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Understanding carbon sources and sinks across the Earth's surface is fundamental in climate science and policy; thus, these topics have been extensively studied but have yet to be fully resolved and are associated with massive debate regarding the sign and magnitude of the carbon budget from global to regional scales. Developing new models and estimates based on state-of-the-art algorithms and data constraints can provide valuable knowledge and contribute to a final ensemble model in which various optimal carbon budget estimates are integrated, such as the annual global carbon budget paper. Here, we develop a new atmospheric inversion system based on the 4D local ensemble transform Kalman filter (4D-LETKF) coupled with the GEOS-Chem global transport model to infer surface-to-atmosphere net carbon fluxes from Orbiting Carbon Observatory-2 (OCO-2) V10r XCO 2 retrievals. The 4D-LETKF algorithm is adapted to an OCO-2-based global carbon inversion system for the first time in this work. On average, the mean annual terrestrial and oceanic fluxes between 2015 and 2020 are estimated as - 2.02 and - 2.34 GtC yr -1 , respectively, compensating for 21 % and 24 %, respectively, of global fossil carbon dioxide (CO 2) emissions (9.80 GtC yr -1). Our inversion results agree with the CO 2 atmospheric growth rates reported by the National Oceanic and Atmospheric Administration (NOAA) and reduce the modeled CO 2 concentration biases relative to the prior fluxes against surface and aircraft measurements. Our inversion-based carbon fluxes are broadly consistent with those provided by other global atmospheric inversion models, although discrepancies still occur in the land–ocean flux partitioning schemes and seasonal flux amplitudes over boreal and tropical regions, possibly due to the sparse observational constraints of the OCO-2 satellite and the divergent prior fluxes used in different inversion models. Four sensitivity experiments are performed herein to vary the prior fluxes and uncertainties in our inversion system, suggesting that regions that lack OCO-2 coverage are sensitive to the priors, especially over the tropics and high latitudes. In the further development of our inversion system, we will optimize the data-assimilation configuration to fully utilize current observations and increase the spatial and seasonal representativeness of the prior fluxes over regions that lack observations. [ABSTRACT FROM AUTHOR]

Which published papers have transformed our understanding of the chemical processes in the troposphere and shaped the field of atmospheric chemistry? By way of expert solicitation and interactive peer review, this paper explores the influence of the ideas in peer-reviewed articles based on input from our community of atmospheric scientists. We explore how these papers have shaped the development of the field of atmospheric chemistry and identify the major landmarks in the field of atmospheric chemistry through the lens of those papers' impact on science, legislation and environmental events. We also explore the ways in which one can identify the papers that have most impacted the field and discuss the advantages and disadvantages of the various approaches. Our work highlights the difficulty of creating a simple list, and we explore the reasons for this difficulty. The paper also provides a history of the development of our understanding of tropospheric chemistry and points some ways for the future. [ABSTRACT FROM AUTHOR]

Understanding carbon dioxide (CO2) biospheric processes is of great importance because the terrestrial exchange drives the seasonal and inter-annual variability of CO2 in the atmosphere. Atmospheric inversions based on CO2 concentration measurements alone can only determine net biosphere fluxes, but not differentiate between photosynthesis (uptake) and respiration (production). Carbonyl sulfide (OCS) could provide an important additional constraint: it is also taken up by plants during photosynthesis but not emitted during respiration, and therefore is a potential mean to differentiate between these processes. Solar absorption Fourier Transform InfraRed (FTIR) spectrometry allows for the retrievals of the atmospheric concentrations of both CO2 and OCS from measured solar absorption spectra. Here, we investigate co-located and quasi-simultaneous FTIR measurements of OCS and CO2 performed at three selected sites located in the Northern Hemisphere. These measurements are compared to simulations of OCS and CO2 using a chemical transport model (GEOS-Chem). The OCS simulations are driven by different land biospheric fluxes to reproduce the seasonality of the measurements. Increasing the plant uptake of Kettle et al. (2002a) by a factor of three resulted in the best comparison with FTIR measurements. However, there are still discrepancies in the latitudinal distribution when comparing with HIPPO (HIAPER Pole-to-Pole Observations) data spanning both hemispheres. The coupled biospheric fluxes of OCS and CO2 from the simple biosphere model (SiB) are used in the study and compared to measurements. The CO2 simulation with SiB fluxes agrees with the measurements well, while the OCS simulation reproduced a weaker drawdown than FTIR measurements at selected sites, and a smaller latitudinal gradient in the Northern Hemisphere during growing season. An offset in the timing of the seasonal cycle minimum between SiB simulation and measurements is also seen. Using OCS as a photosynthesis proxy can help to understand how the biospheric processes are reproduced in models and to further understand the carbon cycle in the real world. [ABSTRACT FROM AUTHOR]

In the framework of the World Meteorological Organisation's Sand and Dust Storm Warning Advisory and Assessment System, we evaluated the predictions of five stateof- the-art dust forecast models during an intense Saharan dust outbreak affecting Western and Northern Europe in April 2011. We assessed the capacity of the models to predict the evolution of the dust cloud with lead-times of up to 72 h using observations of aerosol optical depth (AOD) from the Aerosol Robotic Network (AERONET) and the Moderate Resolution Imaging Spectroradiometer (MODIS), and dust surface concentrations from a ground-based measurement network. In addition, the predicted vertical dust distribution was evaluated with vertical extinction profiles from the Cloud and Aerosol Lidar with Orthogonal Polarization (CALIOP). To assess the diversity in forecast capability among the models, the analysis was extended to wind field (both surface and profile), synoptic conditions, emissions and deposition fluxes. Models predict the onset and evolution of the AOD for all analysed lead-times. On average, differences among the models are larger than differences among lead-times for each individual model. In spite of large differences in emission and deposition, the models present comparable skill for AOD. In general, models are better in predicting AOD than near-surface dust concentration over the Iberian Peninsula. Models tend to underestimate the long-range transport towards Northern Europe. Our analysis suggests that this is partly due to difficulties in simulating the vertical distribution dust and horizontal wind. Differences in the size distribution and wet scavenging efficiency may also account for model diversity in long-range transport. [ABSTRACT FROM AUTHOR]

This paper describes the methods that are used in the SAPRC mechanism generation system, MechGen, to estimate rate constants and derive mechanisms for gas-phase reactions of volatile organic compounds (VOCs) in the lower atmosphere. Versions of this system have been used for over 20 years in the development of the SAPRC mechanisms for air quality models, but this is the first complete documentation of the scientific basis for the chemical mechanisms it derives. MechGen can be used to derive explicit gas-phase mechanisms for most compounds with C, H, O, or N atoms. Included are reactions of organic compounds with hydroxy (OH) and nitrate (NO3) radicals, O3, and O3P; photolysis or unimolecular reactions; and the reactions of the radicals they form in the presence of O2 and oxides of nitrogen (NOx) at lower-atmospheric temperatures and pressures. Measured or theoretically calculated rate constants and branching ratios are used when data are available, but in most cases rate constants and branching ratios are estimated using various structure–reactivity or other estimation methods. Types of reactions include initial reactions of organics with atmospheric oxidants or by photolysis; unimolecular and bimolecular reactions of carbon-centered, alkoxy, and peroxy radicals; and those of Criegee and other intermediates that are formed. This paper documents the methods, assignments, and estimates currently used to derive these reactions and provides examples of MechGen predictions. Many of the estimation methods discussed here have not been published previously, and others have not been used previously in developing comprehensive mechanisms. Our knowledge of atmospheric reactions of organic compounds rapidly and continuously evolves, and therefore mechanism generation systems such as MechGen also need to evolve to continue to represent the current state of the science. This paper points out areas where MechGen may need to be modified when the system is next updated. This paper concludes with a summary of the major areas of uncertainty where further experimental, theoretical, or mechanism development research is most needed to improve predictions of atmospheric reaction mechanisms of volatile organic compounds. [ABSTRACT FROM AUTHOR]

This paper evaluates the current status of global modeling of the organic aerosol (OA) in the troposphere and analyzes the differences between models as well as between models and observations. Thirty-one global chemistry/transport and general circulation models have participated in this intercomparison, in the framework of AeroCom phase II. The simulation of OA varies greatly between models in terms of the magnitude of primary emissions, secondary OA (SOA) formation, the number of OA species used (2 to 62), the complexity of OA parameterizations (gas-particle partitioning, chemical aging, multiphase chemistry, aerosol microphysics), and the OA physical, chemical and optical properties. The diversity of the global OA simulation results has increased since earlier AeroCom experiments, mainly due to the increasing complexity of the SOA parameterization in models, and the implementation of new, highly uncertain, OA sources. Diversity of over an order of magnitude exists in the modeled vertical distribution of OA that deserves a dedicated future study. Furthermore, although the OA/OC ratio depends on OA sources and atmospheric processing and is important for model evaluation against OA and OC observations, it is resolved only by few global models. The median global primary OA (POA) source strength is 56 Tga-1 (range 34- 144 Tga-1) and the median secondary OA source strength (natural and anthropogenic) is 19 Tga-1 (range 13-121 Tga-1). Among the models that take into account the semi-volatile SOA nature, the median source is calculated to be 51 Tga-1 (range 16- 121 Tga-1), much larger than the median value of the models that calculate SOA in a more simplistic way (19 Tga-1; range 13-20 Tga-1, with one model at 37 Tga-1). The median atmospheric burden of OA is 1.4 Tg (24 models in the range of 0.6-2.0 Tg and 4 between 2.4-3.8 Tg) with a median OA lifetime of 5.4 days (range 3.8-9.6 days). In models that reported both OA and sulfate burdens, the median value of the OA/sulfate burden ratio of is calculated to be 0.77; 13 models calculate a ratio lower than 1, and 9 models higher than 1. For 26 models that reported OA deposition fluxes, the median wet removal is 70 Tga-1 (range 28-209 Tga-1), which is on average 85% of the total OA deposition. Fine aerosol organic carbon (OC) and OA observations from continuous monitoring networks and individual field campaigns have been used for model evaluation. At urban locations the model-observation comparison indicates missing knowledge on anthropogenic OA sources, both strength and seasonality. The combined model/measurements analysis suggests the existence of increased OA levels during summer due to biogenic SOA formation over large areas of the USA that can be of the same order of magnitude as the POA, even at urban locations, and contribute to the measured urban seasonal pattern. Global models are able to simulate the high secondary character of OA observed in the atmosphere as a result of SOA formation and of POA aging, although, the amount of OA present in the atmosphere remains largely underestimated, with a mean normalized bias (MNB) equal to -0.62 (-0.51) based on the comparison against OC (OA) urban data of all models at surface, -0.15 (+0.51) when compared with remote measurements, and -0.30 for marine locations with OC data. The correlations overall are low when comparing with OC (OA) measurements: 0.47 (0.52) for urban stations, 0.39 (0.37) for remote, and 0.25 for marine stations with OC data. The combination of high (negative) MNB and higher correlation at urban stations when compared with the low MNB and lower correlation at remote sites suggests that the knowledge about the processes, on top of the sources, are important at the remote stations. There is no clear change in model skill with increasing model complexity with regard to OC or OA mass concentration. However, the complexity is needed in models in order to separate between anthropogenic and natural OA and accurately calculate the impact of OA on climate. [ABSTRACT FROM AUTHOR]

