Abstract
Abstract. The interaction of HO2 radicals with solid films of Arizona Test Dust (ATD) was studied using a low-pressure flow reactor (1–9 Torr) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. The reactive uptake coefficient of HO2 was measured from the kinetics of HO2 consumption on Pyrex rods coated with ATD as a function of HO2 concentration ((0.35–3.30) × 1012 molecule cm−3), relative humidity (RH = 0.02–94%), temperature (T = 275–320 K) and UV irradiance intensity (JNO2 = 0–0.012 s−1). The initial uptake coefficient was found to be independent of concentration of HO2, temperature and irradiation conditions, and to decrease with increasing relative humidity: γ0 = 1.2/(18.7 + RH1.1) (with 30% estimated conservative uncertainty). The uptake coefficient was calculated using geometric surface area and should be considered as an upper limit of γ0. An upper limit of 5% was found for the gaseous H2O2-forming pathway of the HO2 reaction with ATD surface. The results of the measurements indicate that HO2 loss on dust aerosol may be a non-negligible sink for HOx species in the troposphere with the effect depending on specific local conditions.
Highlights
Introduction far for the interaction ofHO2 with mineral dust surfaces
(2011) reviewed the existing laboratory data on HO2 uptake on tropospheric aerosol and derived the global mean uptake coefficient of HO2, γ = 0.028, which is an order of magnitude lower than those considered in earlier works (Jacob, 2000; Thornton et al, 2008).SAnooltihderEcraitirctahl issue concerning the heterogeneous interaction of HO2 with atmospheric
In the present study we report the measurements of the uptake coefficient of HO2 radicals on Arizona Test Dust (ATD) under dark and UV irradiation conditions as a function of relative humidity and temperature
Summary
He circulating water aerosol is the reaction mechanism, especially with regard to the formation of H2O2 as the reaction product. Considering that H2O2 in the atmosphere is produced via self-reaction of HO2 radicals and H2O2 can photolyse to regenerate HOx species, the HO2 uptake on aerosols will have different effects on the concentrations of H2O2 and HO2, depending on whether or not H2O2 is formed in the heterogeneous reaction. Considering the limited kinetic and mechanistic data on HO2–aerosol interaction and that heterogeneous reactions of HO2 still remain a significant source of uncertainty in atmospheric modelling of the HOx budget, it is clear that additional laboratory studies of HO2 uptake, including product study as well as temperature and relative humidity dependence, on various aerosol types are strongly required. In the present study we report the measurements of the uptake coefficient of HO2 radicals on Arizona Test Dust (ATD) under dark and UV irradiation conditions as a function of relative humidity and temperature. For the uptake to the ATD surface, uptake coefficients identical to those on Saharan dust for NO2 and NO3 and somewhat lower uptake coefficients (by a factor of 1.5–2) for N2O5 and HNO3 were reported (Crowley et al, 2010)
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