OH and HO2 Concentrations during PROPHET 2008: Measurement and Theory

Hydroxyl (OH) and hydroperoxy (HO2) radicals are key species driving the gas-phase oxidation of organic trace gases that lead to the formation of ozone and secondary organic aerosols in the troposphere. Previous measurements of OH and HO2 radicals in forest environments have shown serious discrepancies with modeled concentrations of these radicals. These discrepancies bring into question our understanding of the atmospheric chemistry of isoprene and other reactive biogenic emissions. During the summer of 2008, OH and HO2 (HOx) radical measurements were made at the forested PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) site in northern Michigan using a laser-induced fluorescence instrument developed at Indiana University. A suite of additional measurements including isoprene, methyl vinyl ketone, methacrolein, total monoterpenes, glyoxal, formaldehyde, nitrogen oxides (NOx), ozone, and UV actinic flux were measured simultaneously. These measurements were used to constrain a zero-dimensional model based on the Regional Atmospheric Chemistry Mechanism to predict the expected HOx radical concentrations. This analysis investigates individual day and night measured and modeled HOx concentrations, the HO2/OH ratio, and the production and loss processes governing HOx in this remote forested environment. Included in this analysis are 1) efforts to characterize the ozonolysis of unquantified highly reactive terpenes, which may be a potentially significant unaccounted for source of radicals in forested environments, and 2) the impact of recently proposed OH recycling in the isoprene oxidation mechanism that could lead to higher sustained OH concentrations.