Quantifying sources, deposition, transport and radiative forcing of black carbon over the Himalayas and Tibetan Plateau

Black carbon (BC) over the Himalayas and Tibetan Plateau (HTP), both airborne and that deposited on snow, has been shown to affect snowmelt and glacier retreat in this region. Since BC over the HTP may originate from a variety of geographical regions and emission sectors, it is essential to quantify the source-receptor relationships of BC in order to understand the contributions of natural and anthropogenic emissions and provide guidance for potential mitigation actions. In this study, we use the Community Atmosphere Model version 5 (CAM5) with a newly developed source tagging technique to characterize the destiny of BC particles emitted from various geographical regions and sectors, their transport pathways to the HTP, and radiative forcing. Evaluated against observations over the HTP and surrounding regions, the model simulation shows a good agreement in the seasonal variation of the near-surface airborne BC concentrations, providing confidence to use this modeling framework for characterizing BC source-receptor relationships. Our analysis shows that the relative contributions from different geographical regions and source sectors depend on seasons and the locations in HTP. The annual mean BC burden and surface deposition in the entire HTP region is contributed the most by biomass burning and biofuel (BB) emissions in South Asia, followed by fossil fuel (FF) emissions from South Asia and then the FF from East Asia. The same roles hold for all the seasonal means except for the summer when the East Asia FF becomes more important. For BC in multiple finer receptor regions of interest, South Asia BB and FF have the largest impact on BC in Himalayas and Central Plateau, while East Asia FF and BB contribute the most to Northeast Plateau in all seasons and Southeast Plateau in the summer. Central Asia and Middle East FF emissions have relatively more important contribution to BC reaching Northwest Plateau, especially in the summer. Among all the source regions, although the HTP local emissions only contribute about 10%, BC in HTP is found to be extremely sensitive to change in the local emissions. Lastly, we show that the radiative forcing due to BC in snow outweighs BC dimming effect at the surface over the HTP in winter. The seasonal variation of forcing induced by BC and other impurities such as mineral dust particles and its implication for snowmelt will also be discussed.