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. The fine particles (PM 2.5 ) in China have decreased significantly inrecent years as a result of the implementation of Chinese Clean Air Action Plansince 2013, while the O3 pollution is getting worse, especially inmegacities such as Beijing and Shanghai. Better understanding of the elevated O3 pollution in Chinese megacities and its response to emission changeis important for developing an effective emission control strategy inthe future. In this study, we analyze the significant increasing trend of dailymaximum O3 concentration from 2006 to 2015 in the megacity Shanghaiwith the variability of 0.8–1.3 ppbv yr −1 . It could likely be attributed tothe notable reduction in NOx concentrations with the decreasing rate of1.86–2.15 ppbv yr −1 accompanied by the small change in VOCs duringthe same period by excluding the weak trends of meteorological impacts onlocal dispersion (wind speed), regional transport (wind direction), and O3 photolysis (solar radiation). It is further illustrated by using astate-of-the-art regional chemical and dynamical model (WRF-Chem) to explore the O3 variation response to the reduction in NOx emissions inShanghai. The control experiment conducted for September of 2009 showsexcellent performance for O3 and NOx simulations, including boththe spatial distribution pattern and the day-by-day variation throughcomparison with six in situ measurements from the MIRAGE-Shanghai field campaign.Sensitivity experiments with 30 % reduction in NOx emissions from 2009to 2015 in Shanghai estimated by Shanghai Environmental Monitoring Centershows that the calculated O3 concentrations exhibit obvious enhancementby 4–7 ppbv in urban zones with increasing variability of 0.96–1.06 ppbv yr −1 , which is consistent with the observed O3 trend as aresult of the strong VOC-limited condition for O3 production. The largereduction in NOx combined with less change in VOCs in the past 10years promotes the O3 production in Shanghai to move towardsan NOx -limited regime. Further analysis of the WRF-Chem experiments and O3 isopleth diagram suggests that the O3 production downtownis still under a VOC-limited regime after 2015 despite the remarkable NOx reduction, while it moves to the transition regime between NOx -limited and VOC-limited in sub-urban zones. Supposing theinsignificant VOC variation persists, the O3 concentration downtownwould keep increasing until 2020 with the further 20 % reduction in NOx emission after 2015 estimated by Shanghai Clean Air Action Plan.The O3 production in Shanghai will switch from a VOC-limited toan NOx -limited regime after 2020 except for downtown area, which is likelyclose to the transition regime. As a result the O3 concentration willdecrease by 2–3 ppbv in sub-urban zones and by more than 4 ppbv in rural areas as a response to a 20 % reduction in NOx emission after 2020, whereas it is notsensitive to both NOx and VOC changes downtown. This result revealsthat the control strategy of O3 pollution is a very complex processand needs to be carefully studied.

Measurement and model analyses of the ozone variation during 2006 to 2015 and its response to emission change in megacity Shanghai, China