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

Boundary layer concentrations of hydroxyl (OH) and hydroperoxyl (HO2) radic als were measured at 1180 in elevation in a mountainous, forested region of north-western Greece during the AEROsols formation from BIogenic organic C arbon (AEROBIC) field campaign held in July-August 1997. In situ measuremen ts of OH radicals were made by laser-induced fluorescence (LIF) at low pres sure, exciting in the (0, 0) band of the A-X system at 308 nm. HO2 radicals were monitored by chemical titration to OH upon the addition of NO, with s ubsequent detection by LIF. The instrument was calibrated regularly during the field campaign, and demonstrated a sensitivity towards OH and HO2 of 5. 2 x 10(5) and 2.4 x 10(6) molecule cm (-3), respectively, for a signal inte gration period of 2.5 min and a signal-to-noise ratio of 1. Diurnal cycles of OH and HO2 were measured on 10 days within a small clearing of a forest of Greek Fir (Abies Borisi-Regis). In total 4165 OH data points and 1501 HO 2 data points were collected at 30s intervals. Noon-time OH and HO2 concent rations were between 4-12 x 10(6) and 0.4-9 x 10(8) molecule cm(-3), respec tively. The performance of the instrument is evaluated, and the data are in terpreted in terms of correlations with controlling variables. A significan t correlation (r(2) = 0.66) is observed between the OH concentration and th e rate of photolysis of ozone, J((OD)-D-1). However, OH persisted into the early evening when J((OD)-D-1) had fallen to very low values, consistent wi th the modelling study presented in the following paper (Carslaw et al., 20 01, OH and HO2 radical chemistry in a forest region of north-western Greece , Atmospheric Environment 35, 4725-4737) that predicts a significant radica l source from the ozonolysis of biogenic alkenes. Normalisation of the OH c oncentrations for variations in J((OD)-D-1) revealed a bell-shaped dependen ce of OH upon NO, (NO + NO2), which peaked at [NOx] similar to 1.75 ppbv. T he diurnal variation of HO2 was found to be less correlated with J((OD)-D-1 ) compared to OH. (C) 2001 Elsevier Science Ltd. All rights reserved.