Background and Anthropogenic Source Contributions to Surface Ozone Pollution over China

Severe surface ozone pollution over major Chinese cities has become an emerging air quality concern, raising a new challenge for emission control measures in China. Here we explore the source contributions to surface daily maximum 8-h average (MDA8) ozone over China in 2016 and 2017, the two years with high surface ozone pollution over many Chinese cities. We estimate the contributions of anthropogenic, background, and individual natural sources to surface ozone over China, using the GEOS-Chem chemical transport model at 0.25°×0.3125° horizontal resolution. We find that background ozone (defined as ozone that would be presented in the absence of all Chinese anthropogenic emissions) accounts for 90% (49.4 ppbv) of the March-April mean surface MDA8 ozone averaged over 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). Biogenic volatile organic compounds (BVOCs) emissions enhance MDA8 ozone by more than 15 ppbv in eastern China during July-August, while lightning NO_xemissions and ozone transport from the stratosphere both lead to ozone enhancements of over 20 ppbv in western China during March-April. By contrast, 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. We also find background vs. anthropogenic ozone enhancement estimates are significantly different in a regional air quality model (WRF-Chem), and the driving factors will be analyzed.