Radical loss in the atmosphere from Cu-Fe redox coupling in aerosols

Abstract. The hydroperoxyl radical (HO₂) is a major precursor of OH and tropospheric ozone. OH is the main atmospheric oxidant, while tropospheric ozone is an important surface pollutant and greenhouse gas. Standard gas-phase models for atmospheric chemistry tend to overestimate observed HO₂ concentrations, and this has been tentatively attributed to heterogeneous uptake by aerosol particles. It is generally assumed that HO₂ uptake by aerosol involves conversion to H₂O₂, but this is of limited efficacy as an HO₂ sink because H₂O₂ can photolyze to regenerate OH and from there HO₂. Joint atmospheric observations of HO₂ and H₂O₂ suggest that HO₂ uptake by aerosols may in fact not produce H₂O₂. Here we propose a catalytic mechanism involving coupling of the transition metal ions Cu(I)/Cu(II) and Fe(II)/Fe(III) to rapidly convert HO₂ to H₂O in aqueous aerosols. The implied HO₂ uptake and conversion to H₂O significantly affects global model predictions of tropospheric OH, ozone, carbon monoxide (CO) and other species, improving comparisons to observations in the GEOS-Chem model. It represents a previously unrecognized positive radiative forcing of aerosols through the effects on the chemical budgets of major greenhouse gases including methane and hydrofluorocarbons (HFCs).