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

The fine particles (PM2.5) in China have decreased significantly in recent years as a result of the implementation of Chinese Clean Air Action Plan since 2013, while the O-3 pollution is getting worse, especially in megacities such as Beijing and Shanghai. Better understanding of the elevated O-3 pollution in Chinese megacities and its response to emission change is important for developing an effective emission control strategy in the future. In this study, we analyze the significant increasing trend of daily maximum O-3 concentration from 2006 to 2015 in the megacity Shanghai with the variability of 0.8-1.3 ppbv yr(-1). It could likely be attributed to the notable reduction in NOx concentrations with the decreasing rate of 1.86-2.15 ppbv yr(-1) accompanied by the small change in VOCs during the same period by excluding the weak trends of meteorological impacts on local dispersion (wind speed), regional transport (wind direction), and O-3 photolysis (solar radiation). It is further illustrated by using a state-of-the-art regional chemical and dynamical model (WRF-Chem) to explore the O-3 variation response to the reduction in NOx emissions in Shanghai. The control experiment conducted for September of 2009 shows excellent performance for O-3 and NOx simulations, including both the spatial distribution pattern and the day-by-day variation through comparison with six in situ measurements from the MIRAGE-Shanghai field campaign. Sensitivity experiments with 30% reduction in NOx emissions from 2009 to 2015 in Shanghai estimated by Shanghai Environmental Monitoring Center shows that the calculated O-3 concentrations exhibit obvious enhancement by 4-7 ppbv in urban zones with increasing variability of 0.96-1.06 ppbv yr(-1), which is consistent with the observed O-3 trend as a result of the strong VOC-limited condition for O-3 production. The large reduction in NOx combined with less change in VOCs in the past 10 years promotes the O-3 production in Shanghai to move towards an NOx-limited regime. Further analysis of the WRF-Chem experiments and O-3 isopleth diagram suggests that the O-3 production downtown is 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 the insignificant VOC variation persists, the O-3 concentration downtown would keep increasing until 2020 with the further 20% reduction in NOx emission after 2015 estimated by Shanghai Clean Air Action Plan. The O-3 production in Shanghai will switch from a VOC-limited to an NOx-limited regime after 2020 except for downtown area, which is likely close to the transition regime. As a result the O-3 concentration will decrease 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 not sensitive to both NOx and VOC changes downtown. This result reveals that the control strategy of O-3 pollution is a very complex process and needs to be carefully studied.