Occurrence, gas/particle partitioning and carcinogenic risk of polycyclic aromatic hydrocarbons and their oxygen and nitrogen containing derivatives in Xi'an, central China

29 parent- and alkyl-polycyclic aromatic hydrocarbons (PAHs), 15 oxygenated-PAHs (OPAHs), 11 nitrated-PAHs (NPAHs) and 4 azaarenes (AZAs) in both the gaseous and particulate phases, as well as the particulate-bound carbon fractions (organic carbon, elemental carbon, char, and soot) in ambient air sampled in March and September 2012 from an urban site in Xi'an, central China were extracted and analyzed. The average concentrations (gaseous+particulate) of ∑29PAHs, ∑15OPAHs, ∑11NPAHs and ∑4AZAs were 1267.0±307.5, 113.8±46.1, 11.8±4.8 and 26.5±11.8ngm^-3 in March and 784.7±165.1, 67.2±9.8, 9.0±1.5 and 21.6±5.1ngm^-3 in September, respectively. Concentrations of ∑29PAHs, ∑15OPAHs and ∑11NPAHs in particulates were significantly correlated with those of the carbon fractions (OC, EC, char and soot). Both absorption into organic matter in particles and adsorption onto the surface of particles were important for PAHs and OPAHs in both sampling periods, with more absorption occurring in September, while absorption was always the most important process for NPAHs. The total carcinogenic risk of PAHs plus the NPAHs was higher in March. Gaseous compounds, which were not considered in most previous studies, contributed 29 to 44% of the total health risk in March and September, respectively.