Characterization of particulate-bound polycyclic aromatic compounds (PACs) and their oxidations in heavy polluted atmosphere: A case study in urban Beijing, China during haze events.

Characterizations on polycyclic aromatic compounds (PACs) during frequent haze periods have been conducted in an urban site of Beijing, China. Particulate polycyclic aromatic hydrocarbons (PAHs) and oxygenated-PAHs (OPAHs) and other carbonaceous matters were quantified. The average PM2.5 during haze events (256.3 ± 103.7 μg/m3) were one magnitude over than that of clear periods (CRs, 24.7 ± 27.7 μg/m3).The average total quantified PAHs (ΣPAHs) and OPAHs (ΣOPAHs) during haze events were 423.9 ± 178.4 ng/m3 and 581.4 ± 299.8 ng/m3, respectively, which were approximately 10 times higher than those of 40.3 ± 68.2 ng/m3 and 54.4 ± 82.4 ng/m3 in clear days. Four-rings PAHs had the highest compositions. 1,8-Naphthalic anhydride (1,8-NA) is the most abundant OPAHs, accounted for 49.8% of ∑OPAHs, followed by 9,10-anthraquinone (9,10-AQ) (13.8%) and benzo(a)anthracene-7,12-dione (BaAQ) (8.31%). In haze events, the contents of 5- to 7-rings PAHs decreased by 2.32% compared with those of clear days, while lower molecular weight fractions of 3- and 4-rings PAHs increased. The relationships between PAHs, OPAHs and relative humidity (RH) were found to be exponential. High oxygenation rate (R0) ratios of OPAH/PAH represents higher rates in secondary formation or degradation and gas- particle conversion for each PAH or OPAH during the wintertime. Significant positive correlation between BeP and OPAHs (r = 0.97), combined with the results of photochemical aging and negatively correlation with O3, suggest that secondary atmospheric reactions of PAHs played an important role in the burden of OPAHs.