured UV‐visible spectra from aerosols emitted by five wildfires that exhibited curvature not adequately represented by a widely used single mathematical parameter known as the Ångström exponent. The observed spectral curvature differed across the five wildfires. We argue that the differences in absorption spectral curvature, in particular, were driven by different molecular composition of the smoke aerosols. We hypothesize that the observed curvature may eventually be used to optically differentiate wildfire aerosols and to better predict the optical and chemical evolution of wildfire smoke plumes downwind. We hope to encourage more routine measurements of this kind and to motivate development of improved measurement techniques and analytical approaches to maximize the information we can glean from such data. Key Points: Measured in situ atmospheric aerosol hyperspectral (<0.001 μm, resolution) optical properties in biomass burning plumes from 0.3 to 0.7 µmLog‐transformed spectra better fit by second‐order polynomials than Ångström exponents; measured spectra have details missed by fitsSpectral curvature in absorption driven by myriad organic chromophores over full wavelength range [ABSTRACT FROM AUTHOR]

A cruise observation of atmospheric aerosols and gaseous air/seawater compounds was performed in the western North Pacific during the spring pre‐bloom period. The aerosol size distribution and cloud condensation nuclei (CCN) activity were characterized according to the source type: terrestrial/pollution, marine, and mixed. The CCN activity was represented by the aerosol hygroscopicity parameter (ĸCCNC) from the activation diameter (dact) in water‐vapor supersaturation (SS), which is controlled by the composition of particles with sizes around dact. For marine type, whereas a high hygroscopicity parameter value of 0.86 for accumulation mode (ĸaccum) suggests a strong influence of sea salt and sulfate, a lower value of 0.49 in Aitken mode (ĸAit) suggests the contribution of biogenic organic matter (OM). Lower ĸAit and ĸaccum for terrestrial/pollution type (0.12 and 0.62, respectively) suggests greater contributions of OM. Under the influence of marine sources, ĸCCNC at a dact of ∼110 nm (low SS) was not responsive to the chlorophyll a (Chl‐a) concentration, whereas ĸCCNC at ∼50 nm (high SS) correlated negatively with Chl‐a. The hygroscopicity parameter of OM was estimated as 0.13–0.29 considering the aerosol composition. During a new particle formation event, the ĸCCNC at 0.60% SS increased; nucleated particles grew in size to become CCN. Sensitivity analysis indicated that changes in the size distribution and chemical composition of the aerosol contributed to the increase in CCN concentration. This study highlights the high variability in size distribution and CCN activity of off‐coast aerosols from different sources, including marine biological activities. Key Points: The size distribution and hygroscopicity parameter κ of aerosols were characterized by terrestrial and marine sourcesThe κ values of organic matter, presumably of marine origin, varied between 0.13 and 0.29New particle formation was a contributing factor to the high variability in the cloud condensation nuclei concentration [ABSTRACT FROM AUTHOR]

Previous studies on the impacts of severe haze episode on meteorology were generally focused on single episodes. This study aimed to obtain the common features of the influence of aerosol direct radiative effects on meteorology during severe particulate pollution events by composite analyses on model results from the Weather Research and Forecasting Model with online chemistry. Five heavily polluted events (15–17 January 2014, 13–15 January 2015, 8–10 December 2015, 21–23 December 2015, and 29–31 December 2016) in the North China Plain (NCP, 35.4–41.2°N, 113.3–119.3°E) were selected, which were representative of the most frequently observed weather patterns for severe haze. Model results showed that aerosols in the five heavily polluted events led to strong negative changes in radiative flux of 52.1–86.7 W m−2, reductions in the 2‐m temperature by 0.28–0.97°C, and the reductions in planetary boundary layer heights by 23.1–58.5 m, when averaged over the areas with PM2.5 concentrations >150 μg m−3. The magnitudes of aerosol induced changes in the 2‐m temperature were found to be influenced not only by aerosol concentration but also by wind speed, wind direction, and the convergence or divergence of winds. High wind speed diluted the aerosol‐induced cooling, the convergence of wind limited the temperature change to local area, and the intrusion of cold air mass near the surface enhanced the aerosol‐induced cooling. In all the cases, aerosols induced an anti‐cyclone at 500 hPa around NCP and anomalous northerlies or northeasterlies at the surface in the eastern NCP. Plain Language Summary: The impact of aerosol direct radiative effects (DRE) on meteorology are very important, but previous studies on such effects were generally focused on one pollution episode. In order to obtain the common features of the DRE of aerosols on meteorology, we applied the Weather Research and Forecasting‐Chemistry model to simulate five heavily polluted events belonging to the most frequently observed weather patterns for severe particulate pollution in the North China. We found that aerosols lead to the reductions in surface radiative flux, 2‐m temperature, and planetary boundary layer height. In addition, the magnitudes of aerosol‐induced decreases in 2‐m temperature are influenced by winds, including wind speed, wind direction, and the convergence or divergence of winds. High wind speed dilutes the aerosol‐induced cooling, the convergence of wind limits the temperature change to local area, and the intrusion of cold air mass near the surface enhances the aerosol‐induced cooling. Aerosols also can induce an anti‐cyclone at 500 hPa around North China Plain (NCP) and anomalous northerlies or northeasterlies at the surface in the eastern NCP. Key Points: Aerosols induced reductions in surface radiative flux, 2‐m temperature, and planetary boundary layer heightAerosol‐induced changes in surface air temperature were determined by aerosol concentrations and speed and direction of windsAerosols induced an anti‐cyclone at 500 hPa and anomalous northerlies or northeasterlies at the surface in the east of North China [ABSTRACT FROM AUTHOR]

This study validates aerosol properties retrieved using a Satellite Ocean Aerosol Retrieval (SOAR) algorithm applied to Visible Infrared Imaging Radiometer Suite (VIIRS) measurements, from Version 1 of the VIIRS Deep Blue data set. SOAR is the over‐water complement to the over‐land Deep Blue algorithm and has two processing paths: globally, 95% of pixels are processed with the full retrieval algorithm, while the 5% of pixels in shallow or turbid (mostly coastal) waters are processed with a backup algorithm. Aerosol Robotic Network (AERONET) data are used to validate and compare the midvisible (550 nm) aerosol optical depth (AOD), Ångström exponent (AE), and fine mode fraction of AOD at 550 nm (FMF). AOD uncertainty is shown to be approximately ±(0.03 + 10%) for the full and ±(0.03 + 15%) for the backup algorithms, with a small positive median bias around 0.02. When AOD is below about 0.2, the AE and FMF have small negative offsets from AERONET around −0.15 and −0.04, respectively. For higher AOD, AE is less offset and the magnitudes of differences versus AERONET are about ±0.2 and ±0.14, respectively. Aerosol‐type classifications provided by SOAR are found to be reasonable, matching optical‐based classifications from AERONET over 80% of the time. Spatial and temporal patterns of AOD and AE are also compared with those of other contemporary over‐water satellite aerosol data sets; dependent on region, the satellite data sets show varying levels of consistency, with SOAR broadly in‐family, and the largest discrepancies in regions with persistent heavy cloud cover. Plain Language Summary: Aerosols are small particles in the atmosphere like desert dust, volcanic ash, smoke, industrial haze, and sea spray. Understanding them is important for applications such as hazard avoidance, air quality and human health, and climate studies. Satellite instruments provide an important tool to study aerosol loading over the world. However, individual satellites do not last forever and newer satellites often have improved capabilities compared to older ones. This paper evaluates an extension of an algorithm, originally designed to monitor aerosols from an older satellite instrument, to a new satellite instrument called Visible Infrared Imaging Radiometer Suite. The evaluation is performed by comparing to ground truth data which are part of the National Aeronautics and Space Administration's global Aerosol Robotic Network, as well as to other satellite‐based aerosol data sets from different spaceborne instruments. Key Points: Based on comparisons to AERONET, the AOD uncertainty is consistent with expectationsRetrieved parameters related to aerosol particle size and type are also demonstrated to have skillSOAR shows similar regional and temporal patterns to other satellite data sets [ABSTRACT FROM AUTHOR]

