Abstract
Abstract. Hydrogen peroxide (H2O2) and organic peroxides play important roles in the cycle of oxidants and the formation of secondary aerosols in the atmosphere. Recent field observations have suggested that the budget of peroxyacetic acid (PAA, CH3C(O)OOH) is potentially related to the aerosol phase processes, especially to secondary aerosol formation. Here, we present the first laboratory measurements of the uptake coefficient of gaseous PAA and H2O2 onto ambient fine particulate matter (PM2.5) as a function of relative humidity (RH) at 298 K. The results show that the PM2.5, which was collected in an urban area, can take up PAA and H2O2 at the uptake coefficient (γ) of 10−4, and both γPAA and γH2O2 increase with increasing RH. The value of γPAA at 90 % RH is 5.4 ± 1.9 times that at 3 % RH, whereas γH2O2 at 90 % RH is 2.4 ± 0.5 times that at 3 % RH, which suggests that PAA is more sensitive to the RH variation than H2O2 is. Considering the larger Henry's law constant of H2O2 than that of PAA, the smaller RH sensitivity of the H2O2 uptake coefficient suggests that the enhanced uptake of peroxide compounds on PM2.5 under humid conditions is dominated by chemical processes rather than dissolution. Considering that mineral dust is one of the main components of PM2.5 in Beijing, we also determined the uptake coefficients of gaseous PAA and H2O2 on authentic Asian Dust storm (ADS) and Arizona Test Dust (ATD) particles. Compared to ambient PM2.5, ADS shows a similar γ value and RH dependence in its uptake coefficient for PAA and H2O2, while ATD gives a negative dependence on RH. The present study indicates that, in addition to the mineral dust in PM2.5, other components (e.g., soluble inorganic salts) are also important to the uptake of peroxide compounds. When the heterogeneous reaction of PAA on PM2.5 is considered, its atmospheric lifetime is estimated to be 3.0 h on haze days and 7.1 h on non-haze days, values that are in good agreement with the field observations.
Highlights
Peroxide compounds, including hydrogen peroxide (H2O2) and organic peroxides, play an important role in the chemistry of the atmosphere, because they serve as oxidants for the conversion of S(IV) to S(VI) in the atmospheric aqueous phase, resulting in the formation of sulfate aerosol (Calvert et al, 1985; Lind et al, 1987; Stein and Saylor, 2012)
Note: PM2.5h, haze day PM2.5; PM2.5n, non-haze day PM2.5; ADSh and ATDh, the mass of mineral dust about 1.3 mg; ADSl and ATDl, the mass of mineral dust, about 0.3 mg; a uptake coefficient calculated by total surface area of the particles using size distribution, representing the lower limit; b uptake coefficient calculated by BET area, representing the lower limit; the errors represent the relative standard deviation between γ on particles of ascending and descending relative humidity (RH)
The present study is the first to measure the uptake coefficient of gaseous PAA and H2O2 on ambient PM2.5 and on mineral dust over a wide range of RH values (3–90 %)
Summary
Peroxide compounds, including hydrogen peroxide (H2O2) and organic peroxides, play an important role in the chemistry of the atmosphere, because they serve as oxidants for the conversion of S(IV) to S(VI) in the atmospheric aqueous phase, resulting in the formation of sulfate aerosol (Calvert et al, 1985; Lind et al, 1987; Stein and Saylor, 2012). Pradhan et al (2010a) have indicated that the heterogeneous reaction of H2O2 on dust aerosols could compete with its photolysis and significantly affect the HOx radical budget. Our recent study has indicated that H2O2 could enhance the uptake of oxygenated volatile organic compounds (OVOCs) onto the surface of mineral dust particles (Zhao et al, 2014). Our field observation results have suggested that heterogeneous reactions on aerosol particles might be an important removal pathway for PAA in the atmosphere (Zhang et al, 2010; Liang et al, 2013). We use PAA as a representative organic peroxide to investigate the kinetics and mechanisms of its heterogeneous reactions on ambient PM2.5 as well as mineral dust particles over a wide range of relative humidities (3–90 %). We investigate the kinetics of H2O2 uptake on PM2.5
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