Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 6 4 2006 10.5194/acp-6-1135-2006 http://www.atmos-chem-phys.net/6/1135/2006/ http://www.atmos-chem-phys.net/6/1135/2006/acp-6-1135-2006.html http://www.atmos-chem-phys.net/6/1135/2006/acp-6-1135-2006.pdf 1135 1153 2006-04-05 OH and HO₂ chemistry during NAMBLEX: roles of oxygenates, halogen oxides and heterogeneous uptake R. Sommariva W. J. Bloss N. Brough N. Carslaw M. Flynn A.-L. Haggerstone D. E. Heard J. R. Hopkins J. D. Lee A. C. Lewis G. McFiggans P. S. Monks S. A. Penkett M. J. Pilling J. M. C. Plane K. A. Read A. Saiz-Lopez A. R. Rickard P. I. Williams 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 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 HO₂ 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, CH₄ and NMHCs, were able to reproduce [OH] within 25%, but overestimated [HO₂] 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 [HO₂] decreased by up to 30%. The actual impact of halogen monoxides on the modelled concentrations of HO_x was dependant on the uptake coefficients used for HOI, HOBr and HO₂. Better agreement, within the combined uncertainties of the measurements and of the model, was achieved when using high uptake coefficients for HO₂ and HOI (&gamma;_HO₂=1, &gamma;_HOI=0.6). <P style="line-height: 20px;"> A rate of production and destruction analysis of the models allowed a detailed study of OH and HO₂ 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 HO₂ and of HOX photolysis as a source for OH.