Vladimir L. Orkin

High-accuracy measurements of OH reaction rate constants and IR absorption spectra: CH2=CF-CF3 and trans-CHF=CH-CF3.

Rate constants for the gas phase reactions of OH radicals with two isomers of tetrafluoropropene, CH(2)=CF-CF(3) (k(1)) and trans-CHF=CH-CF(3) (k(2)); were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220 to 370 K. The Arrhenius plots were found to exhibit a noticeable curvature. The temperature dependences of the rate constants are very weak and can be represented by the following expressions over the indicated temperature intervals: k(1)(220-298 K) = 1.145 x 10(-12) x exp{13/T} cm(3) molecule(-1) s(-1), k(1)(298-370 K) = 4.06 x 10(-13) x (T/298)(1.17) x exp{+296/T} cm(3) molecule(-1) s(-1), k(2)(220-370 K) = 1.115 x 10(-13) x (T/298)(2.03) x exp{+522/T} cm(3) molecule(-1) s(-1). The overall accuracy of the rate constant measurements is estimated to be ca. 2% to 2.5% at the 95% confidence level. The uncertainty of the measured reaction rate constants is discussed in detail. The atmospheric lifetimes due to reactions with tropospheric OH were estimated to be 12 and 19 days respectively under the assumption of a well mixed atmosphere. IR absorption cross-sections were measured for both compounds and their global warming potentials were estimated.

Determination of rate constants for reactions of some hydrohaloalkanes with OH radicals and their atmospheric lifetimes

Rate constants have been measured for the gas-phase reactions of hydroxyl radical with partly halogenated alkanes using the discharge-flow-EPR technique over the temperature range 298–460 K. The following Arrhenius expressions have been derived (units 10−13 cm3 molecule−1 s−1): (8.1−1.2+1.5) exp{−(1516±53)/T} for CHF2Cl (HCFC-22); (10.3−1.5+1.8) exp{−(1588±52)/T} for CH2FCF3 (HFC-134a); (11.3−1.6+2.1) exp{−(918±52)/T} for CHCl2CF2Cl (HCFC-122); (9.2−2.0+2.5) exp{−(1281±85)/T} for CHFClCF2Cl (HCFC-123a).The atmospheric lifetimes for the substances have been estimated to be 12.6, 12.9, 1.05, and 4.8 years, respectively, and the accuracy of the estimates is discussed.

High-accuracy measurements of OH(•) reaction rate constants and IR and UV absorption spectra: ethanol and partially fluorinated ethyl alcohols.

Rate constants for the gas phase reactions of OH(•) radicals with ethanol and three fluorinated ethyl alcohols, CH(3)CH(2)OH (k(0)), CH(2)FCH(2)OH (k(1)), CHF(2)CH(2)OH (k(2)), and CF(3)CH(2)OH (k(3)) were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220 to 370 K. The Arrhenius plots were found to exhibit noticeable curvature for all four reactions. The temperature dependences of the rate constants can be represented by the following expressions over the indicated temperature intervals: k(0)(220-370 K) = 5.98 × 10(-13)(T/298)(1.99) exp(+515/T) cm(3) molecule(-1) s(-1), k(0)(220-298 K) = (3.35 ± 0.06) × 10(-12) cm(3) molecule(-1) s(-1) [for atmospheric modeling purposes, k(0)(T) is essentially temperature-independent below room temperature, k(0)(220-298 K) = (3.35 ± 0.06) × 10(-12) cm(3) molecule(-1) s(-1)], k(1)(230-370 K) = 3.47 × 10(-14)(T/298)(4.49) exp(+977/T) cm(3) molecule(-1) s(-1), k(2)(220-370 K) = 3.87 × 10(-14)(T/298)(4.25) exp(+578/T) cm(3) molecule(-1) s(-1), and k(3)(220-370 K) = 2.48 × 10(-14)(T/298)(4.03) exp(+418/T) cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH(•) were estimated to be 4, 16, 62, and 171 days, respectively, under the assumption of a well-mixed atmosphere. UV absorption cross sections of all four ethanols were measured between 160 and 215 nm. The IR absorption cross sections of the three fluorinated ethanols were measured between 400 and 1900 cm(-1), and their global warming potentials were estimated.

