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Polycyclic aromatic hydrocarbons (PAHs) have been of health conc due to its carcinogenesis and mutagenesis.In this study,we aimed to assess the variations,sources,and lifetime excessive cancer risk (ECR) attributable to PAHs bound to ambient particulate matters with aerodynamic diameter less than 2.5 μm (PM2.5) in metropolitan Beijing,China.We collected 24-hour integrated PM2.5 samples on daily basis between November 2014 and June 2015 across both central heating (cold months) and non-heating (warm months) seasons,and further analyzed the PAH components in these daily PM2.5 samples.Our results showed that total concentrations of PM2.5-bound PAHs varied between (88.6 ± 75.4) ng/m3 in the cold months and (11.0 ± 5.9) ng/m3 in the warm months.Benzo[a]pyrene (BaP),the carcinogenic marker of PAHs,averaged at 5.7 and 0.4 ng/m3 in the cold and warm months,respectively.Source apportionment analyses illustrated that gasoline,biomass bing,diesel,coal combustion and cooking were the major contributors,accounting for 12.9％,17.8％,24.7％,24.3％ and 6.4％ of PM2.5-bound PAHs,respectively.The BaP equivalent lifetime ECR from inhalation of PM2.5-bound PAHs was 16.2 cases per million habitants.Our results suggested that ambient particulate reduction from energy reconstruction and adaption of clean fuels would result in reductions PM2.5-bound PAHs and its associated cancer risks.However,as only particulate phased PAHs was analyzed in the present study,the concentration of ambient PAHs could be underestimated.