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

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 (γHO2 < 0.004). The mass accommodation coefficient of HO2 radicals onto Cu(II)-doped (NH4)2SO4 aerosols was measured to be αHO2 = 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 γHO2 = 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 (γHO2 = 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.