This thesis investigates measurements of peroxy radicals using PERCA, photolysis frequencies of ozone (jO1D) and NO2 (jNO2 ) using filter radiometers (FR’s) and processes controlling atmospheric chemistry within the West African Monsoon (WAM).\ud The chain length (CL) of the University of Leicester (UoL) ground and aircraft PERCA inlets are shown to decay exponentially with relative humidity, and a humidity interference is observed with the luminol chemiluminescence NO2 detection technique. The altitude dependence of the CL is derived using a chain chemistry model. The aircraft PERCA NO2 sensitivity is derived by calculating the background NO2 concentration from reaction of ambient ozone with reagent NO.\ud Calibration of the UoL aircraft FR’s is described; temperature, solar zenith angle and ozone column bias in the jO1D measurement is estimated. A method is proposed for cross-calibration of FR’s spectral response using spectrally resolved measurements of actinic flux. An estimate for upwelling jO1D is assessed where the instrument failed during fieldwork.\ud Atmospheric composition is dominated by surface emissions and transport within theWAM. A nocturnal flow transports reactive VOC’s and oxidation products north, anthropogenics from coastal cities and biogenics from forested regions. Significant night-time concentrations of RO2, HO2, and potentially OH are observed. The most photochemically active air masses observed are biomass burning (BB) plumes of Central African origin. Variations in radical composition, inferred from VOC reactivity to OH, follow the trends of the University of Leeds DSMACC box model. The ozone balance is NOx limited within air masses where biogenic soil NOx emissions are suggested. PERCA and FR measurements are assessed through comparison to model results. Daytime PERCA measurements compare well with DSMACC RO2 with differences explained through measurement bias – ambient aircraft measurements appear responsive to RO2 only, with no humidity dependence and complete HO2 loss. Median noontime concentrations of RO2 of 35 pptV are observed
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