Parameterisation and impact of aerosol uptake of HO2 on a global tropospheric model

HO2 is an important atmospheric trace gas, whose sink to aerosol is poorly understood yet significant. Previous parameterisations of the rate of uptake have been limited by the lack of laboratory studies. This paper creates a parameterisation for gamma(HO2) based on the available laboratory studies. The calculated global mean gamma(HO2) is 0.028, significantly lower than previous work (0.2). Modelled concentrations of HO2 show significant regional sensitivity to the value of gamma(HO2) (up to +106% at the surface with the parameterisation of gamma(HO2) in this work as compared with a value of 0.2), but global sensitivity is small (+3.2%). The modelled response in O-3 is also highly regional, being up to +27% at the surface over China, and only +0.3% globally (with the parameterisation of gamma(HO2) in this work as compared with a value of 0.2). The impact of gamma(HO2) on sulfate is more complex, with up to +16% over China and -5% over high latitudes, resulting in a global change of +1.2% (with the parameterisation of gamma(HO2) in this work as compared with a value of 0.2). Uncertainty in the reaction mechanism and hence products (previously assumed to be H2O2) impacts the processing of sulfur and hence aerosol loads. Further laboratory studies are desirable to constrain the rate of reaction and to elucidate the reaction mechanism and products.