Uptake of HO₂ radicals on Arizona Test Dust

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 (J_NO2 = 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 + RH_1.1) (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 HO_x species in the troposphere with the effect depending on specific local conditions.