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Models assume that mixed‐phase clouds consist of uniformly mixed ice crystals and liquid cloud droplets when observations have shown that they consist of clusters, or "pockets," of ice crystals and liquid cloud droplets. We characterize the spatial distribution of cloud phase over the Arctic and the Southern Ocean using active satellite observations and determine the relative importance of collocated meteorological parameters and aerosols from reanalysis to predict how uniformly mixed mixed‐phase clouds are for the first time. We performed a multi‐linear regression fit to the data set to predict the spatial distribution of the ice and liquid pockets. Contrary to what models suggest, mixed‐phase clouds are rarely perfectly homogeneous. Our results suggest that high temperatures are associated with homogeneously mixed ice and liquid pockets. We also find that a high mixing ratio of black carbon is associated with heterogeneously mixed ice and liquid pockets. Plain Language Summary: The representation of clouds in numerical models remains one of the largest uncertainties in predicting our future climate. Clouds can consist solely of liquid droplets, ice crystals, or the coexistence of both hydrometeor types. The latter cloud type is referred to as mixed phase. Climate models assume that liquid droplets and ice crystals are uniformly mixed in space in mixed‐phase clouds, but observations show that mixed‐phase clouds are organized in separate pockets of clustered liquid droplets and ice crystals. This difference in representation has a large impact on the lifetime of clouds and on their role in climate change. Using satellite observations over the Arctic and the Southern Ocean, we quantify the spatial distribution of ice and liquid in clouds. We used a statistical method to determine the relationship between meteorology and aerosols and the spatial distribution of ice and liquid. Our results suggest that high temperatures are associated with homogeneously mixed mixed‐phase clouds and high concentrations of soot are associated with heterogeneously mixed mixed‐phase clouds. Furthermore, pockets of liquid within ice clouds are larger than pockets of ice within liquid clouds. These results will improve the representation of mixed‐phase clouds in large‐scale models. Key Points: Space‐based observations and reanalysis are considered to determine the factors that control how mixed mixed‐phase clouds areLiquid dominated clouds contain small and isolated ice pockets whereas ice dominated clouds contain large and isolated liquid pocketsTemperature and black carbon play an important role in controlling the cloud phase spatial distribution and increasing phase heterogeneity [ABSTRACT FROM AUTHOR]

We implement a detailed representation of aerosol mixing state in the Global Environmental Multiscale – Modelling Air quality and CHemistry (GEM-MACH) air quality and weather forecast model. Our mixing-state representation includes three categories: one for more hygroscopic aerosol, one for less hygroscopic aerosol with a high black carbon (BC) mass fraction, and one for less hygroscopic aerosol with a low BC mass fraction. The more detailed representation allows us to better resolve two different aspects of aerosol mixing state: differences in hygroscopicity due to aerosol composition and the amount of absorption enhancement of BC due to non-absorbing coatings. Notably, this three-category representation allows us to account for BC thickly coated with primary organic matter, which enhances the absorption of the BC but has a low hygroscopicity. We compare the results of the three-category representation (1L2B, (one hydrophilic, two hydrophobic)) with a simulation that uses two categories, split by hygroscopicity (HYGRO), and a simulation using the original size-resolved internally mixed assumption (SRIM). We perform a case study that is focused on North America during July 2016, when there were intense wildfires over northwestern North America. We find that the more detailed representation of the aerosol hygroscopicity in both 1L2B and HYGRO decreases wet deposition, which increases aerosol concentrations, particularly of less hygroscopic species. The concentration of PM 2.5 increases by 23 % on average. We show that these increased aerosol concentrations increase cloud droplet number concentrations and cloud reflectivity in the model, decreasing surface temperatures. Using two categories based on hygroscopicity yields only a modest benefit in resolving the coating thickness on black carbon, however. The 1L2B representation resolves BC with thinner coatings than the HYGRO simulation, resulting in absorption aerosol optical depths that are 3 % less on average, with greater differences over strong anthropogenic source regions. We did not find strong subsequent effects of this decreased absorption on meteorology. [ABSTRACT FROM AUTHOR]

