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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-dimensional box model based on the Master Chemical Mechanism ver- sion 3.1, which was constrained with the observed concen- trations of relatively long-lived species. The model included a detailed parameterisation to account for heterogeneous loss of hydroperoxy radicals onto aerosol particles. Quantile- quantile plots were used to assess the model performance in respect of the measured radical concentrations. On av- erage, measured hydroxyl radical concentrations were over- predicted by 24%. Modelled and measured hydroperoxy rad- ical concentrations agreed very well, with the model over- predicting on average by only 7%. The sum of peroxy rad- icals was under-predicted when compared with the respec- tive measurements by 22%. Initiation via OH was dominated

Free radical modelling studies during the UK TORCH Campaign in Summer 2003