The quantification of ammonia (NH3) emissions is essential to the more accurate quantification of atmospheric nitrogen deposition, improved air quality and the assessment of ammonia-related agricultural policy and climate mitigation strategies. The quantity, geographic distribution and historical trends of these emissions remain largely uncertain. In this paper, a new Chinese agricultural fertilizer NH3 (CAF NH3) emissions inventory has been compiled that exhibits the following improvements: (1) a 1 km × 1 km gridded map on the county level was developed for 2008, (2) a combined bottom-up and top-down method was used for the local correction of emission factors (EFs) and parameters, (3) the spatial and temporal patterns of historical time trends for 1978-2008 were estimated and the uncertainties were quantified for the inventories, and (4) a sensitivity test was performed in which a province-level disaggregated map was compared with CAF NH3 emissions for 2008. The total CAF NH3 emissions for 2008 were 8.4 Tg NH3 yr-1 (a 6.6-9.8 Tg interquartile range). From 1978 to 2008, annual NH3 emissions fluctuated with three peaks (1987, 1996 and 2005), and total emissions increased from 3.2 to 8.4 Tg at an annual rate of 3.0 %. During the study period, the contribution of livestock manure spreading increased from 37.0 to 45.5 % because of changing fertilization practices and the rapid increase in egg, milk and meat consumption. The average contribution of synthetic fertilizer, which has a positive effect on crop yields, was approximately 38.3 % (minimum: 33.4 %; maximum: 42.7 %). With rapid urbanization causing a decline in the rural population, the contribution of the rural excrement sector varied widely between 20.3 and 8.5 %. The average contributions of cake fertilizer and straw returning were approximately 3.8 and 4.5 %, respectively, thus small and stable. Collectively, the CAF-NH3 emissions reflect the nation's agricultural policy to a certain extent. An effective approach to decreasing PM2.5 concentrations in China would be to simultaneously decrease NOx, SO2 and NH3 emissions. [ABSTRACT FROM AUTHOR]

Motivated by the need to predict how the Arctic atmosphere will change in a warming world, this article summarizes recent advances made by the research consortium NETCARE (Network on Climate and Aerosols: Addressing Key Uncertainties in Remote Canadian Environments) that contribute to our fundamental understanding of Arctic aerosol particles as they relate to climate forcing. The overall goal of NETCARE research has been to use an interdisciplinary approach encompassing extensive field observations and a range of chemical transport, earth system, and biogeochemical models. Several major findings and advances have emerged from NETCARE since its formation in 2013. (1) Unexpectedly high summertime dimethyl sulfide (DMS) levels were identified in ocean water (up to 75 nM) and the overlying atmosphere (up to 1 ppbv) in the Canadian Arctic Archipelago (CAA). Furthermore, melt ponds, which are widely prevalent, were identified as an important DMS source (with DMS concentrations of up to 6 nM and a potential contribution to atmospheric DMS of 20 % in the study area). (2) Evidence of widespread particle nucleation and growth in the marine boundary layer was found in the CAA in the summertime, with these events observed on 41 % of days in a 2016 cruise. As well, at Alert, Nunavut, particles that are newly formed and grown under conditions of minimal anthropogenic influence during the months of July and August are estimated to contribute 20 % to 80 % of the 30–50 nm particle number density. DMS-oxidation-driven nucleation is facilitated by the presence of atmospheric ammonia arising from seabird-colony emissions, and potentially also from coastal regions, tundra, and biomass burning. Via accumulation of secondary organic aerosol (SOA), a significant fraction of the new particles grow to sizes that are active in cloud droplet formation. Although the gaseous precursors to Arctic marine SOA remain poorly defined, the measured levels of common continental SOA precursors (isoprene and monoterpenes) were low, whereas elevated mixing ratios of oxygenated volatile organic compounds (OVOCs) were inferred to arise via processes involving the sea surface microlayer. (3) The variability in the vertical distribution of black carbon (BC) under both springtime Arctic haze and more pristine summertime aerosol conditions was observed. Measured particle size distributions and mixing states were used to constrain, for the first time, calculations of aerosol–climate interactions under Arctic conditions. Aircraft- and ground-based measurements were used to better establish the BC source regions that supply the Arctic via long-range transport mechanisms, with evidence for a dominant springtime contribution from eastern and southern Asia to the middle troposphere, and a major contribution from northern Asia to the surface. (4) Measurements of ice nucleating particles (INPs) in the Arctic indicate that a major source of these particles is mineral dust, likely derived from local sources in the summer and long-range transport in the spring. In addition, INPs are abundant in the sea surface microlayer in the Arctic, and possibly play a role in ice nucleation in the atmosphere when mineral dust concentrations are low. (5) Amongst multiple aerosol components, BC was observed to have the smallest effective deposition velocities to high Arctic snow (0.03 cm s -1). [ABSTRACT FROM AUTHOR]

Long-term experimental stratospheric NO 2 and NO partial columns measured by means of solar Fourier-transform infrared (FTIR) spectrometry at Zugspitze (47.42° N, 10.98° E; 2964 m a.s.l.), Germany, were used to create a set of experiment-based monthly scaling factors (SF exp). The underlying data set is published in a companion paper (Nürnberg et al., 2024) and comprises over 25 years of measurements depicting the daytime variability of stratospheric NO 2 and NO partial columns with respect to local solar time (LST). In accordance with simulation-based scaling factors recently published by Strode et al. (2022), we created SF exp normalized to SZA =72° for NO 2 and NO for every month of the year as a function of solar zenith angle (SZA). Apart from a boundary value problem at minimum SZA values originating from averaging over different times of the month, the obtained scaling factors SF exp (NO 2) and SF exp (NO) as a function of SZA represent the daytime behavior already shown in model simulations and experiments in the literature very well. This shows a well-pronounced increase in the NO 2 and NO stratospheric partial column with the time of the day and a flattening of this increase after noon. In addition to the discussion of SF exp , we validate the simulation-based scaling factors SF sim (NO 2) (Strode et al., 2022) and present simulation-based scaling factors for NO SF sim (NO). The simulation-based scaling factors show excellent agreement with the experiment-based ones; i.e., for NO 2 and NO the mean value of the modulus between the experiment and simulation over all SZAs and months is only 0.02 %. We show that recently used model simulations can describe the real behavior of nitrogen oxide (NO x) variability in the stratosphere very well. Furthermore, we conclude that ground-based FTIR measurements can be used for validation of the output of photochemistry models and for creating experiment-based data sets describing the daytime stratospheric NO x variability as a function of SZA. This is a contribution to improved satellite validation and a better understanding of stratospheric photochemistry. [ABSTRACT FROM AUTHOR]

An artificial neural network cloud classification scheme is combined with A-Train observations to characterize the physical properties and radiative effects of marine low clouds based on their morphology and type of mesoscale cellular convection (MCC) on a global scale. The cloud morphological categories are (i) organized closed MCC, (ii) organized open MCC and (iii) cellular but disorganized MCC. Global distributions of the frequency of occurrence of MCC types show clear regional signatures. Organized closed and open MCCs are most frequently found in subtropical regions and in mid-latitude storm tracks of both hemispheres. Cellular but disorganized MCC are the predominant type of marine low clouds in regions with warmer sea surface temperature such as in the tropics and trade wind zones. All MCC types exhibit a pronounced seasonal cycle. The physical properties of MCCs such as cloud fraction, radar reflectivity, drizzle rates and cloud top heights as well as the radiative effects of MCCs are found highly variable and a function of the type of MCC. On a global scale, the cloud fraction is largest for closed MCC with mean cloud fractions of about 90%whereas cloud fractions of open and cellular but disorganized MCC are only about 51% and 40 %, respectively. Probability density functions (PDFs) of cloud fractions are heavily skewed and exhibit modest regional variability. PDFs of column maximum radar reflectivities and inferred cloud base drizzle rates indicate fundamental differences in the cloud and precipitation characteristics of different MCC types. Similarly, the radiative effects of MCCs differ substantially from each other in terms of shortwave reflectance and transmissivity. These differences highlight the importance of low cloud morphologies and their associated cloudiness on the shortwave cloud forcing. [ABSTRACT FROM AUTHOR]

Savanna fires contribute approximately 40-50% of total global annual biomass burning carbon emissions. Recent comparisons of emission factors from different savanna regions have highlighted the need for a regional approach to emission factor development, and better assessment of the drivers of the temporal and spatial variation in emission factors. This paper describes the results of open-path Fourier Transform Infrared (OP-FTIR) spectroscopic field measurements at twenty-one fires occurring in the tropical savannas of the Northern Territory, Australia, within different vegetation assemblages and at different stages of the dry season. Spectra of infrared light passing through a long (22-70m) open-path through ground-level smoke released from these fires were collected using an infrared lamp and a field-portable FTIR system. The IR spectra were used to retrieve the mole fractions of fourteen different gases present within the smoke, and these measurements used to calculate the emission ratios and emission factors of the various gases emitted by the burning. Only a handful of previous emission factor measures are available specifically for the tropical savannas of Australia and here we present the first reported emission factors for methanol, acetic acid, and formic acid for this biome. Given the relatively large sample size, it was possible to study the potential causes of the within-biome variation of the derived emission factors. We find that the emission factors vary substantially between different savanna vegetation assemblages; with a majority of this variation being mirrored by variations in the modified combustion efficiency (MCE) of different vegetation classes. We conclude that a significant majority of the variation in the emission factor for trace gases can be explained by MCE, irrespective of vegetation class, as illustrated by variations in the calculated methane emission factor for different vegetation classes using data subsetted by different combustion efficiencies. Therefore, the selection of emission factors for emissions modelling purposes need not necessarily require detailed fuel type information, if data on MCE (e.g. from future spaceborne total column measurements) or a correlated variable were available. From measurements at twenty-one fires, we recommend the following emission factors for Australian tropical savanna fires (in grams of gas emitted per kilogram of dry fuel burned) which are our mean measured values: 1674 gkg-1 of carbon dioxide; 87 gkg-1 of carbon monoxide; 2.1 gkg-1 of methane; 0.11 gkg-1 of acetylene; 0.49 gkg-1 of ethylene; 0.08 gkg-1 of ethane; 1.57 gkg-1 of formaldehyde; 1.06 gkg-1 5 of methanol; 1.54 gkg-1 of acetic acid; 0.16 gkg-1 of formic acid; 0.53 gkg-1 of hydrogen cyanide; and 0.70 gkg-1 of ammonia. [ABSTRACT FROM AUTHOR]

