OH and HO 2 radical chemistry in a midlatitude forest : Measurements and 1 model comparisons 2

Reactions of the hydroxyl (OH) and peroxy radicals (HO2 and RO2) play a central role in the chemistry of the 18 atmosphere. In addition to controlling the lifetimes of many trace gases important to issues of global climate change, OH 19 radical reactions initiate the oxidation of volatile organic compounds (VOCs) which can lead to the production of ozone and 20 secondary organic aerosols in the atmosphere. Previous measurements of these radicals in forest environments characterized 21 by high mixing ratios of isoprene and low mixing ratios of nitrogen oxides (NOx) (typically less than 1-2 ppb) have shown 22 serious discrepancies with modeled concentrations. These results bring into question our understanding of the atmospheric 23 chemistry of isoprene and other biogenic VOCs under low NOx conditions. 24 During the summer of 2015, OH and HO2 radical concentrations as well as total OH reactivity were measured using 25 Laser-Induced Fluorescence Fluorescence Assay by Gas Expansion (LIF-FAGE) techniques as part of the Indiana Radical, 26 Reactivity and Ozone Production Intercomparison (IRRONIC). This campaign took place in a forested area near the Indiana 27 University, Bloomington campus characterized by high mixing ratios of isoprene (average daily maximum of approximately 28 4 ppb at 28C) and low mixing ratios of NO (diurnal average of approximately 170 ppt). Supporting measurements of 29 photolysis rates, VOCs, NOx, and other species were used to constrain a zero-dimensional box model based on the Regional 30 Atmospheric Chemistry Mechanism (RACM2) and the Master Chemical Mechanism (MCM 3.2), including versions of the 31 Leuven Isoprene Mechanism (LIM1) for HOx regeneration (RACM2-LIM1 and MCM 3.3.1). Using an OH chemical scavenger 32 technique, the study revealed the presence of an interference with the LIF-FAGE measurements of OH that increased with 33