Measurements of uptake coefficients for heterogeneous loss of HO2 onto submicron inorganic salt aerosols

Abstract

Laboratory studies were conducted to investigate the kinetics of HO₂ radical uptake onto submicron inorganic salt aerosols. HO₂ reactive uptake coefficients were measured at room temperature using an aerosol flow tube and the Fluorescence Assay by Gas Expansion (FAGE) technique that allowed for measurements to be conducted under atmospherically relevant HO₂ concentrations ([HO₂] = 10⁸ to 10⁹ molecule cm⁻³). The uptake coefficient for HO₂ uptake onto dry inorganic salt aerosols was consistently below the detection limit (γ_HO2 < 0.004). The mass accommodation coefficient of HO₂ radicals onto Cu(II)-doped (NH₄)₂SO₄ aerosols was measured to be α_HO2 = 0.4 ± 0.3 representing the kinetic upper limit to γ. For aqueous (NH₄)₂SO₄, NaCl and NH₄NO₃ aerosols not containing traces of transition metal ions, a range of γ_HO2 = 0.003–0.02 was measured. These values were much lower than γ values previously measured on aqueous (NH₄)₂SO₄ and NaCl aerosols and also those typically used in atmospheric models (γ_HO2 = 0.1–1.0). Evidence is presented showing that the HO₂ uptake coefficients onto aqueous salt aerosol particles are dependent both on the exposure time to the aerosol and on the HO₂ concentration used.