Trends in ambient air pollution levels and PM(2.5) 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 (SO(2)), nitrogen dioxide (NO(2)), and ozone (O(3))] from 1995 to 2017 and PM(2.5) composition for the period of 2000–2017 in Guangzhou, Wuhan, Chongqing, and Lanzhou. We provided socio-economic indicators to help explain the pollution trends. SO(2) and PM (including PM(10) and PM(2.5)) concentrations showed a downward trend in recent years with the most notable reduction in SO(2) in Chongqing and PM(2.5) in Guangzhou. There was an overall flat trend for NO(2), while O(3) showed an upward trend in recent years except in Lanzhou. The majority of PM(2.5) mass was SO(4)(2−) (6.0–30 µg/m(3)) and organic carbon (6.0–38 µg/m(3)), followed by NO(3)(−) (2.0–12 µg/m(3)), elemental carbon (2.1–12 µg/m(3)), NH(4)(+) (1.0–10 µg/m(3)), K(+) (0.2–2.0 µg/m(3)), and Cl(−) (0.2–1.9 µg/m(3)). Except for secondary inorganic aerosols in Wuhan, annual average concentrations of all PM(2.5) constituents showed a declining trend after 2013, corresponding to the trend of PM(2.5). The secondary sources in PM(2.5) 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 PM(2.5) pollution. These trends in ambient air pollution levels and PM(2.5) composition may help understand changes in health outcomes measured at different times within the time period of 1995–2017 in the four cities.