Aerosols and in particular their black carbon (BC) content influence the atmospheric heating rate and fog dissipation. Substantial improvements have been introduced to the solar scheme of the computational fluid dynamic model code_saturne to estimate fluxes and heating rates in the atmosphere. This solar scheme is applied to a well-documented case of a fog that evolves into a low stratus cloud. Different sensitivity tests are conducted. They show that aerosols have a major effect with an overestimation of the direct solar fluxes by 150 W m − 2 when aerosols are not considered and a reduction of the heating of the layers. Aerosols lead to an increase of the heating rate by as much as 55% in the solar infrared (SIR) band and 100% in the Ultra-Violet visible (UV-vis) band. Taking into account the fraction of BC in cloud droplets also accentuates the heating in the layers at the top of the fog layer where water liquid content is maximum. When the BC fraction in cloud droplets is equal to 8.6 × 10 − 6 , there is an increase of approximately 7.3 °C/day in the layers. Increasing the BC fraction leads to an increase of this heating in the layer, especially in the UV-vis band. [ABSTRACT FROM AUTHOR]

Black carbon (BC) aerosol particles in the Arctic heat the atmosphere and snow/ice surfaces and may strengthen the snow-albedo feedback that amplifies Arctic warming. Model simulations of BC concentrations in the Arctic depend strongly on the representation of microphysical processes such as aging, activation, and wet removal. Most BC modeling studies have classified BC particles into hydrophobic BC, which cannot form cloud droplets, and hydrophilic BC, which can form cloud droplets, by assuming a globally constant critical supersaturation threshold value (Sthre), without considering its consistency with cloud maximum supersaturation (Smax). Here we show that it is essential to consider the consistency of Sthre with Smax in global model simulations to reduce uncertainties in near-surface ambientBCconcentrations in the Arctic. Previous studies often obtained good agreement between simulated and observed near-surface Arctic BC mass concentrations when a low Sthre (;0.1%) was assumed in their models. However, this Sthre may be too low (activation and wet removal of BCmay be underestimated) for the Arctic, because some recent observations and our model simulations suggest that Smax may actually be higher (;0.3%) there. We also demonstrate that spatially varying Sthre values and their consistency with Smax, which previous studies did not consider, must be represented in models for more accurate estimation of BC budget in the Arctic. Because both Smax and BC-aging speed depend on climatic conditions, our findings are an important step toward better simulations of BC impacts on past, present, and future Arctic climates. [ABSTRACT FROM AUTHOR]

In this study we introduce an in-cloud wet deposition scheme for liquid and ice phase clouds for global aerosol–climate models which use a size-segregated aerosol description. For in-cloud nucleation scavenging, the scheme uses cloud droplet activation and ice nucleation rates obtained from the host model. For in-cloud impaction scavenging, we used a method where the removal rate depends on the wet aerosol size and cloud droplet radii. We used the latest release version of ECHAM-HAMMOZ (ECHAM6.3-HAM2.3-MOZ1.0) with the Sectional Aerosol module for Large Scale Applications version 2.0 (SALSA) microphysics package to test and compare our scheme. The scheme was compared to a scheme that uses fixed scavenging coefficients. The comparison included vertical profiles and mass and number distributions of wet deposition fluxes of different aerosol compounds and for different latitude bands. Using the scheme presented here, mass concentrations for black carbon, organic carbon, sulfate, and the number concentration of particles with diameters larger than 100 nm are higher than using fixed scavenging coefficients, with the largest differences in the vertical profiles in the Arctic. On the other hand, the number concentrations of particles smaller than 100 nm in diameter show a decrease, especially in the Arctic region. These results could indicate that, compared to fixed scavenging coefficients, nucleation scavenging is less efficient, resulting in an increase in the number concentration of particles larger than 100 nm. In addition, changes in rates of impaction scavenging and new particle formation (NPF) can be the main cause of reduction in the number concentrations of particles smaller than 100 nm. Without further adjustments in the host model, our wet deposition scheme produced unrealistically high aerosol concentrations, especially at high altitudes. This also leads to a spuriously long lifetime of black carbon aerosol. To find a better setup for simulating aerosol vertical profiles and transport, sensitivity simulations were conducted where aerosol emission distribution and hygroscopicity were altered. Vertical profiles of aerosol species simulated with the scheme which uses fixed scavenging rates and the abovementioned sensitivity simulations were evaluated against vertical profiles from aircraft observations. The lifetimes of different aerosol compounds were also evaluated against the ensemble mean of models involved in the Aerosol Comparisons between Observations and Models (AEROCOM) project. The best comparison between the observations and the model was achieved with our wet deposition scheme when black carbon was emitted internally mixed with soluble compounds instead of keeping it externally mixed. This also produced atmospheric lifetimes for the other species which were comparable to the AEROCOM model means. [ABSTRACT FROM AUTHOR]

