The hydroperoxyl radical (HO<sub>2</sub>) 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 HO<sub>2</sub> concentrations, and this has been tentatively attributed to heterogeneous uptake by aerosol particles. It is generally assumed that HO<sub>2</sub> uptake by aerosol involve conversion to H<sub>2</sub>O<sub>2</sub>, but this is of limited efficacy as an HO<sub>2</sub> sink because H<sub>2</sub>O<sub>2</sub> can photolyze to regenerate OH and from there HO<sub>2</sub>. Joint atmospheric observations of HO<sub>2</sub> and H<sub>2</sub>O<sub>2</sub> suggest that HO<sub>2</sub> uptake by aerosols may in fact not produce H<sub>2</sub>O<sub>2</sub>. Here we propose a catalytic mechanism involving coupling of the transition metal ions (TMI) Cu(I)/Cu(II) and Fe(II)/Fe(III) to rapidly convert HO<sub>2</sub> to H<sub>2</sub>O in aerosols. The implied HO<sub>2</sub> uptake significantly affects global model predictions of tropospheric OH, ozone, and other species, improving comparisons to observations, and may have a major and previously unrecognized impact on atmospheric oxidant chemistry.