Seasonal Variation of Wet Deposition of Black Carbon in Arctic Alaska

Black carbon (BC) aerosol deposited in and onto Arctic snow increases the snow's absorption of sunlight and accelerates snowmelt. Wet removal of BC from the atmosphere plays a key role in determining its abundance in the Arctic atmosphere and in Arctic snow. However, this process is poorly understood, mainly due to the scarcity of relevant measurements. To study wet deposition of BC, we made measurements of mass concentration of BC in snow and rain (C_MBC) and of BC in air (M_BC) with high accuracy (16% and 10%, respectively) at the Barrow Atmospheric Baseline Observatory, Alaska, from July 2013 to August 2017 and analyzed them along with routinely measured meteorological parameters from Barrow. Monthly mean M_BC near the surface and C_MBC were poorly correlated from midwinter to early spring, when C_MBC was close to the annual median while M_BC was at its annual peak. Seasonal variations in the altitude distribution of M_BC may lead to these differences in seasonal variation of M_BC near the surface and C_MBC. About 50% of the annual wet deposition of BC occurred in the 3 months of summer, associated with high values of total precipitation and BC originating from biomass burning. Size distributions of BC in snow and rain were stable throughout the year, suggesting that the size distribution of BC in the lower troposphere was similarly stable. Calculations by two global models reproduced the observed seasonal variations of C_MBC and showed that BC from biomass burning dominated C_MBC in summer.