The heterogeneous nucleation of water vapor on insoluble particles affects cloud formation, precipitation, the hydrological cycle, and climate. Despite its importance, heterogeneous nucleation remains a poorly understood phenomenon that relies heavily on empirical information for its quantitative description. Here, we examine the heterogeneous nucleation of water vapor on different types of soots as well as cloud drop activation of different types of soots, including both pure black carbon particles and black carbon particles mixed with secondary organic matter. We show that the recently developed adsorption nucleation theory quantitatively predicts the nucleation of water and droplet formation upon particles of the various soot types. A surprising consequence of this new understanding is that, with sufficient adsorption site density, soot particles can activate into cloud droplets – even when completely lacking any soluble material. [ABSTRACT FROM AUTHOR]

The representation of aerosol activation into cloud droplets in climate models is important for accurate understanding of aerosol radiative impacts on the Arctic climate, but it remains highly uncertain. Here we show that the uncertainty range of subgrid vertical velocity (SVV) and maximum supersaturation (SSmax) in aerosol activation produces fourfold to fivefold differences in the direct radiative effect of black carbon (BC) in the Arctic (0.091–0.40 W m−2) because SVV and SSmax determine the activated fraction and wet removal efficiency of aerosols. Aerosols are particularly sensitive to SVV in remote regions but not near their sources because many aerosols near sources are not yet influenced by wet removal processes. Our results demonstrate that SVV treatment is a major source of uncertainty in Arctic aerosol simulations and may be key for reducing the large discrepancies among global models in estimates of BC and its radiative effects in the Arctic. Plain Language Summary: Black carbon aerosol, emitted mainly in midlatitude regions by combustion of fossil fuel and biomass, is transported to the Arctic and deposited on snow and ice surfaces, where it contributes to Arctic heating. However, estimates of its importance in Arctic warming have large uncertainties. Because global climate models usually use a coarse horizontal grid spacing, they rely on many assumptions to represent the small‐scale atmospheric processes within clouds. This study uses a global climate model to investigate the importance of one of these assumptions, the treatment of "subgrid vertical velocity," to aerosol simulations. We show that varying the subgrid vertical velocity within its uncertainty range changes the calculated heating effect of black carbon in the Arctic by as much as five times. Our results underscore the importance of treating subgrid vertical velocity treatment accurately in estimating how much black carbon from midlatitudes is warming the Arctic. Key Points: The importance of subgrid vertical velocity in activation to aerosol burden and radiative effects was investigated by using a global modelThe current uncertainty in subgrid vertical velocity produces fivefold differences in the radiative effect of black carbon in the ArcticThe subgrid treatment of updraft is important in estimating the long‐range transport of black carbon and its impacts on Arctic climate [ABSTRACT FROM AUTHOR]

