Black carbon (BC) aerosol deposited in and onto Arctic snow increases the snow's absorption of solar radiation and accelerates snowmelt. Concentrations of BC in the Arctic atmosphere and snow are controlled by wet deposition; however, details of this process are poorly understood owing to the scarcity of time‐resolved measurements of BC in hydrometeors. We measured mass concentrations of BC in hydrometeors (CMBC) and in air (MBC) with 16% and 15% accuracies, respectively, at Ny‐Ålesund, Svalbard during 2012–2019. Median monthly MBC and CMBC values showed similar seasonal variations, being high in winter‐spring and low in summer. Median monthly BC wet deposition mass flux (FMBC) was highest in winter and lowest in summer, associated with seasonal patterns of CMBC and precipitation. Seasonally averaged BC size distributions in hydrometeors were similar except for summer. Measurements of MBC and CMBC in spring 2017 showed a size‐independent removal efficiency, indicating that BC‐containing particles were efficiently activated into cloud droplets. These observations at Ny‐Ålesund were compared with observations at Barrow, Alaska, during 2013–2017. The near‐surface MBC at Ny‐Ålesund and Barrow had similar seasonal patterns; however, the two sites differed in CMBC and FMBC. In summer, CMBC was low at Ny‐Ålesund but moderate at Barrow, likely reflecting differences in MBC in the lower troposphere. Seasonally averaged BC size distributions in hydrometeors were similar at both sites, suggesting that average BC size distributions are similar in the Arctic lower troposphere. The efficiency of BC removal tends to be size‐independent during transport, leading to the observed similarity.
In‐cloud wet scavenging dominates the wet removal of aerosols in the atmosphere, but is not well represented in climate models. Aircraft measurements of black carbon (BC) concentrations suggest that models commonly overestimate BC concentrations in the upper troposphere of the tropics by more than one order of magnitude but underestimate BC burdens in polar latitudes. In this study, we improved the in‐cloud wet scavenging parameterizations for convective clouds and mixed‐phase clouds to better characterize BC abundances in the remote atmosphere (remote oceans and polar regions) with a global model, CAM5‐ATRAS2. The modified wet scavenging processes in the model achieved a more realistic simulation of BC concentrations over both the tropics and the Arctic. The new, unified scheme for vertical transport and wet removal during deep convection generally reproduced the observed low mixing ratios (about 0.1 ng kg−1) of BC in the middle and upper troposphere over the tropics, and the Wegener–Bergeron–Findeisen process (WBF) lowered the wet removal efficiency of BC from mixed‐phase clouds and consequently increased BC burdens in the Arctic by about a factor of 2. The BC direct radiative forcings increased by 20% globally (from 0.26 to 0.31 W m−2), and more importantly by a factor of 2 in the Arctic (from 0.09 to 0.18 W m−2). Our results indicated that good agreement between modeled and observed BC concentrations could be obtained in the remote atmosphere without requiring the relatively short global BC lifetime (∼4 days) suggested by previous studies.
