HO_x observations over West Africa during AMMA: impact of isoprene and NO_x

Abstract. Aircraft OH and HO₂ measurements made over West Africa during the AMMA field campaign in summer 2006 have been investigated using a box model constrained to observations of long-lived species and physical parameters. "Good" agreement was found for HO₂ (modelled to observed gradient of 1.23 ± 0.11). However, the model significantly overpredicts OH concentrations. The reasons for this are not clear, but may reflect instrumental instabilities affecting the OH measurements. Within the model, HO_x concentrations in West Africa are controlled by relatively simple photochemistry, with production dominated by ozone photolysis and reaction of O(¹D) with water vapour, and loss processes dominated by HO₂ + HO₂ and HO₂ + RO₂. Isoprene chemistry was found to influence forested regions. In contrast to several recent field studies in very low NO_x and high isoprene environments, we do not observe any dependence of model success for HO₂ on isoprene and attribute this to efficient recycling of HO_x through RO₂ + NO reactions under the moderate NO_x concentrations (5–300 ppt NO in the boundary layer, median 76 ppt) encountered during AMMA. This suggests that some of the problems with understanding the impact of isoprene on atmospheric composition may be limited to the extreme low range of NO_x concentrations.