Response of Summer Ozone to Precursor Emission Controls in the Yangtze River Delta Region
Surface ozone concentrations in the Yangtze River Delta (YRD) region in China have shown a significant increase with the dramatic reduction of anthropogenic nitrogen oxide (NO<sub>x</sub>) emissions since 2013. As the nonlinearity between ozone and its precursors (i.e., NO<sub>x</sub> and volatile organic compounds (VOCs)) varies greatly in time and space, we quantify the monthly changes of surface ozone with the co-control of NO<sub>x</sub> and VOCs anthropogenic emissions in the YRD region from May to October 2017 by using the GEOS-Chem model. Model evaluations show that the GEOS-Chem model exhibits good performance to simulate ozone concentrations in the region. For May–September 2017, most areas in the YRD region are under a transitional regime, but the regions with high anthropogenic emissions including Shanghai and southern Jiangsu are under a VOCs-limited regime. In October, basically, the entire YRD region is under a VOCs-limited regime. Generally, reducing VOCs emissions only is the most effective method for ozone control in the YRD. Nanjing is under a VOCs-limited regime in May, June, September, and October, and under a transition regime from July to August. The VOCs/NO<sub>x</sub> emission reduction ratio of 1:1 is effective for ozone mitigation in Nanjing (Shanghai) in May, June, and September (for May–September); the corresponding ratio is 2:1 in October. Hangzhou belongs to a transitional regime from May to September and is under a VOCs-limited regime in October. Reducing NO<sub>x</sub> emissions only would control ozone in Hangzhou from May to September, while the VOCs/NO<sub>x</sub> emission reduction ratio of 1:1 is favorable to reduce ozone concentrations in October. During high pollution days on July 22–27, 2017, the three cities belong to a transitional regime and reducing NO<sub>x</sub> emissions only is generally the most effective way to control high ozone pollution. GEOS-Chem tagged ozone simulation shows that ozone problem in the region is caused by the joint effect of local generation and regional and long-distance transport. Local generation (19.0–50.7%) is generally the largest contributor to monthly mean ozone concentrations in Jiangsu and Shanghai, Zhejiang, and central eastern China; the contribution of ozone from regions outside the YRD is larger in spring and autumn (42–76.0%) than in summer (23.3–51.8%). Since the annual VOCs (NO<sub>x</sub>) anthropogenic emissions in the region have shown a decline by 8% (11%) from 2017 to 2020 and would continue to reduce by 10% (10%) by 2025 according to the Chinese government requirement, the growth of ozone would be stopped in the YRD for May–September but likely to continue in October. Our study thus would provide a scientific base for guiding the effective emission reduction strategies to control ozone pollution in the YRD region.