Isotopic constraints on heterogeneous sulfate 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 observations of the oxygen-17 excess of PM_2.5 sulfate (Δ¹⁷O(SO₄^2-)) collected in Beijing haze from October 2014 to January 2015 to constrain possible sulfate formation pathways. Throughout the sampling campaign, the 12-hourly averaged PM_2.5 concentrations ranged from 16 to 323 µg m^-3 with a mean of (141 ± 88 (1σ)) µg m^-3, with SO₄^2- representing 8-25 % of PM_2.5 mass. The observed Δ¹⁷O(SO₄^2-) varied from 0.1 to 1.6 ‰ with a mean of (0.9 ± 0.3) ‰. Δ¹⁷O(SO₄^2-) increased with PM_2.5 levels in October 2014 while the opposite trend was observed from November 2014 to January 2015. Our estimate suggested that in-cloud reactions dominated sulfate production on polluted days (PDs, PM_2.5 ≥ 75 µg m^-3) of Case II in October 2014 due to the relatively high cloud liquid water content, with a fractional contribution of up to 68 %. During PDs of Cases I and III-V, heterogeneous sulfate production (P_het) was estimated to contribute 41-54 % to total sulfate formation with a mean of (48 ± 5) %. For the specific mechanisms of heterogeneous oxidation of SO₂, chemical reaction kinetics calculations suggested S(IV) ( = SO₂ ⚫ H₂O + HSO₃^- + SO₃^2-) oxidation by H₂O₂ in aerosol water accounted for 5-13 % of P_het. The relative importance of heterogeneous sulfate production by other mechanisms was constrained by our observed Δ¹⁷O(SO₄^2-). Heterogeneous sulfate production via S(IV) oxidation by O₃ was estimated to contribute 21-22 % of P_het on average. Heterogeneous sulfate production pathways that result in zero-Δ¹⁷O(SO₄^2-), such as S(IV) oxidation by NO₂ in aerosol water and/or by O₂ via a radical chain mechanism, contributed the remaining 66-73 % of P_het. 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. Our local atmospheric conditions-based calculations suggest 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 can be the dominant pathway providing that highly acidic aerosols (pH ≤ 3) exist. Our local atmospheric-conditions-based calculations illustrate the utility of Δ¹⁷O(SO₄^2-) for quantifying sulfate formation pathways, but this estimate may be further improved with future regional modeling work.