Assessment of background ozone concentrations in China and implications for using region-specific volatile organic compounds emission abatement to mitigate air pollution.

Mitigation of ambient ozone (O) pollution is a great challenge because it depends heavily on the background O which has been poorly evaluated in many regions, including in China. By establishing the relationship between O and air temperature near the surface, the mean background O mixing ratios in the clean and polluted seasons were determined to be 35-40 and 50-55 ppbv in China during 2013-2019, respectively. Simulations using the chemical transport model (i.e., the Weather Research and Forecasting coupled with Chemistry model, WRF/Chem) suggested that biogenic volatile organic compounds (VOC) emissions were the primary contributor to the increase in the background O in the polluted season (BOP) compared to the background O in the clean season (BOC), ranging from 8 ppbv to 16 ppbv. More importantly, the BOP continuously increased at a rate of 0.6-8.0 ppbv yr during 2013-2019, while the non-BOP stopped increasing after 2017. Consequently, an additional 2%-16% reduction in anthropogenic VOC emissions is required to reverse the current O back to that measured in the period from 2013 to 2017. The results of this study emphasize the importance of the relative contribution of the background O to the observed total O concentration in the design of anthropogenic precursor emission control strategies for the attainment of O standards.