As the ownership of vehicles and frequency of utilization increase, vehicle emissions have become an important source of air pollution in Chinese cities. An accurate emission inventory for on-road vehicles is necessary for numerical air quality simulation and the assessment of implementation strategies. This paper presents a bottom-up methodology based on the local emission factors, complemented with the widely used emission factors of Computer Programme to Calculate Emissions from Road Transport (COPERT) model and near real time (NRT) traffic data on road segments to develop a high temporal-spatial resolution vehicle emission inventory (HTSVE) for the urban Beijing area. To simulate real-world vehicle emissions accurately, the road has been divided into segments according to the driving cycle (traffic speed) on this road segment. The results show that the vehicle emissions of NOx, CO, HC and PM were 10.54×104, 42.51×104 and 2.13×104 and 0.41×104 Mg, respectively. The vehicle emissions and fuel consumption estimated by the model were compared with the China Vehicle Emission Control Annual Report and fuel sales thereafter. The grid-based emissions were also compared with the vehicular emission inventory developed by the macro-scale approach. This method indicates that the bottom-up approach better estimates the levels and spatial distribution of vehicle emissions than the macro-scale method, which relies on more information. Additionally, the on-road vehicle emission inventory model and control effect assessment system in Beijing, a vehicle emission inventory model, was established based on this study in a companion paper (He et al., 2015). [ABSTRACT FROM AUTHOR]

The presence of polar mesospheric clouds (PMCs) at high latitudes could affect the retrieval of ozone profiles using backscattered ultraviolet (BUV) measurements. PMC-induced errors in ozone profile retrievals from Ozone Monitoring Instrument (OMI) BUV measurements are investigated through comparisons with Microwave Limb Sounder (MLS) ozone measurements. This comparison demonstrates that the presence of PMCs leads to systematic biases at altitudes above 6 hPa in summer high latitudes; the biases increase from ∼ -2 % at 2 hPa to ∼ -20 % at 0.5 hPa on average, and are significantly correlated with brightness of PMCs. Sensitivity studies show that the radiance sensitivity to PMCs strongly depends on wavelengths, increasing by a factor of ∼ 4 from 300 to 265 nm. It also strongly depends on the PMC scattering, thus depending on viewing geometry. The optimal estimation-based retrieval sensitivity analysis shows that PMCs located at 80-85 km have the greatest effect on ozone retrievals at ∼ 0.2 hPa (∼ 60 km), where the retrieval errors range from -2.5 % with PMC optical depth (POD) of 10-4 to -20 % with 10-3 at back scattering angles, and the impacts increase by a factor of ∼ 5 at forward scattering angles due to stronger PMC sensitivities. To reduce the interference of PMCs on ozone retrievals, we perform simultaneous retrievals of POD and ozone with a loose constraint of 10-3 for POD, which results in retrieval errors of 1-4 × 10-4. It is demonstrated that the negative bias of OMI ozone retrievals relative to MLS could be improved by including the PMC in the forward model calculation and retrieval. [ABSTRACT FROM AUTHOR]

This paper assesses the reasons for high ice number concentrations observed in orographic clouds by comparing in-situ measurements from the Ice NUcleation Process Investigation And Quantification field campaign (INUPIAQ) at Jungfraujoch, Switzerland (3570 m a.s.l.) with the Weather Research and Forecasting model (WRF) simulations over real terrain surrounding Jungfraujoch. During the 2014 winter field campaign, between the 20 January and 28 February, the model simulations regularly underpredicted the observed ice number concentration by 103 L-1. Previous literature has proposed several processes for the high ice number concentrations in orographic clouds, including an increased ice nuclei (IN) concentration, secondary ice multiplication and the advection of surface ice crystals into orographic clouds. We find that increasing IN concentrations in the model prevents the simulation of the mixed-phase clouds that were witnessed during the INUPIAQ campaign at Jungfraujoch. Additionally, the inclusion of secondary ice production upwind of Jungfraujoch into the WRF simulations cannot consistently produce enough ice splinters to match the observed concentrations. A surface flux of hoar crystals was included in the WRF model, which simulated ice concentrations comparable to the measured ice number concentrations, without depleting the liquid water content (LWC) simulated in the model. Our simulations therefore suggest that high ice concentrations observed in mixed-phase clouds at Jungfraujoch are caused by a flux of surface hoar crystals into the orographic clouds. [ABSTRACT FROM AUTHOR]

Biomass burning releases trace gases and aerosol particles that significantly affect the composition and chemistry of the atmosphere. Australia contributes approximately 8% of gross global carbon emissions from biomass burning, yet there are few previous measurements of emissions from Australian forest fires available in the literature. This paper describes the results of field measurements of trace gases emitted during hazard reduction burns in Australian temperate forests using open-path Fourier transform infrared spectroscopy. In a companion paper, similar techniques are used to characterise the emissions from hazard reduction burns in the savanna regions of the Northern Territory. Details of the experimental methods are explained, including both the measurement set-up and the analysis techniques employed. The advantages and disadvantages of different ways to estimate whole-fire emission factors are discussed and a measurement uncertainty budget is developed. Emission factors for Australian temperate forest fires are given for the first time for many trace gases. We recommend the following emission factors for Australian temperate forest fires (in grams of gas emitted per kilogram of dry fuel burned) which are our mean measured values: 1620 gkg-1 of carbon dioxide; 118 gkg-1 of carbon monoxide; 3.5 gkg-1 of methane; 1.3 gkg-1 of ethylene; 1.7 gkg-1 of formaldehyde; 2.4 gkg-1 of methanol; 3.8 gkg-1 of acetic acid; 0.4 gkg-1 of formic acid; 1.7 gkg-1 of ammonia; 0.15 gkg-1 of nitrous oxide and 0.5 gkg-1 of ethane. [ABSTRACT FROM AUTHOR]

Open burning of household solid waste is a large source of air pollutants worldwide, especially in the Global South. However, waste burning emissions are either missing or have large uncertainties in local, regional, or global emission inventories due to limited emission factor (EF) and activity data. Detailed particulate matter (PM) chemical speciation data are even less available. This paper reports source profiles and EFs for PM 2.5 species as well as acidic and alkali gases measured from laboratory combustion of 10 waste categories that represent open burning in South Africa. Carbonaceous materials contributed more than 70 % of PM 2.5 mass. Elemental carbon (EC) was most abundant from flaming materials (e.g., plastic bags, textiles, and combined materials), and its climate forcing exceeded the corresponding CO 2 emissions by a factor of 2–5. Chlorine had the highest EFs among elements measured by X-ray fluorescence (XRF) for all materials. Vegetation emissions showed high abundances of potassium, consistent with its use as a marker for biomass burning. Fresh PM 2.5 emitted from waste burning appeared to be acidic. Moist vegetation and food discards had the highest hydrogen fluoride (HF) and PM fluoride EFs due to fluorine accumulation in plants, while burning rubber had the highest hydrogen chloride (HCl) and PM chloride EFs due to high chlorine content in the rubber. Plastic bottles, plastic bags, rubber, and food discards had the highest EFs for polycyclic aromatic hydrocarbons (PAHs) and nitro-PAHs as well as their associated toxicities. Distinct differences between odd and even carbon preferences were found for alkanes from biological and petroleum-based materials: dry vegetation, paper, textiles, and food discards show preference for the odd-numbered alkanes, while the opposite is true for plastic bottles, plastic bags, and rubber. As phthalates are used as plasticizers, their highest EFs were found for plastic bottles and bags, rubber, and combined materials. Data from this study will be useful for health and climate impact assessments, speciated emission inventories, source-oriented dispersion models, and receptor-based source apportionment. [ABSTRACT FROM AUTHOR]

Transport pathways of air originating in the upper tropospheric Asian monsoon anticyclone are investigated based on three-dimensional trajectories. The Asian monsoon anticyclone emerges in response to persistent deep convection over India and southeast Asia in northern summer, and this convection is associated with rapid transport from the surface to the upper troposphere, and possibly into the stratosphere. Here, we investigate the fate of air that originates within the upper tropospheric anticyclone from the outflow of deep convection, using trajectories driven by ERA-interim reanalysis data. Calculations include isentropic estimates, plus fully three-dimensional results based on kinematic and diabatic transport calculations. Isentropic calculations show that air parcels are typically confined within the anticyclone for 10-20 days, and spread over the tropical belt within a month of their initialization. However, only few parcels (3 % at 360 K, 8 % at 380 K) reach the extratropical stratosphere by isentropic mixing. When considering vertical transport we find that 31 % (48%) of the trajectories reach the stratosphere within 60 days when using vertical velocities or diabatic heating rates to calculate vertical transport, respectively. In both cases, most parcels that reach the stratosphere are transported upward within the anticyclone and enter the stratosphere in the tropics, typically 10-20 days after their initialization at 360 K. This suggests that trace gases, including pollutants, that are transported into the stratosphere via the Asian monsoon system are in a position to enter the tropical pipe and thus be transported into the deep stratosphere. Sensitivity calculations with respect to the initial altitude of the trajectories showed that air needs to be transported to levels of 360 K or above by deep convection to likely (≥50 %) reach the stratosphere through transport by the large-scale circulation. [ABSTRACT FROM AUTHOR]

The fluorescent nature of aerosol at a high Alpine site was studied using a wide-band integrated bioaerosol (WIBS-4) single particle multi-channel ultra violet-light induced fluorescence (UV-LIF) spectrometer. This was supported by comprehensive cloud microphysics and meteorological measurements with the aims of cataloguing concentrations of bio-fluorescent aerosols at this high altitude site and also investigating possible influences of UV-fluorescent particle types on cloud-aerosol processes. Analysis of background free tropospheric air masses, using a total aerosol inlet, showed there to be a minor but statistically insignificant increase in the fluorescent aerosol fraction during in-cloud cases compared to out of cloud cases. The size dependence of the fluorescent aerosol fraction showed the larger aerosol to be more likely to be fluorescent with 80% of 10 μm particles being fluorescent. Whilst the fluorescent particles were in the minority (NFl=NAll = 0.27±0.19), a new hierarchical agglomerative cluster analysis approach, Crawford et al. (2015) revealed the majority of the fluorescent aerosol were likely to be representative of fluorescent mineral dust. A minor episodic contribution from a cluster likely to be representative of primary biological aerosol particles (PBAP) was also observed with a wintertime baseline concentration of 0.1±0.4 L-1. Given the low concentration of this cluster and the typically low ice active fraction of studied PBAP (e.g. pseudomonas syringae) we suggest that the contribution to the observed ice crystal concentration at this location is not significant during the wintertime. [ABSTRACT FROM AUTHOR]