MODerate resolution Imaging Spectroradiometer (MODIS) data can play an important role in aerosol retrieval at the global scale due to their short revisit period and long‐term observations. The operational MODIS aerosol optical depth (AOD) products are severely limited in air quality studies at the city or local scales due to their coarse spatial resolutions. Therefore, an improved aerosol retrieval algorithm for MODIS images at 1‐km spatial resolution is proposed in this paper. This algorithm is based on the high‐resolution aerosol retrieval algorithm with a priori land surface reflectance database support (HARLS), which was developed over bright urban areas and was subsequently modified and validated over land. For this study, an eight‐day surface reflectance database and a seasonal aerosol‐type database over land are constructed using the MODIS surface reflectance and aerosol products. Four typical regions (in Europe, North America, Beijing‐Tianjin‐Hebei, and the Sahara) with different underlying surfaces and aerosol types are selected to perform the aerosol retrieval experiments. The AOD retrievals are validated against the AERosol RObotic NETwork (AERONET) version 2 level 2.0 AOD measurements and compared with the operational MODIS AOD product at 3‐km resolution (MOD04_3K). The results show that AOD retrievals adequately match with AERONET AOD measurements, with 79.56%, 72.69%, 74.71%, and 61.01% of the collections falling within the MOD04_3K expected error over land [±(0.05 + 20%)] for each of the above‐listed regions, respectively. The Improved HARLS (I‐HARLS) algorithm performs well overall under different surface conditions, but the data quality gradually decreases with the increase in surface reflectance. Moreover, the I‐HARLS algorithm is robust with less bias and can provide more detailed aerosol spatial distributions than those provided by the MOD04_3K AOD product. These results suggest that the I‐HARLS algorithm can be used for air‐pollution‐ and climate‐related studies at medium or small scales. Key Points: An I‐HARLS AOD retrieval algorithm (1 km) for MODIS images is proposed over landI‐HARLS AOD retrievals agree well with AERONET AOD measurements over landI‐HARLS AOD retrievals are overall better and less biased than MOD04_3K AOD products [ABSTRACT FROM AUTHOR]

Atmospheric aerosols or atmospheric particulate matter affects climate variables like temperature and rainfall, agricultural productivity, soil, and human health. We evaluated aerosol lifecycle over India via simulations (2005–2014) from three general circulation models under the COALESCE project (carbonaceous aerosol emissions, source apportionment, and climate impacts; Venkataraman et al., 2020, 10.1175/bams‐d‐19‐0030.1). The ECHAM6.3‐HAM2.3, CAM5.3, and NICAM‐SPRINTARS simulations use identical regional emissions (from the Speciated Multi‐pollutant generator, SMoG‐India‐v1). Satisfactory model simulations of meteorological variable magnitudes and seasonal cycle have been achieved partly from the adoption of nudging. Estimations of anthropogenic aerosol, aerosol optical depth (AOD), and particulate matter surface concentrations are significantly improved from (a) dust tuning (b) use of satellite‐derived organic aerosol to carbon ratio, and (c) nudged meteorology to capture variables influencing the production of secondary sulfate. Larger wintertime under prediction (−30% to −60%) results from over prediction of seasonal planetary boundary layer height and the absence of secondary ammonium nitrate and organic aerosols. Vertical dispersion to higher altitudes than in observations calls for improved modeling of vertical mass flux representation. Carbonaceous aerosol residence time and AOD fraction larger than global mean values in India, with a seasonal predominance in the autumn and winter seasons can be explained by enhanced regional emissions from residential biofuel cooking, agricultural stubble burning, and traditional informal industries like brick production. Plain Language Summary: Air pollution, climate change and agricultural productivity are all affected by atmospheric pollutants called aerosols. Models must accurately estimate aerosol particle amounts seasonally, spatially and with altitude to capture their impacts. In this work, we use test and identify approaches to improve aerosol estimation by global climate models over India. We diagnose reasons for model disagreement with observations and suggest some ways to improve them. Larger abundances of carbonaceous aerosols (black carbon or soot, which causes warming and organic aerosol, which has high toxicity) are found over India, than globally, especially in autumn and winter. These result from enhanced regional emissions from residential biofuel cooking, agricultural stubble burning, and traditional informal industries like brick production. They could cause possible near‐term climate warming over India, in contrast to consistent aerosol induced cooling in other world regions. Key Points: Simulated aerosol mass improves from use of a regional emissions inventory, organic aerosol to carbon ratio and dust tuningModel improvements are needed in estimation of vertical mass flux, secondary nitrate and organic aerosolLarger carbonaceous aerosol residence times and AOD fraction over India, than global estimates, result from larger regional emissions [ABSTRACT FROM AUTHOR]

This study investigates the autumn precipitation in Sichuan Basin (SB), China in combination with reanalysis data sets of aerosol and meteorological fields from 2008 to 2018. To identify the aerosol impacts on precipitation, we carefully selected and divided the data into multiple groups according to vorticity, precipitation amount category, aerosol pollution level, and the geopotential height (GH) in the upper troposphere. Our data show that the precipitation in SB are generally reduced and weakened in afternoon to evening, but enhanced and intensified after midnight under the aerosol polluted condition. By analyzing data contrasts between different groups, we find that aerosol pollution in SB reduces shortwave heating near surface, which enhances the tropospheric stability and inhibits the vertical motion and rainfall in the afternoon to early evening. As the stability reduces within the basin, the vertical motions are invigorated to enhance and intensify the rain after midnight. Therefore, we conclude that the aerosol pollution in SB and the rapid adjustment of aerosol‐radiation interaction amplify the diurnal features of precipitation in autumn. Plain Language Summary: The present study illustrates that aerosols modify atmospheric thermodynamic states by direct effects and further impact diurnal variation of precipitation over the Sichuan Basin (SB). Key Points: Precipitation in Sichuan Basin are reduced and weakened in the afternoon and evening but enhanced and intensified after midnightAerosol pollution reduces shortwave heating at the surface, enhances tropospheric stability, and inhibits vertical motion in the afternoonThe rapid adjustment of aerosol‐radiation interaction amplifies the diurnal evolution of thermodynamic and precipitation features [ABSTRACT FROM AUTHOR]

