Sources of black carbon in the atmosphere and in snow in the Arctic

We systematically identify sources of black carbon (BC) in the Arctic, including BC in the troposphere, at surface and in snow, using tagged tracer technique implemented in a 3D global chemical transport model GEOS-Chem. We validate modeled BC sources (fossil fuel combustion versus biomass burning) against carbon isotope measurements at Barrow (Alaska), Zeppelin (Norway), Abisko (Sweden), Alert (Canada) and Tiksi (Russia) in the Arctic. The model reproduces the observed annual mean fraction of biomass burning (f_bb, %) at the five sites within 20% and the observed and modeled monthly f_bb values agree within a factor of two. Model results suggest that fossil fuel combustion is the major source of BC in the troposphere (50-94%, vary with sub-regions), at surface (55-68%) and in snow (58-69%) in the Arctic as annual mean, but biomass burning dominates at certain altitudes (600-800 hPa) and during periods of time between April to September. The model shows that BC in the troposphere, in deposition and in snow in different Arctic sub-regions have distinctively different sources and source regions. We find that long-range transport of Asian emissions has a stronger influence on BC in the atmosphere than on BC deposition. In contrast, contributions from Russian and European emissions are larger for BC deposition than for BC in the atmosphere. Specifically, Asian fossil fuel combustion emissions dominate BC loading in all Arctic sub-regions in both winter (Oct.-Mar., 35-54%) and summer (Apr.-Sep., 34-56%). For BC deposition, Siberian fossil fuel emissions are the largest contributors in Russia both in winter (62%) and summer (44%), and European fossil fuel emissions dominate in Ny-Ålesund (44% in winter) and Tromsø (71% in winter and 46% in summer). For BC deposition in the North American sector, Asian fossil fuel emissions are the largest contributors in winter (25-38%) and North American biomass burning emissions (38-72%) dominate in summer.