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The aim of this Ph.D. was to develop and deploy a broadband cavity enhanced absorption spectrometer for use in the field, to test the instrument in a variety of locations, and to contribute to the body of knowledge both on spectroscopic measurement techniques and on the chemical composition of the troposphere through analysis of datasets from both this instrument and others. The first ship-deployment of the cavity instrument provided challenging conditions but retrieved a detection limit for IO of 20 pptv over 10 min and measurements of NO₂ with a detection limit of 0.9 ppbv over 10 min. Additional IO measurements were attempted at a coastal site thought to be a greater IO source but IO was not observed above the detection limit. The final deployment detailed here was for the measurement of the sum of NO₃ and N₂O₃ concentrations on the BT tower at 160m above London. A highly variable, month-long dataset was obtained showing a strong diurnal cycle with very low daytime concentrations measured. The maximum night-time mixing ratio was approximately 800 pptv, with a limit of detection of 2 pptv over 15 s. The results from this campaign were compared with models to aid our understanding of nocturnal processes occurring in this region. It was found that night-time enhancements in NO₃ and N₂O₅often coincided with air masses containing little NO and moderate O₃. Those of low turbulence also favoured the night-time enhancement. N₂O₃ was found to be very reactive and calculated lifetimes were short. Estimates of the heterogeneous uptake of N₂O₅ onto water suggest a small contribution of around 8 ppbv per night of HNO₃ via this route.

The Cambridge 3D chemical and transport model (TOMCAT) has been developed and used to separate the two sources of O₃, in situ O₃ production and downward transport of O₃ from the stratosphere/upper troposphere. The main source of tropospheric O₃ in the model is from downward transport from the stratosphere/upper troposphere. Photochemical production of O₃ depends on downward transport from the stratosphere/troposphere, where this transport is determined by the tropopause definition used. These results have been compared with other CTMs and reasonable agreement is found. Analyses of the TOMCAT model integrations are done for key tropospheric species (e.g. O₃, CO, NMHCs and NOy) by comparison with measurements obtained over the North Atlantic region between August and October 1997. The period contains several international aircraft measurement campaign: ACSOE, NARE, TACIA and SONEX. The model was used to predict the O₃ budget over the North Atlantic region for this flight campaign period. The O₃ budget was found to be determined by an influx of O₃ rich air from North America and photochemical O₃ destruction occurring over the North Atlantic, resulting in a negative O₃ budget overall. The model was used to examine the effect of continental emissions on O₃ production. The long-range transport of continental emissions emitted from North America, Europe and Asia were investigated to determine how long-range the effects of these emissions can be. Results shown is this thesis suggest that Asian emissions have a global impact on O₃ production, and that North American emissions have a significant impact on O₃ production in the Northern Hemisphere.