Contrasting Responses of Peak Summer Surface Ozone to Anthropogenic Emission Changes Across Two Major Emission Hotspots in Eastern China

Peak summer surface ozone (O3) threatens human health, crop productivity, and ecosystem stability in eastern China, but similar O3 trends may reflect contrasting anthropogenic drivers. Using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) and the Multi-Resolution Emission Inventory for China (MEIC), we examined July maximum daily 8 h average (MDA8) O3 over the Bohai Plain and the Yangtze River Delta (YRD) from 2014 to 2020. Emission substitutions under fixed 2014 meteorology isolated the O3 responses to changes in total emissions and individual pollutants. Although O3 levels were high in both regions by 2019 and declined sharply in 2020, the two regions showed opposite responses to emission changes. Over the Bohai Plain, substituted inventories reduced MDA8 O3 in all later scenarios, with a 4.57 ppb decrease for the 2020 inventory. Over the YRD, emission changes increased O3 by up to 4.70 ppb. VOC substitution produced the largest decrease over the Bohai Plain at 5.46 ppb, whereas substituting 2020 NOx emissions increased YRD O3 by 4.22 ppb. The joint NOx and VOC substitution increased YRD O3 by 4.97 ppb, revealing a nonadditive response. Formation sensitivity also diverged, with the western Bohai Plain shifting toward VOC-sensitive conditions and NOx sensitivity strengthening in the northern YRD. These results show that similar peak summer O3 evolution can conceal fundamentally different emission responses. These findings support region-specific multipollutant control strategies for the two regions.