Rate Coefficients For Reactions Of OH Research Articles

Rate coefficients for the gas‐phase OH + furan (C4H4O) reaction between 273 and 353 K

AbstractRate coefficients, k1, for the gas‐phase OH radical reaction with the heterocyclic ether C4H4O (1,4‐epoxybuta‐1,3‐diene, furan) were measured over the temperature range 273–353 K at 760 Torr (syn. air). Experiments were performed using: (i) the photochemical smog chamber THALAMOS (thermally regulated atmospheric simulation chamber, IMT NE, Douai‐France) equipped with Fourier Transform Infrared (FTIR) and Selected Ion Flow Tube Mass Spectrometry (SIFT‐MS) detection methods and (ii) a photochemical reactor coupled with FTIR spectroscopy (PCR, University of Crete, Greece). k1(273–353 K) was measured using a relative rate (RR) method, in which the loss of furan was measured relative to the loss of reference compounds with well‐established OH reaction rate coefficients. k1(273–353 K) was found to be well represented by the Arrhenius expression (1.30 ± 0.12) × 10−11 exp[(336 ± 20)/T] cm3 molecule−1 s−1, with k1(296 K) measured to be (4.07 ± 0.32) × 10−11 cm3 molecule−1 s−1. The k1(296 K) and pre‐exponential quoted error limits are 2σ and include estimated systematic errors in the reference rate coefficients. The observed negative temperature dependence is consistent with a reaction mechanism involving the OH radical association to a furan double bond. Quantum mechanical molecular calculations show that OH addition to the α‐carbon (ΔHr(296 K) = −121.5 kJ mol−1) is thermochemically favored over the β‐carbon (ΔHr(296 K) = −52.9 kJ mol−1) addition. The OH‐furan adduct was found to be stable over the temperature range of the present measurements. Maleic anhydride (C4H2O3) was identified as a minor reaction product, 3% lower‐limit yield, demonstrating a non‐ring‐opening active reaction channel. The present results are critically compared with results from previous studies of the OH + furan reaction rate coefficient. The infrared spectrum of furan was measured as part of this study and its estimated climate metrics are reported.

OH reaction rate coefficients, infrared spectra, and climate metrics for (E)‐ and (Z)‐ 2‐perfluoroheptene (2‐C7F14) and 3‐perfluoroheptene (3‐C7F14)

AbstractIn this work, perfluoroheptene 2‐ and 3‐C7F14 stereoisomer specific gas‐phase OH reaction rate coefficients, k, were measured at 296 K in ∼600 Torr (He bath gas) using a relative rate (RR) method. Gas‐chromatography (GC) with electron capture detection (ECD) was used for the separation and detection of the stereoisomers. Rate coefficients for (E)‐2‐C7F14, (Z)‐2‐C7F14, (E)‐3‐C7F14, and (Z)‐3‐C7F14 were measured to be (in units of 10−13 cm3 molecule−1 s−1) (3.60 ± 0.51), (2.22 ± 0.21), (3.43 ± 0.47), and (1.48 ± 0.19), respectively, where the uncertainties include estimated systematic errors. Rate coefficients for the (E)‐ stereoisomers were found to be systematically greater than the (Z)‐ stereoisomers by a factor of 1.6 and 2.3 for 2‐C7F14 and 3‐C7F14, respectively. Atmospheric lifetimes with respect to OH radical reaction for (E)‐2‐C7F14, (Z)‐2‐C7F14, (E)‐3‐C7F14, (Z)‐3‐C7F14 were estimated to be ∼33, ∼56, ∼36, and ∼86 days, respectively, for an average OH radical concentration of 1 × 106 molecule cm−3. Quantitative infrared absorption spectra were measured as part of this work. Complimentary theoretically calculated infrared absorption spectra using density functional theory (DFT) are included in this work. The theoretical spectra were used to evaluate stereoisomer climate metrics. Radiative efficiencies (adjusted) and global warming potentials (GWPs, 100‐year time‐horizon), were estimated to be 0.12, 0.19, 0.12, and 0.23 W m−2 ppb−1 and 1.9, 5.1, 2.1, 9.3 for (E)‐2‐C7F14, (Z)‐2‐C7F14, (E)‐3‐C7F14, (Z)‐3‐C7F14, respectively. Atmospheric degradation mechanisms are discussed.

