Evidence for Deposition of Black Carbon in the Springtime Arctic

Blue light PMT-Photo detectors Red light Pump diode laser Motivation ARCPAC: Aerosol, Radiation and Cloud Processes A ecting Arctic Climate Vertical profiles of black carbon (BC) mass were observed from the surface to near 7-km altitude in April 2008 during flights on the NOAA WP-3D research aircraft from Fairbanks, Alaska using a Single-Particle Soot Photometer (SP2). In the free troposphere, the Arctic air mass was often influenced by long-range transport from biomass-burning and anthropogenic source regions at lower latitudes with BC mass loadings reaching maximum values near 4-km altitude. In the boundary layer (BL) over the snow and ice north of Alaska, the air mass was largely decoupled from the advected pollution aloft. In this shallow layer, BC mass loadings decreased from the top of the BL to near the surface by up to a factor of five. The BC gradients in the BL were well correlated with ozone depletion events in all the observations, suggesting that BC particles were removed by dry deposition on the snow or ice in these cases because the short-lived ozone destruction precursor, Br2, is likely emitted from the sea-ice. Open leads in the sea-ice may facilitate the removal of BC through surface contact by enhancing mixing in the BL. We estimate the deposition flux of BC mass to the snow using a box model constrained by the vertical profiles of BC in the Arctic BL. Understanding the removal of BC in the Arctic BL is crucial for evaluating the impact of anthropogenic and natural sources of BC on Arctic climate. Introduction