Uptake of HO2 Radicals Onto Dust Aerosols

OH and HO2 radicals play an important role in the troposphere by controlling its oxidative capacity and therefore the concentration of many trace species. Several field studies have observed significantly lower concentrations of HO2 radicals than predicted using box models (1,2). HO2 loss onto aerosols has been suggested as a possible sink. Mineral dust has an estimated annual flux of 2000 Tg year-1 (3). However, there has only been one study of HO2 uptake onto Arizona Test Dust (ATD) surfaces (4) and there are currently no published studies for dust aerosols. Therefore, the aim of this study was to measure the HO2 uptake coefficient onto ATD aerosols over a range of humidities and for different HO2 concentrations, as well as investigating the uptake as a function of the exposure time to the aerosol, for which a dependence had been observed for aqueous salt aerosols (5). Uptake coefficients were measured for ATD aerosols at atmospheric pressure and at 291 K using a Fluorescence Assay by Gas Expansion (FAGE) detector combined with a flow tube. HO2 was formed from the photolysis of water vapour and was injected into the flow tube using a moveable injector, which was placed in six different positions along the flow tube. The non stable aerosol output was produced by stirring ATD in a bottle producing a dust cloud which was entrained into a flow. The aerosol number concentration was measured using a Condensation Particle Counter (CPC) and was converted into a surface area using the average radius of one aerosol. The uptake coefficient was then able to be calculated by assuming first order kinetics. The HO2 uptake coefficient was measured at a relative humidity of between 6 and 75% and at initial HO2 concentrations of ~ 0.3 - 1 × 10^9 molecule cm-3. Average uptake coefficients of 0.018 × 0.006 and 0.031 × 0.008 were measured for the higher and lower HO2 concentrations respectively, and the impact investigated using a constrained box model. A time dependence was also observed with larger uptakes at shorter times. The combination of the HO2 concentration dependence and time dependence may suggest a surface saturation mechanism. References (1) Sommariva, R. et al. Atmos. Chem. Phys. 2006, 6, 1135-1153. (2) Whalley, L. K. et al. Atmos. Chem. Phys. 2010, 10, 1555-1576. (3) Textor, C. et al. Atmos. Chem. Phys. 2006, 6, 1777-1813. (4) Bedjanian, Y. Et al. Atmos. Chem. Phys. Discuss. 2013, 13, 8873-8900. (5) George, I. J. et al. Phys. Chem. Chem. Phys. 2013, 15,12829-12845.