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Sulfur dioxide (SO2) can enhance the formation of secondary aerosols from biogenic volatile organic compounds (VOCs), but its influence on secondary aerosol formation from anthropogenic VOCs, particularly complex mixtures like vehicle exhaust, remains uncertain. Gasoline vehicle exhaust (GVE) and SO2, a typical pollutant from coal burning, are directly co-introduced into a smog chamber, in this study, to investigate the formation of secondary organic aerosols (SOA) and sulfate aerosols through photooxidation. New particle formation was enhanced, while substantial sulfate was formed through the oxidation of SO2 in the presence of high concentration of SO2. Homogenous oxidation by OH radicals contributed a negligible fraction to the conversion of SO2 to sulfate, and instead the oxidation by stabilized Criegee intermediates (sCIs), formed from alkenes in the exhaust reacting with ozone, dominated the conversion of SO2. After 5 h of photochemical aging, GVE's SOA production factor revealed an increase by 60–200 % in the presence of high concentration of SO2. The increase could principally be attributed to acid-catalyzed SOA formation as evidenced by the strong positive linear correlation (R2 = 0.97) between the SOA production factor and in situ particle acidity calculated by the AIM-II model. A high-resolution time-of-flight aerosol mass spectrometer (HR-TOF-AMS) resolved OA's relatively lower oxygen-to-carbon (O : C) (0.44 ± 0.02) and higher hydrogen-to-carbon (H : C) (1.40 ± 0.03) molar ratios for the GVE / SO2 mixture, with a significantly lower estimated average carbon oxidation state (OSc) of −0.51 ± 0.06 than −0.19 ± 0.08 for GVE alone. The relative higher mass loading of OA in the experiments with SO2 might be a significant explanation for the lower SOA oxidation degree.

Water-soluble organic carbon (WSOC) is a major component of carbonaceous aerosols. However, the detailed information of WSOC origins is still unclear. In the current study, fine particles (PM2.5) were collected at one sub-urban and two rural sites in the Pearl River Delta (PRD) region, south China during fall–winter 2008 to measure WSOC and organic tracers of biomass burning (BB) and secondary organic aerosols (SOA) from isoprene, monoterpenes, β-caryophyllene, aromatics and 2-ring polycyclic aromatic hydrocarbons (PAHs). WSOC concentrations ranged from 7.63 to 11.5 μg C m−3 and accounted for 38.8–57.9% of organic carbon (OC). Both WSOC and water-insoluble organic carbon (WIOC) exhibited higher levels at the sub-urban site than the rural sites. Subtracting BB-derived WSOC (WSOCBB) from measured WSOC, secondary OC (SOC) and primary OC (POC) were estimated that POC exhibited dominance over SOC and contributed 68–79% to OC. Significant correlation between WSOC and EC was observed, suggesting that BB could have important contributions to ambient WSOC in the PRD region during fall–winter. Organic tracers were applied to do source apportionment of WSOC, which further confirmed that BB was the dominant contributor, accounting for 42–47% of measured WSOC. SOC estimated by SOA tracers totally contributed 22–40% of WSOC, among which anthropogenic SOC (sum of aromatics and 2-ring PAHs, 18–25%) exhibited dominance over biogenic SOC (sum of isoprene, monoterpenes and β-caryophyllene, 4–15%). The unexplained WSOC (18–31%) showed a positive correlation with POC, indicating that this portion might be associated with POC aging.