This study statistically evaluated the aerosol impact on the temperature error in the lower‐level troposphere in short‐range numerical weather prediction (NWP). The Global Ensemble Forecast System version 12 (GEFSv12) reforecast exhibited large‐temperature errors in high‐loading areas (North India, Africa, South America, and China). In 1‐day GEFSv12 forecasts, the largest average temperature error occurred in the aerosol optical depth (AOD) peak month, and the daily error distribution corresponded to the daily AOD distribution. Even though the temperature error in the 1‐day operational forecasts was smaller than that in the GEFSv12 forecasts, the forecast uncertainties in the operational forecasts were comparable to those in 3‐day GEFSv12 forecasts over high‐loading areas. The daily temperature errors in all NWP models exhibited a correlation coefficient of ∼0.5–0.6 for the AOD over Central Africa and northern South America and ∼0.3–0.6 for AOD anomalies over China and northern South America. These results indicated that the interannual aerosol variability contributed 25–36% to errors, and the daily variability contributed 10%–36% to temperature errors in 3‐day forecasts. Although the correlation was low, aerosol impacts also emerged in North India and Central Africa. Partial correlation and composite analysis suggested that the direct effect mainly influenced temperature forecast errors over northern South America, whereas both direct and indirect effects influenced temperature errors over China. Model intercomparison revealed that operational NWP models could experience common forecast errors associated with aerosols in high‐loading areas. Plain Language Summary: Atmospheric aerosols significantly impact weather and climate system via direct (aerosol‐radiation interactions) and indirect (aerosol‐cloud interactions) effects. Aerosol influences are, however, treated via a monthly climatology (i.e., real‐time spatiotemporal variations are not included) in short‐to medium‐range numerical weather predictions. This study assessed the aerosol influences on temperature forecasts in the lower‐level troposphere on short‐range timescales. The results indicated that the variability in monthly and daily temperature errors almost corresponded to the aerosol optical depth (AOD) variability in high‐loading areas (North India, Africa, South America, and China). The correlation coefficients of the temperature error and AOD ranged from ∼0.5 to 0.6 over central Africa and northern South America, indicating that the interannual aerosol variability contributed ∼25%–36% to this error. In addition, the correlation between the error and AOD anomalies reached 0.6 over northern South America and 0.3 over China. These results suggested a 10%–36% contribution of the daily aerosol variability. Additionally, the direct effect dominated over northern South America, while the both direct and indirect effects were important over China. Model intercomparison in this study revealed that state‐of‐the‐art numerical weather prediction (NWP) models are subject to a common error source associated with the aerosol variability in high‐loading regions. Key Points: The lower‐troposphere temperature error was correlated with the aerosol optical depth (AOD) (R was ∼0.6) over Africa and northern South America (N. S. America)The error was correlated with AOD anomaly over N. S. America and China due to direct and both direct and indirect effects, respectivelyOperational short‐range forecasts experienced common errors associated with interannual and daily aerosol variabilities in these regions [ABSTRACT FROM AUTHOR]

With the continuation of the Coronavirus Disease 2019 (Covid‐19) pandemic, the impacts of this catastrophe on anthropogenic emissions are no longer limited to its early stage. This study quantitatively estimates effective radiative forcings (ERFs) due to anthropogenic well‐mixed greenhouse gases (WMGHGs) and aerosols for the period 2020–2050 under the three latest Covid‐19 economic‐recovery scenarios using an aerosol‐climate model. The results indicate that reductions in both WMGHG and aerosol emissions under the Covid‐19 green recoveries lead to increases ranging from 0 to 0.3 W m−2 in global annual mean anthropogenic ERF over the period 2020–2050 relative to the Shared Socioeconomic Pathway 2‐4.5 scenario (the baseline case). These positive ERFs are mainly attributed to the rapid and dramatic decreases in atmospheric aerosol content that increase net shortwave radiative flux at the top of atmosphere via weakening the direct aerosol effect and low cloud cover. At the regional scale, reductions in aerosols contribute to positive ERFs throughout the Northern Hemisphere, while the decreased WMGHGs dominate negative ERFs over the areas away from aerosol pollution, such as the Southern Hemisphere oceans. This drives a strong interhemispheric contrast of ERFs. In contrast, the increased anthropogenic emissions under the fossil‐fueled recovery scenario lead to an increase of 0.3 W m−2 in global annual mean ERF in 2050 compared with the baseline case, primarily due to the contribution of WMGHG ERFs. The regional ERF changes are highly dependent on local cloud radiative effects. Plain Language Summary: As a result of the Covid‐19 pandemic, the unprecedented containment has imposed on energy demand and other structural changes in transport globally. These changes may further affect future emission levels, thus inducing climate changes at the global and regional scales. In this study, we estimated effective radiative forcings (ERFs) due to anthropogenic forcing agents, such as well‐mixed greenhouse gases (WMGHGs), and aerosols, under the three latest Covid‐19 economic recovery scenarios (fossil‐fueled, moderate green, and strong green recoveries). Despite large decreases in both WMGHG and aerosol emissions under two green recovery scenarios, we found the aerosol ERF that mainly occurs over the Northern Hemisphere is the dominant contributor to the total positive ERFs over the period 2020–2050. Conversely, the ERFs over the Southern Hemisphere oceans turn negative due to the reduction in WMGHG emissions, which partly offset the effects of the abovementioned changes in aerosols, and result in asymmetric interhemispheric patterns in Earth's radiation budget in future. Key Points: Reductions in aerosol emissions under two green recoveries dominate the increases in total effective radiative forcing (ERF) from 2020 to 2050Compared to 2020, the total ERF increases in 2050 due to increased greenhouse gas under a fossil‐fueled recoveryInterhemispheric contrasts of ERF are striking due to the simultaneous decreased anthropogenic forcing agents [ABSTRACT FROM AUTHOR]

Using ensemble forecasts from the Global Ensemble Forecast System (GEFS) and controlled integrations with the Weather Research and Forecasting (WRF) model, we explore the stratospheric influence on the development of the 2018 European severe cold air outbreak (CAO). The 2018 CAO set in on 22 February, and its evolution was closely related to a growing tropospheric blocking ridge over the North Atlantic in late February that brought cold air mass into Europe. The event was also preceded by a split of the stratospheric polar vortex on 12 February, after which one child vortex persisted over northeastern Canada. GEFS forecasts initialized 1 week prior to this CAO substantially underestimated its severity, exhibiting large spread correlated with the strength and location of the Atlantic ridge, while forecasts initialized closer to the event captured its severity with higher fidelity. Based on a set of nudged simulations, we show that capturing the evolution of the stratospheric child vortex over northeastern Canada, particularly in the lowermost stratosphere, is of great importance for predicting this CAO, since it substantially affects the strength and location of the Atlantic ridge that in turn determines the evolution and severity of this CAO. When the stratosphere is nudged toward the observed evolution, the northward flow between the child vortex and the Atlantic ridge advects more low potential vorticity air into the Atlantic ridge, and therefore amplifies it. Further experiments demonstrate that the impact of the stratosphere on the development of the ridge is particularly strong over the North Atlantic. Key Points: Global Ensemble Forecast System forecasts initialized one week prior to the 2018 European severe cold air outbreak (CAO) underestimated its severityControlled Weather Research and Forecasting simulations demonstrate that the stratospheric polar vortex evolution is essential to the synoptic development of the CAOThe impact of the stratospheric polar vortex is particularly strong over the North Atlantic [ABSTRACT FROM AUTHOR]

