Atmospheric chemistry of 2,3-pentanedione: photolysis and reaction with OH radicals

The kinetics of the overall reaction between OH radicals and 2,3-pentanedione (1) were studied using both direct and relative kinetic methods at laboratory temperature. The low pressure fast discharge flow experiments coupled with resonance fluorescence detection of OH provided the direct rate coefficient of (2.25 ± 0.44) × 10(-12) cm(3) molecule(-1) s(-1). The relative-rate experiments were carried out both in a collapsible Teflon chamber and a Pyrex reactor in two laboratories using different reference reactions to provide the rate coefficients of 1.95 ± 0.27, 1.95 ± 0.34, and 2.06 ± 0.34, all given in 10(-12) cm(3) molecule(-1) s(-1). The recommended value is the nonweighted average of the four determinations: k(1) (300 K) = (2.09 ± 0.38) × 10(-12) cm(3) molecule(-1) s(-1), given with 2σ accuracy. Absorption cross sections for 2,3-pentanedione were determined: the spectrum is characterized by two wide absorption bands between 220 and 450 nm. Pulsed laser photolysis at 351 nm was used and the depletion of 2,3-pentanedione (2) was measured by GC to determine the photolysis quantum yield of Φ(2) = 0.11 ± 0.02(2σ) at 300 K and 1000 mbar synthetic air. An upper limit was estimated for the effective quantum yield of 2,3-pentanedione applying fluorescent lamps with peak wavelength of 312 nm. Relationships between molecular structure and OH reactivity, as well as the atmospheric fate of 2,3-pentanedione, have been discussed.

Hydroxyl radical regeneration in the photochemical oxidation of glyoxal: kinetics and mechanism of the HC(O)CO + O(2) reaction

Glyoxal, HC(O)CHO, is an important trace component of the Earth's atmosphere, formed in biomass burning and in the photooxidation of volatile organic compounds (VOCs) like isoprene and aromatic hydrocarbons. The HC(O)CO free radical is the primary product of the glyoxal + OH reaction, and this study uses computational chemistry to show that the HC(O)CO radical can react with O(2) to regenerate the hydroxyl radical (OH) in the atmosphere. Master equation simulations indicate that the HC(O)C(O)O(2) peroxy radical adduct proceeds directly to CO(2) + CO + OH in a chemically activated mechanism, with minor collisional deactivation of the relatively unstable HC(O)C(O)O(2) peroxy radical. The reaction of HC(O)CO with O(2) is found to be competitive with thermal decomposition to HCO + CO at tropospheric temperatures and pressures, accounting for ca. 40% or more of the total yield. The present process provides a new mechanism for OH regeneration in the troposphere, which involves the decomposition of unstable alpha-formylperoxy radicals.