Seasonal variations of Asian black carbon outflow to the Pacific: Contribution from anthropogenic sources in China and biomass burning sources in Siberia and Southeast Asia

The Community Multiscale Air Quality model with a source and process tagged method (CMAQ/PASCAL) was used to understand source regions and types (anthropogenic (AN) and biomass burning (BB)) of Asian black carbon (BC) outflow to the Pacific during 2008-2010. The model simulations generally reproduced absolute concentrations and temporal (seasonal, monthly, and day-to-day) variations of BC mass concentrations, observed by both surface and aircraft measurements in outflow regions in East Asia. These model simulations show that both the total eastward flux and transport efficiency (fractions transported from sources) of BC are highest during spring (26kgs(-1) and 33% at 150 degrees E) and lowest during summer (8kgs(-1) and 20% at 150 degrees E). These seasonal variations of Asian BC outflow are generally controlled by transport patterns (monsoons, frontal passages, and convection) and emissions from the following three sources: (1) AN emissions from China (China AN), (2) BB emissions from Southeast Asia and South China (SEA BB) during February-April, and (3) BB emissions from Siberia and Kazakhstan (Siberia BB) during April-July. In our simulations, China AN dominates the total eastward BC flux on a 3year average (61%, 17%, and 6% from China AN, Siberia BB, and SEA BB, respectively, at 150 degrees E). In contrast, SEA and Siberia BB account for 30-50% of the total eastward BC flux (150 degrees E and 175 degrees E) during spring and summer, and they increase the seasonal variability of the Asian BC outflow flux. BC from Siberia BB is also found to be transported to the Pacific more efficiently than BC from other sources. Although the magnitudes of BB emissions are highly uncertain, our results suggest that the control of Siberia BB will be important in terms of the transboundary transport of BC to the Pacific, North America, and the Arctic.