Significant Contributions of Regional Transport to Sulfate Aerosols During Haze Events in Northern China Plain: Constrained by Sulfur Isotopic Compositions

Abstract Regional transport is a significant source of sulfate (SO 4 2− ) aerosols, which frequently induces haze formation in the Northern China Plain (NCP). Quantification of regional transport to SO 4 2− using chemical transport models (CTMs) is debated due to uncertainties in SO 2 emissions and SO 4 2− formation mechanisms incorporated in the CTMs. In this study, sulfur isotope ratios in sulfate (δ 34 S‐SO 4 2− ) were measured at Gucheng, a heavily polluted city in the NCP, to differentiate local emissions from regional transport to sulfate. The δ 34 S‐SO 4 2− values varied from 2.5 to 6.5‰, with a mean of 4.6 ± 1.1‰. The lower δ 34 S‐SO 4 2− values were observed in the haze events, attributed to regional transport from high‐SO 2 emission regions, where coal materials typically exhibit declined δ 34 S values. The fractionation factors (α 34 S g→p ) of δ 34 S for SO 2 to SO 4 2− conversion were estimated to be 3.6 ± 1.1‰. Considering the α 34 S g→p , the quantified formation mechanisms indicated that transition metallic ions (TMIs)‐catalyzed O 2 oxidation dominated sulfate formation, contributing 34%, followed by NO 2 (27%), OH (23%), and H 2 O 2 /O 3 oxidation pathways (16%). Local coal burning, vehicle emissions, and oil combustion contributed 58%, 16%, and 9% to sulfate, respectively. The remaining 17% of sulfate was contributed by regional transport (mainly coal burning) with enhancements in the specific haze events. Although uncertainties remained in the estimations, our findings did suggest that strict control in SO 2 emissions from local and regional coal combustion along with TMIs emissions from the steel industry was crucial for reducing sulfate concentrations in this NCP city.