Abstract.A DUALER (dual-channel airborne peroxy radical chemical amplifier) instrument has been developed and optimised for the airborne measurement of the total sum of peroxy radicals during the AMMA (African Monsoon Multidisciplinary Analyses) measurement campaign which took place in Burkina Faso in August 2006. The innovative feature of the instrument is that both reactors are sampling simultaneously from a common pre-reactor nozzle while the whole system is kept at a constant pressure to ensure more signal stability and accuracy.Laboratory experiments were conducted to characterise the stability of the NO 2 detector signal and the chain length with the pressure. The results show that airborne measurements using chemical amplification require constant pressure at the luminol detector. Wall losses of main peroxy radicals HO 2 and CH 3 O 2 were investigated. The chain length was experimentally determined for different ambient mixtures and compared with simulations performed by a chemical box model.The DUALER instrument was successfully mounted within the German DLR-Falcon. The analysis of AMMA data utilises a validation procedure based on the O 3 mixing ratios simultaneously measured onboard. The validation and analysis procedure is illustrated by means of the data measured during the AMMA campaign. The detection limit and the accuracy of the ambient measurements are also discussed.

[1] The objective of this study has been to investigate the origin of the water vapor effect on the chain length (CL) of peroxy radical chemical amplifiers (PERCA). Results of the investigation of the water interference in the determination of peroxy radicals by using the PERCA technique are presented. The experimental conditions have been analyzed and modeled. A nonlinear dependence of the CL on the relative humidity (RH) has been accurately determined. The combined analysis of experimental and simulated results rules out wall loses as a single explanation of the CL variation observed and indicates three reactions, which possibly account for this water effect:À À À À À! nonradical products (e.g., HNO 3 ). Assuming a mechanism involving the formation of a HO 2 -nH 2 O complex, the corresponding rate coefficients and their water dependence have been estimated. The quadratic dependence of these rate coefficients upon the RH implies the participation of two H 2 O molecules in the proposed reactions. The study has shown that at a RH of 80% an effective secondorder rate coefficient of 10 À15 cm 3 molecule À1 s À1 for the reaction of CO with HO 2 , or 10À13 cm 3 molecule À1 s À1 for the reaction of HO 2 with NO, explains the observed behavior. Both these complex reactions have potential significance for the chemistry of the marine boundary layer (MBL) and their atmospheric implications are discussed.

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Abstract. 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 convection and biomass burning. Generally, the total sum of peroxy radical mixing ratios, measured in the outflow of convective clouds, are quite variable but occasionally are coupled with the NO variations indicating the coexistence or simultaneous emission of NO x , with a potential radical precursor (i.e. formaldehyde, acetone or peroxides), which has likely been transported to higher atmospheric altitudes. Based on the measurements, significant O 3 production rates around 1 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 backtrajectory densities confirm the origin of these air masses being a biomass burning 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 maximum concentrations between 2 and 4 km, in agreement with other literature values obtained under similar conditions.

Abstract. A DUALER (dual-channel airborne peroxy radical chemical amplifier) instrument has been developed and optimised for the airborne measurement of the total sum of peroxy radicals during the AMMA (African Monsoon Multidisciplinary Analyses) measurement campaign which took place in Burkina Faso in August 2006. The innovative feature of the instrument is that both reactors are sampling simultaneously from a common pre-reactor nozzle while the whole system is kept at a constant pressure to ensure more signal stability and accuracy. Laboratory experiments were conducted to characterise the stability of the NO2 detector signal and the chain length with the pressure. The results show that airborne measurements using chemical amplification require constant pressure at the luminol detector. Wall losses of main peroxy radicals HO2 and CH3O2 were investigated. The chain length was experimentally determined for different ambient mixtures and compared with simulations performed by a chemical box model. The DUALER instrument was successfully mounted within the German DLR-Falcon. The analysis of AMMA data utilises a validation procedure based on the O3 mixing ratios simultaneously measured onboard. The validation and analysis procedure is illustrated by means of the data measured during the AMMA campaign. The detection limit and the accuracy of the ambient measurements are also discussed.