OH reactivity and concentrations of biogenic volatile organic compounds in a Mediterranean forest of downy oak trees

Total OH reactivity, defined as the total loss fre- quency of the hydroxyl radical in the atmosphere, has proved to be an excellent tool to identify the total loading of reac- tive species in ambient air. High levels of unknown reactiv- ity were found in several forests worldwide and were often higher than at urban sites. Our study presents atmospheric mixing ratios of biogenic compounds and total OH reactivity measured during late spring 2014 at the forest of downy oak trees of the Obser- vatoire de Haute Provence (OHP), France. Air masses were sampled at two heights: 2 m, i.e., inside the canopy, and 10 m, i.e., above the canopy, where the mean canopy height is 5 m. We found that the OH reactivity at the site mainly de- pended on the main primary biogenic species emitted by the forest, which was isoprene and to a lesser extent by its degradation products and long-lived atmospheric compounds (up to 26 % during daytime). During daytime, no significant missing OH reactivity was reported at the site, either inside or above the canopy. However, during two nights we deter- mined a missing fraction of OH reactivity up to 50 %, pos- sibly due to unmeasured oxidation products. We confirmed that no significant oxidation of the primary species occurred within the canopy; primary compounds emitted by the for- est were fast transported to the atmosphere. Finally, the OH reactivity at this site was maximum 69 s 1 , which is a high value for a forest characterized by a temperate climate. Ob- servations in various and diverse forests in the Mediterranean region are therefore needed to better constrain the impact of reactive gases over this area.