The work in this thesis describes measurements of peroxy radicals from a ground-based\ud platform using a PERCA (Peroxy Radical Chemical amplifier) at two field campaigns,\ud TORCH I and II (Tropospheric Organic Chemistry Experiment). TORCH I took place during\ud July and August of 2003 in Writtle, Essex, 25 miles northeast of London. TORCH II took\ud place one year later in April and May at the Weybourne Atmospheric Observatory, 120 miles\ud northeast of London. Originally conceived to compare relatively fresh air masses from\ud London with more processed air masses some distance from London, the two TORCH\ud campaigns rarely experienced wind coming from the London direction. During TORCH I\ud record temperatures hit much of the UK and Europe with ozone mixing ratios exceeding 150\ud ppbv at the measurement site and peroxy radicals having average midday mixing ratios during\ud the heatwave over three times non-heatwave averages. The relationship between the high\ud ozone and peroxy radical mixing ratios is explored. A comparison of measured, modelled and\ud PSS (Photostationary state) peroxy radicals for the TORCH I campaign is presented.\ud Although during TORCH II the desired air masses from London did not come and there was\ud no such heatwave as in TORCH I, the measurement site did experience air masses with a\ud variety of histories. A particular event from the TORCH II field campaign involved high\ud concentrations of propene (3 orders of magnitude higher than the previous or subsequent\ud hourly measurements) at 20:00 hours and concomitant five-fold increase in peroxy radical\ud mixing ratio. Radical production is attributed to ozone-alkene reactions and although the\ud importance of such reactions for radical production is well-known, the case study presented\ud represents a clear example occurring in the planetary boundary layer. In order to enhance the\ud capabilities of the PERCA instrument a further inlet was added to it with a view to the\ud seperate measurement of HO2 from SRiO2 (the sum of organic peroxy radicals). A calibration\ud source for generating HO2 radicals was built and tested. Laboratory tests showed that the\ud system could measure either the sum of peroxy radicals or organic peroxy radicals only but\ud results from deployment at TORCH I proved inconclusive
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