Rapid improvement in air quality due to aerosol-pollution control during 2012-2018: An evidence observed in Kunshan in the Yangtze River Delta, China

China's severe air pollution was widely concerned in the past decade. A rigorous emission control has been implemented by the Chinese government since 2013. It is essential to evaluate changes of air pollutants to understand effectiveness of present air-pollution control on improving air quality. Here, we investigate temporal trends of air pollutants, including PM2.5, SO2, NO2 and O-3, observed at 12 sites over the Kunshan area in the Yangtze River Delta region during 2012-2018 using Mann-Kendall statistical test. Overall, significant reduction trends of monthly PM2.5(- 7.4% yr(-1)) were observed, together with reduction in the PM2.5 to CO ratio ( - 5.8% yr(-1)), an indicator of secondary aerosol production. Secondary aerosol precursors, SO2 ( - 10.3% yr(-1)) and NO2 ( - 4.4% yr(-1)) also presented statistically significant reduction trends. These results reflect the consequence of emission control that leads to substantial reduction in the bulk PM2.5 concentration, as well as secondary aerosols likely formed from SO2 and NO2. However, O-3 had statistically significant increase trend ( + 3.4% yr(-1)) during 2014-2018. Limited formation of O-3 under high PM2.5 and NO2 concentrations condition was found at daytime in summer, which might reflect one of the reasons causing the increase trend of O-3 under the current reduction scenario of PM2.5 and NO2. Potential source contribution function analysis demonstrated that the transport from the regions located to northwest of Kunshan could contribute to high concentrations of PM2.5 in all seasons, while the south and southeast could be the high potential as source areas for O-3.