Source Attribution of Black Carbon and its Radiative Forcing in China

Black carbon (BC) is an air pollutant that has adverse effects on air quality. It can also heat the atmosphere through absorbing solar radiation and influences the Earth’s climate. BC concentrations are relatively high in East Asia, especially China, due to strong anthropogenic emissions associated with the accelerated urbanization and rapid economic growth. In this study, the source attributions for mass concentration, haze formation, transport, and direct radiative forcing of BC in various regions of China are quantified using the Community Earth System Model (CESM) with a newly implemented source-tagging technique. Anthropogenic emissions used in our model simulations are from the Community Emissions Data System (CEDS, http://www.globalchange.umd.edu/CEDS/) developed for the Coupled Model Intercomparison Project Phase 6 (CMIP6). Simulated BC concentrations in China show seasonal high in winter and low in summer. Radiative forcing estimated shows that only half of the annual mean BC direct radiative forcing (1.66 W m^-2) in China results from local emissions, with the other half contributed by emissions from outside China. Over north China where the air quality is often poor, nearly 70% of column burden BC is contributed by local emissions. About 30% of BC over south China in winter can be attributed to emissions from north China and 40% comes from sources outside China in spring. For other regions in China, more than 50% of BC mainly results from non-local emissions. We further investigated potential factors that contribute to the poor air quality in China. During hazy days, local increases in BC burden are mainly caused by inflows of BC transported from other source regions associated with anomalous winds. BC-containing particles emitted in East Asia can also be transported across the Pacific. Our model results show that emissions from inside and outside China are equally important for the BC outflow from East Asia, while emissions from China account for about 20% of BC burden in western United States.