Articles | Volume 16, issue 3
Research article
11 Feb 2016
Research article |  | 11 Feb 2016

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

N. Zannoni, V. Gros, M. Lanza, R. Sarda, B. Bonsang, C. Kalogridis, S. Preunkert, M. Legrand, C. Jambert, C. Boissard, and J. Lathiere

Abstract. Total OH reactivity, defined as the total loss frequency of the hydroxyl radical in the atmosphere, has proved to be an excellent tool to identify the total loading of reactive species in ambient air. High levels of unknown reactivity 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 Observatoire 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 depended 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 determined a missing fraction of OH reactivity up to 50 %, possibly due to unmeasured oxidation products. We confirmed that no significant oxidation of the primary species occurred within the canopy; primary compounds emitted by the forest 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. Observations in various and diverse forests in the Mediterranean region are therefore needed to better constrain the impact of reactive gases over this area.

Short summary
Our manuscript shows results of OH reactivity and Biogenic Volatile Organic Compounds (BVOCs) concentration during a field experiment conducted in late spring 2014 at the Observatoire de Haute Provence (OHP) site. We found that OH reactivity is among the highest measured in forests globally (69 s−1) and it is mainly due to isoprene. No missing reactivity was present during daytime inside or above the canopy, while 50 % missing reactivity was found by night at both heights.
Final-revised paper