9625437 Hynes This project has three components that relate to the tropospheric cycling of the hydroxyl radical, the most important oxidant in the troposphere. The first project involves laboratory measurements of the quantum yield for production of singlet oxygen atoms in ozone photolysis at long wavelengths (295-350 nm). In the atmosphere, these singlet oxygen atoms react with water vapor to form hydroxyl. In the laboratory, the singlet oxygen atoms will be monitored directly via Resonance Enhanced Multiphoton Ionization (REMPI) or indirectly through Laser-Induced Fluorescence (LIF) of hydroxyl radicals and by monitoring the appearance of triplet (ground state) oxygen atoms via 2-photon LIF. These types of measurements are needed in order to accurately model hydroxyl radical production in the atmosphere under all conditions. The second component addresses the kinetics and photochemistry of the hydroperoxyl radical. Many of its reactions appear to proceed via complex mechanisms and several display complex pressure and water vapor dependencies. Because of the difficulties involved with high pressure kinetic studies of this radical, its kinetic database is limited. The photofragment-LIF technique will be used for kinetic studies of the reactions of hydroperoxyl with nitric oxide, nitrogen dioxide, and carbonyl compounds. Temperature and pressure dependencies will be determined under realistic atmospheric conditions. The extension of this technique to the study of radical-radical reactions, in particular the reactions of hydroperoxyl with peroxy species, will be investigated. The third component addresses the kinetics and mechanism of the reaction of hydroxyl radical with isoprene. Isoprene is a significant sink for hydroxyl in clean and moderately polluted environments and the oxidation appears to proceed via the formation of an addition complex. These complexes can further react with molecular oxygen, ozone, nitrogen dioxide, or nitric oxide, and the effective rates and temperature depen dencies of the reactions can be a complex function of gas composition and pressure. The pulsed laser photolysis-pulsed laser induced fluorescence technique will be used to perform direct kinetic and mechanistic studies of this reaction under realistic conditions of atmospheric composition and temperature. ***
