ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-6-10523-2006 Free radical modelling studies during the UK TORCH Campaign in summer 2003 Emmerson K. M. 1 7 Carslaw N. 1 Carslaw D. C. 2 Lee J. D. 3 McFiggans G. 4 Bloss W. 5 Gravestock T. 5 Heard D. E. 5 Hopkins J. 3 Ingham T. 5 Pilling M. J. 5 Smith S. C. 5 Jacob M. J. 6 8 Monks P. S. 6 Environment Department, University of York, York, YO10 5DD, UK Institute for Transport Studies, University of Leeds, Leeds, LS2 9JT, UK Department of Chemistry, University of York, York, YO10 5DD, UK School of Earth, Atmospheric and Environmental Science, University of Manchester, Manchester, M60 1QD, UK School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK Department of Chemistry, University of Leicester, Leicester, LE1 7RH, UK now at: School of Earth and Environment, University of Leeds, LS2 9JT, UK now at: IÖZ (Interdisciplinary Environmental Research Centre), TU Bergakademie Freiberg, Brennhausgasse 14, 09599 Freiberg, Germany 18 10 2006 6 5 10523 10565 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/6/10523/2006/acpd-6-10523-2006.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/6/10523/2006/acpd-6-10523-2006.pdf

The Tropospheric ORganic CHemistry experiment (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 experiment was one of the most highly instrumented to date. A combination of a large number of days of simultaneous, collocated measurements, a consequent wealth of model constraints and a highly detailed chemical mechanism, allowed the atmospheric chemistry of this site to be studied in detail. The concentrations of the hydroxyl radical, the hydroperoxy radical and the sum of peroxy radicals, were measured between 25 July and 31 August using laser-induced fluorescence at low pressure and the peroxy radical chemical amplifier techniques. The concentrations of the radical species were predicted using a zero-dimensional box model based on the Master Chemical Mechanism version&nbsp;3.1, which was constrained with the observed concentrations 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 average, measured hydroxyl radical concentrations were over-predicted by 24%. Modelled and measured hydroperoxy radical concentrations agreed very well, with the model over-predicting on average by only 7%. The sum of peroxy radicals was under-predicted when compared with the respective measurements by 22%. OH initiation was dominated by the reactions of excited oxygen atoms with water, nitrous acid photolysis and the ozone reaction with alkene species. Photolysis of aldehyde species was the main initiation route for HO<sub>2</sub> and RO<sub>2</sub>. Termination, under all conditions, primarily involved reactions with NO<sub>x</sub> for OH and heterogeneous chemistry on aerosol surfaces for HO<sub>2</sub>. The OH chain length varied between 2 and 8 cycles, the longer chain lengths occurring before and after the most polluted part of the campaign. Peak local ozone production of 17 ppb hr<sup>&minus;1</sup> occurred on 3 and 5 August, signifying the importance of local chemical processes to ozone production on these days. On the whole, agreement between model and measured radicals is good, giving confidence that our understanding of atmospheres influenced by nearby urban sources is adequate.