Quantifying the anthropogenic and meteorological influences on summertime surface ozone in China over 2012-2017

We applied the global 3-D chemical transport model GEOS-Chem to examine the anthropogenic and meteorological contributions in driving summertime (JJA) surface ozone (O₃) trend in China during the Clean Air Action period 2012-2017. The model captures the observed spatial distribution of summertime O₃ concentrations in China (R = 0.78) and reproduces the observed increasing trends in two most populated city clusters: North China Plain (NCP) and Yangtze River Delta (YRD). Trend of simulated maximum daily 8-h average (MDA8) O₃ concentration is 0.58 ppbv yr^-1 in NCP and 1.74 ppbv yr^-1 in YRD in JJA 2012-2017. Sensitivity studies show that both changes in anthropogenic emissions and meteorology favored the MDA8 O₃ increases in these two regions with respective contributions of 39% and 49% in NCP, and 13% and 84% in YRD. In NCP, the 49% meteorology impact includes a considerable contribution from natural emissions (19%). Changes in biogenic VOCs, soil NO_x, and lightning NO_x emissions are estimated to enhance MDA8 O₃ in NCP with a rate of 0.14, 0.10, and 0.14 ppbv yr^-1, respectively. In YRD, natural emissions made small contributions to the MDA8 O₃ trend. Statistical analysis shows that higher temperatures and anomalous southerlies at 850 hPa in 2017 relative to 2012 are the two major meteorological drivers in NCP that favored the O₃ increases, while weaker wind speed and lower relative humidity are those for YRD. We further examined the trend of fourth highest daily maximum 8-h average (4MDA8) O₃ among a specific month that linked with extreme pollution episodes. Trends of simulated 4MDA8 O₃ in NCP and YRD are 34-46% higher than those of MDA8 O₃ and are found more meteorology-induced. Our results suggest an important role of meteorology in driving summertime O₃ increases in China in recent years.