Peroxy radical measurements made on board the DLR-Falcon research aircraft over West Africa within the African Monsoon Multidisciplinary Analysis (AMMA) campaign during the 2006 wet monsoon are presented in this study. The analysis of data focuses on the photochemical activity of air masses sampled during episodes of intense 5 convection and biomass burning. Generally, the total sum of peroxy radical mixing ratios, measured in the outflow of convective clouds, are quite variable but

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... are coupled with the NO variations indicating the coexistence, or simultaneously emission of NO x , with a potential radical precursor (i.e., formaldehyde, acetone or peroxides) which has likely been transported to higher atmospheric layers. Based on 10 the measurements, significant O 3 production rates up to 2 ppb/h in the MCS outflow are estimated by using a box model with simplified chemistry. Peroxy radicals having mixing ratios around 20-25 pptv and with peak values of up to 60-70 pptv are measured within biomass burning plumes, detected at the coast in Ghana. Calculations of back-trajectory densities confirm the origin of these air masses being a biomass burn-15 ing region at southern latitudes and close to the Gulf of Guinea, according to satellite pictures. Measured peroxy radical concentrations agree reasonably with modelled estimations taking into account simple local chemistry. Moreover the vertical profiles taken at the aircraft base in Ouagadougou, Burkina Faso, indicate the common feature of having 20 maximum concentrations between 2 and 4 km, in agreement with other literature values obtained under similar conditions.