Amplification of light absorption of black carbon associated with air pollution

The impacts of black carbon (BC) aerosols on air quality, boundary layer dynamics and climate depend not only on the BC mass concentration but also on the light absorption capability of BC. It is well known that the light absorption capability of BC depends on the amount of coating materials (namely other species that condense and coagulate on BC). However, the difference of light absorption capability of ambient BC-containing particles under different air pollution conditions (e.g., clean and polluted conditions) remains unclear due to the complex aging process of BC in the atmosphere. In this work, we investigated the evolution of light absorption capability for BC-containing particles with changing pollution levels in urban Beijing, China. During the campaign period (17 to 30 November 2014), with an increase in PM1 concentration from similar to 10 to similar to 230 mu g m(-3), we found that the mass-weighted averages of the aging degree and theoretical light absorption capability of BC-containing particles increased by similar to 33% and similar to 18 %, respectively, indicating stronger light absorption capability of BC-containing particles under more polluted conditions due to more coating materials on the BC surface. By using an effective emission intensity (EEI) model, we further found that aging during regional transport plays an important role in the difference in the light absorption capability of BC-containing particles under different air pollution levels. During the pollution episode, similar to 63% of the BC over Beijing originated from regional sources outside of Beijing. These regionally sourced BC-containing particles were characterized by more coating materials on BC surfaces due to more coating precursors within more polluted air, which contributed similar to 75% of the increase in theoretical light absorption capability of BC observed in Beijing during the polluted period (PM1 of similar to 230 mu g m(-3)) compared to that in the clean period (PM1 of similar to 10 mu g m(-3)). Due to the increase in theoretical light absorption capability of BC associated with air pollution, the direct radiative forcing of BC was estimated to be increased by similar to 18% based on a simple radiation transfer model. Our work identified an amplification of theoretical light absorption and direct radiative forcing under a more polluted air environment due to more coating materials on BC. The air pollution control measures may, however, break the amplification effect by reducing emissions of both BC and the coating precursors and achieve co-benefits of both air quality and climate.