Understanding summertime H₂O₂ chemistry in North China Plain through observations and modelling studies

Abstract. Hydrogen peroxide (H₂O₂) is a key atmospheric oxidant, crucial for oxidation capacity and sulfate production. However, its chemistry remains understudied compared to ozone (O₃), limiting our understanding of photochemical pollution. In summer 2016, atmospheric peroxides and trace gases were measured at a rural site in the North China Plain. H₂O₂ was the dominant peroxide (0.62 ppb), constituting 69 % of total peroxides. It exhibited diurnal variation similar to peroxyacetyl nitrate (PAN) and O₃, indicating photochemical production. The O₃/H₂O₂ ratio was higher on high-particle days, suggesting H₂O₂ uptake by particles reduces its concentration. A box model with default gas-phase chemistry overestimated H₂O₂ by a factor of 2.7, but including particle uptake (uptake coefficient: 6×10⁻⁴) improved agreement with observations.

HO₂ recombination contributed 91 % of H₂O₂ production, with a peak rate of 1 ppb h^-1. Major removal pathways included particle uptake (69 %), dry deposition (25 %), OH reaction (4 %), and photolysis (2%). Relative incremental reactivity (RIR) analysis showed that reducing NO_x, PM_2.5, and alkanes increased H₂O₂, while reducing alkenes, aromatics, CO, and HONO decreased it, with alkenes having the strongest effect. H₂O₂/NO_z ratios (>0.15 in 82 % of cases) indicated O₃ formation was in a transition and NO_x-sensitive regime, emphasizing the need for VOC and further NO_x reductions. These findings improve our understanding of H₂O₂ chemistry and provide insights for mitigating photochemical pollution in rural North China.