Rate constants for reactions of OH radicals with some Br-containing haloalkanes

Rate constants have been measured for the gas-phase reactions of hydroxyl radical with two halons and three of their proposed substitutes and also with CHClBr-CF3 using the discharge-flow-EPR technique over the temperature range 298–460 K. The following Arrhenius expressions have been derived (units are 10−13 cm3 molecule−1 s−1): (9.3−0.9+1.0) exp{−(1326±33)/T} for CHF2Br; (7.2−0.6+0.7) exp{−(1111±32)/T} for CHFBrCF3; (8.5−0.8+0.9) exp{−(1113±35)/T} for CH2BrCF3; (12.8−1.2+1.5) exp{−(995±38)/T} for CHClBrCF3. The rate constants at 298 K have been estimated to be <2×10−17 cm3 molecule−1 s−1 for CF3Br and CF2Br—CF2Br. The atmospheric lifetimes due to hydroxyl attack have been estimated to be 5.5, 3.3, 2.8, and 1.2 years for CHF2Br, CHFBr—CF3, CH2Br—CF3 and CHClBr—CF3, respectively.

Measurements of rate constants for the OH reactions with bromoform (CHBr3), CHBr2Cl, CHBrCl2, and epichlorohydrin (C3H5ClO).

Measurements of the rate constants for the gas phase reactions of OH radicals with bromoform (CHBr3) and epichlorohydrin (C3H5ClO) were performed using a flash photolysis resonance-fluorescence technique over the temperature range 230-370 K. The temperature dependences of the rate constants can be represented by the following expressions: kBF(230-370 K) = (9.94 ± 0.76) × 10(-13) exp[-(387 ± 22)/T] cm(3) molecule(-1) s(-1) and kECH(230-370 K) = 1.05 × 10(-14)(T/298)(5.16) exp(+1082/T) cm(3) molecule(-1) s(-1). Rate constants for the reactions of OH with CHCl2Br and CHClBr2 were measured between 230 and 330 K. They can be represented by the following expressions: kDCBM(230-330 K) = (9.4 ± 1.3) × 10(-13) exp[-(513 ± 37)/T] cm(3) molecule(-1) s(-1) and kCDBM(230-330 K) = (9.0 ± 1.9) × 10(-13) exp[-(423 ± 61)/T] cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH were estimated to be 57, 39, 72, and 96 days, respectively. The total atmospheric lifetimes of the Br-containing methanes due to both reaction with OH and photolysis were calculated to be 22, 50, and 67 days for CHBr3, CHClBr2, and CHCl2Br, respectively.

High Accuracy Measurements of OH Reaction Rate Constants and IR Absorption Spectra: Substituted 2-Propanols

Rate constants for the gas phase reactions of OH radicals with 2-propanol and three fluorine substituted 2-propanols, (CH(3))(2)CHOH (k(0)), (CF(3))(2)CHOH (k(1)), (CF(3))(2)C(OH)CH(3) (k(2)), and (CF(3))(3)COH (k(3)), were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220-370 K. The Arrhenius plots were found to exhibit noticeable curvature for all four reactions. The temperature dependences of the rate constants can be represented by the following expressions: k(0)(T) = 1.46 × 10(-11) exp{-883/T} + 1.30 × 10(-12) exp{+371/T} cm(3) molecule(-1) s(-1); k(1)(T) = 1.19 × 10(-12) exp{-1207/T} + 7.85 × 10(-16) exp{+502/T } cm(3) molecule(-1) s(-1); k(2)(T) = 1.68 × 10(-12) exp{-1718/T} + 7.32 × 10(-16) exp{+371/T} cm(3) molecule(-1) s(-1); k(3)(T) = 3.0 × 10(-20) × (T/298)(11.3) exp{+3060/T} cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH were estimated to be 2.4 days and 1.9, 6.3, and 46 years, respectively. UV absorption cross sections were measured between 160 and 200 nm. The IR absorption cross sections of the three fluorinated compounds were measured between 450 and 1900 cm(-1), and their global warming potentials were estimated.

Deriving Global OH Abundance and Atmospheric Lifetimes for Long-Lived Gases: A Search for CH3CCl3 Alternatives

An accurate estimate of global hydroxyl radical (OH) abundance is important for projections of air quality, climate, and stratospheric ozone recovery. As the atmospheric mixing ratios of methyl chloroform (CH₃CCl₃) (MCF), the commonly used OH reference gas, approaches zero, it is important to find alternative approaches to infer atmospheric OH abundance and variability. The lack of global bottom‐up emission inventories is the primary obstacle in choosing a MCF alternative. We illustrate that global emissions of long‐lived trace gases can be inferred from their observed mixing ratio differences between the Northern Hemisphere (NH) and Southern Hemisphere (SH), given realistic estimates of their NH‐SH exchange time, the emission partitioning between the two hemispheres, and the NH versus SH OH abundance ratio. Using the observed long‐term trend and emissions derived from the measured hemispheric gradient, the combination of HFC‐32 (CH₂F₂), HFC‐134a (CH₂FCF₃, HFC‐152a (CH₃CHF₂), and HCFC‐22 (CHClF₂), instead of a single gas, will be useful as a MCF alternative to infer global and hemispheric OH abundance and trace gas lifetimes. The primary assumption on which this multispecies approach relies is that the OH lifetimes can be estimated by scaling the thermal reaction rates of a reference gas at 272 K on global and hemispheric scales. Thus, the derived hemispheric and global OH estimates are forced to reconcile the observed trends and gradient for all four compounds simultaneously. However, currently, observations of these gases from the surface networks do not provide more accurate OH abundance estimate than that from MCF.