Data from the standard cloud physics payload during the NSF/NCAR High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole-to-Pole Observations (HIPPO) campaigns provide a snapshot of unusual wintertime microphysical conditions in the boundary layer over the Southern Ocean. On 29 June 2011, the HIAPER sampled the boundary layer in a region of pre-frontal warm air advection between 58 and 48° S to the south of Tasmania. Cloud droplet number concentrations were consistent with climatological values in the northernmost profiles but were exceptionally high for wintertime in the Southern Ocean at 100-200 cm-3 in the southernmost profiles. Sub-micron (0.06-3. Analysis of back trajectories and atmospheric chemistry observations revealed that while conditions in the troposphere were more typical of a clean remote ocean airmass, there was some evidence of continental or anthropogenic influence. However, the hypothesis of long range transport of continental aerosol fails to explain the magnitude of the aerosol and cloud droplet concentration in the boundary layer. Instead, the gale force surface winds in this case (wind speed at 167 m above sea level was >25 m s-1) were most likely responsible for production of sea spray aerosol which influenced the microphysical properties of the boundary layer clouds. The smaller size and higher number concentration of cloud droplets is inferred to increase the albedo of these clouds, and these conditions occur regularly, and are expected to increase in frequency, over windy parts of the Southern Ocean. [ABSTRACT FROM AUTHOR]

A 30-year (1980–2010) climatology of the major variables and terms of the transformed Eulerian-mean (TEM) momentum and thermodynamic equations is constructed by using four global atmospheric reanalyses: the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2); the Japanese 55-year Reanalysis (JRA-55); the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim); and the Climate Forecast System Reanalysis (CFSR). Both the reanalysis ensemble mean (REM) and the differences in each reanalysis from the REM are investigated in the latitude–pressure domain for December–January–February and for June–July–August. For the REM investigation, two residual vertical velocities (the original one and one evaluated from residual meridional velocity) and two mass streamfunctions (from meridional and vertical velocities) are compared. Longwave (LW) radiative heating and shortwave (SW) radiative heating are also shown and discussed. For the TEM equations, the residual terms are also calculated and investigated for their potential usefulness, as the residual term for the momentum equation should include the effects of parameterized processes such as gravity waves, while that for the thermodynamic equation should indicate the analysis increment. Inter-reanalysis differences are investigated for the mass streamfunction, LW and SW heating, the two major terms of the TEM momentum equation (the Coriolis term and the Eliassen–Palm flux divergence term), and the two major terms of the TEM thermodynamic equation (the vertical temperature advection term and the total diabatic heating term). The spread among reanalysis TEM momentum balance terms is around 10 % in Northern Hemisphere winter and up to 50 % in Southern Hemisphere winter. The largest uncertainties in the thermodynamic equation (about 50 %) are found in the vertical advection, for which the structure is inconsistent with the differences in heating. The results shown in this paper provide basic information on the degree of agreement among recent reanalyses in the stratosphere and upper troposphere in the TEM framework. [ABSTRACT FROM AUTHOR]

Nitrogen oxides (NOx = NO + NO2) are air pollutants which are co-emitted with CO2 during high-temperature combustion processes. Monitoring NOx emissions is crucial for assessing air quality and for providing proxy estimates of CO2 emissions. Satellite observations, such as those from the TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel-5P satellite, provide global coverage at high temporal resolution. However, satellites measure only NO2 , necessitating a conversion to NOx. Previous studies have applied a constant NO2 -to- NOx conversion factor. In this paper, we develop a more realistic model for NO2 -to- NOx conversion and apply it to TROPOMI data of 2020 and 2021. To achieve this, we analysed plume-resolving simulations from the MicroHH large-eddy simulation model with chemistry for the Bełchatów (PL), Jänschwalde (DE), Matimba (ZA) and Medupi (ZA) power plants, as well as a metallurgical plant in Lipetsk (RU). We used the cross-sectional flux method to calculate NO , NO2 and NOx line densities from simulated NO and NO2 columns and derived NO2 -to- NOx conversion factors as a function of the time since emission. Since the method of converting NO2 to NOx presented in this paper assumes steady-state conditions and that the conversion factors can be modelled by a negative exponential function, we validated the conversion factors using the same MicroHH data. Finally, we applied the derived conversion factors to TROPOMI NO2 observations of the same sources. The validation of the NO2 -to- NOx conversion factors shows that they can account for the NOx chemistry in plumes, in particular for the conversion between NO and NO2 near the source and for the chemical loss of NOx further downstream. When applying these time-since-emission-dependent conversion factors, biases in NOx emissions estimated from TROPOMI NO2 images are greatly reduced from between - 50 % and - 42 % to between only - 9.5 % and - 0.5 % in comparison with reported emissions. Single-overpass estimates can be quantified with an uncertainty of 20 %–27 %, while annual NOx emission estimates have uncertainties in the range of 4 %–21 % but are highly dependent on the number of successful retrievals. Although more simulations covering a wider range of meteorological and trace gas background conditions will be needed to generalise the approach, this study marks an important step towards a consistent, uniform, high-resolution and near-real-time estimation of NOx emissions – especially with regard to upcoming NO2 -monitoring satellites such as Sentinel-4, Sentinel-5 and CO2M. [ABSTRACT FROM AUTHOR]

Independent estimation and verification of fossil CO2 emissions on a regional and national scale are crucial for evaluating the fossil CO2 emissions and reductions reported by countries as part of their nationally determined contributions (NDCs). Top-down methods, such as the assimilation of in situ and satellite observations of different tracers (e.g., CO2 , CO , Δ14CO2 , XCO2), have been increasingly used for this purpose. In this paper, we use the Lund University Modular Inversion Algorithm (LUMIA) to estimate fossil CO2 emissions and natural fluxes by simultaneously inverting in situ synthetic observations of CO2 and Δ14CO2 over Europe. We evaluate the inversion system by conducting a series of observing system simulation experiments (OSSEs). We find that in regions with a dense sampling network, such as western/central Europe, adding Δ14CO2 observations in an experiment where the prior fossil CO2 and biosphere fluxes are set to zero allows LUMIA to recover the time series of both categories. This reduces the prior-to-truth root mean square error (RMSE) from 1.26 to 0.12 TgC d−1 in fossil CO2 and from 0.97 to 0.17 TgC d−1 in biosphere fluxes, reflecting the true total CO2 budget by 91 %. In a second set of experiments using realistic prior fluxes, we find that in addition to retrieving the time series of the optimized fluxes, we are able to recover the true regional fossil CO2 budget in western/central Europe by 95 % and in Germany by 97 %. In all experiments, regions with low sampling coverage, such as southern Europe and the British Isles, show poorly resolved posterior fossil CO2 emissions. Although the posterior biosphere fluxes in these regions follow the seasonal patterns of the true fluxes, a significant bias remains, making it impossible to close the total CO2 budget. We find that the prior uncertainty of fossil CO2 emissions does not significantly impact the posterior estimates, showing similar results in regions with good sampling coverage like western/central Europe and northern Europe. Finally, having a good prior estimate of the terrestrial isotopic disequilibrium is important to avoid introducing additional noise into the posterior fossil CO2 fluxes. [ABSTRACT FROM AUTHOR]

General circulation models (GCMs), unlike other lines of evidence, indicate that anthropogenic aerosols cause a global-mean increase in cloud liquid water path (L) and thus a negative adjustment to radiative forcing of the climate by aerosol–cloud interactions. In part 1 of this series of papers, we showed that this is true even in models that reproduce the negative correlation observed in present-day internal variability in L and cloud droplet number concentration (Nd). We studied several possible confounding mechanisms that could explain the noncausal cloud–aerosol correlations in GCMs and that possibly contaminate observational estimates of radiative adjustments. Here, we perform single-column and full-atmosphere GCM experiments to investigate the causal model-physics mechanisms underlying the model radiative adjustment estimate. We find that both aerosol–cloud interaction mechanisms thought to be operating in real clouds – precipitation suppression and entrainment evaporation enhancement – are active in GCMs and behave qualitatively in agreement with physical process understanding. However, the modeled entrainment enhancement has a negligible global-mean effect. This raises the question of whether the GCM estimate is incorrect due to parametric or base-state representation errors or whether the process understanding gleaned from a limited set of canonical cloud cases is insufficiently representative of the diversity of clouds in the real climate. Regardless, even at limited resolution, the GCM physics appears able to parameterize the small-scale microphysics–turbulence interplay responsible for the entrainment enhancement mechanism. We suggest ways to resolve tension between current and future (storm-resolving) global modeling systems and other lines of evidence in synthesis climate projections. [ABSTRACT FROM AUTHOR]

The paper evaluates the long-term (2007–2016) impact of biogenic volatile organic compound (BVOC) emissions on urban ozone patterns over central Europe, specifically focusing on the contribution of urban vegetation using a regional climate model coupled offline to a chemistry transport model. BVOCs are emitted by terrestrial ecosystems, and their impact is considered especially important over NOx-rich environments such as urban areas. The study evaluates the impact of BVOC emissions on ozone (O3), formaldehyde (HCHO), and hydroxyl radical (OH) near-surface concentrations, showing an increase in summer ozone by 6 %–10 % over large areas in central Europe due to their emissions. It also demonstrates a substantial increase in formaldehyde concentrations. Additionally, the impact of BVOC emissions on hydroxyl radical concentrations shows a decrease over most of the modeled region by 20 %–60 %, with some increases over urban areas. Impacts on peroxy radicals (HO2 and higher RO2) are shown too. Importantly, the study explores the partial role of urban vegetation in modulating ozone and evaluates its contribution to the overall ozone formation due to all BVOC emissions. The findings reveal that urban BVOC emissions contribute to around 10 % of the total impact on ozone and formaldehyde concentrations in urban areas, indicating their significant but localized influence. The study also conducts sensitivity analyses to assess the uncertainty arising from the calculation of the urban fraction of BVOC emissions. The results show that the impact of urban BVOC emissions responds to their magnitude nearly linearly, with variations up to 4-fold, emphasizing the importance of accurately quantifying the urban BVOC fluxes. Overall, the study sheds light on the intricate relationship between urban vegetation, BVOC emissions, and their impact on atmospheric chemistry, providing valuable insights into the regional chemistry of BVOC emissions over central Europe and the causes of urban ozone pollution. [ABSTRACT FROM AUTHOR]

