Response And Source Analysis Of Ozone Reduction In The Yangtze River Delta Region

Since the implementation of the"Air Pollution Prevention and Control Action Plan"policy in 2013,anthropogenic nitrogen oxides(NO_x)emissions in the Yangtze River Delta(YRD)region have decreased significantly,while surface ozone concentrations have shown an increasing trend,which largely depends on the nonlinearity between ozone and its precursors(i.e.,NO_x and volatile organic compounds(VOCs)).Due to ozone sensitivity to its precursor varies greatly in time and space,in this paper we analyze the responses of surface ozone to the co-control of NO_x and VOCs anthropogenic emissions in the YRD region(especially Nanjing,Shanghai,and Hangzhou)from May to October in 2015 and 2017 to obtain the most effective VOCs/NO_x reduction ratios in different months,different regions,and high pollution event by using the GEOS-Chem model;we conduct the GEOS-Chem tagged ozone technique to qualify the contributions of ozone from local emissions and regional transport to ozone in the YRD region.The main conclusions are as follows:(1)For May–September 2017(2015),most areas in the YRD region are under a transitional regime but the regions with high anthropogenic emissions including Shanghai and southern Jiangsu(Shanghai,southern Jiangsu,and northern Zhejiang)are under a VOCs-limited regime.In October,basically the entire YRD is under a VOCs-limited regime.Generally,reducing VOCs emissions is the most effective method for ozone control in the YRD.In addition,the YRD region is changing from a VOCs-limited regime to a transition regime due to the reduction of NO_x anthropogenic emissions in recent years.(2)Nanjing is under a VOCs-limited regime in May,June,September,and October 2017(from May to October 2015),and under a transition regime from July to August.The VOCs/NO_xemission reduction ratio of 1:1 is effective for ozone mitigation in Nanjing in May,June,and September 2017(from May to September 2015);the corresponding ratio is 2:1 in October.Shanghai belongs to a VOCs-limited regime from May–October,and the VOCs/NO_xemission reduction ratio of 1:1 is effective for ozone mitigation from May to September 2017(from July to September 2015),the corresponding ratio is 2:1 in other months.Hangzhou belongs to a transitional regime from May to September and is under a VOCs-limited regime in October.Reducing NO_x emissions only would control ozone in Hangzhou from May to September,while the VOCs/NO_xemission reduction ratio of 1:1 is favor to reduce ozone concentrations in October.The analysis on sensitivity of ozone to its precursors in urban and suburban areas in three cities shows that urban area of Nanjing(Shanghai)belongs to a VOCs-limited regime from May to June and from August to October(from May to October)while belongs to a transitional regime in July.Nanjing suburb is controlled by a VOCs-limited regime from May to June and from September to October but belongs to a transitional regime from July to August.Shanghai suburb is under a VOCs-limited regime in May and October,but under a transition regime from June to September.Urban and suburban areas of Hangzhou are under a transitional regime for May–September while under a VOCs-limited regime in October.During the high pollution event on July 22–27,2017,the three cities belong to a transitional regime and reducing VOCs or NO_x emissions can control high ozone pollution.(3)GEOS-Chem tagged ozone simulation shows that ozone problem in the YRD 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%).