How have the characteristics of air quality in a typical large Chinese city changed between 2011 and 2017

Great efforts have been made to control air pollution in China, and these have markedly changed air quality. Here, a detailed analysis of changes in air pollution in Nanjingbetween 2011 and 2017 based on hourly concentration measurements of six air pollutants is presented. The concentrations of most of these pollutants have decreased since 2013. This has resulted in the annual proportion of days meeting the new Chinese air quality standards increasing by 17.5%, as well as the proportion of days of severe or strong pollution decreasing by 8.0%. However, the ozone (O3) concentration increased between 2011 and 2017, and the number of days that O3 concentrations exceeded the Chinese standard dramatically increased from 8 days in 2011 to 60 days in 2017. Clearly, O3 has replaced fine particulate matter (PM2.5) as the most frequent dominant pollutant since 2016. Our results indicate that air pollution in Nanjing has evolved from being dominated by primary pollutants to being dominated by secondary pollutants, reflecting interaction of multiple emissions and chemical processes. Current policies and strategies need to be reassessed and modified to deal with interactions between emissions from different sources under variable meteorological conditions.