The PM_(2.5)air quality of most cities in China failed to meet Grade II national air quality standards,and PM_(2.5)pollution level varied greatly among different types of cities.In order to improve the overall air quality across the country,it is necessary to set phased-in targets for the cities at different pollution levels.Firstly,ambient air quality standards and guidelines for PM_(2.5)of WHO,the European Union and some developed countries were reviewed systematically,and target indicators of PM_(2.5)air...