We have compiled and analyzed a comprehensive data set of field observations of iodine speciation in marine aerosol. The soluble iodine content of fine aerosol (PM1) is dominated by soluble organic iodine (SOI; ∼50%) and iodide (∼30%), while the coarse fraction is dominated by iodate (∼50%), with nonnegligible amounts of iodide (∼20%). The SOI fraction shows an equatorial maximum and minima coinciding with the ocean "deserts," which suggests a link between soluble iodine speciation in aerosol and ocean productivity. Among the major aerosol ions, organic anions and non‐sea‐salt sulfate show positive correlations with SOI in PM1. Alkali cations are positively correlated to iodate and negatively correlated with SOI and iodide in coarse aerosol. These relationships suggest that under acidic conditions iodate is reduced to HOI, which reacts with organic matter to form SOI, a possible source of iodide. In less acidic sea‐salt or dust‐rich coarse aerosols, HOI oxidation to iodate and reaction with organic matter likely compete. Plain Language Summary: Iodine has a profound impact on tropospheric chemistry and plays a key role in in mammalian metabolism. Marine aerosol is an atmospheric iodine reservoir and its carrier to continental food chains. However, the chemistry behind the varying concentrations of iodine‐bearing species observed in aerosol is poorly understood, which hinders model predictions about iodine recycling to the gas phase and accumulation in aerosol. To shed some light on this problem, we have compiled and analyzed a global data set of field observations of iodine speciation in aerosol over the ocean between 1983 and 2018. Comparison of the spatial trends of the relative amounts of soluble iodine species in aerosol and ocean surface remote sensing variables reveals a link to ocean productivity, which points to an important role of organics emitted by the ocean and condensed on aerosol surfaces. Furthermore, the speciation of iodine differs between coarse and fine aerosol, which are originated, respectively, from condensation of gases derived from ocean surface emissions (sulfate aerosol) and aeolian processes (sea salt and dust) and, as a consequence of their different composition, have different pH. Overall, organic matter and acidity emerge from our study as the factors controlling iodine speciation in marine aerosol. Key Points: Iodide and SOI are more abundant in PM1, while the coarse fraction is dominated by iodate; iodide is only 30% lower in the coarse fractionIodine speciation in fine aerosol is related to ocean productivity and present a distinct latitudinal variationDifferent chemical regimes correlated with acidic and alkali ions prevail in fine and coarse aerosol speciation [ABSTRACT FROM AUTHOR]

The backscattering optical properties of an ensemble of randomly oriented dust particles at a wavelength of 355 nm were comprehensively studied by examining the invariant imbedding T‐matrix results of the super‐spheroid dust model. In particular, we focused on the lidar ratio (S $S$) and depolarization ratio (δ $\delta $) relations of dust aerosols to aid interpretation of data from the Atmospheric Lidar (ATLID) instrument that will be onboard the Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) satellite. Super‐spheroid models with various aspect ratios (α $\alpha $), roundness parameters (n) $(n)$, and refractive indices were investigated over a wide range of particle sizes and compared to the observation data of the National Aeronautics and Space Administration (NASA) Langley 355‐nm airborne high spectral resolution lidar. We found that super‐spheroid dust particles with different sets of n $n$ and α $\alpha $ could be used to model almost the entire range of the observed joint distributions of S $S$ and δ $\delta $. The S−δ $S-\delta $ relation could effectively discriminate among dust particle types. The observed S $S$ and δ $\delta $ values with the largest population density were best covered by models with n > 2, especially by those with n $n$ varying from 2.4 to 3.0. Plain Language Summary: Dust aerosols are irregularly shaped. Dust shape has a significant impact on the modeling of lidar observations. In this work, super‐spheroid models (the conventional spheroid model is a special case) were used to study the impacts of nonsphericity and irregularity on lidar observations. In particular, we were interested in the lidar and depolarization ratios, because these ratios have proven useful for distinguishing among aerosol types in the atmosphere and will be measurable by the future Earth Cloud, Aerosol, and Radiation Explorer (EarthCARE) satellite. Because the lidar and depolarization ratios can be affected by multiple factors, including dust shape, refractive indices, and particle sizes, systematic theoretical studies related to these factors were conducted. Furthermore, we compared the theoretical simulations with NASA Langley airborne high spectral resolution lidar data, and found that super‐spheroid models could be used to explain the wide variation of dust optical properties found in the lidar data. The present study confirmed that dust particles represented by super‐spheroid models can be useful in lidar remote sensing applications. Based on this, the optimal parameters of super‐spheroid models can be used to parameterize dust shapes. Key Points: The backscattering optical properties of dust aerosols were modeled systematically using homogeneous super‐spheroidsA comprehensive survey of the lidar ratio‐depolarization ratio relations produced by super‐spheroids at 355 nm wavelength was conductedSuper‐spheroids could be used to explain the wide variation of dust optical properties found in the high spectral resolution lidar data [ABSTRACT FROM AUTHOR]

Ambient nitrogen dioxide (NO2) not only has adverse health effects on humans but also contributes to the production of two major secondary atmospheric pollutants, ozone (O3) and fine particulate matter (PM2.5). In this study, surface NO2 concentrations in East Asia from 2001 to 2016 were estimated by combining an ensemble backpropagation neural network method, satellite NO2 column data, and reanalysis data. The estimated monthly and annual mean NO2 concentrations were well‐correlated with the observations, with R (correlation coefficient) values of 0.89 and 0.91, respectively. Our results indicate that the NO2 concentrations in most areas of East Asia peaked during 2011–2013. The NO2 concentrations in autumn and winter, especially in the eastern and northern parts of China, were much higher than those in summer. In terms of population NO2 exposure, over 25 million South Korea residents (∼45% of the population) were exposed to NO2 concentrations higher than the 2005 World Health Organization's annual standard (40 μg/m3, ∼22 ppbv at 25°C) in 2015. In contrast, the entire populations of some developing countries, such as Vietnam, Cambodia, Myanmar, the Philippines, and Lao PDR, were exposed to NO2 concentrations less than 14 ppbv. Based on the estimation, NO2‐related asthma cases in East Asia increased by 1.37% annually from 2001 to 2015, reaching 139,000 cases (95% confidence interval: 37,400–263,400) in 2015. NOx emission inventories vary from country to country in East Asia; thus, more targeted NOx emission‐control policies are urgently required. Plain Language Summary: As one of the major atmospheric pollutants, NO2 contributes to the formation of several secondary atmospheric pollutants and the development of respiratory diseases. Satellite NO2 data and meteorological factors are commonly used to retrieve surface NO2 concentrations. Several regression methods have been adopted to construct models to estimate NO2 pollution concentrations; however, limitations such as low resolution and relatively large bias hinder the performance of these simulations. In this study, we applied a machine learning method to estimate surface NO2 concentrations over East Asia, improving the estimation accuracy and resolution compared with previous models. The spatial distribution pattern revealed that NO2 concentrations were relatively high in most territories in South Korea and the North China Plain. Based on the estimated NO2 concentrations, population NO2 exposure and NO2‐related asthma incidence were analyzed for each country. Residents of South Korea and China were exposed to higher NO2 concentrations than those in other countries, and NO2‐related asthma incidence increased by 1.37% annually in East Asia from 2001 to 2015. Therefore, more stringent NOx emission‐control policies are urgently needed in East Asian countries. Key Points: An ensemble machine learning method with satellite NO2 and reanalysis data was adopted to estimate surface NO2 concentration in East AsiaThe populations of South Korea and North China were exposed to the highest NO2 levels, which peaked in 2010 and decreased slightly in 2015NO2‐related asthma incidence increased by 1.37% annually from 2001 to 2015 in East Asia, reaching 139,000 cases in 2015 [ABSTRACT FROM AUTHOR]

