The ambient fine particle matters (PM_2.5) were collected from November 2008 till March 2009 (wintertime) in an urban site of Xi'an to compare with the chemical compositions according to the air quality grades guidelines (HJ 633-2012). All of PM_2.5 mass concentrations were in excess of the 75 μg/m³ level in the 24 h Ambient Air Quality Standards (GB 3095-2012). The concentrations of PM_2.5 mass and their corresponding chemical compositions were elevated to show a generally increasing trend of the AQI except for a few elements. The concentrations of organic carbon (OC) and water-soluble inorganic ions on six-grade days were approximately 4.5 and 2.7 times those of three-grade days. The enrichment factors of anthropogenic elements (e.g., As, Zn and Pb) on six-grade days were 1.6-2.0 times those of three-grade days, while the elements from crustal sources were not observed along with any surge and rule of air pollution levels. PAHs and n-alkanes mainly were contributed by anthropogenic sources. During the occurrence of heavy air pollution in winter, biomass burning and coal combustion emissions showed a larger contribution to PAHs than gasoline vehicle emissions.

PM_2.5  /  chemical composition  /  PAHs  /  n-alkane  /  air quality grades