Above canopy OH and HO2 during PROPHET 2008 and CABINEX 2009: Measurement and theory

Hydroxyl (OH) and hydroperoxy (HO2) radicals are key species driving the oxidation of organic trace gases leading to the production of ozone and secondary organic aerosols. Previous measurements of these radicals in forest environments have shown serious discrepancies with modeled concentrations, bringing into question the current understanding of isoprene oxidation chemistry in these environments. During the summers of 2008 and 2009, OH and HO2 (HOx) radical measurements were made as part of the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) and CABINEX (Community Atmosphere-Biosphere INteractions EXperiment) campaigns at a forested site in northern Michigan. Supporting measurements of photolysis rates, volatile organic compounds, NOx (NO+NO2) and other inorganic species were used to constrain a zero-dimensional box model based on the Regional Atmospheric Chemistry Mechanism, modified to include recently proposed mechanisms of OH recycling. In this presentation, we will discuss the main production and loss processes governing the concentrations of OH and HO2 above the forest canopy, and the agreement observed between measured and modeled HOx concentrations and HO2/OH ratios in this environment.