Quantifying the role of PM_2.5 dropping in variations of ground-level ozone: Inter-comparison between Beijing and Los...

In recent decade the ambient fine particle (PM_2.5) 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_2.5 and ozone decreasing together over past decades. In this study, we utilize observation-based methods to establish the parametric relationship between PM_2.5 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_2.5 decline in enhancing ground-level ozone pollution over a large PM_2.5 concentration range (10-120 μg m^-3). We find that the significance of ozone enhancement due to PM_2.5 dropping depends on both the PM_2.5 levels and optical properties of particles. Ozone formation increased by 37% in 2006-2016 due to PM_2.5 dropping in Beijing, while it becomes less important (7%) as PM_2.5 reaches below 40 μg/m³, 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_2.5 reductions in Beijing, as experienced in Los Angeles.