© Author(s) 2007. This work is licensed under a Creative Commons License. Atmospheric Chemistry and Physics Free radical modelling studies during the UK TORCH Campaign in

Abstract. The Tropospheric ORganic CHemistry experi-ment (TORCH) took place during the heatwave of summer 2003 at Writtle College, a site 2 miles west of Chelmsford in Essex and 25 miles north east of London. The experi-ment was one of the most highly instrumented to date. A combination of a large number of days of simultaneous, col-located measurements, a consequent wealth of model con-straints and a highly detailed chemical mechanism, allowed the atmospheric chemistry of this site to be studied in de-tail. Between 25 July and 31 August, the concentrations of the hydroxyl radical and the hydroperoxy radical were mea-sured using laser-induced fluorescence at low pressure and the sum of peroxy radicals was measured using the peroxy radical chemical amplifier technique. The concentrations of the radical species were predicted using a zero-dimensiona