Radical loss in the atmosphere from Cu-Fe redox coupling in aerosols
- ISSN: 16807316
- DOI: 10.5194/acp-13-509-2013
The hydroperoxyl radical (HO sub(2)) 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) concentrations, and this has been tentatively attributed to heterogeneous uptake by aerosol particles. It is generally assumed that HO sub(2) uptake by aerosol involves conversion to H sub(2)O sub(2), but this is of limited efficacy as an HO sub(2) sink because H sub(2)O sub(2) can photolyze to regenerate OH and from there HO sub(2). Joint atmospheric observations of HO sub(2) and H sub(2)O sub(2) suggest that HO sub(2) uptake by aerosols may in fact not produce H sub(2)O sub(2). 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 HO sub(2) to H sub(2)O in aqueous aerosols. The implied HO sub(2) uptake and conversion to H sub(2)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).