2 research outputs found

    VUV Photoionization Cross Sections of HO<sub>2</sub>, H<sub>2</sub>O<sub>2</sub>, and H<sub>2</sub>CO

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    The absolute vacuum ultraviolet (VUV) photoionization spectra of the hydroperoxyl radical (HO<sub>2</sub>), hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>), and formaldehyde (H<sub>2</sub>CO) have been measured from their first ionization thresholds to 12.008 eV. HO<sub>2</sub>, H<sub>2</sub>O<sub>2</sub>, and H<sub>2</sub>CO were generated from the oxidation of methanol initiated by pulsed-laser-photolysis of Cl<sub>2</sub> in a low-pressure slow flow reactor. Reactants, intermediates, and products were detected by time-resolved multiplexed synchrotron photoionization mass spectrometry. Absolute concentrations were obtained from the time-dependent photoion signals by modeling the kinetics of the methanol oxidation chemistry. Photoionization cross sections were determined at several photon energies relative to the cross section of methanol, which was in turn determined relative to that of propene. These measurements were used to place relative photoionization spectra of HO<sub>2</sub>, H<sub>2</sub>O<sub>2</sub>, and H<sub>2</sub>CO on an absolute scale, resulting in absolute photoionization spectra

    Gas-Phase Reactions of Isoprene and Its Major Oxidation Products

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    Isoprene carries approximately half of the flux of non-methane volatile organic carbon emitted to the atmosphere by the biosphere. Accurate representation of its oxidation rate and products is essential for quantifying its influence on the abundance of the hydroxyl radical (OH), nitrogen oxide free radicals (NO<sub><i>x</i></sub>), ozone (O<sub>3</sub>), and, via the formation of highly oxygenated compounds, aerosol. We present a review of recent laboratory and theoretical studies of the oxidation pathways of isoprene initiated by addition of OH, O<sub>3</sub>, the nitrate radical (NO<sub>3</sub>), and the chlorine atom. From this review, a recommendation for a nearly complete gas-phase oxidation mechanism of isoprene and its major products is developed. The mechanism is compiled with the aims of providing an accurate representation of the flow of carbon while allowing quantification of the impact of isoprene emissions on HO<sub><i>x</i></sub> and NO<sub><i>x</i></sub> free radical concentrations and of the yields of products known to be involved in condensed-phase processes. Finally, a simplified (reduced) mechanism is developed for use in chemical transport models that retains the essential chemistry required to accurately simulate isoprene oxidation under conditions where it occurs in the atmosphereî—¸above forested regions remote from large NO<sub><i>x</i></sub> emissions