Exploring 2016–2017 surface ozone pollution over China: source contributions and meteorological influences

Abstract. Severe surface ozonepollution over major Chinese cities has become an emerging air quality concern, raising a new challenge for emission control measures in China. In this study, we explore the source contributions to surface daily maximum 8-h average (MDA8) ozoneover China in 2016 and 2017, the two years with the highest surface ozoneaveraged over Chinese cities in record. We estimate the contributions of anthropogenic, background, and individual natural sources to surface ozoneover China, using the GEOS-Chem chemical transport modelat 0.25° × 0.3125° horizontal resolution with the most up-to-date Chinese anthropogenic emission inventory. Model results are evaluated with concurrent surface ozonemeasurements at 169 cities over China and show general good agreement. We find that background ozone(defined as ozonethat would be presented in the absence of all Chinese anthropogenic emissions) accounts for 90 % (49.4 ppbv) of the national March–April mean surface MDA8 ozoneover China and 80 % (44.5 ppbv) for May–August. It includes large contributions from natural sources (80 % in March–April and 72 % in May–August). Among them, biogenic volatile organic compounds (BVOCs) emissions enhance MDA8 ozoneby more than 15 ppbv in eastern China during July–August, while lightning NO x emissions and ozonetransport from the stratosphere both lead to ozoneenhancements of over 20 ppbv in western China during March–April. Over major Chinese city clusters, domestic anthropogenic sources account for about 30 % of the May–August mean surface MDA8 ozone, and reach 39–73 ppbv (38 %–69 %) for days with simulated MDA8 ozone> 100 ppbv in the Northern China Plain, Fenwei Plain, Yangtze River Delta, and Pearl River Delta city clusters. These high ozoneepisodes are usually associated with high temperatures, which induce large BVOCs emissions and enhance ozonechemical production. Our results indicate that there would be no days with MDA8 ozone> 80 ppbv in these major Chinese cities in the absence of domestic anthropogenic emissions. We find that the 2017 ozoneincreases relative to 2016 are largely due to higher background ozonedriven by hotter and dryer weather conditions, while changes in domestic anthropogenic emissions alone would have led to ozonedecreases in 2017. Meteorological conditions in 2017 favor natural source contributions (particularly soil NO x and BVOCs ozoneenhancements) and ozonechemical production, increase thermal decomposition of peroxyacetyl nitrate(PAN), and further decrease ozonedry deposition velocity. More stringent emission control measures are thus required to offset the adverse effects of unfavorable meteorology such as high temperature on surface ozoneair quality.