Observations of OH and HO₂ radicals over West Africa

Abstract. The hydroxyl radical (OH) plays a key role in the oxidation of trace gases in the troposphere. However, observations of OH and the closely related hydroperoxy radical (HO₂) have been sparse, especially in the tropics. Based on a low-pressure laser-induced fluorescence technique (FAGE – Fluorescence Assay by Gas Expansion), an instrument has been developed to measure OH and HO₂ aboard the Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft. During the African Monsoon Multidisciplinary Analyses (AMMA) campaign, observations of OH and HO₂ (HO_x) were made in the boundary layer and free troposphere over West Africa on 13 flights during July and August 2006. Mixing ratios of both OH and HO₂ were found to be highly variable, but followed a diurnal cycle: OH varied from 1.3 pptv to below the instrumental limit of detection, with a median mixing ratio of 0.17 pptv. HO₂ varied from 42.7 pptv to below the limit of detection, with a median mixing ratio of 8.0 pptv. A median HO₂/OH ratio of 95 was observed. Daytime OH observations were compared with the primary production rate of OH from ozone photolysis in the presence of water vapour. Daytime HO₂ observations were generally reproduced by a simple steady-state HO_x calculation, where HO_x was assumed to be formed from the primary production of OH and lost through HO₂ self-reaction. Deviations between the observations and this simple model were found to be grouped into a number of specific cases: (a) within cloud, (b) in the presence of high levels of isoprene in the boundary layer and (c) within a biomass burning plume. HO₂ was sampled in and around cloud, with significant short-lived reductions of HO₂ observed. Up to 9 pptv of HO₂ was observed at night, with HO₂ above 6 pptv observed at altitudes above 6 km. In the forested boundary layer, HO₂ was underestimated by a steady state calculation at altitudes below 500 m but overestimated between 500 m and 2 km. In a biomass burning plume, observed HO₂ concentrations were significantly below those calculated.