Assessment of the H₂O₂ budget at an urban site concerning the HO₂ underprediction and the vertical transport from residual layers

Field observation of H₂O₂ was conducted at an urban site in Beijing from Sep.23rd to Nov.4th of 2018. The daily peak concentrations of H₂O₂ and organic peroxides (OPs) during the measurement were 0.57 ppbv (Range: 0.07–1.89 ppbv) and 0.16 ppbv (Range 0.06–0.35 ppbv). Using an observation based model, the modeled HO₂ level reproduced a much lower H₂O₂ production rate than the observation, especially in the morning under the weak wind, non-turbulent surface winds conditions. With the modeled-HO₂ concentration corrected based on the previously reported observed/modeled HO₂ ratio in this city, the daily integrated H₂O₂ production rate increased by 144%–164%, which is important but still not enough to explain the observed high H₂O₂ in the lightly polluted and polluted days. Heterogeneous production of H₂O₂ on aerosols and the overestimation of H₂O₂ removal rates were examined not the underlying causes of this additional H₂O₂ missing source. H₂O₂ concentrations in residual layers were evaluated. The residual layer was revealed a stronger reservoir of H₂O₂ than its reservoir effect on O₃. In the morning of non-turbulent conditions at the urban site, the residual layers H₂O₂ were estimated about 20–80 times of the ground surface H₂O₂ before the air layers vertical mixing. The vertical mixing significantly affects gaseous H₂O₂ concentration on the ground in the morning. This work highlights the importance to consider the HO₂ underestimation and air mass vertical mixing in atmospheric H₂O₂ studies.