This paper explores several aspects of the chemistry of a forested region i
n north-western Greece, from data collected during the AEROBIC97 campaign.
An observationally constrained box model has been constructed to enable com
parisons between modelled concentrations of OH and HO2 and those determined
by the fluorescence assay by gas expansion (FAGE) technique. These results
represent the first comparison of measured and modelled OH concentrations
in such an environment. The modelled OH concentrations are, on average, sim
ilar to 50% of those measured (range of 16-61 %) over 4 days of model and m
easurement comparison. Possible reasons for the model-measurement discrepan
cy are discussed. A rate of production analysis illustrates the dominance o
f isoprene and the monoterpenes on OH loss, as well as the significance of
the ozonolysis of biogenic species as an OH source. The measured and modell
ed [HO2]/[OH] ratio averaged between 11:00 and 15:00 h is much higher than
has been found previously for similar NO, concentrations, similar to 75 and
340, respectively, cf. 10-20. The high ratio reflects the rapid recycling
through the OH-HO2 oxidation chain, involving biogenic species. The high bi
ogenic concentrations result in a midday OH lifetime of similar to 0.15 s.
Finally, for the conditions encountered during the campaign, there is high
net photochemical ozone production, peaking at similar to 20 ppbv h(-1) aro
und 09:00h. (C) 2001 Elsevier Science Ltd. All rights reserved.