ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-6-1135-2006 OH and HO<sub>2</sub> chemistry during NAMBLEX: roles of oxygenates, halogen oxides and heterogeneous uptake Sommariva R. 1 7 Bloss W. J. 1 Brough N. 3 Carslaw N. 4 Flynn M. 6 Haggerstone A.-L. 2 Heard D. E. 1 Hopkins J. R. 2 Lee J. D. 1 8 Lewis A. C. 2 McFiggans G. 6 Monks P. S. 5 Penkett S. A. 3 Pilling M. J. 1 Plane J. M. C. 3 Read K. A. 1 Saiz-Lopez A. 3 Rickard A. R. 5 9 Williams P. I. 6 School of Chemistry, University of Leeds, Leeds, UK Department of Chemistry, University of York, York, UK School of Environmental Sciences, University of East Anglia, Norwich, UK Environment Department, University of York, York, UK Department of Chemistry, University of Leicester, Leicester, UK School of Earth, Atmospheric & Environmental Sciences, University of Manchester, Manchester, UK now at: Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA now at: Department of Chemistry, University of York, York, UK now at: School of Chemistry, University of Leeds, Leeds, UK 05 04 2006 6 4 1135 1153 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.net/6/1135/2006/acp-6-1135-2006.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/6/1135/2006/acp-6-1135-2006.pdf

Several zero-dimensional box-models with different levels of chemical complexity, based on the Master Chemical Mechanism (MCM), have been used to study the chemistry of OH and HO<sub>2</sub> in a coastal environment in the Northern Hemisphere. The models were constrained to and compared with measurements made during the NAMBLEX campaign (Mace Head, Ireland) in summer 2002. <P style="line-height: 20px;"> The base models, which were constrained to measured CO, CH<sub>4</sub> and NMHCs, were able to reproduce [OH] within 25%, but overestimated [HO<sub>2</sub>] by about a factor of 2. Agreement was improved when the models were constrained to oxygenated compounds (acetaldehyde, methanol and acetone), highlighting their importance for the radical budget. When the models were constrained to measured halogen monoxides (IO, BrO) and used a more detailed, measurements-based, treatment to describe the heterogeneous uptake, modelled [OH] increased by up to 15% and [HO<sub>2</sub>] decreased by up to 30%. The actual impact of halogen monoxides on the modelled concentrations of HO<sub>x</sub> was dependant on the uptake coefficients used for HOI, HOBr and HO<sub>2</sub>. Better agreement, within the combined uncertainties of the measurements and of the model, was achieved when using high uptake coefficients for HO<sub>2</sub> and HOI (&gamma;<sub>HO<sub>2</sub></sub>=1, &gamma;<sub>HOI</sub>=0.6). <P style="line-height: 20px;"> A rate of production and destruction analysis of the models allowed a detailed study of OH and HO<sub>2</sub> chemistry under the conditions encountered during NAMBLEX, showing the importance of oxygenates and of XO (where X=I, Br) as co-reactants for OH and HO<sub>2</sub> and of HOX photolysis as a source for OH.