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

The first measurements of HO2 uptake coefficients (γHO2) 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 γHO2 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, γHO2 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 γHO2 ≈ αHO2, the mass accommodation coefficient, which increases very slightly from γHO2 = 0.62 ± 0.05 to 0.71 ± 0.06 between 292 and 263 K with lowering temperature. New measurements of γHO2 are also reported for ammonium sulphate particles doped with a range of Fe2+ and Fe3+ concentrations. The dependence of γHO2 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 γHO2 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 γHO2 as a function of time for particles containing transition metal ions. For atmospherically relevant transition metal ion concentrations in aerosol particles, γHO2 can take a range of values depending on pH and the particle size from γHO2 < 0.04 to γHO2 = αHO2. γHO2 for larger particles (radius >= 0.5 μm) can be significantly reduced by gas-diffusion limitations.