Nitrous acid (HONO) is a main precursor of hydroxyl radicals (OH), which contribute to the formation of numerous secondary air pollutants in the troposphere. Despite its importance in atmospheric chemistry, HONO chemistry has not been fully incorporated into many chemical transport models (CTMs). Due to the lack of atmospheric HONO processes, CTM simulations often tend to underestimate atmospheric mixing ratios of HONO. This study was undertaken because simulations with the current Community Multiscale Air Quality (CMAQ) model have a strong tendency to underestimate the HONO mixing ratio. In search of missing sources of atmospheric HONO, we attempted to sequentially incorporate the following potential HONO sources and processes into the CMAQ modeling framework: (i) gas-phase HONO reactions, (ii) traffic HONO emissions, (iii) soil HONO emissions, (iv) heterogeneous HONO production on the surfaces of aerosols, (v) heterogeneous HONO formation on tree leaf and building surfaces, and (vi) photolysis reactions of particulates and deposited HNO3 /nitrates called "renoxification". The simulation performances of the modified CMAQ models were then evaluated by comparing the modeled HONO mixing ratios with the HONO mixing ratios observed at the Olympic Park station in Seoul, South Korea. When HONO processes were fully added to the CMAQ model, average daily HONO mixing ratios increased from 0.06 to 1.18 ppb. The daytime HONO mixing ratios produced from the CMAQ model run with a full account of atmospheric HONO processes were found to be in better agreement with observations than those from the original CMAQ model (CMAQv5.2.1) runs with improved statistical metrics (e.g., the index of agreement (IOA) increased from 0.59 to 0.68, while the mean bias (MB) decreased dramatically from -0.57 to -0.34 ppb). In addition, we investigated the contributions of individual atmospheric HONO processes to HONO mixing ratios, as well as the impacts of HONO atmospheric processes on the concentrations of other atmospheric species in South Korea. All these issues are also discussed in this paper. [ABSTRACT FROM AUTHOR]

Mineral dust significantly influences the Earth's climate system by affecting the radiative balance through the emission, absorption and scattering of solar and terrestrial radiation. Estimating the dust radiative effect remains challenging due to the lack of detailed information on the physical and chemical properties of dust. High-spectral-resolution instruments in the infrared (IR) spectrum, such as the Infrared Atmospheric Sounding Instrument (IASI), have demonstrated the ability to quantify these aerosol properties. A crucial parameter for characterizing mineral dust from space is the complex refractive index (CRI), as it links the dust's physical and chemical properties to its optical properties. This paper examines the impact of six prior laboratory CRI datasets to improve the characterization of dust microphysical properties using IASI. The CRIs include older measurements obtained through the classical pellet method, commonly employed in mineral dust applications, as well as newer datasets that incorporate the latest advancements in laboratory measurement techniques for aerosol generation. These datasets are tested on IASI measurements during a dust storm event over the Gobi Desert in May 2017. We evaluate the sensitivity of IASI to different CRI datasets using the Atmospheric Radiation Algorithm for High-Spectral Resolution Measurements from Infrared Spectrometer (ARAHMIS) radiative transfer algorithm and explore the datasets' impact on retrieving size distribution parameters by mapping their spatial distributions. The results indicate that the laboratory CRI datasets decrease the total error in the covariance matrix by 30 %. In addition, we assess the ability to accurately reconstruct IASI detections and the extent to which we can retrieve the microphysical properties of dust particles. The choice of CRI significantly impacts the accuracy of dust detection and characterization from satellite observations. Notably, datasets that incorporate recent aerosol generation techniques with higher spectral resolution and samples from the case study region show improved compatibility with IASI observations. The outcomes of this research emphasize two key points: the crucial link between chemical composition of dust and its optical properties and the importance of considering the specific composition of the CRI dataset for improved retrieval of the microphysical parameters. Furthermore, this study highlights the critical role of ongoing enhancements in CRI measurement approaches, as well as the potential of high-spectral-resolution infrared sounders for aerosol atmospheric investigation and for understanding the radiative impacts of sounders. [ABSTRACT FROM AUTHOR]

The ammonia cycle and nitrate particle formation have been introduced in the LMDz- INCA global model. Both fine nitrate particles formation in the accumulation mode and coarse nitrate forming on existing dust and sea-salt particles are considered. The model simulates distributions of nitrates and related species in agreement with previous studies and observations. The calculated present-day total nitrate direct radiative forcing since the pre-industrial is -0.056Wm-2. This forcing has the same magnitude than the forcing associated with organic carbon particles and represents 18% of the sulfate forcing. Fine particles largely dominate the nitrate forcing representing close to 90% of this value. The model has been used to investigate the future changes in nitrates and direct radiative forcing of climate based on snapshot simulations for the four Representative Concentration Pathway (RCP) scenarios and for the 2030, 2050 and 2100 time horizons. Due to a decrease in fossil fuel emissions in the future, the concentrations of most of the species involved in the nitrate-ammonium-sulfate system drop by 2100 except for ammonia which originates from agricultural practices and for which emissions significantly increase in the future. Despite the decrease of nitrate surface levels in Europe and Northern America, the global burden of accumulation mode nitrates increases by up to a factor of 2.6 in 2100. This increase in nitrate in the future arises despite decreasing NOx emissions due to increased availability of ammonia to form ammonium nitrate. The total aerosol direct forcing decreases from its present-day value of -0.234Wm-2 to a range of -0.070 to -0.130Wm-2 in 2100 based on the considered scenario. The direct forcing decreases for all aerosols except for nitrates for which the direct negative forcing increases to a range of -0.060 to -0.115Wm-2 in 2100. Including nitrates in the radiative forcing calculations increases the total direct forcing of aerosols by a factor of 1.3 in 2000, by a factor of 1.7-2.6 in 2030, by 1.9-4.8 in 2050 and by 6.4-8.6 in 2100. These results show that agricultural emissions of ammonia will play a key role in the future mitigation of climate change with nitrates becoming the dominant contributor to the anthropogenic aerosol optical depth during the second half of the XXIst century and significantly increasing the calculated aerosol direct forcing. This significant increase in the influence that nitrate exert on climate in the future will at the same time affect regional air quality and nitrogen deposition to the ecosystems. [ABSTRACT FROM AUTHOR]

The impact of climate and emissions changes on the deposition of reactive nitrogen (Nr) over Europe was studied using four offline regional chemistry transport models (CTMs) driven by the same global projection of future climate over the period 2000- 2050. Anthropogenic emissions for the years 2005 and 2050 were used for simulations of both present and future periods in order to isolate the impact of climate change, hemispheric boundary conditions and emissions, and to assess the robustness of the result across the different models. The results from these four CTMs clearly show that the main driver of future N deposition changes is the specified emission change. Under the specified emission scenario for 2050, emissions of oxidised nitrogen were reduced substantially, whereas emissions of NH3 increase to some extent, and these changes are largely reflected in the modelled concentrations and depositions. The lack of sulphur and oxidised nitrogen in the future atmosphere results in a much larger fraction of NHx being present in the form of gaseous ammonia. Predictions for wet and total deposition were broadly consistent, although the three fine-scale models resolve European emission areas and changes better than the hemispheric-scale model. The biggest difference in the models is for predictions of individual N-compounds. One model (EMEP) was used to explore changes in critical 20 loads, also in conjunction with speculative climate-induced increases in NH3 emissions. These calculations suggest that the area of ecosystems which exceed critical loads is reduced from 64% for year 2005 emissions levels to 50% for currently estimated 2050 levels. A possible climate-induced increase in NH3 emissions could worsen the situation, with areas exceeded increasing again to 57%(for a 30%NH3 emission increase). [ABSTRACT FROM AUTHOR]

Mineral dust interacts with incoming/outgoing electromagnetic radiation in the atmosphere. This interaction depends on the microphysical properties of the dust particles, including size, mineral composition, external morphology, and internal structure. Ideally all these properties should be accounted for in dust remote sensing, the modeling of single-scattering properties, and radiative effect assessment. There have been many reports on the microphysical characterizations of mineral dust, but no investigations of the internal structures or mineral composition of individual dust particles. We explored the interiors of Asian dust particles using the combined application of focused ion beam thin-slice preparation and high-resolution transmission electron microscopy. The results showed that individual dust particles consisted of numerous mineral grains, which were organized into several types of internal structure: single and polycrystalline cores of quartz, feldspars, calcite, and amphibole often with oriented clay coatings; individual clay agglomerates of nano-thin clay platelets showing preferred to random orientations commonly with coarser mineral inclusions; and platy coarse phyllosilicates (muscovite, biotite, and chlorite). Micron to submicron pores were scattered throughout the interior of particles. Clays in the coatings and agglomerates were dominated by nano-thin platelets of the clay minerals of illite-smectite series including illite, smectite, and their mixed layers with subordinate kaolinite and clay-size chlorite. Submicron iron oxide grains, dominantly goethite, were distributed throughout the clay agglomerates and coatings. Unlike the common assumptions and simplifications, we found that the analyzed dust particles were irregularly shaped with birefringent, polycrystalline, and polymineralic heterogeneous compositions. Accounting for this structural and mineralogical makeup may improve the remote sensing retrieval of dust and the evaluation of radiation effects, but will also require sophisticated single-scattering modeling. In particular, the observed internal structures of dust particles such as clay coatings, preferred orientation, embedded grains in clays, and pores, likely have a great impact on the light scattering of dust particles. The distribution and size of structural components with contrasting dielectric properties, such as iron oxides, should also be explicitly accounted for. [ABSTRACT FROM AUTHOR]

This study examines N2O emission estimates from 5 different atmospheric inversion frameworks. The 5 frameworks differ in the choice of atmospheric transport model, meteorological data, prior uncertainties and inversion method but use the same prior emissions and observation dataset. The mean emissions for 2006 to 2008 are compared in terms of the spatial distribution and seasonality. Overall, there is a good agreement among the inversions for the mean global total emission, which ranges from 16.1 to 18.7 Tg N yr-1 and is consistent with previous estimates. Ocean emissions represent between 31% and 38% of the global total compared to widely varying previous estimates of 24% to 38%. Emissions from the northern mid to high latitudes are likely to be more important, with a consistent shift in emissions from the tropics and subtropics to the mid to high latitudes in the Northern Hemisphere; the emission ratio for 0-30° N to 30-90° N ranges from 1.5 to 1.9 compared with 2.9 to 3.0 in previous estimates. The largest discrepancies across inversions are seen for the regions of South and East Asia and for tropical and South America owing to the poor observational constraint for these areas and to considerable differences in the modelled transport, especially inter-hemispheric exchange rates and tropical convection. Estimates of the seasonal cycle in N2O emissions are also sensitive to errors in modelled stratosphere-to-troposphere transport in the tropics and southern extra-tropics. Overall, the results show a convergence in the global and regional emissions compared to previous independent studies. [ABSTRACT FROM AUTHOR]

