Polycyclic aromatic hydrocarbons in atmospheric PM2.5 and PM10 in the semi-arid city of Xi'an, Northwest China: Seasonal variations, sources, health risks, and relationships with meteorological factors

Atmospheric particulate matter (PM2.5 and PM10) samples were collected at urban and suburban sites in Xi'an City from December 2016 to November 2017. Sixteen priority polycyclic aromatic hydrocarbons (PAHs) associated with PM2.5 and PM10 were analyzed for their seasonal variations, sources, health risks, and influencing factors. The results showed that the annual average concentrations of PM2.5 and PM10 were 111 and 185 mu g/m(3), respectively, exceeding the National Ambient Air Quality Standard of China (35 mu g/m(3) for PM2.5 and 70 mu g/m(3) for PM10 in Grade II). The annual averages of PM2.5- and PM10-bound total PAHs were 63.1 and 66.8 ng/m(3), respectively, with a PM2.5 decrease in the order of winter (115 ng/m(3)) > > spring (47.6 ng/m(3)) > summer (33.2 ng/m(3)) > autumn (30.8 ng/m(3)) and a PM10 decrease in the order of winter (127 ng/m(3)) > > spring (55.6 ng/m(3)) > autumn (32.6 ng/m(3)) > summer (30.2 ng/m(3)). The most abundant PAHs were benzo[a]anthracene, benzo[b]fluoranthene, and fluoranthene. The PM2.5- and PM10-bound PAHs were originated mainly from traffic emissions (51.0% and 43.4%), followed by combustion of biomass (20.4% and 23.6%) and coal (16.8% and 23.1%). Pressure and relative humidity were positively correlated with PM2.5 and PM10 as well as PM2.5- and PM10-bound PAHs, while temperature, visibility and wind speed had negative correlations. The annual means of TEQs (toxic equivalency quantities) for the 16 PAHs in PM2.5 and PM10 were 10.1 and 10.2 ng/m(3), respectively, being attributed to 7 carcinogenic PAHs (> 95%). The ECRs (lifetime excess cancer risks) for PM2.5- and PM10-bound PAHs were 1.12 x 10(-3) and 1.17 x 10(-3), i.e., 1125 and 1169 lung cancer cases per million, respectively. The ILCRs (incremental lifetime cancer risks) due to PM2.5- and PM10-bound PAHs for adults (1.21 x 10(-6) and 1.26 x 10(-6)) were larger than those for children (2.09 x 10(-7) and 2.17 x 10(-7)), with acceptable carcinogenic risks. The ambient levels of PM and most PM-bound PAHs as well as their TEQs, ECRs and ILCRs exhibited a spatial pattern of the suburban site > the urban site and a seasonality with winter and spring > summer and autumn. The results suggest that it is important to control PM2.5 and PM10 for local air quality improvement and special attention should be paid to PM and PM-bound PAHs in suburban areas, particularly in winter and spring.