In this study we introduce an in-cloud wet deposition scheme for liquid and ice phase clouds for global aerosol-climate models which use a size-segregated aerosol description. For in-cloud nucleation scavenging, the scheme uses cloud droplet activation and ice nucleation rates obtained from the host model. For in-cloud impaction scavenging, we used a method where the removal rate depends on the aerosol size and cloud droplet radii. The scheme was compared to a scheme that uses fixed scavenging coefficients. The comparison included vertical profiles and mass and number distributions of wet deposition fluxes of different aerosol compounds and for different latitude bands. Using the scheme presented here, mass concentrations for black carbon, organic carbon, sulfate, and the number concentration of particles with diameters larger than 100nm are higher than using fixed scavenging coefficients, with the largest differences in the vertical profiles in the Arctic. On the other hand, the number concentrations of small particles show a decrease, especially in the Arctic region. These results indicate that, compared to using fixed scavenging coefficients, nucleation scavenging is less efficient and impaction scavenging is increased in the scheme introduced here. Without further adjustments in the host model, our wet deposition scheme produced unrealistically high aerosol concentrations, especially at high altitudes. This also leads to a spuriously long lifetime of black carbon aerosol. To find a better setup for simulating aerosol vertical profiles and transport, sensitivity simulations were conducted where aerosol emission distribution and hygroscopicity were altered. The simulated vertical profiles of aerosol in these sensitivity studies were evaluated against aircraft observations. The lifetimes of different aerosol compounds were also evaluated against the ensemble mean of models involved in the Aerosol Comparisons between Observations and Models (AEROCOM) project. The best comparison between the observations and the model was achieved with the new wet deposition scheme when black carbon was emitted internally mixed with soluble compounds instead of keeping it externally mixed. This also produced atmospheric lifetimes for the other species which were comparable to the AEROCOM model means. [ABSTRACT FROM AUTHOR]

Wet scavenging of black carbon (BC) has been subject to large uncertainty, which importantly determines its atmospheric lifetime and indirect forcing impact on cloud microphysics. This study reveals the complex BC‐hydrometeor interactions in mixed‐phase clouds via single particle measurements in the real‐world environment, by capturing precipitation processes throughout cloud formation, cold rain/graupel, and subsequent snow events at a mountain site influenced by anthropogenic sources in wintertime. We found highly efficient BC wet scavenging during cloud formation, with large and thickly coated BC preferentially incorporated into droplets. During snow processes, BC core sizes in the interstitial phase steadily increased. A mechanism was proposed whereby the BC mass within each droplet was accumulated through droplet collision, leading to larger BC cores, which were then released back to the interstitial air through the Wegener‐Bergeron‐Findeisen processes when ice dominated. These results provide fundamental basis for constraining BC wet scavenging. Plain Language Summary: The wet removal of black carbon (BC) is crucial to determine its atmospheric lifetime and indirect radiative effects. However, the mechanism of BC‐cloud particle interactions is still largely unknown, especially for mixed‐phase clouds. This study conducted in situ single particle measurements to reveal this microphysical process throughout an entire precipitation event. The BC particles with large core sizes and thick coatings were observed to be preferentially incorporated into liquid droplets when the cloud was formed. The subsequent droplet collision further resulted in fewer but larger droplets with larger BC cores inside. Afterward, when a significant amount of ice was present, the water vapor could be transferred from droplets to ice crystals. As a result, the BC contained in the droplet could be released back to the interstitial phase, leading to larger BC cores in the air than those before the precipitation event. This is an important process when considering the wet scavenging of BC in mixed‐phase clouds, which is first observed in the real atmosphere in this study. Key Points: This study provides the first detailed and continuous observations of black carbon interaction with mixed‐phased cloudsIn‐cloud scavenging by droplets preferentially removes larger and thickly coated black carbonBlack carbon with a larger core size in water droplets is released back to air through the WBF process during a snow event [ABSTRACT FROM AUTHOR]

