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
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
Rate
coefficients for the gas-phase reaction of the OH radical
with (<i>E</i>)- and (<i>Z</i>)-CF<sub>3</sub>CHî—»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, HCÂ(O)ÂCl, 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>CHî—»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>CHî—»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>CHî—»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
Rate
coefficients for the gas-phase reaction of the OH radical
with (<i>E</i>)- and (<i>Z</i>)-CF<sub>3</sub>CHî—»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, HCÂ(O)ÂCl, 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>CHî—»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>CHî—»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>CHî—»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
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
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>)