Lightning production is described using thermodynamic hypotheses mainly according to convective activity in the atmosphere. However, existing formal thermodynamic descriptions are unable to fully explain the profound difference in the flash rate between tropical Africa and South America and between land and ocean. Aerosols are shown to be regulators of lightning in regional studies, but their influence on lightning production at the global scale is not described. I analyzed spatial patterns of the satellite annual global flash rate and simulated, annually averaged cloud condensation nuclei (CCN) distribution and found consistent positive correlation between them for land, ocean and continents. I developed a simple model of lightning production that is based solely on an aerosol hypothesis. The central premise of this model is that concentration of graupel pellets and concentration of ice crystals, which both determine flash rate, are monotonically increasing by CCN concentration up to a critical value (around 2000 cm-3). However, ice crystal concentration falls rapidly after the threshold due to lowering in the number of large cloud droplets effective for rime-splintering ice multiplication. Comparison of the model with a model of the flash rate based on thermodynamic hypotheses demonstrates that the aerosol hypothesis explains the global annual spatial distribution of lightning production consistently better over land and over oceans. My results emphasize importance of aerosols for lightning production and point to the existence of a global aerosol-lightning feedback, which affects both the climate system and the land surface. [ABSTRACT FROM AUTHOR]

This study presents results from the European Centre for Medium-Range Weather Forecasts (ECMWF) carbon dioxide (CO2) analysis system where the atmospheric CO2 is controlled through the assimilation of column-average dry-air mole fractions of CO2 (XCO2) from the Greenhouse gases Observing Satellite (GOSAT). The analysis is compared to a free run simulation and they are both evaluated against XCO2 data from the Total Carbon Column Observing Network (TCCON). We show that the assimilation of the GOSAT XCO2 product from the Bremen Optimal Estimation DOAS (BESD) algorithm during the year 2013 provides XCO2 fields with an improved station-to-station bias deviation of 0.7 parts per million (ppm) compared to the free run (1.4 ppm) and an improved estimated precision of ∼ 1 ppm compared to the used GOSAT data (3.4 ppm). We also show that the analysis has skill for synoptic situations in the vicinity of frontal systems where the GOSAT retrievals are sparse due to cloud contamination. We finally computed the 10 day forecast from each analysis at 00:00 UTC. Compared to its own analysis the CO2 forecast shows synoptic skill for the largest scale weather patterns even up to day 5 according to the anomaly correlation coefficient. [ABSTRACT FROM AUTHOR]

Transit properties across the tropical tropopause layer are studied using extensive forward and backward Lagrangian trajectories between cloud tops and the reference surface 380 K. The tropical domain being subdivided into 11 sub-regions according to the distribution of land and convection, we estimate the contribution of each region to the upward mass flux across the 380 K surface, the vertical distribution of convective sources and of transit times over the period 2005-2008. The good agreement between forward and backward statistics is the basis of the results presented here. It is found that about 85% of the tropical parcels at 380K originate from convective sources all along the year. From November to April, the sources are dominated by the warm pool which accounts for up to 70% of the upward flux. During summer, Asian monsoon region is the largest contributor with similar contributions from oceanic regions and Asian mainland, although the signature in vertical distribution and transit time is very different, Asian mainland displaying higher sources and smaller transit times. The Tibetan plateau, although a minor overall contributor, is found to be the region with the highest impact of convection at 380 K due to its central location beneath the Asian upper level anticyclone. We show the robustness of our main results to uncertainties in data and methods but we alsofind some sensitivity of the vertical distribution of sources and transit times to the representation of cloud tops and heating rates. [ABSTRACT FROM AUTHOR]

It is often implicitly assumed that over suitably long periods the mean of observations and models should be comparable, even if they have different temporal sampling. We assess the errors incurred due to ignoring temporal sampling and show they are offsimilar magnitude as (but smaller than) actual model errors (20-60%). Using temporal sampling from remote sensing datasets (the satellite imager MODIS and the ground-based sun photometer network AERONET) and three different global aerosol models, we compare annual and monthly averages of full model data to sampled model data. Our results show that sampling errors as large as 100% in AOT (Aerosol Optical Thickness), 0.4 in AE (Ångström Exponent) and 0.05 in SSA (Single Scattering Albedo) are possible. Even in daily averages, sampling errors can be significant. More-over these sampling errors are often correlated over long distances giving rise to artificial contrasts between pristine and polluted events and regions. Additionally, we provide evidence that suggests that models will underestimate these errors. To prevent sampling errors, model data should be temporally collocated to the observations before any analysis is made. We also discuss how this work has consequences for in-situ measurements (e.g. aircraft campaigns or surface measurements) in model evaluation. [ABSTRACT FROM AUTHOR]

Clouds are the dominant source of variability in surface solar radiation and uncertainty in its prediction. However, the increasing share of solar energy in the world-wide electric power supply increases the need for accurate solar radiation forecasts. In this work, we present results of a shortest-term global horizontal irradiance (GHI) forecast experiment based on hemispheric sky images. A two month dataset with images from one sky imager and high resolutive GHI measurements from 99 pyranometers distributed over 10km by 12km is used for validation. We developed a multi-step model and processed GHI forecasts up to 25min with an update interval of 15 s. A cloud type classification is used to separate the time series in different cloud scenarios. Overall, the sky imager based forecasts do not outperform the reference persistence forecasts. Nevertheless, we find that analysis and forecast performance depend strongly on the predominant cloud conditions. Especially convective type clouds lead to high temporal and spatial GHI variability. For cumulus cloud conditions, the analysis error is found to be lower than that introduced by a single pyranometer if it is used representatively for the whole area in distances from the camera larger than 1-2 km. Moreover, forecast skill is much higher for these conditions compared to overcast or clear sky situations causing low GHI variability which is easier to predict by persistence. In order to generalize the cloud-induced forecast error, we identify a variability threshold indicating conditions with positive forecast skill. [ABSTRACT FROM AUTHOR]

A simultaneous analysis of 13 years of remotely sensed data of land cover, fires, precipitation, and aerosols from the MODIS, TRMM, and MISR satellites and the AERONET network over Southeast Asia is performed, leading to a set of robust relationships be- tween land-use change and fire being found on inter-annual and intra-annual scales over Southeast Asia, reflecting the heavy amounts of anthropogenic influence over land use change and fires in this region of the world. First, we find that fires occur annually, but with a considerable amount of variance in their onset, duration, and intensity from year to year, and from two separate regions within Southeast Asia from each other. This variability is already partially understood from previous works, including the impacts of both inter-annually and intra-annually occurring influences such as the Monsoon and El-Nino events, but yet there are other as of yet unknown influences that also are found to strongly influence the results. Second, we show that a simple regression-model of the land-cover, fire, and precipitation data can be used to recreate a robust representation of the timing and magnitude of measured AOD from multiple measurements sources of this region using either 8-day (better for onset and duration) or monthly based (better for magnitude) measurements, but not daily measurements. We find that the reconstructed AOD matches the timing and intensity from AERONET measurements to within 70 to 90% and the timing and intensity of MISR measurements from to within 50 to 95%. This is a unique finding in this part of the world, since could-covered regions are large, yet the robustness of the model is still capable of holding over many of these regions, where otherwise no fires are observed and hence no emissions source contribution to AOD would otherwise be thought to occur. Third, we determine that while Southeast Asia is a source region of such intense smoke emissions, that it is also impacted by transport of smoke from other regions as well. There are regions in northern Southeast Asia which have two annual AOD peaks, one during the local fire season, and the second smaller peak corresponding to a combination of some local smoke sources as well as transport of aerosols from fires in southern Southeast Asia, and possibly even from anthropogenic sources in South Asia. Conversely, we show that southern Southeast Asia is affected exclusively by its own local fire sources during its own local fire season. Overall, this study highlights the importance of taking into account a simultaneous use of land-use, fire, and precipitation for understanding the impacts of fires on the atmospheric loading and distribution of aerosols in Southeast Asia over both space and time. [ABSTRACT FROM AUTHOR]

In this paper, we studied the frequency of occurrence and shortwave direct radiative effects (DRE) of above-cloud aerosols (ACAs) over global oceans using eight years of collocated CALIOP and MODIS observations. Similar to previous work, we found high ACA occurrence in four regions: Southeast (SE) Atlantic region where ACAs are mostly light-absorbing aerosols, i.e., smoke and polluted dust according to CALIOP classification, originating from biomass burning over African Savanna; Tropical Northeast Atlantic and Arabian Sea where ACAs are predominantly windblown dust from the Sahara and Arabian desert, respectively; and Northwest Pacific where ACAs are mostly transported smoke and polluted dusts from Asian. From radiative transfer simulations based on CALIOP-MODIS observations and a set of the preselected aerosol optical models, we found the DREs of ACAs at the top of atmosphere (TOA) to be positive (i.e., warming) in the SE Atlantic and NW Pacific regions, but negative (i.e., cooling) in TNE Atlantic and Arabian Sea. The cancellation of positive and negative regional DREs results in a global ocean annual mean diurnally averaged cloudy-sky DRE of 0.015Wm-2 (range of -0.03 to 0.06W-2) at TOA. The DREs at surface and within atmosphere are -0.15W-2 (range of -0.09 to -0.21W-2), and 0.17W-2 (range of 0.11 to 0.24W-2), respectively. The regional and seasonal mean DREs are much stronger. For example, in the SE Atlantic region the JJA (July ∼ August) seasonal mean cloudy-sky DRE is about 0.7W-2 (range of 0.2 to 1.2W-2) at TOA. The uncertainty in our DRE computations is mainly cause by the uncertainties in the aerosol optical properties, in particular aerosol absorption, and uncertainties in the CALIOP operational aerosol optical thickness retrieval. In situ and remotely sensed measurements of ACA from future field campaigns and satellite missions, and improved lidar retrieval algorithm, in particular vertical feature masking, would help reduce the uncertainty. [ABSTRACT FROM AUTHOR]

