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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]

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]

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]