Impacts of aerosols on the chemistry of atmospheric trace gases: a case study of peroxides and HO₂ radicals

Field measurements of atmospheric peroxides were obtained during the summer on two consecutive years over urban Beijing, which highlighted the impacts of aerosols on the chemistry of peroxide compounds and hydroperoxyl radicals (HO₂). The major peroxides were determined to be hydrogen peroxide (H₂O₂), methyl hydroperoxide (MHP), and peroxyacetic acid (PAA). A negative correlation was found between H₂O₂ and PAA in rainwater, providing evidence for a conversion between H₂O₂ and PAA in the aqueous phase. A standard gas phase chemistry model based on the NCAR Master Mechanism provided a good reproduction of the observed H₂O₂ profile on non-haze days but greatly overpredicted the H₂O₂ level on haze days. We attribute this overprediction to the reactive uptake of HO₂ by the aerosols, since there was greatly enhanced aerosol loading and aerosol liquid water content on haze days. The discrepancy between the observed and modeled H₂O₂ can be diminished by adding to the model a newly proposed transition metal ion catalytic mechanism of HO₂ in aqueous aerosols. This confirms the importance of the aerosol uptake of HO₂ and the subsequent aqueous phase reactions in the reduction of H₂O₂. The closure of HO₂ and H₂O₂ between the gas and aerosol phases suggests that the aerosols do not have a net reactive uptake of H₂O₂, because the conversion of HO₂ to H₂O₂ on aerosols compensates for the H₂O₂ loss. Laboratory studies for the aerosol uptake of H₂O₂ in the presence of HO₂ are urgently required to better understand the aerosol uptake of H₂O₂ in the real atmosphere