Trends in ambient air pollution levels and PM2.5 chemical compositions in four Chinese cities from 1995 to 2017

An in-depth analysis of the specific evolution of air pollution in a given city can provide a better understanding of the chronic effects of air pollution on human health. In this study, we reported trends in ambient concentrations of particulate matter (PM) and gaseous pollutants [sulfur dioxide (SO2), nitrogen dioxide (NO2), and ozone (O-3)] from 1995 to 2017 and PM2.5 composition for the period of 2000-2017 in Guangzhou, Wuhan, Chongqing, and Lanzhou. We provided socio-economic indicators to help explain the pollution trends. SO2 and PM (including PM10 and PM2.5) concentrations showed a downward trend in recent years with the most notable reduction in SO2 in Chongqing and PM2.5 in Guangzhou. There was an overall flat trend for NO2, while O-3 showed an upward trend in recent years except in Lanzhou. The majority of PM2.5 mass was SO42- (6.0-30 mu g/m(3)) and organic carbon (6.0-38 mu g/m(3)), followed by NO3- (2.0-12 mu g/m(3)), elemental carbon (2.1-12 mu g/m(3)), NH4+ (1.0-10 mu g/m(3)), K+ (0.2-2.0 mu g/m(3)), and Cl- (0.2-1.9 mu g/m(3)). Except for secondary inorganic aerosols in Wuhan, annual average concentrations of all PM2.5 constituents showed a declining trend after 2013, corresponding to the trend of PM2.5. The secondary sources in PM2.5 were found to be most prominent in Wuhan, while the most abundant EC and Cl- in Lanzhou was attributed to the use of coal. Despite temporal and spatial variabilities across the four cities, coal combustion, traffic emissions, and secondary pollution have been the major sources of PM2.5 pollution. These trends in ambient air pollution levels and PM2.5 composition may help understand changes in health outcomes measured at different times within the time period of 1995-2017 in the four cities.