Response of aerosol chemistry to clean air action in Beijing, China: Insights from two-year ACSM measurements and model simulations.

Despite substantial mitigation of particulate matter (PM) pollution during the past decade in Beijing, the response of aerosol chemistry to clean air action and meteorology remains less understood. Here we characterized the changes in aerosol composition as responses to emission reductions by using two-year long-term measurements in 2011/2012 and 2017/2018, and WRF-Chem model. Our results showed substantial decreases for all aerosol species except nitrate from 2011/2012 to 2017/2018. Chloride exhibited the largest decrease by 65-89% followed by organics (37-70%), mainly due to reductions in coal combustion emissions in winter and agriculture burning in June. Primary and secondary organic aerosol (SOA) showed comparable decreases by 61-70% in fall and winter, and 34-63% in spring and summer, suggesting that reductions in primary emissions might also suppress SOA formation. The changes in nitrate were negligible and even showed increases due to less reductions in NOx emissions and increased formation potential from N2O5 heterogeneous reactions. As a result, nitrate exceeded sulfate and became the major secondary inorganic aerosol species in PM with the contribution increasing from 14-21% to 22-32%. Further analysis indicated that the reductions in aerosol species from 2011/2012 to 2017/2018 were mainly caused by the decreases of severely polluted events (PM1 > 100 μg m-3). WRF-Chem simulations suggested that the decreases in OA and sulfate in fall and winter were mainly resulted from emission reductions (27-36% and 25-43%) and favorable meteorology (4-10% and 19-30%), while they were dominantly contributed by emission changes in spring and summer. Comparatively, the changes in nitrate were mainly associated with meteorological variations while the contributions of emissions changes were relatively small. Our results highlight different chemical responses of aerosol species to emission changes and meteorology, suggesting that future mitigation of air pollution in China needs species-targeted control policy.