Atmospheric hydrogen peroxide (H 2 O 2 ) plays an important role in sulfate formation. To explore the contribution of the H 2 O 2 oxidation pathway to atmospheric sulfate in winter in Beijing, three field campaigns of atmospheric H 2 O 2 measurements were conducted at an urban site (Beijing) and a rural site (Wangdu) during the winter in 2016, 2017, and 2018. The H 2 O 2 concentrations were usually around the detection limit (0.05 ppbv) during clean and severely polluted periods, whereas the highest H 2 O 2 concentration of 0.90 ppbv was observed during moderately polluted periods. Obvious increases in the concentration of H 2 O 2 could be observed after sunset at the urban site during each moderately polluted day, which was mainly attributed to transportation of H 2 O 2 -rich air from the rural areas in the south of Beijing. Coincident increases in the concentrations of H 2 O 2 and PM 2.5 were also observed during the day at high NO concentrations, implying that heterogeneous reactions might contribute to the formation of H 2 O 2 under polluted conditions. In addition, the contrast between urban and rural measurements also provides some support for the potential formation of H 2 O 2 from heterogeneous reactions. On the basis of the data measured in this study, sulfate formation through H 2 O 2 oxidation was found to be the dominant pathway rather than the NO 2 oxidation pathway.

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