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Aircraft OH and HO2 measurements made over West Africa during the AMMA fieldcampaign in summer 2006 have been investigated using a box model constrained to observationsof long-lived species and physical parameters. "Good" agreement was found forHO2 (modelled to observed gradient of 1.23 ± 0.11). However, the modelsignificantly overpredicts OH concentrations. The reasons for this are not clear, but mayreflect instrumental instabilities affecting the OH measurements. Within themodel, HOx concentrations in West Africa are controlled by relatively simplephotochemistry, with production dominated by ozone photolysis and reaction of O(1D)with water vapour, and loss processes dominated by HO2 + HO2 andHO2 + RO2. Isoprene chemistry was found to influence forestedregions. In contrast to several recent field studies in very low NOx and highisoprene environments, we do not observe any dependence of model success for HO2 onisoprene and attribute this to efficient recycling of HOx through RO2 + NOreactions under the moderate NOx concentrations (5–300 ppt NO in the boundary layer, median 76 ppt)encountered during AMMA. This suggests that some of the problems with understandingthe impact of isoprene on atmospheric composition may be limited to the extreme low range ofNOx concentrations

atmospheric chemistry and physics

HO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; observations over West Africa during AMMA: impact of isoprene and NO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt;