PM 2.5 levels, chemical composition and health risk assessment in Xinxiang, a seriously air-polluted city in North China.

Seventeen PM _2.5 samples were collected at Xinxiang during winter in 2014. Nine water-soluble ions, 19 trace elements and eight fractions of carbonaceous species in PM _2.5 were analyzed. PM _2.5 concentrations and elements species during different periods with different pollution situations were compared. The threat of heavy metals in PM _2.5 was assessed using incremental lifetime cancer risk. During the whole period, serious regional haze pollution persisted, and the averaged concentration of PM _2.5 was 168.5 μg m ^-3 , with 88.2 % of the daily samples exhibiting higher PM _2.5 concentrations than the national air quality standard II. The high NO ₃ ^- /SO ₄ ^2- ratio suggested that vehicular exhaust made an important contribution to atmospheric pollution. All of organic carbon and elemental carbon ratios in this study were above 2.0 for PM _2.5 , which might reflect the combined contributions from coal combustion, motor vehicle exhaust and biomass burning. Mean 96-h backward trajectory clusters indicated that more serious air pollution occurred when air masses transported from the Hebei, Shanxi and Zhengzhou. The concentrations of the water-soluble ions and trace elements on haze days were 2 and 1.8 times of those on clear days. The heavy metals in PM _2.5 might not cause non-cancerous health issues by exposure through the human respiratory system. However, lifetime cancer risks of heavy metals obviously exceeded the threshold (10 ^-6 ) and might have a cancer risk for residents in Xinxiang. This study provided detailed composition data and comprehensive analysis of PM _2.5 during the serious haze pollution period and their potential impact on human health in Xinxiang.