Uptake of HO2 radicals on Arizona Test Dust.

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: ɣ0 = 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 ɣ0. 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 HO_x species in the troposphere with the effect depending on specific local conditions. [ABSTRACT FROM AUTHOR]