Severe surface ozone pollution over major Chinese cities has become anemerging air quality concern, raising a new challenge for emission controlmeasures in China. In this study, we explore the source contributions tosurface daily maximum 8 h average (MDA8) ozone over China in 2016 and 2017,the 2 years with the highest surface ozone averaged over Chinese cities inrecord. We estimate the contributions of anthropogenic, background, andindividual natural sources to surface ozone over China using the GEOS-Chemchemical transport model at 0.25 ∘ × 0.3125 ∘ horizontal resolution with the most up-to-date Chinese... anthropogenicemission inventory. Model results are evaluated with concurrent surfaceozone measurements at 169 cities over China and show generally good agreement.We find that background ozone (defined as ozone that would be present inthe absence of all Chinese anthropogenic emissions) accounts for 90 %(49.4 ppbv) of the national March–April mean surface MDA8 ozone over Chinaand 80 % (44.5 ppbv) for May–August. It includes large contributions fromnatural sources (80 % in March–April and 72 % in May–August). Amongthem, biogenic volatile organic compound (BVOC) emissions enhance MDA8ozone by more than 15 ppbv in eastern China during July–August, whilelightning NO x emissions and ozone transport from the stratosphere bothlead to ozone enhancements of over 20 ppbv in western China duringMarch–April. Over major Chinese city clusters, domestic anthropogenicsources account for about 30 % of the May–August mean surface MDA8 ozoneand reach 39–73 ppbv (38 %–69 %) for days with simulated MDA8 ozone > 100 ppbv in the North China Plain, Fenwei Plain, YangtzeRiver Delta, and Pearl River Delta city clusters. These high ozone episodesare usually associated with high temperatures, which induce large BVOCemissions and enhance ozone chemical production. Our results indicate thatthere would be no days with MDA8 ozone > 80 ppbv in these majorChinese cities in the absence of domestic anthropogenic emissions. We findthat the 2017 ozone increases relative to 2016 are largely due to higherbackground ozone driven by hotter and drier weather conditions, whilechanges in domestic anthropogenic emissions alone would have led to ozonedecreases in 2017. Meteorological conditions in 2017 favor natural sourcecontributions (particularly soil NO x and BVOC ozone enhancements) andozone chemical production, increase the thermal decomposition of peroxyacetylnitrate (PAN), and further decrease ozone dry deposition velocity. Morestringent emission control measures are thus required to offset the adverseeffects of unfavorable meteorology, such as high temperature, on surface ozoneair quality.