Unexpectedly High Levels of H₂O₂ Drive Sulfate Formation over the Residual Layer in Beijing

Hydrogen peroxide (H₂O₂) plays a key role in atmospheric chemistry, but knowledge of its variation, sources, and impact on sulfate formation remains incomplete, especially in the urban boundary layer aloft. Here, we conducted a field campaign with measurements of H₂O₂ and related species at a tower-based site (∼528 m above the ground surface) of Beijing in spring of 2022. The observed hourly H₂O₂ concentration reached up to 21.2 ppbv with an average value of 3.4 ± 3.7 ppbv during the entire observation period, which was higher than values from previous observations throughout the world. The H₂O₂ budget revealed that the two known sources (self-reaction of HO₂ radicals and ozonolysis of alkenes) could not account for the significant formation of H₂O₂, leading to a considerable unknown source strength (∼0.14-0.53 ppbv h^-1) of H₂O₂ at noon and after sunset. Based on the levoglucosan signal, distribution of fire points, and backward trajectories, biomass burning emissions from the southwest of Beijing (e.g., North China Plain) were found to contribute greatly to H₂O₂ formation. Besides, photochemical aging of PM_2.5 might also have a potential impact on H₂O₂ production at noon. The unexpectedly high concentrations of H₂O₂ aloft made a vital contribution to sulfate production (0.2-1.1 μg m^-3 h^-1), which could be transported to the ground surface during the turbulent mixing. Our findings provide an improved understanding of the H₂O₂ chemistry in the boundary layer aloft in a megacity, as well as its impact on sulfate formation.