Liquid clouds form by condensation of water vapour on aerosol particles in the atmosphere. Even black carbon (BC) particles, which are known to be slightly hygroscopic, have been shown to readily form cloud droplets once they have acquired water-soluble coatings by atmospheric aging processes. Accurately simulating the life cycle of BC in the atmosphere, which strongly depends on the wet removal following droplet activation, has recently been identified as a key element for accurate prediction of the climate forcing of BC. Here, to assess BC activation in detail, we performed in situ measurements during cloud events at the Jungfraujoch high-altitude station in Switzerland in summer 2010 and 2016. Cloud droplet residual and interstitial (unactivated) particles as well as the total aerosol were selectively sampled using different inlets, followed by their physical characterization using scanning mobility particle sizers (SMPSs), multi-angle absorption photometers (MAAPs) and a single-particle soot photometer (SP2). By calculating cloud droplet activated fractions with these measurements, we determined the roles of various parameters on the droplet activation of BC. The half-rise threshold diameter for droplet activation (Dhalfcloud), i.e. the size above which aerosol particles formed cloud droplets, was inferred from the aerosol size distributions measured behind the different inlets. The effective peak supersaturation (SS peak) of a cloud was derived from Dhalfcloud by comparing it to the supersaturation dependence of the threshold diameter for cloud condensation nuclei (CCN) activation measured by a CCN counter (CCNC). In this way, we showed that the mass-based scavenged fraction of BC strongly correlates with that of the entire aerosol population because SS peak modulates the critical size for activation of either particle type. A total of 50 % of the BC-containing particles with a BC mass equivalent core diameter of 90 nm was activated in clouds with SS peak≈0.21 %, increasing up to ∼80 % activated fraction at SS peak≈0.50 %. On a single-particle basis, BC activation at a certain SS peak is controlled by the BC core size and internally mixed coating, which increases overall particle size and hygroscopicity. However, the resulting effect on the population averaged and on the size-integrated BC scavenged fraction by mass is small for two reasons: first, acquisition of coatings only matters for small cores in clouds with low SS peak ; and, second, variations in BC core size distribution and mean coating thickness are limited in the lower free troposphere in summer. Finally, we tested the ability of a simplified theoretical model, which combines the κ -Köhler theory with the Zdanovskii–Stokes–Robinson (ZSR) mixing rule under the assumptions of spherical core–shell particle geometry and surface tension of pure water, to predict the droplet activation behaviour of BC-containing particles in real clouds. Predictions of BC activation constrained with SS peak and measured BC-containing particle size and mixing state were compared with direct cloud observations. These predictions achieved closure with the measurements for the particle size ranges accessible to our instrumentation, that is, BC core diameters and total particle diameters of approximately 50 and 180 nm, respectively. This clearly indicates that such simplified theoretical models provide a sufficient description of BC activation in clouds, as previously shown for activation occurring in fog at lower supersaturation and also shown in laboratory experiments under controlled conditions. This further justifies application of such simplified theoretical approaches in regional and global simulations of BC activation in clouds, which include aerosol modules that explicitly simulate BC-containing particle size and mixing state. [ABSTRACT FROM AUTHOR]

The complex microphysical characteristics of black carbon aerosol (BC) results in large uncertainties in the evaluation of BC's climatic and environmental effects. In this study, the microphysical properties were investigated in fresh BC emitted from traffic sources (gasoline vehicle and diesel) and aged BC emitted from solid fuel burning sources (coal, cotton stem, rice stem and mulberry). For the BC core size distribution, the number size distribution of traffic sources peaks at a lower value than that of aged solid fuel burning sources. Although traffic sources emit a greater number of small BC cores, traffic sources could also emit BC aerosols with large BC cores (diameter larger than 400 nm) not found in solid fuel sources. The coating thickness of BC from traffic sources is significantly lower than that of aged BC from solid fuel sources. Such differences in size distribution and coating thickness cause large variations in other microphysical parameters, such as light absorption and hygroscopicity, for these two types of BC. The mass absorption cross section (MAC) of aged BC from cotton stems reached 11.69 m2/g, nearly 2 times higher than the MAC (6.08 m2/g) of fresh BC from gasoline vehicles. The hygroscopicity of aged BC from solid fuel burning is also much higher than that of fresh BC from traffic. The critical supersaturation needed to activate BC to become cloud droplets is 0.03% for the former and 0.42% for the latter. This study reports the BC emission characteristics of traffic sources and emphasizes the large property difference between fresh traffic emission BC and aged solid fuel burning BC. The microphysical properties of BC should be carefully considered to better resolve BC concentrations and optical properties in climatic models. • BC number size distribution from traffic peaks at lower value than that from solid fuel burning. • The absorption ability of aged BC from solid fuel burning could be nearly two times higher than that from traffic. • The effective density of aged BC exhibited a unimodal pattern peaking at 0.9–1.4 g/cm3. [ABSTRACT FROM AUTHOR]

