Chemical characteristics and source apportionment of PM2.5 between heavily polluted days and other days in Zhengzhou, China

PMsamples were collected in Zhengzhou during 3 years of observation, and chemical characteristics and source contribution were analyzed. Approximately 96% of the daily PMconcentrations and annual average values exceeded the Chinese National Ambient Air Quality Daily and Annual Standards, indicating serious PMpollution. The average concentration of water-soluble inorganic ions was 2.4 times higher in heavily polluted days(daily PM32.5 concentrations > 250 μg/mand visibility < 3 km) than that in other days, with sulfate, nitrate, and ammonium as major ions. According to the ratio of NO-3/SO2-4,stationary sources are still the dominant source of PMand 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 PMwere 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 PMin the study area was aggravated.