Towards a better understanding of the HO2 uptake coefficient to aerosol particles measured during laboratory experiments

The first measurements of HO₂ uptake coefficients (γ_HO2) onto suspended aerosol particles as a function of temperature are reported in the range 314 K to 263 K. For deliquesced ammonium nitrate (AN) particles γ_HO2 increases from 0.005 ± 0.002 to 0.016 ± 0.005 as the temperature is lowered over this range. For effloresced sodium chloride and ammonium sulphate particles, γ_HO2 decreases slightly from 0.004 ± 0.002 to 0.000 ± 0.002 and 0.002 ± 0.003, respectively, between 314 and 263 K. For AN particles doped with Cu²⁺ ions, we find γ_HO2 ≈ α_HO2, the mass accommodation coefficient, which increases very slightly from α_HO2 = 0.62 ± 0.05 to 0.71 ± 0.06 between 292 and 263 K with lowering temperature. New measurements of γ_HO2 are also reported for ammonium sulphate particles doped with a range of Fe²⁺ and Fe³⁺ concentrations. The dependence of γ_HO2 on Cu and Fe concentrations are reconciled with published rate coefficients using the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB). The model shows that in experimental studies using aerosol flow tubes, a time dependence is expected for γ_HO2 onto aerosol particles which do not contain transition metal ions due to a decrease in the gas-phase concentration of HO₂ as a function of time. The model also demonstrates that Fenton-like chemistry has the potential to decrease γ_HO2 as a function of time for particles containing transition metal ions. For atmospherically relevant transition metal ion concentrations in aerosol particles, γ_HO2 can take a range of values depending on pH and the particle size from γ_HO2 < 0.04 to γ_HO2 = α_HO2. γ_HO2 for larger particles (radius ≥ 0.5 μm) can be significantly reduced by gas-diffusion limitations.