Atmospheric Chemistry of c-C5HF7 and c-C5F8: Temperature-Dependent OH Reaction Rate Coefficients, Degradation Products, Infrared Spectra, and Global Warming Potentials.

c-C5HF7 (1H-heptafluorocyclopentene) and c-C5F8 (perfluorocyclopentene) are potent greenhouse gases presently used as replacement compounds in Si etching. A thorough understanding of their potential impact on climate and air quality necessitates studies of their atmospheric reactivity, radiative properties, and atmospheric degradation pathways. The predominant atmospheric removal process for these compounds is expected to be via reaction with the OH radical. In this study, rate coefficients, k, for the gas-phase reaction of the OH radical with c-C5HF7 and c-C5F8 were measured over a range of temperatures (242-370 K) and pressures (50-100 Torr, He) using a pulsed laser photolysis-laser-induced fluorescence technique. In addition, a complementary relative rate technique, employing multiple reference compounds, was used to study the reactions between 273 and 372 K at 100 Torr (He) total pressure. Reaction rate coefficients were found to be independent of pressure over this range of conditions with k1(296 K) = (4.59 ± 0.10) × 10-14 cm3 molecule-1 s-1 and k1(T) = (4.00 ± 0.40) × 10-13 exp(-(631 ± 30)/T) cm3 molecule-1 s-1 for c-C5HF7 and k2(296 K) = (4.90 ± 0.14) × 10-14 cm3 molecule-1 s-1 and k2(T) = (3.59 ± 0.4) × 10-13 exp(-(591 ± 25)/T) cm3 molecule-1 s-1 for c-C5F8. Stable end-products were measured following the OH radical-initiated degradation of c-C5HF7 and c-C5F8 in the presence of O2. F(O)CCF2CF2CF2CH(O), CF2O, and CO2 were observed as the major end-products in the oxidation of c-C5HF7 with molar yields of 0.64, 1.27, and 0.53, respectively. For c-C5F8, F(O)CCF2CF2CF2CF(O), CF2O, and CO2 were observed with molar yields of 0.66, 0.63, and 0.43, respectively. The total carbon mass balance in both systems was 1.0 ± 0.15. The high yield of a C5-dicarbonyl end-product is consistent with a ring opening at the carbon-carbon double bond site for both c-C5HF7 and c-C5F8. A comparison of the present kinetic and degradation product results with previously published studies is presented. A rate coefficient upper limit for the gas-phase reaction of O3 with c-C5HF7 and c-C5F8 of 1 × 10-21 cm3 molecule-1 s-1 was measured as part of this work. Atmospheric lifetimes for c-C5HF7 and c-C5F8 are estimated to be 252 and 236 days, respectively. Infrared absorption spectra of c-C5HF7 and c-C5F8 were also measured and found to agree, to within 5%, with results from previous studies. The well-mixed and lifetime adjusted radiative efficiencies (RE, W m-2 ppb-1) and 100 year time horizon global warming potential (GWP) for c-C5HF7 are 0.35, 0.24, and 46.7 and for c-C5F8 are 0.38, 0.25, and 46.2, respectively.

(CH3)3COOH (tert-butyl hydroperoxide): OH reaction rate coefficients between 206 and 375 K and the OH photolysis quantum yield at 248 nm

Rate coefficients, k, for the gas-phase reaction of the OH radical with (CH(3))(3)COOH (tert-butyl hydroperoxide) were measured as a function of temperature (206-375 K) and pressure (25-200 Torr (He, N(2))). Rate coefficients were measured under pseudo-first-order conditions using pulsed laser photolysis to produce OH and laser induced fluorescence (PLP-LIF) to measure the OH temporal profile. Two independent methods were used to determine the gas-phase infrared cross sections of (CH(3))(3)COOH, absolute pressure and chemical titration, that were used to determine the (CH(3))(3)COOH concentration in the LIF reactor. The temperature dependence of the rate coefficients is described, within the measurement precision, by the Arrhenius expression k(1)(T) = (7.0 ± 1.0) × 10(-13) exp[(485 ± 20)/T] cm(3) molecule(-1) s(-1) where k(1)(296 K) was measured to be (3.58 ± 0.54) × 10(-12) cm(3) molecule(-1) s(-1). The uncertainties are 2σ (95% confidence level) and include estimated systematic errors. UV absorption cross sections of (CH(3))(3)COOH were determined at 185, 214, 228, and 254 nm and over the wavelength range 210-300 nm. The OH quantum yield following the 248 nm pulsed laser photolysis of (CH(3))(3)COOH was measured relative to the OH quantum yields of H(2)O(2) and HNO(3) using PLP-LIF and found to be near unity.

A study of the IR and UV-Vis absorption cross-sections, photolysis and OH-initiated oxidation of CF3CHO and CF3CH2CHO

Infrared and ultraviolet-visible absorption cross-sections, effective quantum yields of photolysis and OH reaction rate coefficients for CF3CHO and CF3CH2CHO are reported. Relative rate measurements at 298(2) K and 1013(10) hPa, give k(OH + CF3CHO)/k(OH + CH3CH3) = 2.00(13), k(OH + CF3CH2CHO)/k(OH + CH3CH2OH) = 1.21(5) and k(OH + CF3CH2CHO)/k(OH + HC(O)OC2H5) = 3.51(9) (2σ). The effective quantum yield of photolysis was measured under pseudo-natural conditions in the European simulation chamber, Valencia, Spain (EUPHORE). Over the wavelength range 290–400 nm, the effective quantum yields of photolysis for CF3CHO and CF3CH2CHO are less than 2 × 10−2 and 4 × 10−2, respectively. The tropospheric lifetimes are estimated to be: τOH(CF3CHO) ∼ 26 days; τphotol(CF3CHO) > 27 days; τOH(CF3CH2CHO) ∼ 4 days; τphotol(CF3CH2CHO) > 15 days.

