PAHs and nitro-PAHs in urban Beijing from 2017 to 2018: Characteristics, sources, transformation mechanism and risk assessment

Polycyclic aromatic hydrocarbons (PAHs) and their nitrated derivatives (NPAHs) attract continuous attention due to their distinct carcinogenicity and mutagenicity. To investigate the characteristics, sources, formation mechanism and health risk assessment of PAHs and NPAHs, PM2.5 were collected at an urban site in Beijing from 2017 to 2018. The highest PAHs and NPAHs concentrations were 77.92 ± 54.62 ng/m3 and 963.71 ± 695.06 pg/m3 in the winter campaign, which were several times larger than those in other seasonal campaigns. Distinct diurnal variations of nocturnal levels higher than daytime levels were shown for PAHs and NPAHs. Source analysis indicated that besides vehicle exhaust, biomass burning and coal combustion were important sources of PAHs and NPAHs in the fall and winter campaigns. Secondary formation in atmosphere was another source of NPAHs especially in the spring and summer campaigns. NO2 and RH could positively influence the heterogeneous formation of NPAHs when RH was less than 60%. Quantum calculation results confirmed the formation pathway of 2N-FLA from the OH/NO3-initiated oxidation of FLA. The results of health risk assessment showed the potential health risks for the residents, especially in the winter campaign. These results indicated that PAHs and NPAHs still deserve attention following with the decrease concentrations of particulate matter.