Photochemical properties of some Cl-containing halogenated alkanes.

Rate constants for the gas-phase reactions of OH radicals with three partially halogenated alkanes, CH3Cl (kMC), CHFClCFCl2 (k122a), and CH2FCFCl2 (k132c), were measured using a discharge flow-electron paramagnetic resonance technique over the temperature range from 298 to 460 K. The temperature dependences of the rate constants can be represented by the expressions kMC(298-460 K) = (3.09 ± 0.94) × 10(-12) exp[-(1411 ± 85)/T] cm(3) molecule(-1) s(-1), k122a(298-460 K) = (1.26 ± 0.24) × 10(-12) exp[-(1298 ± 66)/T] cm(3) molecule(-1) s(-1), and k132c(298-370 K) = (8.1 ± 2.2) × 10(-13) exp[-(1247 ± 89)/T] cm(3) molecule(-1) s(-1). The atmospheric lifetimes of CH3Cl, CHFClCFCl2, and CH2FCFCl2 due to their reaction with OH were estimated to be 1.6, 3.5, and 4.5 years, respectively. The UV absorption cross sections of halogenated ethanes, CHFClCFCl2, and CH2FCFCl2, were measured at T = 295 K between 190 and 240 nm, as were those for CHCl2CF2Cl (HCFC-122), CHCl2CF3 (HCFC-123), CHFClCF2Cl (HCFC-123a), and CH3CFCl2 (HCFC-141b). The atmospheric lifetimes due to stratospheric photolysis were also estimated.

Photochemical Properties of CH2═CH-CFCl-CF2Br (4-Bromo-3-chloro-3,4,4-trifluoro-1-butene) and CH3-O-CH(CF3)2 (Methyl Hexafluoroisopropyl Ether): OH Reaction Rate Constants and UV and IR Absorption Spectra

Rate constants for the reactions of hydroxyl radicals (OH) with 1,1,1,3,3,3-hexafluoroisopropyl methyl ether (CH3-O-CH(CF3)2) and 4-bromo-3-chloro-3,4,4-trifluoro-1-butene (CH2═CH-CFCl-CF2Br) have been measured over the temperature range 230-370 K to give the following Arrhenius expressions: kCH3OCH(CF3)2(T) = 7.69 × 10-14 × (T/298)2.99 × exp(+342/T), cm3 molecule-1 s-1, and kCH2CHCFClCF2Br(T) = (6.45 ± 0.72) × 10-13 × exp{+(424 ± 32)/T}, cm3 molecule-1 s-1. Atmospheric lifetimes of compounds were estimated to be 67 days and 4.5 days, respectively. UV absorption spectrum of CH2═CH-CFCl-CF2Br between 164 and 260 nm and IR absorption spectra of both compounds between 450 and 1600 cm-1 were measured at room temperature.

Photochemical properties of hydrofluoroethers CH3OCHF2, CH3OCF3, and CHF2OCH2CF3: reactivity toward OH, IR absorption cross sections, atmospheric lifetimes, and global warming potentials.

Rate constants for the gas phase reactions of OH radicals with three partially fluorinated ethers, CH3OCF3 (kHFE-143a), CH3OCHF2 (kHFE-152a), and CHF2OCH2CF3 (kHFE-245fa2), were measured using a discharge flow-electron paramagnetic resonance technique over the temperature range 298-460 K. The temperature dependences of the rate constants can be represented by the following expressions: kHFE-143a(T) = (1.10 ± 0.20) × 10(-12) × exp{-(1324 ± 61)/T} cm(3) molecule(-1) s(-1); kHFE-152a(T) = (11.6 ± 4.2) × 10(-12) × exp{-(1728 ± 133)/T} cm(3) molecule(-1) s(-1); and kHFE-245fa2(T) = (3.04 ± 0.57) × 10(-12) × exp{-(1665 ± 66)/T} cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH were estimated to be 5.2, 1.9, and 5.6 years, respectively. The IR absorption cross sections of these fluorinated ethers were measured between 400 and 2000 cm(-1), and their global warming potentials were estimated.