* Corresponding author. Present address: The Environment Department, University of York, Heslington, York YO10 5DD, UK. E-mail address: nc12@york.ac.uk (N. Carslaw). Atmospheric Environment 35 (2001) 4725}4737 OH and HO  radical chemistry in a forested region of north-western Greece N. Carslaw*, D.J. Creasey, D. Harrison,D.E.Heard, M.C. Hunter, P.J. Jacobs, M.E. Jenkin, J.D. Lee, A.C. Lewis, M.J. Pilling, S.M. Saunders, P.W. Seakins School of Chemistry, University of Leeds, Leeds, England, UK AEA Technology, National Environmental Technology Centre, Culham, England, UK Received 4 August 2000; received in revised form 14 November 2000; accepted 21 November 2000 Abstract This paper explores several aspects of the chemistry of a forested region in north-western Greece, from data collected during the AEROBIC97 campaign. An observationally constrained box model has been constructed to enable compari- sons between modelled concentrations of OH and HO  and those determined by the #uorescence assay by gas expansion (FAGE) technique. These results represent the "rst comparison of measured and modelled OH concentrations in such an environment. The modelled OH concentrations are, on average,&50% of those measured (range of 16}61%) over 4 days of model and measurement comparison. Possible reasons for the model-measurement discrepancy are discussed. A rate of production analysis illustrates the dominance of isoprene and the monoterpenes on OH loss, as well as the signi"cance of the ozonolysis of biogenic species as an OH source. The measured and modelled [HO  ]/[OH] ratio averaged between 11:00 and 15:00 h is much higher than has been found previously for similar NO  concentrations,&75 and 340, respectively, cf. 10}20. The high ratio re#ects the rapid recycling through the OH}HO  oxidation chain, involving biogenic species. The high biogenic concentrations result in a midday OH lifetime of&0.15 s. Finally, for the conditions encountered during the campaign, there is high net photochemical ozone production, peaking at&20ppbvh around 09:00h.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Isoprene; Monoterpenes; AEROBIC; Hydroxyl radicals; Ozone 1. Introduction The hydroxyl radical (OH) is the major daytime tropospheric oxidant and is responsible for removing many anthropogenic pollutants, as well as a signi"cant number of biogenic species. The OH radical is formed in the troposphere primarily through the photolysis of ozone (O  ) at wavelengths below&340 nm, followed by reaction of the resulting O(D) atom with water vapour. The chemistry of OH has been described in detail before (e.g., Logan et al., 1981) and is only summarised here. In forested regions, the non-methane hydrocarbon (NMHC) composition is dominated by vegetative emis- sions of biogenic species, such as isoprene and mono- terpenes, which undergo rapid reaction with the OH radical. Although these species are structurally complex, the OH-initiated oxidation mechanism is believed to follow the general pattern of hydrocarbon oxidation in the presence of su$cient NO  (sum of NO and NO  ): OH#hydrocarbon(#O  )PRO  (R1) RO  #NOPRO#NO  (R2) ROPPcarbonyl compound(s)#HO  (R3) HO  #NOPOH#NO  (R4) 1352-2310/01/$-see front matter  2001 Elsevier Science Ltd. All rights reserved. PII:S1352-2310(01)00089-9