We use the Global Modeling Initiative (GMI) modeling framework to assess the utility of cosmogenic beryllium-7 (7Be), a natural aerosol tracer, for evaluating crosstropopause transport in global models. The GMI chemical transport model (CTM) was used to simulate atmospheric Be7 distributions using four different meteorological data sets (GEOS1-STRAT DAS, GISS II' GCM, fvGCM, and GEOS4-DAS), featuring significantly different stratosphere-troposphere exchange (STE) characteristics. The simulations were compared with the upper troposphere/lower stratosphere (UT/LS) Be7 climatology constructed from ∼ 25 years of aircraft and balloon data, as well as clima tological records of surface concentrations and deposition fluxes. Comparison of the fraction of surface air of stratospheric origin estimated from the Be7 simulations with observationally-derived estimates indicates excessive cross-tropopause transport at middle latitudes in simulations using GEOS1-STRAT and at high latitudes using GISS II0 meteorological data. These simulations also overestimate Be7 deposition fluxes at middle latitudes (GEOS1-STRAT) and at high latitudes (GISS II0), respectively. We show that excessive cross-tropopause transport of Be7 corresponds to overestimated stratospheric contribution to tropospheric ozone. Our perspectives on STE in these meteorological fields based on Be7 simulations are consistent with previous modeling studies of tropospheric ozone using the same meteorological fields. We further apply observational constraints to other global models including GFDL AM2 and GEOSChem (driven by GEOS3-DAS and GEOS5-DAS). We conclude that the observational constraints for Be7 and observed Be7 total deposition fluxes can be used routinely as a first-order assessment of cross-tropopause transport in global models. [ABSTRACT FROM AUTHOR]

Small-scale nonlinear chemical and physical processes over pollution source regions affect the global ozone (O3) chemistry, but these processes are not captured by current global chemical transport models (CTMs) and chemistry-climate models that are limited by coarse horizontal resolutions (100-500 km, typically 200 km). These models tend to contain large (and mostly positive) tropospheric O3 biases in the Northern Hemisphere. Here we use a recently built two-way coupling system of the GEOS-Chem CTM to simulate the global tropospheric O3 in 2009. The system couples the global model (at 2.5° long. × 2° lat.) and its three nested models (at 0.667° long. × 0.5° lat.) covering Asia, North America and Europe, respectively. Benefiting from the high resolution, the nested models better capture small-scale processes than the global model alone. In the coupling system, the nested models provide results to modify the global model simulation within respective nested domains while taking the lateral boundary conditions from the global model. Due to the "coupling" effects, the two-way system significantly improves the tropospheric O3 simulation upon the global model alone, as found by comparisons with a suite of ground (1420 sites from WDCGG, GMD, EMEP, and AQS), aircraft (HIPPO and MOZAIC), and satellite measurements (two OMI products). Compared to the global model alone, the two-way coupled simulation enhances the correlation in day-to-day variation of afternoon mean O3 with the ground measurements from 0.53 to 0.68, and it reduces the mean model bias from 10.8 to 6.7 ppb in annual average afternoon O3. Regionally, the coupled model reduces the bias by 4.6 ppb over Europe, 3.9 ppb over North America, and 3.1 ppb over other regions. The two-way coupling brings O3 vertical profiles much closer to the HIPPO (for remote areas) and MOZAIC (for polluted regions) data, reducing the tropospheric (0-9 km) mean bias by 3-10 ppb at most MOZAIC sites and by 5.3 ppb for HIPPO profiles. The two-way coupled simulation also reduces the global tropospheric column ozone by 3.0 DU (9.5 %, annual mean), bringing them closer to the OMI data in all seasons. Simulation improvements are more significant in the northern hemisphere, and are primarily a result of improved representation of urban-rural contrast and other small-scale processes. The two-way coupled simulation also reduces the global tropospheric mean hydroxyl radical by 5 % with enhancements by 5 % in the lifetimes of methyl chloroform (from 5.58 to 5.87 yr) and methane (from 9.63 to 10.12 yr), bringing them closer to observation-based estimates. Improving model representations of small-scale processes are a critical step forward to understanding the global tropospheric chemistry. [ABSTRACT FROM AUTHOR]

During the Carbonaceous Aerosols and Radiative Effects Study (CARES) the DOE G-1 aircraft was used to sample aerosol and gas phase compounds in the Sacramento, CA plume and surrounding region. We present data from 66 plume transects obtained during 13 flights in which southwesterly winds transported the plume towards the foothills of the Sierra Nevada Mountains. Plume transport occurred partly over land with high isoprene emission rates. Our objective is to empirically determine whether organic aerosol (OA) can be attributed to anthropogenic or biogenic sources, and to determine whether there is a synergistic effect whereby OA concentrations are enhanced by the simultaneous presence of high concentrations of CO and either isoprene, MVK+MACR (sum of methyl vinyl ketone and methacrolein) or methanol, which are taken as tracers of anthropogenic and biogenic emissions, respectively. Linear and bilinear correlations between OA, CO, and each of three biogenic tracers, "Bio", for individual plume transects indicate that most of the variance in OA over short time and distance scales can be explained by CO. For each transect and species a plume perturbation, (i.e., ΔOA, defined as the difference between 90th and 10th percentiles) was defined and regressions done amongst Δ values in order to probe day to day and location dependent variability. Species that predicted the largest fraction of the variance in ΔOA were ΔO3 and ΔCO. Background OA was highly correlated with background methanol and poorly correlated with other tracers. Because background OA was ~ 60 % of peak OA in the urban plume, peak OA should be primarily biogenic and therefore non-fossil. Transects were split into subsets according to the percentile rankings of ΔCO and ΔBio, similar to an approach used by Setyan et al. (2012) and Shilling et al. (2013) to determine if anthropogenic-biogenic interactions enhance OA production. As found earlier, ΔOA in the data subset having high ΔCO and high ΔBio was several-fold greater than in other subsets. Part of this difference is consistent with a synergistic interaction between anthropogenic and biogenic precursors and part to an independent linear dependence of ΔOA on precursors. Highest values of ΔO3 also occur in the high ΔCO-high ΔBio data set, raising the possibility that the coincidence of high concentrations of anthropogenic and biogenic tracers as well as OA and O3 may be associated with high temperatures, clear skies, and poor ventilation in addition to specific interaction between anthropogenic and biogenic compounds. [ABSTRACT FROM AUTHOR]

This work presents for the first time a classification of shrinkage events based on the aerosol processes that precede them. To this end, 3.5 years of continuous measurements (from 2009 to 2012) of aerosol size distributions, obtained with a Scanning Mobility Particle Sizer (SMPS) at an urban background site in Southern Europe, have been interpreted. 48 shrinkage events were identified and analysed, all occurring during spring and summer when the atmospheric conditions are more favourable for their development. In this study the shrinkage events took place mostly towards the end of the day, and their occurrence could be associated to atmospheric dilution conditions and a reduction in photochemical activity. The shrinkage rate (SR) varied between -1.0 and -11.1 nm h-1 (average value of -4.7 ± 2.6 nm h-1). Changes in particle concentrations corresponding to the nucleation and Aitken modes were detected, whereby an increase in the number of particles in the nucleation mode often coincided with a reduction in the Aitken mode. The accumulation mode did not undergo significant changes during these processes. In addition, in some cases, a dilution of the total particle number concentration in the ambient air was observed. Following the proposed methodology, three groups of events have been identified: Group I (NPF + shrinkage), Group II (aerosol growth process + shrinkage) and Group III (pure shrinkage events). The largest number of shrinkage events has been observed in the absence of prior processes, i.e. pure shrinkage events, followed by Group I events and finally Group II events. Although this analysis has confirmed that the triggering of shrinkage events is clearly linked to the atmospheric situation and the characteristics of the measurement area, this classification may contribute to a better understanding of the processes involved and the features that characterize shrinkage events. [ABSTRACT FROM AUTHOR]

In order to study the imprints of solar-ENSO-geomagnetic activity on the Indian Subcontinent, we have applied the Singular Spectral Analysis (SSA) and wavelet analysis to the tree ring temperature variability record from the western Himalayas. The data used in the present study are the Solar Sunspot Number (SSN), Geomagnetic Indices (aa Index), Southern Oscillation Index (SOI) and tree ring temperature record from western Himalayas (WH), for the period of 1876-2000. The SSA and wavelet spectra reveal the presence of 5 years short term ENSO variations to 11 year solar cycle indicating the influence of both the solar-geomagnetic and ENSO imprints in the tree ring data. The presence of 33-year cycle periodicity suggests the Sun-temperature variability probably involving the induced changes in the basic state of the atmosphere. Our wavelet analysis for the SSA reconstructed time series agrees with our previous results and also enhance the amplitude of the signals by removing the noise and showing a strong influence of solar-geomagnetic and ENSO patterns throughout the record. The solar flares are considered to be responsible for cause in the circulation patterns in the atmosphere. The net effect of solar-geomagnetic processes on temperature record thus appears to be the result of counteracting influences on shorter (about 5-6 years) and longer (about 11-12 years) time scales. The present analysis thus suggests that the influence of solar processes on Indian temperature variability operates in part indirectly through ENSO, but on more than one time scale. The analyses hence provides credible evidence for teleconnections of tropical pacific climatic variability with Indian climate ranging from interannual-decadal time scales and also demonstrate the possible role of exogenic triggering in reorganizing the global earth-ocean-atmospheric systems. [ABSTRACT FROM AUTHOR]

The online coupled Weather Research and Forecasting-Chemistry (WRF-Chem) model was applied to simulate a haze event that happened in January 2010 in the North China Plain (NCP), and was validated against various types of measurements. The evaluations indicate that WRF-Chem provides reliable simulations for the 2010 haze event in the NCP. This haze event is mainly caused by high emissions of air pollutants in the NCP and stable weather conditions in winter. Secondary inorganic aerosols also played an important role and cloud chemistry had important contributions. Air pollutants outside Beijing contributed about 47.8% to the PM2.5 levels in Beijing during this haze event, and most of them are from south Hebei, Shandong and Henan provinces. In addition, aerosol feedback has important impacts on surface temperature, Relative Humidity (RH) and wind speeds, and these meteorological variables affect aerosol distribution and formation in turn. In Shijiazhuang, Planetary Boundary Layer (PBL) decreased about 300m and PM2.5 increased more than 20 μgm-3 due to aerosol feed back. Feedbacks associated to Black Carbon (BC) account for about 50% of the PM2.5 increases and 50% of the PBL decreases in Shijiazhuang, indicating more attention should be paid to BC from both air pollution control and climate change perspectives. [ABSTRACT FROM AUTHOR]

