OH and HO2 radical chemistry in a forested region of north-western Greece

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 comparisons 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, 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 significance of the ozonolysis of biogenic species as an OH source. The measured and modelled [HO2]/[OH] ratio averaged between 11:00 and 15:00 h is much higher than has been found previously for similar NOx concentrations, 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 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 20 ppbv h−1 around 09:00 h.