The chemistry of OH and HO₂ radicals in the boundary layer over the tropical Atlantic Ocean

Fluorescence Assay by Gas Expansion (FAGE) has been used to detect ambient levels of OH and HO₂ radicals at the Cape Verde Atmospheric Observatory, located in the tropical Atlantic marine boundary layer, during May and June 2007. Midday radical concentrations were high, with maximum concentrations of 9×10⁶ molecule cm^−3 and 6×10⁸ molecule cm^−3 observed for OH and HO₂, respectively. A box model incorporating the detailed Master Chemical Mechanism, extended to include halogen chemistry, and constrained by all available measurements including halogen and nitrogen oxides, has been used to assess the chemical and physical parameters controlling the radical chemistry. IO and BrO, although present only at a few pptv, constituted ~23% of the instantaneous sinks for HO₂. Modelled HO₂ was sensitive to both HCHO concentration and the rate of heterogeneous loss to the ocean surface and aerosols. However, a unique combination of these parameters could not be found that gave optimised (to within 15%) agreement during both the day and night. The results imply a missing nighttime source of HO₂. The model underpredicted the daytime (sunrise to sunset) OH concentration by 12%. Photolysis of HOI and HOBr accounted for ~13% of the instantaneous rate of OH formation. Taking into account that halogen oxides increase the oxidation of NO_x (NO→NO₂), and in turn reduce the rate of formation of OH from the reaction of HO₂ with NO, OH concentrations were estimated to be 10% higher overall due to the presence of halogens. The increase in modelled OH from halogen chemistry gives an estimated 10% shorter lifetime for methane in this region, and the inclusion of halogen chemistry is necessary to model the observed daily cycle of ozone destruction that is observed at the surface. Due to surface losses, we hypothesise that HO₂ concentrations increase with height and therefore contribute a larger fraction of the ozone destruction than at the surface.