Hydroxyl (OH) and hydroperoxyl (HO2) strongly influence the atmosphere's oxidation of gases emitted from Earth's surface and the formation and aging of aerosol particles. Thus, understanding OH and HO2 chemistry is essential for examining the impact of human activity on future atmospheric composition and climate. Using the OH and HO2 data set collected with the Penn State Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) during nine aircraft missions over the past 20 years, we compare observed OH and HO2 to that modeled using the same near‐explicit photochemical box model. In general, the agreement is well within the combined uncertainties of the observations and models, even when the model is constrained only with a common data set of simultaneous measurements. However, in environments such as cities, forests, and pollution plumes, the model chemical mechanism and size of the data set of constraining measurements do matter. The model constrained by the full set of measurements better simulates OH loss and OH in these regions than the model constrained by the common set of measurements. In cleaner regions, the differences between observed and modeled OH and HO2 found in previous studies generally remain and do not appear to be systematic, indicating that the differences are driven by measurement issues for ATHOS and/or other instruments. Thus, these comparisons indicate that the oxidation chemistry in most of the free troposphere is understood to as well as current measurements can determine. The focus of future research needs to be on regions rich in volatile organic compounds, where observed‐to‐modeled differences are more persistent, and on improving measurement consistency. Plain Language Summary: Understanding the atmospheric chemistry that results in the removal of important greenhouse gases, such as methane, is critical in assessing the impact of human activity on future climate. Using airborne atmospheric data collected over a twenty‐year period, we compare observed and modeled hydroxyl and hydroperoxyl, two important reactive gases, to evaluate our ability to model this oxidation chemistry. Overall, these comparisons show agreement between observed and modeled hydroxyl and hydroperoxyl to within combined instrument and model uncertainties although the level of agreement degrades over polluted or forested regions and is variable from mission to mission and even within missions. The lack of systematic disagreement between observed and modeled hydroxyl and hydroperoxyl when using the same model for all the studies suggests that unexpected changes in the performance of multiple instruments are likely responsible for this variable level of agreement. Key Points: Observed and modeled OH and HO2 generally agree throughout the free troposphere, based on analysis of nine airborne studiesMeasuring more chemical species may not improve observed‐to‐modeled agreement for OH and HO2, except in polluted or forested environmentsDisagreements between observed and modeled OH and HO2 are likely due to measurement issues and not the different models used [ABSTRACT FROM AUTHOR]

Aerosol deposition is a major source of soluble Fe in open oceans, affecting marine biogeochemistry and primary production. However, Fe fractional solubility, a key parameter in estimating deposition fluxes of soluble aerosol Fe, is still highly uncertain. Abundance and fractional solubility of aerosol Fe in fine (<1 μm) and coarse (>1 μm) particles was measured at Qingdao (a coastal city in northern China) in November‐December 2019. Average concentrations of total and soluble Fe were found to be 798 ± 466 and 7.7 ± 14.5 ng/m3 in coarse particles, and 801 ± 534 and 7.3 ± 7.6 ng/m3 in fine particles. Fe solubility was significantly lower in coarse particles (average: 0.80 ± 1.03%) than fine particles (average 1.29 ± 1.41%). Compared to clean days, total Fe concentration was substantially increased during dust and haze days; however, Fe solubility was significantly reduced in dust days and elevated in haze days. Acid processing significantly enhanced Fe solubility in both fine and coarse particles, and the contribution of primary emission to Fe solubility enhancement was important for fine particles but minor for coarse particles. Higher Fe solubility (>1%) in fine and coarse particles was usually observed at high aerosol acidity (pH < 4) and high RH (>60%), suggesting critical roles of aerosol acidity and water content in regulating aerosol Fe solubility. Key Points: Fe solubility was significantly lower in coarse particles than fine particlesContribution of primary emission to Fe solubility enhancement was important for fine particles but minor for coarse particlesAerosol acidity and water content played critical roles in regulating Fe solubility in both fine and coarse particles [ABSTRACT FROM AUTHOR]

Delhi metropolitan area suffers from extreme haze during the post‐monsoon and winter season, impacting climate, public health, and economy. We used a high‐resolution time‐of‐flight aerosol mass spectrometer (HR‐ToF‐AMS) and aethalometer at two urban locations in Delhi to capture non‐refractory PM2.5 (NR‐PM2.5) and black carbon (BC) during the post‐monsoon and winter season of 2019–2020. Four haze periods with high composition based‐PM2.5 (C‐PM2.5 = NR‐PM2.5 + BC) concentration and distinct chemical composition were identified, during all of which organics dominated but with varying contribution (∼[50%–70%] of C‐PM2.5). Biomass burning organic aerosol (BBOA) was dominant in all periods (∼[31%–45%] of OA), but the majority of it was highly aged ∼(45%–50%) with high O/C (0.71 and 0.46 at the two sites), formed most likely through rapid dark oxidation of freshly emitted and partially oxidized BBOA. High polycyclic aromatic hydrocarbons (PAH) signals in the fresh BBOA mass spectra suggest incomplete combustion activities such as open biomass burning emissions as major source. During an agricultural burning event in north‐western India, we estimated that ∼(44%–53%) of total C‐PM2.5 (combined contribution of aged BBOA and oxygenated OA) measured in Delhi was influenced by long‐range transported biomass burning emissions. During winter, secondary inorganics constituted a significant fraction apart from organics ∼(48%–55%), mainly in the form of ammonium nitrate (NH4NO3; up to ∼[19%–25%] of C‐PM2.5) and ammonium sulfate (NH4SO4; up to ∼[27%–38%]). Enhanced formation of NH4NO3 and related‐secondary organic aerosol (SOA) were linked to nighttime oxidation of BBOA, while NH4SO4 and related‐SOA were linked to heterogeneous aqueous phase oxidation under high RH conditions (>90%). Plain Language Summary: During post‐monsoon and winter seasons, Delhi experiences high concentration of fine particles leading to haze events. To design targeted mitigation policies, an understanding of key factors responsible for haze events is needed. We used an advanced suite of instrumentation to capture temporal trends of real‐time particle chemical composition at two urban locations in Delhi. Using advanced receptor modelling and molecular level source signatures, the open burning of biomass material was estimated to be the largest contributor to haze events during post‐monsoon and winter haze events. The post‐monsoon haze pollution was largely affected by the aged biomass burning particles likely from the long‐range transport of agricultural residue burning emissions in the upwind states of Delhi. Winter haze episodes were also highly affected by biomass burning emissions, likely from the local burning of wood, coal and/or roadside trash for heating and/or cooking purposes. However, the winter haze particles contained significant aged inorganic material including ammonium nitrate, ammonium sulfate and ammonium chloride aerosol apart from aged organics. This study also suggests that the nighttime aging of biomass burning emissions contributed in the rapid formation of aged organic and inorganic particles, causing haze events. Key Points: Four haze periods with distinct PM2.5 chemical composition were identified during post‐monsoon and winter season in DelhiBiomass burning organic aerosol was dominant in all haze periods with the majority of it present in highly aged formOpen biomass burning emissions undergo rapid nighttime oxidation resulting in high yields of secondary organic and inorganics [ABSTRACT FROM AUTHOR]

