5 research outputs found

    <i>cis-</i>Pinonic Acid Oxidation by Hydroxyl Radicals in the Aqueous Phase under Acidic and Basic Conditions: Kinetics and Mechanism

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    Aqueous-phase oxidation of <i>cis-</i>pinonic acid (CPA) by hydroxyl radicals (OH) was studied using a relative rate technique under acidic and basic conditions. Liquid chromatography (LC) coupled to the negative electrospray ionization (ESI) quadrupole tandem mass spectrometry (MS/MS) was used to monitor the concentrations of CPA and reference compounds. The measured second order reaction rate coefficients of CPA with OH were: 3.6 ± 0.3 × 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup> (pH 2) and 3.0 ± 0.3 × 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup> (pH 10) - combined uncertainties are 2σ. These results indicated that the lifetimes of CPA in the atmosphere are most likely independent from the aqueous-phase pH. LC-ESI/MS/MS was also used to tentatively identify the CPA oxidation products. Formation of carboxylic acids with molecular weight (MW) 216 Da (most likely C<sub>10</sub>H<sub>16</sub>O<sub>5</sub>) and MW 214 Da (C<sub>10</sub>H<sub>14</sub>O<sub>5</sub>) was confirmed with LC-ESI/MS/MS. When the initial CPA concentration was increased from 0.3 to 10 mM, formation of additional products was observed with MW 188, 200, 204, and 232 Da. Hydroperoxy, hydroxyl and carbonyl-substituted CPA derivatives were tentatively identified among the products. Similar products were formed by the CPA oxidation by OH in the gas-phase, at the air–water interface as well as in the solid phase (dry film). Formation of the stable adduct of CPA and H<sub>2</sub>O<sub>2</sub> was also observed when the reaction mixture was evaporated to dryness and redissolved in water. Acquired mass spectrometric data argues against formation of oligomers

    OH + (<i>E</i>)- and (<i>Z</i>)‑1-Chloro-3,3,3-trifluoropropene‑1 (CF<sub>3</sub>CHCHCl) Reaction Rate Coefficients: Stereoisomer-Dependent Reactivity

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    Rate coefficients for the gas-phase reaction of the OH radical with (<i>E</i>)- and (<i>Z</i>)-CF<sub>3</sub>CHCHCl (1-chloro-3,3,3-trifluoropropene-1, HFO-1233zd) (<i>k</i><sub>1</sub>(<i>T</i>) and <i>k</i><sub>2</sub>(<i>T</i>), respectively) were measured under pseudo-first-order conditions in OH over the temperature range 213–376 K. OH was produced by pulsed laser photolysis, and its temporal profile was measured using laser-induced fluorescence. The obtained rate coefficients were independent of pressure between 25 and 100 Torr (He, N<sub>2</sub>) with <i>k</i><sub>1</sub>(296 K) = (3.76 ± 0.35) × 10<sup>–13</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> and <i>k</i><sub>2</sub>(296 K) = (9.46 ± 0.85) × 10<sup>–13</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> (quoted uncertainties are 2σ and include estimated systematic errors). <i>k</i><sub>2</sub>(<i>T</i>) showed a weak non-Arrhenius behavior over this temperature range. The (<i>E</i>)- and (<i>Z</i>)- stereoisomer rate coefficients were found to have opposite temperature dependencies that are well represented by <i>k</i><sub>1</sub>(<i>T</i>) = (1.14 ± 0.15) × 10<sup>–12</sup> exp[(−330 ± 10)/<i>T</i>] cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> and <i>k</i><sub>2</sub>(<i>T</i>) = (7.22 ± 0.65) × 10<sup>–19</sup> × <i>T</i><sup>2</sup> × exp[(800 ± 20)/<i>T</i>] cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>. The present results are compared with a previous room temperature relative rate coefficient study of <i>k</i><sub>1</sub>, and an explanation for the discrepancy is presented. CF<sub>3</sub>CHO, HC­(O)­Cl, and CF<sub>3</sub>CClO, were observed as stable end-products following the OH radical initiated degradation of (<i>E</i>)- and (<i>Z</i>)-CF<sub>3</sub>CHCHCl in the presence of O<sub>2</sub>. In addition, chemically activated isomerization was also observed. Atmospheric local lifetimes of (<i>E</i>)- and (<i>Z</i>)-CF<sub>3</sub>CHCHCl, due to OH reactive loss, were estimated to be ∼34 and ∼11 days, respectively. Infrared absorption spectra measured in this work were used to estimate radiative efficiencies and well-mixed global warming potentials of ∼10 and ∼3 for (<i>E</i>)- and (<i>Z</i>)-CF<sub>3</sub>CHCHCl, respectively, on the 100-year time horizon

    OH + (<i>E</i>)- and (<i>Z</i>)‑1-Chloro-3,3,3-trifluoropropene‑1 (CF<sub>3</sub>CHCHCl) Reaction Rate Coefficients: Stereoisomer-Dependent Reactivity