Many field experiments have found high nitrous acid (HONO) mixing ratios in both urban and rural areas during daytime, but these high daytime HONO mixing ratios cannot be explained well by gas-phase production, suggesting that an unknown daytime HONO source (Punknown) could exist. The formula Punknown ≈ 19.60x NO2 x J(NO2) was obtained using observed data from 13 field experiments across the globe. The additional HONO sources (i.e. the Punknown, HONO emissions, and nighttime hydrolysis conversion of nitrogen dioxide (NO2) on aerosols) were coupled into the WRFChem model (Weather Research and Forecasting model coupled with Chemistry) to assess the P unknown (hydroxyl, hydroperoxyl, and organic peroxy) radicals (ROx) (= OH + HO2 + RO2) in the coastal regions of China. Results indicated that the additional HONO sources produced a significant improvement in HONO and OH simulations, particularly in the daytime. Elevated daytime-mean Punknown values were found in the coastal regions of China, with a maximum of 2.5 ppb h-1 in the Beijing-Tianjin-Hebei region. The Punknown produced a 60-250% increase of OH, HO2 and RO2 near the ground in the major cities of the coastal regions of China, and a 5-48% increase of OH, HO2 and RO2 in the daytime meridional-mean mixing ratios within 1000 m above the ground. When the additional HONO sources were included, the photolysis of HONO was dominated in the OH production rate in Beijing, Shanghai and Guangzhou before 10:00 LST with a maximum of 10.01 [7.26 due to the Punknown] ppbh-1 in Beijing, whereas the reaction of HO2 + NO (nitric oxide) was dominated after 10:00 LST with a maximum of 9.38 [7.23] ppb h in Beijing. The whole ROx cycle was accelerated by the additional HONO sources, especially the Punknown. The OH production rate was enhanced by 0.67 [0.64] to 4.32 [3.86]ppbh-1 via the reaction of HO2 + NO, and by 0.85 [0.69] to 4.11 [3.61]ppbh-1 via the photolysis of HONO, and the OH loss rate was enhanced by 0.58 [0.55] to 2.03 [1.92] ppbh-1 via the reaction of OH + NO2 and by 0.31 [0.28] to 1.78 [1.64] ppb h - 1 via the reaction of OH + CO (carbon monoxide) in Beijing, Shanghai and Guangzhou. Similarly, the additional HONO sources produced an increase of 0.31 [0.28] to 1.78 [1.64] ppbh- 1 via the reaction of OH + CO and 0.10 [0.09] to 0.63 [0.59] ppbh-1 via the reaction of CH3O2 (methylperoxy radical) + NO in the HO2 production rate, and 0.67 [0.61] to 4.32 [4.27]ppbh-1 via the reaction of HO2 + NO in the HO2 loss rate in Beijing, Shanghai and Guangzhou. The above results suggest that the Punknown considerably enhanced the ROx concentrations and accelerated ROx cycles in the coastal regions of China, and could produce significant increases in concentrations of inorganic aerosols and secondary organic aerosols and further aggravate haze events in these regions. [ABSTRACT FROM AUTHOR]

Vertical redistribution of HNO3 through condensation, sedimentation and evaporation of large HNO3-containing particles inside polar stratospheric clouds (PSCs) plays an important role in the chemistry of the Arctic winter stratosphere. In situ observations by the particle probe FSSP-100 during the RECONCILE campaign indicate unexpected large potential NAT (nitric acid trihydrate) particles inside PSCs. The observations can hardly be explained assuming particles with compact morphology and spherical shape due to limited growing time at temperatures below the existence temperature of NAT (TNAT). Utilizing simulations by the CLaMS and measurements by the airborne Fourier transform infrared spectrometer MIPAS-STR we study the impact of reduced settling velocities of NAT particles on vertical HNO3 redistribution. Reduced settling velocities are expected for spherical NAT particles with low mass density or aspheric NAT particles that might explain the maximum sizes of the particles observed in situ. The results of our study support the hypothesis that denitrification is produced by significantly aspheric (i.e. columnar) compact NAT particles which are characterised by reduced settling velocities. [ABSTRACT FROM AUTHOR]

A Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) was used to evaluate the hygroscopic properties of aerosol particles measured at the Puy de Dôme research station in central France from September 2008 to December 2012. This high-altitude site is ideally situated to allow for both the upper part of the planetary boundary layer and the lower free troposphere to be sampled. The aim of the study is to investigate both the influence of year-to-year, seasonal, and diurnal cycles, as well as the influence of air mass type on particle hygroscopicity and mixing state. Results show that particle hygroscopicity increases with particle size and depends both on air mass type and on season. Average growth factor values are lowest in winter (1.21±0.13, 1.23±0.18 and 1.38±0.25 for 25, 50 and 165nm particles, respectively) and highest in autumn (1.27±0.11, 1.32±0.12 and 1.49±0.15 for 25, 50 and 165nm particles, respectively). Particles are generally more hygroscopic at night than during the day. The seasonal and diurnal variations are likely to be strongly in fluenced by boundary layer dynamics. Furthermore, particles originating from oceanic and continental regions tend to be more hygroscopic than those measured in African and local air masses. The high hygroscopicity of marine aerosol may be explained by large proportions of inorganic aerosol and sea salts, and it is speculated that continental particles are more hygroscopic than local and African ones due to ageing of fresh combustion aerosol. Aerosol measured at the Puy de Dôme display a high degree of external mixing, and hygroscopic growth spectra can be divided into three different hygroscopic modes: a less hygroscopic mode (GF< 1.3), a hygroscopic mode (GF 1.3-1.7) and a more hygroscopic mode (GF> 1.7). The majority of particles measured can be classified 25 as being in either the less hygroscopic mode or the hygroscopic mode, and only few of them have more hygroscopic properties. The degree of external mixing, evaluated as the fraction of time when the aerosol is found with two or more populations with different hygroscopic properties, is found to increase with particle size (average yearly values are 22, 33 and 49% for 25, 50, and 165nm particles, respectively). The degree of external mixing is more sensitive to season than to air mass type, and it is higher in the cold seasons than in the warm seasons. This study gathers the results from one of the longest data sets of hygroscopic growth factor measurements to date, allowing a statistically relevant hygroscopic growth parameterization to be determined as a function of both air mass type and season. [ABSTRACT FROM AUTHOR]

Ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of sulfur dioxide (SO2) have been performed at the Xianghe station (39.75° N, 116.96° E) located at ~50 km southeast of Beijing from March 2010 to February 2013. Tropospheric SO2 vertical profiles and corresponding vertical column densities (VCDs), retrieved by applying the Optimal Estimation Method to the MAX-DOAS observations, have been used to study the seasonal and diurnal cycles of SO2, in combination to correlative measurements from in situ instruments, as well as meteorological data. A marked seasonality was observed in both SO2 VCD and surface concentration, with a maximum in winter (February) and a minimum in summer (July). This can be explained by the larger emissions in winter due to the domestic heating and more favorable meteorological conditions for the accumulation of SO2 close to the ground during this period. Wind speed and direction are also found to be two key factors in controlling the level of the SO2-related pollution at Xianghe. In the case of east or southwest wind, the SO2 concentration rises with the increase of the wind speed, since heavy polluting industries are located to the east and southwest of the station. In contrast, when wind comes from other directions, the stronger the wind, the less SO2 is observed. Regarding the diurnal cycle, the SO2 amount is larger in the early morning and late evening and lower at noon, in line with the diurnal variation of pollutant emissions and atmospheric stability. The observed diurnal cycles of MAX-DOAS SO2 surface concentration are also in very good agreement (correlation coefficient close to 0.9) with those from collocated in-situ data, demonstrating the reliability and robustness of our retrieval. [ABSTRACT FROM AUTHOR]

High frequency, in situ observations from the Advanced Global Atmospheric Gases Experiment (AGAGE), for the period 2003 to 2012, combined with archive flask measurements dating back to 1977, have been used to capture the rapid growth of HFC-143a (CH3CF3) and HFC-32 (CH2F2) mole fractions and emissions into the atmosphere. Here we report the first in situ global measurements of these two gases. HFC-143a and HFC-32 are the third and sixth most abundant HFCs respectively and they currently make an appreciable contribution to the HFCs in terms of atmospheric radiative forcing (1.7 and 0.7mWm² in 2012, respectively). In 2012 the global average mole fraction of HFC-143a was 13.4±0.3 ppt (1-sigma) in the lower troposphere and its growth rate was 1.4±0.04 pptyr-1; HFC-32 had a global mean mole fraction of 6.2±0.2 ppt and a growth rate of 1.1±0.04 pptyr-1 in 2012. The extensive observations presented in this work have been combined with an atmospheric transport model to simulate global atmospheric abundances and derive global emission estimates. It is estimated that 23±3Ggyr-1 of HFC-143a and 21±11Ggyr-1 of HFC-32 were emitted globally in 2012, and the emission rates are estimated to be increasing 7±5%yr-1 for HFC-143a and 14±11%yr-1 for HFC-32. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAH) concentration in air of the central and eastern Mediterranean in summer 2010 was 1.45 (0.30-3.25) ngm-3, with ≈ 8% in the particulate phase, associated with particles< 0.25 µm. The diffusive air-sea exchange fluxes of fluoranthene and pyrene were mostly found net-depositional or close to phase equilibrium, while retene was net-volatilisational in a large sea region. Regional fire activity records in combination with box model simulations suggest that seasonal depositional input of retene from biomass burning into the surface waters during summer is followed by an annual reversal of air-sea exchange, while inter-annual variability is dominated by the variability of the fire season. It is concluded that future negative emission trends or interannual variability of regional sources may trigger the sea to become a secondary PAH source through reversal of diffusive air-sea exchange. [ABSTRACT FROM AUTHOR]

Lidar and in situ observations performed during POLARCAT campaign are reported here in terms of statistics to characterize aerosol properties over northern Europe using daily airborne measurements conducted between Svalbard Island and Scandinavia from 30 March to 11 April 2008. It is shown that during this period, a rather large number of aerosol layers was observed in the troposphere, with a backscatter ratio at 532 nm of 1.2 (1.5 below 2 km, 1.2 between 5 and 7 km and a minimum in-between). Their sources were identified using multispectral backscatter and depolarization airborne lidar measurements after careful calibration analysis. Transport analysis and comparisons between in situ and airborne lidar observations are also provided to assess the quality of this identification. Comparison with level 1 backscatter observations of the spaceborne CALIOP lidar were done to adjust CALIOP multispectral observations to airborne observations on a statistical basis. Re-calibration for CALIOP daytime 1064 nm signals led to an increase of their values by about 30% in agreement with previous analyses. No re-calibration is made at 532 nm, but scattering ratios appear to be biased low. Regional analyses in the European Arctic then performed as a test, emphasize the potential of the CALIOP spaceborne lidar to further monitor more in depth properties of the aerosol layers over Arctic using infrared and depolarization observations. The CALIOP April 2008 global distribution of the aerosol backscatter reveal two regions with large backscatter below 2 km: the Northern Atlantic between Greenland and Norway, and Northern Siberia. The aerosol color ratio increase between the sources regions and the observations at latitudes above 70° N is consistent with a growth of the aerosol size once transported to the Arctic. The distribution of the aerosol optical properties in the mid troposphere supports the known main transport pathways between mid-latitudes and the Arctic. [ABSTRACT FROM AUTHOR]