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 H2 O2 , but this is of limited efficacy as an HO2 sink because H2 O2 can photolyze to regenerate OH and from there HO2 . Joint atmospheric observations of HO2 and H2 O2 suggest that HO2 uptake by aerosols may in fact not produce H2 O2 . 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 H2 O in aqueous aerosols. The implied HO2 uptake and conversion to H2 O 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).