Measurements of peroxy radicals, chemistry and transport in the atmosphere

This thesis explores the atmospheric chemistry of peroxy radicals (HO2 + SigmaiRiO 2) via a series of new measurements. Peroxy radical measurement techniques are discussed, and the University of Leicester PERCA instrument is described in detail. Three field campaigns are described.;Peroxy radical concentrations during ITOP displayed an increase with altitude that is likely to have been impacted by the effects of long-range transport. Enhanced peroxy radical concentrations were observed within air-masses traversing the Atlantic indicating significant photochemical activity during transport. Net ozone production at clear sky limit is in general negative, and the summer mid-Atlantic troposphere is at limit net ozone destructive. There is evidence of positive ozone production within air masses undergoing long-range transport. Ozone production was NOx limited throughout the airmasses measured during ITOP.;Two contrasting periods of meteorology were encountered at Jungfraujoch -- days with and without heavy snowfall. Peroxy radicals were significantly suppressed during 'snowy' days, with neither increased NO x nor reduced j(O1D) accounting for the difference. To reconcile the difference a pseudo first order loss rate of kex = 0.0063 s-1 is required. Ozone production rates show that overall the Jungfraujoch was net neutral to marginally ozone destructive. Overall ozone production through the campaign was peroxy radical limited. The ozone compensation point for 'snow free' days was 28 pptv.;The formation of secondary organic aerosol was studied at a simulation chamber. High NOx, O3 and peroxy radical concentrations caused problems with peroxy radical measurements, but modelling studies showed that measurements under atmospheric conditions are only negligibly affected. Further experiments at low concentrations of NOx, O3 and peroxy radicals and constant HONO concentration showed peroxy radicals within the chamber began to build up prior to aerosol nucleation, indicating the potential role of organic hydroperoxides in the formation of secondary organic aerosol. Preliminary model results are also discussed.