Reduction in black carbon light absorption due to multi-pollutant emission control during APEC China 2014

Reducing black carbon (BC) emissions has been recognized as an efficient way to simultaneously improve air quality and mitigate climate change. However, the benefits of BC emission controls are not well quantified, partly due to a lack of understanding of the changes in BC light absorption as a result of emission reductions. In this work, we discuss the effects of multi-pollutant emission reductions on BC light absorption based on a field campaign study conducted before, during and after the 2014 APEC (Asia-Pacific Economic Cooperation) meeting in Beijing, China. When emission restrictions were in place during APEC, we found that the reduction in the light absorption of BC-containing particles was driven by both the decrease in BC mass concentration and the weakened light-absorption capability of BC. Compared with that before and after APEC, the daytime light absorption of BC-containing particles during APEC was reduced by similar to 56 %, of which similar to 48% was contributed by the decrease in BC mass concentration and the remaining similar to 8% was contributed by a weakening of light-absorption capability for BC. Based on single-particle soot photometer (SP2) measurements and Mie calculations, we estimated that the light-absorption capability of BC-containing particles with similar to 80-200 nm refractory BC (rBC) cores in daytime during APEC was reduced by similar to 6-15% and similar to 10-20% compared with those before and after APEC, respectively. The decrease in BC light-absorption capability could be attributed to less coating material on BC surfaces as a result of the decreased chemical production of secondary aerosols. Compared with that before and after APEC, the mass ratio between the coating materials and rBC core (similar to 80-200 nm) during APEC decreased by similar to 10-30% and similar to 31-53 %, respectively, due to reductions in coating precursor emissions, e.g., SO2 and NO2. The results reveal the benefits of emission control on BC light absorption by simultaneously reducing the mass concentration and light-absorption capability of BC, implying that synergetic reduction in multiple-pollutant emissions could benefit both air quality and climate.