Factors controlling black carbon distribution in the Arctic

• Published in: Atmospheric Chemistry and Physics
• Main Authors: L. Qi, Q. Li, Y. Li, C. He
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2017
• Subjects: Physics; QC1-999; Chemistry; QD1-999; Arctic; black carbon; Extreme North of Russia
• Online Access: https://doi.org/10.5194/acp-17-1037-2017; https://doaj.org/article/3442cd38784d4a54abae46151fb3a0a9

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 ...