Factors controlling black carbon distribution in the Arctic

Abstract. We investigate the sensitivity of black carbon (BC) in the Arctic, including BC concentration in snow (BC_snow, ng g⁻¹) and surface air (BC_air, ng m⁻³), as well as emissions, dry deposition, and wet scavenging using the global three-dimensional (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 (the western side of the 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 the GEOS-Chem) are too small. We apply the resistance-in-series method to compute a dry deposition velocity (v_d) that varies with local meteorological and surface conditions. The resulting velocity is significantly larger and varies by a factor of 8 in the Arctic (0.03–0.24 cm s⁻¹), which 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 clouds for 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. Overall, flaring emissions increase BC_air in the Arctic (by ∼ 20 ng m⁻³), the updated v_d more than halves BC_air (by ∼ 20 ng m⁻³), and the WBF effect increases BC_air 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 BC_air are much smaller during the 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. In addition, we find that the poorly constrained precipitation in the Arctic may introduce large uncertainties in estimating BC_snow. Doubling precipitation introduces a positive bias approximately as large as the overall effects of flaring emissions and the WBF effect; halving precipitation produces a similarly large negative bias.