Isotopic constraints on heterogeneous sulphate production in Beijing haze

Discerning mechanisms of sulfate formation during fine-particle pollution (referred to as haze hereafter) in Beijing is important for understanding the rapid evolution of haze and for developing cost-effective air pollution mitigation strategies. Here we present the first observations of the oxygen-17 excess of PM2.5 sulfate (Δ17O(SO42−)) collected in Beijing haze from October 2014 to January 2015, to constrain possible sulfate formation pathways. Throughout the sampling campaign, the 12h-averaged PM2.5 concentrations ranged from 16 to 323 μg m−3 with a mean of (141 ± 88 (1σ)) μg m−3, with SO42− representing 8–25 % of PM2.5 mass. The observed Δ17O(SO42−) varied from 0.1 ‰ to 1.6 ‰ with a mean of (0.9 ± 0.3) ‰.Δ17O(SO42−)increased with PM2.5 levels in October 2014 while the opposite trends were observed in November 2014 to January 2015. Heterogeneous sulfate production rate (Phet) on aerosols was estimated to enhance with PM2.5 levels, generally dominating sulfate formation during haze days when cloud liquid water content (LWC) was low. When LWC was high, however, in-cloud reactions would dominate haze sulfate formation with a fractional contribution up to 68 %. For the specific mechanisms of heterogeneous oxidation of SO2, chemical reaction kinetics calculations suggest S(IV) (= SO2•H2O + HSO3− + SO32−) oxidation by H2O2 in aerosol water accounted for 5–13 % of Phet. The relative importance of heterogeneous sulfate production by other mechanisms was constrained by our observed Δ17O(SO42−). Heterogeneous sulfate production via S(IV) oxidation by O3 was estimated to contribute 21–22 % of Phet on average. Heterogeneous sulfate production pathways that result in zero-Δ17O(SO42−), such as S(IV) oxidation by NO2 in aerosol water and/or by O2 on acidic microdroplets via a radical chain mechanism, contributed the remain 66–73 % of Phet. The assumption about the thermodynamic state of aerosols (stable or metastable) was found to significantly influence the calculated aerosol pH (7.6 ± 0.1 or 4.7 ± 1.1, respectively), and thus influence the relative importance of heterogeneous sulfate production via S(IV) oxidation by NO2 and by O2 on acidic microdroplets. Our calculation suggests sulfate formation via NO2 oxidation can be the dominant pathway in aerosols at high pH-conditions calculated assuming stable state while S(IV) oxidation by O2 on acidic microdroplets can be the dominant pathway providing that highly acidic aerosols (pH ≤ 3) exist. Our results also illustrate the utility of Δ17O(SO42−) for quantifying sulfate formation pathways and its inclusion in models may improve our understanding of rapid sulfate formation during haze events