Background Polycyclic aromatic hydrocarbons (PAHs) are one of the most widely distributed and harmful organic pollutants in the atmosphere.

Objective To investigate the distribution characteristics and composition sources of PAHs in the atmosphere of two districts of Lanzhou from 2019 to 2020 and evaluate the health risks of PAHs via inhalation to different populations.

Methods The PAHs concentrations in two urban areas (Chengguan District and Xigu District) of Lanzhou City from January 2019 to December 2020 were regularly monitored. Mann-Whitney U test was used to compare differences in target pollutant concentrations between the two areas. Diagnostic ratio method and principal component analysis were adopted for source identification. Incremental lifetime cancer risk (ILCR) model was applied to evaluate the health risks of PAHs.

Results The M (P₂₅, P₇₅) PAHs concentrations in Chengguan District and Xigu District were 24.04 (14.59, 41.81) ng·m⁻³ and 25.97 (18.59, 42.56) ng·m⁻³, respectively, with no significant difference (Z=−0.970, P>0.05). As to seasonal distribution, most PAHs monomer concentrations in Chengguan District were higher than those in Xigu District in summer, and the concentrations of benzoaanthracene and benzog,h,iperylene in Chengguan District were also higher than those in Xigu District in spring and autumn (P<0.05), but there were no significant differences in PAHs monomer concentrations between the two urban areas in winter (P>0.05). Ring number of PAHs exhibited seasonal fluctuations. In winter and spring, the highest proportions in Chengguan District and Xigu District were both 4-ring PAHs (37.32%-41.73%, 35.20%-39.66%), and in summer and autumn, the highest proportions were both 2- and 3-ring PAHs (39.38%-49.54%, 47.17%-51.23%). The results of diagnostic ratio method revealed mixed atmospheric PAHs sources in the two urban areas, including fossil fuel, coal, and biomass combustion. The results of principal component analysis showed that the cumulative contribution rate of the four principal component factors reached 79.54%. Principal component 1 included phenanthrene, anthracene, fluoranthene, pyrene, chrysene, benzoaanthracene, benzobfluoranthene, benzokfluoranthene, and benzoapyrene; principal component 2 included acenaphthene and dibenzoa,hanthracene; principal component 3 was fluorene; principal component 4 was naphthalene. The results of health risk assessment showed that the ILCR values of adult males, adult females, and children in Chengguan District were 2.30×10⁻⁶, 2.16×10⁻⁶, and 1.73×10⁻⁶, respectively; and those in Xigu District were 1.58×10⁻⁶, 1.48×10⁻⁶, and 1.19×10⁻⁶, respectively; all were greater than 10⁻⁶.

Conclusion PAHs pollution exists in the atmosphere of the two urban areas of Lanzhou City, mainly comes from mixed sources of fossil fuels, coal, and biomass burning, and may pose potential carcinogenic risks to the population.