This standard is formulated in a bid to implement the Environmental Protection Law of the People’s Republic of China and the Law of the People's Republic of China on the Prevention and Control of Atmospheric Pollution, protect and improve living environment and ecological environment as well as safeguard human health. The standard stipulates the classification of ambient air functional zones, categories of standards, items of pollutants, limit of average time and concentration, monitoring methods, stipulation on the validity of statistics as well as relevant implementation and supervision. The people’s governments of various provinces, autonomous regions and municipalities can formulate local ambient air quality standards over pollutant items that haven’t been specified in this standard. All the concentration of pollutants in this standard is mass concentration. This standard was first issued in 1982, first amended in 1996 and second amended in 2000, and this is the third amendment, and it will be duly amended according to national economic and social development situation and the requirements of environmental protection. The standards of Ambient Air Quality Standard (GB 3095-1996), Ambient Air Quality Standard (GB 3095-1996 Revision) (Huanfa  No. 1) and Maximum Allowable Concentration of Pollutants in Atmosphere for Protection of Crops (GB 9137-88) shall be abolished upon the enforcement date of this standard.