Bimolecular reactions in Earth's atmosphere are generally assumed to proceed between reactants whose internal quantum states are fully thermally relaxed. Here, we highlight a dramatic role for vibrationally excited bimolecular reactants in the oxidation of acetylene. The reaction proceeds by preliminary adduct formation between the alkyne and OH radical, with subsequent O(2) addition. Using a detailed theoretical model, we show that the product-branching ratio is determined by the excited vibrational quantum-state distribution of the adduct at the moment it reacts with O(2). Experimentally, we found that under the simulated atmospheric conditions O(2) intercepts ~25% of the excited adducts before their vibrational quantum states have fully relaxed. Analogous interception of excited-state radicals by O(2) is likely common to a range of atmospheric reactions that proceed through peroxy complexes.

The kinetics of the OH + glyoxal, (HCO)2, reaction have been studied in N2 and N2/O2 bath gas from 5-80 Torr total pressure and 212-295 K, by monitoring the OH decay via laser induced fluorescence (LIF) in excess (HCO)2. The following rate coefficients, kOH+(HCO)2 = (9.7 ± 1.2), (12.2 ± 1.6), and (15.4 ± 2.0) × 10(-12) cm(3) molecule(-1) s(-1) (where errors represent a combination of statistical errors at the 2σ level and estimates of systematic errors) were measured in nitrogen at temperatures of 295, 250, and 212 K, respectively. Rate coefficient measurements were observed to be independent of total pressure but decreased following the addition of O2 to the reaction cell, consistent with direct OH recycling. OH yields, ΦOH, for this reaction were quantified experimentally for the first time as a function of total pressure, temperature, and O2 concentration. The experimental results have been parametrized using a chemical scheme where a fraction of the HC(O)CO population promptly dissociates to HCO + CO, the remaining HC(O)CO either dissociates thermally or reacts with O2 to give CO2, CO, and regenerate OH. A maximum ΦOH of (0.38 ± 0.02) was observed at 212 K, independent of total pressure, suggesting that ∼60% of the HC(O)CO population promptly dissociates upon formation. Qualitatively similar behavior is observed at 250 K, with a maximum ΦOH of (0.31 ± 0.03); at 295 K, the maximum ΦOH decreased further to (0.29 ± 0.03). From the parametrization, an OH yield of ΦOH = 0.19 is calculated for 295 K and 1 atm of air. It is shown that the proposed mechanism is consistent with previous chamber studies. While the fits are robust, experimental evidence suggests that the system is influenced by chemical activation and cannot be fully described by thermal rate coefficients. The atmospheric implications of the measurements are briefly discussed.

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The kinetics of the OH + glyoxal, (HCO) 2 , reaction have been studied in N 2 and N 2 /O 2 bath gas from 5 80 Torr total pressure and 212 295 K, by monitoring the OH decay via laser induced fluorescence (LIF) in excess (HCO) 2 . The following rate coefficients, k OH + (HCO)2 = (9.7 ± 1.2), (12.2 ± 1.6) and (15.4 ± 2.0) × 10 -12 cm 3 molecule -1 s -1 (where errors represent a combination of statistical error errors) were measured in nitrogen at temperatures of 295, 250 and 212 K, respectively. Rate coefficient measurements were observed to be independent of total pressure, but decreased following addition of O 2 to the reaction cell, consistent with direct OH recycling.OH yields, OH , for this reaction were quantified experimentally for the first time as a function of total pressure, temperature and O 2 concentration. The experimental results have been parameterised using a chemical scheme where a fraction of the HC(O)CO population promptly dissociates to HCO + CO, the remaining HC(O)CO either dissociates thermally or reacts with O 2 to give CO 2 , CO and regenerate OH. A maximum OH of (0.38 ± 0.02) was observed at 212 K, independent of total pressure, suggesting that ~60 % of the HC(O)CO population promptly dissociates upon formation. Qualitatively similar behaviour is observed at 250 K, with a maximum OH of (0.31 ± 0.03); at 295 K the maximum OH decreased further to (0.29 ± 0.03). F OH OH = 0.19 is calculated for 295 K and 1 atm of air. It is shown that the proposed mechanism is consistent with previous chamber studies. Whilst the fits are robust, experimental evidence suggests that the system is influenced by chemical activation and cannot be fully described by thermal rate coefficients. The atmospheric implications of the measurements are briefly discussed.

The perturbation of the temperature field caused by a quartz sampling probe has been investigated in a fuel-rich low-pressure premixed ethylene/oxygen/argon/krypton flame using Xray fluorescence. The experiments were performed at the 7-BM beamline at the Advanced Photon Source (APS) at the Argonne National Laboratory where a continuous beam of X-rays at 15 keV was used to excite krypton atoms that were added to the unburnt flame gases in a concentration of 5% (by volume). The resulting krypton X-ray fluorescence at 12.65 keV was collected and the spatially resolved signal was subsequently converted into the local temperature of the imaged spot. One and two dimensional scans of the temperature field were obtained by translating the entire flame chamber through a pre-programmed sequence of positions on high precision translation stages and measuring the X-ray fluorescence at each location. Multiple measurements were performed at various separations between the burner surface and probe tip, representing sampling positions from the preheat, reaction, and postflame zones of the lowpressure flame. Distortions of up to 1000 K of the burner-probe centerline flame temperature were found with the tip of the probe in the preheat zone and distortions of up to 500 K were observed with it in the reaction and postflame zones. Furthermore, perturbations of the temperature field have been revealed that reach radially as far as 20 mm from the burner-probe centerline and about 3 mm in front of the probe tip. These results clearly reveal the limitations of one-dimensional models for predicting flame-sampling experiments and comments are made with regard to model developments and validations based on quantitative speciation data from low-pressure flames obtained via intrusive sampling techniques.