Amplification of light absorption of black carbon associated with air pollution.

The impacts of black carbon (BC) aerosols on air quality, boundary layer dynamic and climate depend not only on the BC mass concentration but also on the light absorption capability of BC. It well known that the light absorption capability of BC depends on the amount of coating materials (namely other species on BC by condensation and coagulation). However, the difference of light absorption capability of ambient BC-containing particles under different air pollution conditions (e.g., the air 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 the growth of PM₁ concentration from ~ 10 μg m⁻³ to ~ 230 μg m⁻³, we found that the aging degree and light absorption capability of BC-containing particles with refractory BC cores of ~ 75–200 nm increased by 26–73 % and 13–44 % 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 effective emission intensity (EEI) model, we further found that aging during the regional transport plays an important role in the difference among the light absorption capability of BC-containing particles under different air pollution levels. During the pollution episode, ~ 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 surface due to accelerated aging process within more polluted air, which contributed ~ 78 % of the increase in light absorption capability of BC observed in Beijing during the polluted period (PM₁ of ~ 230 μg m⁻³) comparing to that in the clean period (PM₁ of ~ 10 μg m⁻3). Due to the increase of light absorption capability of BC associated with air pollution, the direct radiative forcing of BC was estimated to be increased by ~ 20 % based on a simple radiation transfer model. Our work identified an amplification of light absorption and direct radiative forcing under more air polluted environment due to more coating pollutants on BC. The air pollution control measures may, on the other hand, break the amplification effect by reducing both emissions of BC and the coating materials and achieve co-benefits of both air quality and climate. [ABSTRACT FROM AUTHOR]