Direct measurements of urban OH reactivity during Nashville SOS in summer 1999.

Emissions of volatile chemicals control the hydroxyl radical (OH), the atmosphere's main cleansing agent, and thus the production of secondary pollutants. Accounting for all of these chemicals can be difficult, especially in environments with mixed urban and forest emissions. The first direct measurements of the atmospheric OH reactivity, the inverse of the OH lifetime, were made as part of the Southern Oxidant Study (SOS) at Cornelia Fort Airpark in Nashville, TN in summer 1999. Measured OH reactivity was typically 11 s(-1). Measured OH reactivity was 1.4 times larger than OH reactivity calculated from the sum of the products of measured chemical concentrations and their OH reaction rate coefficients. This difference is statistically significant at the 1sigma uncertainty level of both the measurements and the calculations but not the 2sigma uncertainty level. Measured OH reactivity was 1.3 times larger than the OH reactivity from a model that uses measured ambient concentrations of volatile organic compounds (VOCs), NO, NO2, SO2, and CO. However, it was within approximately 10% of the OH reactivity from a model that includes hydrocarbon measurements made in a Nashville tunnel and scaled to the ambient CO at Cornelia Fort Airpark. These comparisons indicate that 30% of the OH reactivity in Nashville may come from short-lived highly reactive VOCs that are not usually measured in field intensive studies or by US EPA's Photochemical Assessment Monitoring Stations.

Atmospheric degradation and global warming potentials of three perfluoroalkenes

The vapour phase reactions of perfluoropropene, CF 3CFCF 2, and perfluorobuta-1,3-diene, CF 2CFCFCF 2, with OH, NO 3 and O 3 were studied at 298±4 K and 740±5 Torr using long-path FT-IR detection. The reactions with ozone are very slow, k CF 3CFCF 2+O 3 =(6.2±1.5)×10 −22 and k CF 2CFCFCF 2+O 3 =(6.5±0.2)×10 −21 cm 3 molecules −1 s −1, and upper limits of 3×10 −15 cm 3 molecules −1 s −1 are reported for the NO 3 reaction rate coefficients. The OH reaction rate coefficients were determined as k CF 3CFCF 2+OH =(2.6±0.7)×10 −12 and k CF 2CFCFCF 2+OH =(1.1±0.3)×10 −11 cm 3 molecules −1 s −1; perfluoropropene gave a nearly quantitative yield of CF 3CFO and CF 2O as organic products, while perfluorobuta-1,3-diene gave from 130% to 170% of CF 2O. A chemistry transport model was applied to calculate the atmospheric distributions and lifetimes of the perfluoroalkenes; the global and yearly averaged lifetimes were calculated as 1.9 day for C 2F 4 and C 4F 6 and 6 days for C 3F 6. Quantitative infrared cross-sections of perfluoroethene, perfluoropropene, and perfluorobuta-1,3-diene have been obtained at 298 K in the region 100–2600 cm −1. Radiative forcing calculations have been performed for these gases assuming either constant vertical profiles or the distribution derived from the chemistry transport model. The results show that the Global Warming Potentials are totally negligible for these compounds.

Atmospheric fate of CF3Br, CF2Br2, CF2ClBr, and CF2BrCF2BR

The temperature dependent UV absorption cross sections of the haloalkanes CF3Br, CF2ClBr, CF2Br2, and CF2BrCF2Br are reported. The UV absorption cross sections were measured over the temperature range 210 to 296 K and the wavelength range 190 to 320 run. Upper limits of the rate coefficients for the reactions of OH with CF3Br, CF2Br2, CF2ClBr, and CF2BrCF2Br were also determined using both pulsed photolysis and flow tube techniques. The rate coefficients at 296 K were found to be <1.2×10−16, <1.5×10−16, <5.0×10−16, and <1.5×10−16 cm3 molec−1 s−1 for CF3Br, CF2ClBr, CF2Br2, and CF2BrCF2Br, respectively. The UV absorption cross‐section data and OH reaction rate coefficients of these species were combined with a one‐dimensional model to yield atmospheric lifetimes of 65, 16, 3.2, and <20 years for CF3Br, CF2ClBr, CF2Br2, and CF2BrCF2Br, respectively.

Revised predictions of the effect on stratospheric ozone of increasing atmospheric N 2O and chlorofluoromethanes: A two-dimensional model study

The perturbation of stratospheric ozone caused by a doubling of atmospheric N 2O and by an increase of stratospheric chlorine of about 0.9 ppbv due to continued release of chlorofluoromethanes is simulated with the aid of a two-dimensional photochemical model and new measurements of OH reaction rate coefficients. The global mean O 3 decrease is revised upward from about 8.5% to about 14% for N 2O doubling, but is reduced from about 1.6% to about 0.8% for 0.9 ppbv chlorine increase. The latitude variation of the O 3 perturbations as well as predictions of stratospheric OH abundance are also discussed.