    No full text
    Rate coefficients for the gas-phase reaction of the OH radical with (<i>E</i>)- and (<i>Z</i>)-CF<sub>3</sub>CHCHCl (1-chloro-3,3,3-trifluoropropene-1, HFO-1233zd) (<i>k</i><sub>1</sub>(<i>T</i>) and <i>k</i><sub>2</sub>(<i>T</i>), respectively) were measured under pseudo-first-order conditions in OH over the temperature range 213–376 K. OH was produced by pulsed laser photolysis, and its temporal profile was measured using laser-induced fluorescence. The obtained rate coefficients were independent of pressure between 25 and 100 Torr (He, N<sub>2</sub>) with <i>k</i><sub>1</sub>(296 K) = (3.76 ± 0.35) × 10<sup>–13</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> and <i>k</i><sub>2</sub>(296 K) = (9.46 ± 0.85) × 10<sup>–13</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> (quoted uncertainties are 2σ and include estimated systematic errors). <i>k</i><sub>2</sub>(<i>T</i>) showed a weak non-Arrhenius behavior over this temperature range. The (<i>E</i>)- and (<i>Z</i>)- stereoisomer rate coefficients were found to have opposite temperature dependencies that are well represented by <i>k</i><sub>1</sub>(<i>T</i>) = (1.14 ± 0.15) × 10<sup>–12</sup> exp[(−330 ± 10)/<i>T</i>] cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> and <i>k</i><sub>2</sub>(<i>T</i>) = (7.22 ± 0.65) × 10<sup>–19</sup> × <i>T</i><sup>2</sup> × exp[(800 ± 20)/<i>T</i>] cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>. The present results are compared with a previous room temperature relative rate coefficient study of <i>k</i><sub>1</sub>, and an explanation for the discrepancy is presented. CF<sub>3</sub>CHO, HC­(O)­Cl, and CF<sub>3</sub>CClO, were observed as stable end-products following the OH radical initiated degradation of (<i>E</i>)- and (<i>Z</i>)-CF<sub>3</sub>CHCHCl in the presence of O<sub>2</sub>. In addition, chemically activated isomerization was also observed. Atmospheric local lifetimes of (<i>E</i>)- and (<i>Z</i>)-CF<sub>3</sub>CHCHCl, due to OH reactive loss, were estimated to be ∼34 and ∼11 days, respectively. Infrared absorption spectra measured in this work were used to estimate radiative efficiencies and well-mixed global warming potentials of ∼10 and ∼3 for (<i>E</i>)- and (<i>Z</i>)-CF<sub>3</sub>CHCHCl, respectively, on the 100-year time horizon

    Methyl-Perfluoroheptene-Ethers (CH<sub>3</sub>OC<sub>7</sub>F<sub>13</sub>): Measured OH Radical Reaction Rate Coefficients for Several Isomers and Enantiomers and Their Atmospheric Lifetimes and Global Warming Potentials

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    Mixtures of methyl-perfluoroheptene-ethers (CH<sub>3</sub>OC<sub>7</sub>F<sub>13</sub>, MPHEs) are currently in use as replacements for perfluorinated alkanes (PFCs) and poly-ether heat transfer fluids, which are persistent greenhouse gases with lifetimes >1000 years. At present, the atmospheric processing and environmental impact from the use of MPHEs is unknown. In this work, rate coefficients at 296 K for the gas-phase reaction of the OH radical with six key isomers (including stereoisomers and enantiomers) of MPHEs used commercially were measured using a relative rate method. Rate coefficients for the six MPHE isomers ranged from ∼0.1 to 2.9 × 10<sup>–12</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> with a strong stereoisomer and −OCH<sub>3</sub> group position dependence; the (<i>E</i>)-stereoisomers with the −OCH<sub>3</sub> group in an α- position relative to the double bond had the greatest reactivity. Rate coefficients measured for the <i>d</i><sub>3</sub>-MPHE isomer analogues showed decreased reactivity consistent with a minor contribution of H atom abstraction from the −OCH<sub>3</sub> group to the overall reactivity. Estimated atmospheric lifetimes for the MPHE isomers range from days to months. Atmospheric lifetimes, radiative efficiencies, and global warming potentials for these short-lived MPHE isomers were estimated based on the measured OH rate coefficients along with measured and theoretically calculated MPHE infrared absorption spectra. Our results highlight the importance of quantifying the atmospheric impact of individual components in an isomeric mixture

    Low-Pressure Photolysis of 2,3-Pentanedione in Air: Quantum Yields and Reaction Mechanism

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    Dicarbonyls in the atmosphere mainly arise from secondary sources as reaction products in the degradation of a large number of volatile organic compounds (VOC). Because of their sensitivity to solar radiation, photodissociation of dicarbonyls can dominate the fate of these VOC and impact the atmospheric radical budget. The photolysis of 2,3-pentanedione (PTD) has been investigated for the first time as a function of pressure in a static reactor equipped with continuous wave cavity ring-down spectroscopy to measure the HO<sub>2</sub> radical photostationary concentrations along with stable species. We showed that (i) Stern–Volmer plots are consistent with low OH-radical formation yields in RCO + O<sub>2</sub> reactions, (ii) the decrease of the photodissociation rate due to pressure increase from 26 to 1000 mbar is of about 30%, (iii) similarly to other dicarbonyls, the Stern–Volmer analysis shows a curvature at the lower pressure investigated, which may be assigned to the existence of excited singlet and triplet PTD states, (iv) PTD photolysis at 66 mbar leads to CO<sub>2</sub>, CH<sub>2</sub>O and CO with yields of (1.16 ± 0.04), (0.33 ± 0.02) and (0.070 ± 0.005), respectively, with CH<sub>2</sub>O yield independent of pressure up to 132 mbar and CO yield in agreement with that obtained at atmospheric pressure by Bouzidi et al. (2014), and (v) the PTD photolysis mechanism remains unchanged between atmospheric pressure and 66 mbar. As a part of this work, the O<sub>2</sub> broadening coefficient for the absorption line of HO<sub>2</sub> radicals at 6638.21 cm<sup>–1</sup> has been determined (γ<sub>O2</sub> = 0.0289 cm<sup>–1</sup> atm<sup>–1</sup>)
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