This work serves as the second of a two‐part study to improve surface PM2.5 forecasts in the continental U.S. through the integrated use of multisatellite aerosol optical depth (AOD) products (MODIS Terra/Aqua and VIIRS DT/DB), multichemical transport model (CTM) (GEOS‐Chem, WRF‐Chem, and CMAQ) outputs, and ground observations. In Part I of the study, an ensemble Kalman filter (KF) technique using three CTM outputs and ground observations was developed to correct forecast bias and generate a single best forecast of PM2.5 for next day over nonrural areas that have surface PM2.5 measurements in the proximity of 125 km. Here, with AOD data, we extended the bias correction into rural areas where the closest air quality monitoring station is at least 125–300 km away. First, we ensembled all of satellite AOD products to yield the single best AOD. Second, we corrected daily PM2.5 in rural areas from multiple models through the AOD spatial pattern between these areas and nonrural areas, referred to as "extended ground truth" or EGT, for the present day. Lastly, we applied the KF technique to reduce the forecast bias for next day using the EGT. Our results find that the ensemble of bias‐corrected daily PM2.5 from three CTMs for both today and next day show the best performance. Together, the two‐part study develops a multimodel and multi‐AOD bias‐correction technique that has the potential to improve PM2.5 forecasts in both rural and nonrural areas in near real time, and be readily implemented at state levels. Plain Language Summary: The U.S. Environmental Protection Agency's AirNow program reports current or forecasted air quality to the general public in the form of Air Quality Index (AQI). The forecasted AQI is made available by local and state air quality agencies across more than 500 cities across the U.S. However, since surface observations of particulate matter (PM) are primarily located in the urban areas, observation‐based AQI in the rural areas is limited, and either the current or the forecasted AQI from AirNow has large uncertainties that are difficult to assess, especially during the fire season. Satellite observation with large spatial coverage provides a promising opportunity to fill in the gaps in areas where observations are spare. Building upon our previous work, here we develop a statistical technique to improve surface PM forecasts in the rural areas of continental U.S. through the use of satellite observations of aerosols, surface observations, and air quality forecasting models. Assessment with the data from Interagency Monitoring of Protected Visual Environments (IMPROVE) network shows the promise of our technique. The technique is designed with the consideration of the forecast in near real time, and is efficient with minimal requirement of computing. Key Points: Ensemble‐based AOD from different sensors and algorithms shows better performance than individual ensemble memberA multimodel and multi‐AOD ensemble bias correction via Kalman filter improves PM2.5 forecasts in rural areas lack of ground observationsAn ensemble framework for producing the single best PM2.5 forecast for next day in near real time via bias correction is established [ABSTRACT FROM AUTHOR]

Stratospheric aerosol injection is one of the most widely studied solar radiation modification methods to reduce some effects of anthropogenic warming. We perform idealized simulations to analyze climate response to different latitudinal and altitudinal distributions of additional stratospheric sulphate aerosols. We analyze climate response to both large volcanic‐size aerosols that are representative of sulphate aerosols which form when SO2 is injected into the stratosphere by major volcanic eruptions or by proposed stratospheric sulphate aerosol geoengineering option, and small background‐size aerosols which are present in the stratosphere during quiescent periods with no major volcanic eruptions. Our simulations show that under the climate state with doubled atmospheric CO2, for the same total mass, independent of size‐distributions, aerosols concentrated at higher latitudes produce less negative effective radiative forcing but larger surface cooling as a result of larger forcing efficacy. For the same latitudinal distribution, small background‐size aerosols added at lower altitudes cause larger surface cooling as a result of hygroscopic growth that increases scattering of sunlight. In contrast, large volcanic‐size aerosols added at lower altitudes cause smaller surface cooling than at higher altitudes as a result of enhanced stratospheric heating and associated water vapor feedback that enhance absorption of solar and terrestrial radiation. For both background‐size and volcanic‐size aerosols, asymmetric distributions about the equator induce substantial cross‐equatorial energy transport, causing the Intertropical Convergence Zone to move to the warmer hemisphere with less aerosol loading. Our study helps to understand the climate consequences of different spatial patterns and size distributions of stratospheric aerosol perturbation. Plain Language Summary: It has been proposed that addition of particles such as sulphate aerosols in the stratosphere could reduce anthropogenic warming by scattering sunlight back to space. Here we use a global climate model to examine climate response to different latitudinal and altitudinal profiles of additional stratospheric aerosols. We test climate response to both small background‐size aerosols and large volcanic‐size aerosols. The small background‐size aerosols are present in the stratosphere during quiescent periods with no major volcanic eruptions while injection of SO2 into the stratosphere by major volcanic eruptions or by the proposed stratospheric sulphate aerosol geoengineering option would form H2SO4 droplets with a typical size similar to that of volcanic‐size aerosols. We find that relative to the high‐CO2 climate without aerosol perturbation, for the same mass, aerosols concentrated at poles produce more surface cooling than do aerosols concentrated in the tropics. Small background‐size aerosols produce larger surface cooling when they are placed at lower altitudes, and the opposite is true for large volcanic‐size aerosols. When aerosols are asymmetrically distributed about the equator, the location of Intertropical Convergence Zone shifts to the relatively warmer hemisphere. Different latitudinal and altitudinal aerosol distributions also have important implications for the response of land primary productivity. Key Points: We use CESM model to analyze climate response to different sizes and different spatial patterns of stratospheric aerosol perturbationAerosols concentrated at higher latitude induce less negative radiative forcing but larger surface cooling due to larger forcing efficacyDependence of climate response on heights of prescribed background aerosols in the stratosphere is opposite to that of volcanic aerosols [ABSTRACT FROM AUTHOR]