Interactions between clouds and black carbon (BC) represent a significant uncertainty in aerosol radiative forcing. To investigate the influence of cloud processing on the scavenging of BC, concurrent measurement of individual cloud droplet residue particles (cloud RES) and interstitial particles (cloud INT) throughout a cloud event was deployed at Mt. Tianjing (1690 m a.s.l.) in southern China. An aethalometer (AE-33), a single particle aerosol mass spectrometer (SPAMS) and a scanning mobility particle sizer (SMPS) were used to investigate the mass concentration of equivalent BC (EBC), size-resolved number of BC-containing particles, and size-resolved number concentration of submicron particles in real-time, respectively. The number-based SEs of the submicron particles varied between 2.7 and 31.1%. Mass scavenging efficiency (MSE) ranged from 4.7% to 52.6% for EBC, consistent with the number-based SE (from 11.3% to 59.6%) of the BC-containing particles throughout the cloud event. Several factors that may influence the SEs of the BC-containing particles are considered and examined. SEs are most likely determined by a single factor, i.e., liquid water content (LWC), with R2 > 0.8 in a power function throughout the cloud event. Stage-resolved investigation of SEs further reveals that particle size matters more than other factors in the cloud formation stage, whereas there is an increasing role of the mixing state in the development and stability stage. We also observed lower SEs for the BC-containing particles internally mixed organics, consistent with previous literature. • The stage-resolved scavenging efficiency of BC particles is firstly reported. • Scavenging efficiency (SE) is dominantly controlled by liquid water content (LWC). • Key factors influencing the in-cloud SE of BC-containing particles are assessed. [ABSTRACT FROM AUTHOR]

Black carbon (BC) aerosol is of great importance not only for its strong potential in heating air and impacts on cloud, but also because of its hazards to human health. Wet deposition is regarded as the main sink of BC, constraining its lifetime and thus its impact on the environment and climate. However, substantial controversial and ambiguous issues in the wet scavenging processes of BC are apparent in current studies. Despite of its significance, there are only a small number of field studies that have investigated the incorporation of BC-containing particles into cloud droplets and influencing factors, in particular, the in-cloud scavenging, because it was simplicitly considered in many studies (as part of total wet scavenging). The mass scavenging efficiencies (MSEs) of BC were observed to be varied over the world, and the influencing factors were attributed to physical and chemical properties (e.g., size and chemical compositions) and meteorological conditions (cloud water content, temperature, etc.). In this review, we summarized the MSEs and potential factors that influence the in-cloud and below-cloud scavenging of BC. In general, MSEs of BC are lower at low-altitude regions (urban, suburban, and rural sites) and increase with the rising altitude, which serves as additional evidence that atmospheric aging plays an important role in the chemical modification of BC. Herein, higher altitude sites are more representative of free-tropospheric conditions, where BC is usually more aged. Despite of increasing knowledge of BC–cloud interaction, there are still challenges that need to be addressed to gain a better understanding of the wet scavenging of BC. We recommend that more comprehensive methods should be further estimated to obtain high time-resolved scavenging efficiency (SE) of BC, and to distinguish the impact of in-cloud and below-cloud scavenging on BC mass concentration, which is expected to be useful for constraining the gap between field observation and modeling simulation results. [ABSTRACT FROM AUTHOR]

In the present study, a ground-based counterflow virtual impactor (GCVI) was used to sample cloud droplet residual (cloud RES) particles, while a parallel PM2.5 inlet was used to sample cloud-free or cloud interstitial (cloud INT) particles. The mixing state of black carbon (BC)-containing particles and the mass concentrations of BC in the cloud-free, RES and INT particles were investigated using a single particle aerosol mass spectrometer (SPAMS) and two aethalometers, respectively, at a mountain site (1690 m a.s.l.) in southern China. The measured BC-containing particles were extensively internally mixed with sulfate, and were scavenged into cloud droplets (0.05–0.45) to a similar (or slightly lower) extent as all the measured particles (0.07–0.6) over the measured size range of 0.1–1.6 µm. The results indicate the preferential activation of larger particles and/or that the production of secondary compositions shifts the BC-containing particles towards larger sizes. BC-containing particles with an abundance of both sulfate and organics were scavenged less than those with sulfate but limited organics, implying the importance of the mixing state on the incorporation of BC-containing particles into cloud droplets. The mass scavenging efficiency of BC with an average of 33% was similar for different cloud events independent of the air mass. This is the first time that both the mixing state and cloud scavenging of BC in China have been reported. Our results would improve the knowledge on the concentration, mixing state, and cloud scavenging of BC in the free troposphere. [ABSTRACT FROM AUTHOR]