HOx Radical Chemistry Above a Forest Canopy: Measurements and Model Comparison

Reactions of the hydroxyl (OH) and peroxy (HO2 and RO2) radicals play a central role in the chemistry of the atmosphere. OH radical reactions initiate the oxidation of volatile organic compounds (VOCs) which can lead to the production of ozone and secondary organic aerosols in the atmosphere. Previous measurements of these radicals in forest environments, especially under high isoprene, low NOx conditions, have shown serious discrepancies with modeled concentrations, bringing into question the current understanding of the oxidation chemistry of isoprene and other biogenic volatile organic compounds. Measurements of these radicals were made using laser-induced fluorescence techniques above a northern Michigan forest as part of the CABINEX 2009 (Community Atmosphere-Biosphere INteractions EXperiment) campaign. Supporting measurements of photolysis rates, volatile organic compounds, nitrogen oxides, and other inorganic species were used to constrain a zero-dimensional box model based on the Regional Atmospheric Chemistry Mechanism with the Mainz Isoprene Mechanism (RACM-MIM). In general, the base RACM-MIM model is able to reproduce the observed OH concentrations for average isoprene concentrations between 1-2 ppb, while the model tends to overpredict the measured concentration of peroxy radicals. The impact of including recently proposed mechanisms of radical recycling in isoprene oxidation on the modeled results will be discussed, and a chemical coordinate analysis will be used to gain insight into the discrepancies between the measured and modeled radical concentrations.