Uptake of HO₂ radicals onto Arizona test dust particles using an aerosol flow tube

Abstract. Uptake coefficients for HO₂ radicals onto Arizona test dust (ATD) aerosols were measured at room temperature and atmospheric pressure using an aerosol flow tube and the sensitive fluorescence assay by gas expansion (FAGE) technique, enabling HO₂ concentrations in the range 3–10 × 10⁸ molecule cm⁻³ to be investigated. The uptake coefficients were measured as 0.031 ± 0.008 and 0.018 ± 0.006 for the lower and higher HO₂ concentrations, respectively, over a range of relative humidities (5–76%). A time dependence for the HO₂ uptake onto the ATD aerosols was observed, with larger uptake coefficients observed at shorter reaction times. The combination of time and HO₂ concentration dependencies suggest either the partial saturation of the dust surface or that a chemical component of the dust is partially consumed whilst the aerosols are exposed to HO₂. A constrained box model is used to show that HO₂ uptake to dust surfaces may be an important loss pathway of HO₂ in the atmosphere.