Quantifying the role of PM dropping in variations of ground-level ozone: Inter-comparison between Beijing and Los Angeles.

In recent decade the ambient fine particle (PM) levels have shown a trend of distinct dropping in China, while ground-level ozone concentrations have been increasing in Beijing and many other Chinese mega-cities. The variation pattern in Los Angeles was markedly different, with PM and ozone decreasing together over past decades. In this study, we utilize observation-based methods to establish the parametric relationship between PM concentration and key aerosol physical properties (including aerosol optical depth and aerosol surface concentration), and an observation-based 1-D photochemical model to quantify the response of PM decline in enhancing ground-level ozone pollution over a large PM concentration range (10-120 μg m). We find that the significance of ozone enhancement due to PM dropping depends on both the PM levels and optical properties of particles. Ozone formation increased by 37% in 2006-2016 due to PM dropping in Beijing, while it becomes less important (7%) as PM reaches below 40 μg/m, similar to Los Angeles since 1980s. Therefore, the two cities show the convergence of air pollutant characteristics. Hence a control strategy prioritizing reactive volatile organic compound abatement is projected to yield simultaneous ozone and PM reductions in Beijing, as experienced in Los Angeles.