Accurate fire emissions inventories are crucial to predict the impacts of wildland fires on air quality and atmospheric composition. Two traditional approaches are widely used to calculate fire emissions: a satellite‐based top‐down approach and a fuels‐based bottom‐up approach. However, these methods often considerably disagree on the amount of particulate mass emitted from fires. Previously available observational datasets tended to be sparse, and lacked the statistics needed to resolve these methodological discrepancies. Here, we leverage the extensive and comprehensive airborne in situ and remote sensing measurements of smoke plumes from the recent Fire Influence on Regional to Global Environments and Air Quality (FIREX‐AQ) campaign to statistically assess the skill of the two traditional approaches. We use detailed campaign observations to calculate and compare emission rates at an exceptionally high‐resolution using three separate approaches: top‐down, bottom‐up, and a novel approach based entirely on integrated airborne in situ measurements. We then compute the daily average of these high‐resolution estimates and compare with estimates from lower resolution, global top‐down and bottom‐up inventories. We uncover strong, linear relationships between all of the high‐resolution emission rate estimates in aggregate, however no single approach is capable of capturing the emission characteristics of every fire. Global inventory emission rate estimates exhibited weaker correlations with the high‐resolution approaches and displayed evidence of systematic bias. The disparity between the low‐resolution global inventories and the high‐resolution approaches is likely caused by high levels of uncertainty in essential variables used in bottom‐up inventories and imperfect assumptions in top‐down inventories. Plain Language Summary: Smoke emitted by wildland fires is dangerous to human health and contributes to climate change. To predict and evaluate the impacts of fires, we need to know how much smoke is emitted into the atmosphere. There are two state‐of‐the‐art methods used to estimate the mass of smoke emitted by fires, but they often disagree. In this study, we use unusually detailed measurements collected using an aircraft that flew within wildland fire smoke plumes to calculate the amount of smoke emitted from fires in the Western United States. We compare emission rates derived from the exceptionally high spatial and temporal resolution approach to the two traditional, lower resolution approaches to understand why they sometimes diverge. Key Points: In situ measurements of wildland fire smoke plumes provide emission rates for evaluating emissions inventories at unprecedented resolutionFire emissions inventories struggle to capture the emissions rate characteristics of individual fires but may perform well in the aggregateBottom‐up inventories suffer from major uncertainty in key variables, while top‐down inventories may have bias from imperfect assumptions [ABSTRACT FROM AUTHOR]

An investigation of aerosol effects on the Arctic summer clouds is carried out by using a fully‐coupled version of GEM‐MACH, an online air quality forecast model. Model simulations are carried out for the July 2014 NETCARE field campaign based from Resolute, Nunavut, Canada. For a pristine period, the model simulated thin water clouds, with liquid water path <50 g m−2, at relatively low altitudes (<200 m). The model simulated more and smaller droplets with higher liquid water content (LWC) during a polluted period. Further sensitivity analysis indicates that the increased aerosol loading from long‐range transport during the polluted period is responsible for the increase in CDNC and decreased droplet size, which also led to an increase in cloud liquid water content (LWC), a decrease in precipitation, and increased cloudiness in the area. The model agrees with observation showing that aerosols smaller than 100 nm are commonly activated in the summer Arctic, with even smaller aerosols (<50 nm) being activated during the pristine period. The impact of atmospheric dimethyl sulfide (DMS) upon the Arctic summer cloud microphysics was also investigated. The inclusion of DMS leads to an increase in CDNC and smaller droplet sizes. It also improves the modeled aerosol size distribution considerably compared to the observation, particularly in the size range between 60 and 200 nm. The enhancement of CDNC due to DMS is more significant (e.g., >50%) in the regions with higher DMS(g) mixing ratios, resulting in higher sulphate mass increases in these regions. Key Points: Model is able to simulate low‐level thin clouds in the Arctic during the pristine period where aerosols smaller than 50 nm are activatedModel demonstrated that the enhanced aerosol loading from long‐range transport led to increased cloud amount and decreased precipitationDimethyl sulfide oxidation led to increased aerosols between 60–200 nm and a 50% increase in droplet number in some regions of the Arctic [ABSTRACT FROM AUTHOR]

While the particulate matters (PM) concentration has been reduced across North China in recent years, the surface ozone level has been increasing in many cities in the region. There is a widely held speculation that the elevated ozone level was caused by increased solar radiation due to the reduced PM level in recent years. This study quantifies the radiation effect of PM on O3 by analyzing data from years 2013 to 2018 in six major cities in North China. We first evaluate PM's effect on the radiation by controlling meteorological factors to radiation, which include the cloud cover, the humidity and time of the day. The PM's effect on the radiation was statistically significant in all six cities, but was the least significant among all significant factors. The PM's radiative effect on the ozone was also statistically significant, but the average ozone increase due to the decreased PM level was rather small, ranging from 0 to 0.12 μg/m3 (0–0.056 ppb) in the spring and 0–0.36 μg/m3 (0–0.168 ppb) in the summer per 10 μg/m3 PM reduction. Therefore, the wide spread spring‐summer increase of the ozone level in North China should not be attributed to the radiation increase caused by the reduced PM. Key Points: The effect of particular matters (PM) on the radiation is significant, but was the least significant among all significant factorsThe PM's radiative effect on the ozone was also significant, but the average ozone increase due to the decreased PM level was rather smallThe wide spread increase of the ozone level in North China should not be attributed to the radiation increase caused by the reduced PM [ABSTRACT FROM AUTHOR]

Aerosol particles undergo physical and chemical changes during cloud processes. In this work, we quantified the changes in aerosol mixing state using a particle‐resolved model. To this end, we coupled the particle‐resolved aerosol model PartMC‐MOSAIC with the aqueous chemistry module CAPRAM 2.4 and designed cloud parcel simulations that mimicked several cloud cycles that a particle population may be exposed to in polluted urban environments. With ammonium nitrate and ammonium sulfate added to the activated particles, after the cloud evaporated, the activation potential of the resuspended aerosol particles increased for supersaturation thresholds lower than the maximum supersaturation attained in the cloud. Formation of sulfate and nitrate increased the internally mixed state of all particle populations, quantified by the mixing state index χ. The change of aerosol mixing state due to aqueous‐phase chemistry was related to the fraction of activated particles. For a case with low aerosol number concentration, where the activated fraction was up to 60%, χ increased by up to 50 percentage points after cloud processing, reaching an almost completely internal mixture. In contrast, for a case with high aerosol emissions and activated fraction of less than 20%, the increase in χ was less than 20 percentage points, and χ remained below 80% after cloud processing. The change in aerosol mixing state caused by coagulation within the cloud parcel was negligible. These findings highlight the complex influence of cloud processing on particle properties. Plain Language Summary: Every cloud droplet contains at least one aerosol nucleus. The composition and mass of this nucleus can be changed during a cloud's lifetime by several chemical and physical processes. Once the cloud evaporates, a modified aerosol population is released into the atmosphere compared to the population that formed the cloud, which may also have different impacts on climate. In this work, we used a particle‐resolved process model to study the effects of cloud processing on aerosols within clouds. This modeling approach is suitable for this research because each particle is tracked individually, which allows us to quantify changes in particle composition without simplifying assumptions, such as averaging composition within predescribed size ranges. Because of the formation of ammonium sulfate and nitrate in the cloud, particles that formed cloud droplets grew larger and were more likely to form cloud droplets in future cloud cycles. Overall, cloud processing by aqueous‐phase chemistry produced aerosol populations where the particles look more similar to each other in composition. Key Points: Aqueous‐phase chemistry processes cause aerosol populations to be more internally mixedThe change of aerosol mixing state due to aqueous phase chemistry was related to the fraction of activated particlesCoagulation within clouds has a negligible impact on aerosol mixing state [ABSTRACT FROM AUTHOR]

