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

Boundary layer concentrations of hydroxyl (OH) and hydroperoxyl (HO2) radicals were measured at 1180 m elevation in a mountainous, forested region of north-western Greece during the AEROsols formation from BIogenic organic Carbon (AEROBIC) field campaign held in July–August 1997. In situ measurements of OH radicals were made by laser-induced fluorescence (LIF) at low pressure, 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 subsequent detection by LIF. The instrument was calibrated regularly during the field campaign, and demonstrated a sensitivity towards OH and HO2 of 5.2×105 and 2.4×106 molecule cm−3, respectively, for a signal integration 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 HO2 data points were collected at 30 s intervals. Noon-time OH and HO2 concentrations were between 4–12×106 and 0.4–9×108 molecule cm−3, respectively. The performance of the instrument is evaluated, and the data are interpreted in terms of correlations with controlling variables. A significant correlation (r2=0.66) is observed between the OH concentration and the rate of photolysis of ozone, J(O1D). However, OH persisted into the early evening when J(O1D) had fallen to very low values, consistent with the modelling study presented in the following paper (Carslaw et al., 2001, OH and HO2 radical chemistry in a forest region of north-western Greece. Atmospheric Environment 35, 4725–4737) that predicts a significant radical source from the ozonolysis of biogenic alkenes. Normalisation of the OH concentrations for variations in J(O1D) revealed a bell-shaped dependence of OH upon NOx (NO+NO2), which peaked at [NOx] 1.75 ppbv. The diurnal variation of HO2 was found to be less correlated with J(O1D) compared to OH.