PM2.5 samples were collected in Zhengzhou during 3 years of observation,and chemical characteristics and source contribution were analyzed.Approximately 96％ of the daily PM2.5 concentrations and annual average values exceeded the Chinese National Ambient Air Quality Daily and Annual Standards,indicating serious PM2.5 pollution.The average concentration of water-soluble inorganic ions was 2.4 times higher in heavily polluted days (daily PM2.5 concentrations ＞ 250 μg/m3 and visibility ＜3 km) than that in other days,with sulfate,nitrate,and ammonium as major ions.According to the ratio of NO3-/SO42-,stationary sources are still the dominant source of PM2.5 and vehicle emission could not be ignored.The ratio of secondary organic carbon to organic carbon indicated that photochemical reactivity in heavily polluted days was more intense than in other days.Crustal elements were the most abundant elements,accounting for more than 60％ of 23 elements.Chemical Mass Balance results indicated that the contributions of major sources (i.e.,nitrate,sulfate,biomass,carbon and refractory material,coal combustion,soil dust,vehicle,and industry) of PM2.5 were 13％,16％,12％,2％,14％,8％,7％,and 8％ in heavily polluted days and 20％,18％,9％,2％,27％,14％,15％,and 9％ in other days,respectively.Extensive combustion activities were the main sources of polycyclic aromatic hydrocarbons during the episode (Jan 1-9,2015) and the total benzo[a]pyrene equivalency concentrations in heavily polluted days present significant health threat.Because of the effect of regional transport,the pollution level of PM2.5 in the study area was aggravated.