The major fraction of intermediate and semi‐volatility organic compounds (I/SVOCs) is still unresolved by traditional analyses, leaving large unresolved complex mixture (UCM) and limiting the estimation of secondary organic aerosol (SOA). This study addressed the UCM by employing two‐dimensional gas chromatography−time‐of‐flight mass spectrometry. The ratios of UCM in I/SVOCs emitted from household burning of biomass and coal were reduced to 1.0 ± 0.3% and 2.1 ± 2.0%, respectively, and these levels are one order of magnitude less than those reported in previous studies. Phenols, polycyclic aromatic hydrocarbons, and ketones made the major contribution to I/SVOCs emission factors (EFs) (65.9 ± 9.6%) for biomass burning, while amides, acids, and esters constituted the majority of EFs (56.5 ± 45.0%) for coal burning. Furthermore, SOA production can be predicted via the highly identified I/SVOCs compounds based on volatility distributions of each speciated species. These majority compounds contribute the predicted SOA production with 76.0 ± 12.7% and 82.0 ± 60.3% for biomass and coal burning, respectively. Underestimated SOA production with the ratio of 62.5 ± 25.2% to 80.9 ± 2.8% via previous Bins method has been well addressed. The obtained results suggest that the nontarget analysis can significantly improve the accuracy of I/SVOCs estimation and environmental impacts by addressing chemical components at the molecular level. Plain Language Summary: Intermediate and semi‐volatility organic compounds (I/SVOCs) have already been recognized as important precursors of secondary organic aerosol in the atmosphere and are harmful to human health. Their major chemical components still constitute a largely unresolved complex mixture. This knowledge gap in chemical analysis limits further understanding of the environmental impacts of I/SVOCs, including estimates of secondary organic aerosol produced. This study addressed the chemical complexity of unresolved complex mixture by employing two‐dimensional gas chromatography−time‐of‐flight mass spectrometry. Many unresolved complex mixtures in I/SVOCs were effectively identified by the nontarget analysis. Furthermore, secondary organic aerosol production can be predicted via the highly identified I/SVOCs compounds based on their volatility distributions, which were obtained from comprehensive analyses of I/SVOCs. These results suggest that the nontarget analysis developed herein significantly improves the I/SVOCs estimates used in identifying chemical compounds at the molecular level and environmental impacts. Key Points: The unresolved ratios of intermediate and semi‐volatility organic compounds emitted from household biomass and coal burning are reduced to about 1.0% and 2.1%, respectivelyThe predictions of secondary organic aerosol production are increased by 62.5%–80.9% using non‐target analysis method, as compared with traditional 1D‐GC‐MSThe improvement in identifying intermediate and semi‐volatility organic compounds species benefits the impact estimation of source emissions on the atmospheric environment [ABSTRACT FROM AUTHOR]

Agricultural fires affecting grass‐, crop‐ and shrublands represent a major, mainly anthropogenically driven disturbance of many ecosystems. In addition to emissions of carbonaceous aerosol, they were found to inject also mineral dust particles into the atmosphere. The fires can significantly modulate the near‐surface wind patterns so that conditions suitable for dust emission occur. However, the exact emission mechanism has not been investigated so far, but is inevitable for the understanding of its impacts on the Earth system. Here, we test two dust emission parameterizations representing saltation bombardment (SALT) and direct aerodynamic dust entrainment by (convective) turbulence (convective turbulent dust emission, CTDE) in the context of fire‐modulated wind patterns using large‐eddy simulation with an idealized setup to represent typical agricultural fire settings. Favorable aerodynamic preconditions for the initialization of both emission processes are found, however, with sometimes significant differences in dust emission flux depending on specific wind and fire properties. The strong fire‐induced modulations of the instantaneous momentum flux suggest that CTDE can be a very potent emission process in the fire vicinity. Nevertheless, fire impacts on the friction velocity can be significant too, so that dust emission through SALT is facilitated as well. Ultimately, the specific aerodynamic conditions within pyro‐convectively modulated wind patterns require the development of a parameterization that can describe these unique fire‐related dust emissions and their influencing factors properly. This will finally allow for considering fire‐induced dust emissions in aerosol‐atmosphere models and an investigation of its atmospheric impacts such as on the radiation budget. Plain Language Summary: Mineral dust particles are frequently observed within smoke plumes originating from agricultural fires of mostly semi‐arid regions. Such fires can modulate the near‐surface wind patterns and thus foster dust emission. However, the exact emission mechanism has not been investigated so far. Here, two conceptually different dust emission parameterizations are tested in the context of fire‐modulated wind patterns using high‐resolution model simulation. The results show that wind conditions favorable for the initialization of both processes occur, however, with partly substantial differences depending on the environmental preconditions. The direct entrainment of dust particles appears to be an important emission process, particularly under weak wind conditions, while another emission process including larger sand‐sized grains becomes more important under strong wind conditions. The unique fire‐related aerodynamic preconditions require the development of an adjusted parameterization to describe pyro‐convective dust emission together with its influencing factors sufficiently to enable a proper investigation of its atmospheric impacts. Key Points: Aerodynamic preconditions to favor dust emission through saltation and direct aerodynamic entrainment exist in fire vicinityStrength of relative contribution of both dust emission processes can vary significantly depending on wind, fire and soil‐surface conditionsAccurate estimation of global importance of fire‐related dust emissions requires development of an adjusted parameterization approach [ABSTRACT FROM AUTHOR]

To better understand the responses of particle number concentrations (PNCs) and new particle formation (NPF) to the largely reduced air pollutant emissions in urban atmospheres, we investigated the particle number size distributions in Qingdao, a coastal megacity in northern China, during two separate periods, 2010–2012 and 2016–2018. The results show only an average of 5% decrease in the total PNCs in 2016–2018 relative to 2010–2012, although the PM2.5 mass concentration decreased by over 40%. Nucleation‐mode PNCs decreased by 20%, which is attributable to reductions in primary emissions from on‐road vehicles and secondary sources. Unexpectedly, the accumulation‐mode PNCs increased by 11% in 2016–2018 relative to 2010–2012. The SO2 concentrations strongly decreased by ∼60%, and the NPF frequencies slightly decreased from 34% in 2010–2012 to 25% in 2016–2018. However, there were no significant changes in the apparent new particle formation rate (FR) or net maximum increase in nucleation‐mode PNCs (NMINP). The increased concentrations of other precursors may substantially negate the effect of SO2 reduction, leading to the invariant FR and NMINP. The maximum sizes of grown new particles decreased by 50%; combined with the decreased NPF frequency, the contribution of grown new particles to accumulation‐mode PNCs expectedly decreased in 2016–2018 relative to 2010–2012. Alternatively, the increased accumulation‐mode PNCs in 2016–2018 mainly attribute to the enhanced primary combustion emissions excluding on‐road vehicle emissions. The increase in accumulation‐mode PNCs in 2016–2018 provides insights on the inter‐annual variation in satellite‐based cloud properties, which appeared insensitive to the large decrease in satellite‐based aerosol optical depth. Key Points: Total particle number concentrations (PNCs) decreased by 5% in 2016–2018 relative to 2010–2012, despite the PM2.5 decreased by over 40%In 2016–2018, nucleation‐mode PNCs decreased by 20% relative to 2010–2012, whereas accumulation‐mode PNCs increased by 11%Maximum sizes of grown new particles were reduced by 50%, leading to a decreased contribution of new particles to accumulation mode PNCs [ABSTRACT FROM AUTHOR]