Factors Controlling Black Carbon Distribution in the Arctic.

We investigate the sensitivity of black carbon (BC) in the Arctic, including BC in snow (BC_snow, ng g^-1) and surface air (BCair, μg m^-3), to emissions, dry deposition and wet scavenging using a global 3-D chemical transport model (CTM) GEOS-Chem. We find that the model underestimates BC_snow in the Arctic by 40% on average (median = 11.8 ng g^-1). Natural gas flaring substantially increases total BC emissions in the Arctic (by ~ 70%). The flaring emissions lead to up to 49% increases (0.1-8.5 ng g^-1) in Arctic BC_snow, dramatically improving model comparison with observations (50% reduction in discrepancy) near flaring source regions (Western Extreme North of Russia). Ample observations suggest that BC dry deposition velocities over snow and ice in current CTMs (0.03 cm s^-1 in GEOS-Chem) are exceedingly small. We apply the resistance-in-series method to compute the dry deposition velocity that varies with local meteorological and surface conditions. The resulting velocity is significantly larger and varies by a factor of eight in the Arctic (0.03-0.24 cm s^-1), increases the fraction of dry to total BC deposition (16% to 25%), yet leaves the total BC deposition and BC_snow in the Arctic unchanged. This is largely explained by the offsetting higher dry and lower wet deposition fluxes. Additionally, we account for the effect of the Wegener-Bergeron-Findeisen (WBF) process in mixed-phase clouds, which releases BC particles from condensed phases (water drops and ice crystals) back to the interstitial air and thereby substantially reduces the scavenging efficiency of BC (by 43-76% in the Arctic). The resulting BC_snow is up to 80% higher, BC loading is considerably larger (from 0.25 to 0.43 mg m^-2), and BC lifetime is markedly prolonged (from 9 to 16 days) in the Arctic. Over all, flaring emissions increase BCair in the Arctic (by ~ 20 ng m^-3), the updated dry deposition velocity more than halves BCair (by ~ 20 ng m^-3), and the WBF effect increases BCair by 25-70% during winter and early spring. The resulting model simulation of BC_snow is substantially improved (within 10% of the observations) and the discrepancies of BCair are much smaller during snow season at Barrow, Alert and Summit (from -67%--47% to -46%-3%). Our results point toward an urgent need for better characterization of flaring emissions of BC (e.g. the emission factors, temporal and spatial distribution), extensive measurements of both the dry deposition of BC over snow and ice, and the scavenging efficiency of BC in mixed-phase clouds. [ABSTRACT FROM AUTHOR]