Hydroxyl and hydroperoxy radical chemistry in forested and urban areas: measurements, modeling, and implications for atmospheric chemistry

Hydroxyl (OH) and hydroperoxy (HO2) radicals are key species that drive the oxidation of organic trace gases leading to the production of ozone and secondary organic aerosols in the atmosphere. Previous efforts to measure these radicals in low NOx, (NO + NO 2) isoprene rich forest environments have shown serious discrepancies with modeled concentrations consistently underestimating OH radical concentrations. These results have important implications to issues of regional air quality and climate change as isoprene contributes to approximately one-third of total VOC emissions globally (C yr-1). Previous efforts to measure OH and HO2 (HOx) radicals in urban environments characterized by higher NOx conditions have shown mixed agreement with modeled concentrations, bringing into question our understanding of the chemistry of the polluted atmosphere.;The work detailed by this research describes important developments in the measurement of OH and HO2 radicals by the Indiana University Fluorescence Assay by Gas Expansion Laser-Induced Fluorescence instrument (IU-FAGE). This instrument was deployed as part of three field campaigns between 2008 and 2010, with two campaigns in a forested site in Northern Michigan and one in Los Angeles, California. Supporting measurements of photolysis rates, volatile organic compounds, NOx and other inorganic species were used to constrain a zero-dimensional box model of the radical chemistry, comparisons of the measured and modeled HOx concentrations are discussed. Measurements from the forest environment are compared to the results of the base model, and the impact of several newly proposed HOx recycling mechanisms are discussed. The results from the campaign in Los Angeles include a large weekend/weekday effect in the measured and model HOx values that are examined and discussed in the context of our current understanding of the chemistry of ozone production in urban areas.