Abstract
Abstract. 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.
Highlights
Peroxides play important roles in atmospheric chemical processes
It is well known that H2O2, together with organic peroxides including methyl hydroperoxide (MHP) and peroxyacetic acid (PAA), can oxidize the dissolved S(IV) to S(VI)
A negative correlation was observed between H2O2 and PAA in rainwater, reflecting the conversion of H2O2 to PAA in the aqueous phase
Summary
Peroxides play important roles in atmospheric chemical processes. They serve as oxidants both in their own right and as a reservoir species for HOx (OH and HO2) radicals, the principle oxidants in the troposphere (Lee et al, 2000; Reeves and Penkett, 2003). It is well known that H2O2, together with organic peroxides including MHP and PAA, can oxidize the dissolved S(IV) to S(VI). This results in the formation of sulfate aerosols in the atmospheric aqueous phase, under low pH conditions (Penkett et al, 1979; Calvert et al, 1985; Lind et al, 1987; Gaffney et al, 1987), and contributes to the formation of up to 60 % of the sulfate aerosols in the global atmosphere (Feichter et al, 1996). The combination of the methyl peroxy radical (CH3O2) and the HO2 radical will yield either MHP (R1) or HCHO (R2) as it undergoes different reaction channels. CH3O2 can react with NO to yield HCHO (Reactions R3–R4) in high NOx regions
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