Spatially and Temporally Refined Sources of Black Carbon Aerosols in the Arctic in spring
Abstract:Black carbon (BC) aerosols play a substantial role in the rapid warming of the Arctic. We systematically evaluate the simulation of BC vertical profile (ARCTAS and ARCPAC flights), surface concentrations and concentrations in snow (Doherty et al., 2010) in the Arctic in spring using a global chemical transport model GEOS-Chem. We also provide highly spatially (2° lat ×2.5° lon) and temporally resolved source estimates of surface BC concentrations using GEOS-Chem adjoint. Results showed that the vertical profiles of BC agree with the aircraft observations from surface to 10 km within 50%. The monthly mean regional averaged BC concentrations in snow in Alaska, Arctic Ocean, Canada, Greenland and Russia agree with the observations within 40%, while the mean BC concentrations in snow in Svalbard and Norway agree with the observations within a factor of 3 because of inaccurate simulation of snow precipitation in GEOS5.
We found Asian anthropogenic sources contributed 20-50% of BC concentrations through troposphere and in snow in the Arctic and are the major contributors in most regions. Adjoint analysis showed that BC concentrations of about 10 ng/m3 (30-100% of the total Asian contribution) observed at the surface stations are attributed to emissions emitted by Asian anthropogenic sources 30 days before the observation, and these background BC aerosols were transported to the station by circum-polar-vortex. Previous source estimates that repeated identified Europe as the major contributor of BC in the Arctic only trace back for 5 to 10 days and do not capture the background contribution from Asia. Russian biomass burning is important in mid-troposphere and accounts for 60% of total BC at 4-5 km. The relative contribution from different sources estimated by the adjoint of GEOS-Chem agrees with the forward estimate within 10%. Moreover, the adjoint analysis enables us to pinpoint the major sources to grid box level. Results showed that for surface observations, contribution from the top-ten gird box accounts for more than 70% of the total global contribution.