Atmospheric Lifetimes of Halogenated Hydrocarbons: Improved Estimations from an Analysis of Modeling Results.

Detailed results of computer modeling of halocarbon removal rates from the atmosphere are analyzed to find simple correlations useful for improving estimations of the atmospheric lifetimes of industrial chemicals based on the rate constants for their reactions with OH and O(1D) and their UV absorption spectra. Ths analysis is limited to relatively long-lived chemicals that are well mixed in the troposphere.

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 trans-1-chloro-3,3,3-trifluoropropene (trans-CHCl═CHCF3): OH reaction rate constant, UV and IR absorption spectra, global warming potential, and ozone depletion potential.

Measurements of the rate constant for the gas-phase reactions of OH radicals with trans-1-chloro-3,3,3-trifluoropropene (trans-CHCl═CHCF3) were performed using a flash photolysis resonance-fluorescence technique over the temperature range 220-370 K. The reaction rate constant exhibits a noticeable curvature of the temperature dependence in the Arrhenius plot, which can be represented by the following expression: kt-CFP (220-370 K) = 1.025 × 10(-13) × (T/298)(2.29) exp(+384/T) cm(3 )molecule(-1) s(-1). The room-temperature rate constant was determined to be kt-CFP (298 K) = (3.29 ± 0.10) × 10(-13) cm(3) molecule(-1) s(-1), where the uncertainty includes both two standard errors (statistical) and the estimated systematic error. For atmospheric modeling purposes, the rate constant below room temperature can be represented by the following expression: kt-CFP (220-298 K) = (7.20 ± 0.46) × 10(-13) exp[-(237 ± 16)/T] cm(3) molecule(-1) s(-1). There was no difference observed between the rate constants determined at 4 kPa (30 Torr) and 40 kPa (300 Torr) at both 298 and 370 K. The UV and IR absorption cross sections of this compound were measured at room temperature. The atmospheric lifetime, global warming potential, and ozone depletion potential of trans-CHCl═CHCF3 were estimated.

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.

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 the reactions between OH and perfluorinated alkenes.

The rate constants for the reactions of OH radicals with fully fluorinated alkenes containing different numbers of -CF(3) groups next to olefinic carbon, CF(2)═CF(2), CF(2)═CFCF(3), CF(3)CF═CFCF(3), and (CF(3))(2)C═CFC(2)F(5), were measured between 230 and 480 K using the flash photolysis resonance fluorescence technique to give the following expressions: k(C(2)F(4))(250-480 K) = 1.32 × 10(-12) × (T/298 K)(0.9) × exp(+600 K/T) cm(3) molecule(-1) s(-1), k(C(3)F(6))(230-480 K) = 9.75 ×10(-14) × (T/298 K)(1.94) × exp(+922 K/T) cm(3) molecule(-1) s(-1), k(trans-C(4)F(8))(230-370 K) = 7.50 × 10(-14) × (T/298 K)(1.68) × exp(+612 K/T) cm(3) molecule(-1) s(-1), k(cis-C(4)F(8))(230-370 K) = 2.99 × 10(-14) × (T/298 K)(2.61) × exp(+760 K/T) cm(3) molecule(-1) s(-1), and k(C(6)F(12))(250-480 K) = 2.17 × 10(-15) × (T/298 K)(3.90) × exp(+1044 K/T) cm(3) molecule(-1) s(-1). The kinetics of the OH reaction in an industrial sample of octofluoro-2-propene (a mixture of the cis- and trans-isomers of CF(3)CF═CFCF(3)) was studied to determine the "effective" reaction rate constant for the typically industrial mixture: k()(230-480 K) = 7.89 × 10(-14) × (T/298 K)(1.71) × exp(+557 K/T) cm(3) molecule(-1) s(-1). On the basis of these results, the atmospheric lifetimes were estimated to be 1.2, 5.3, 21, 34, and 182 days for CF(2)═CF(2), CF(3)CF═CF(2), trans-CF(3)CF═CFCF(3), cis-CF(3)CF═CFCF(3), and (CF(3))(2)C═CFC(2)F(5), respectively. The general pattern of halolalkene reactivity toward OH is discussed.

Rate constant for the reaction of OH with H2 between 200 and 480 K.

The rate constant for the reaction of OH radicals with molecular hydrogen was measured using the flash photolysis resonance-fluorescence technique over the temperature range of 200-479 K. The Arrhenius plot was found to exhibit a noticeable curvature. Careful examination of all possible systematic uncertainties indicates that this curvature is not due to experimental artifacts. The rate constant can be represented by the following expressions over the indicated temperature intervals: k(H2)(250-479 K) = 4.27 x 10(-13) x (T/298)2.406 x exp[-1240/T] cm3 molecule(-1) (s-1) above T = 250 K and k(H2)(200-250 K) = 9.01 x 10(-13) x exp[-(1526 +/- 70)/T] cm3 molecule(-1) s(-1) below T = 250 K. No single Arrhenius expression can adequately represent the rate constant over the entire temperature range within the experimental uncertainties of the measurements. The overall uncertainty factor was estimated to be f(H2)(T) = 1.04 x exp[50 x /(1/T) - (1/298)/]. These measurements indicate an underestimation of the rate constant at lower atmospheric temperatures by the present recommendations. The global atmospheric lifetime of H2 due to its reaction with OH was estimated to be 10 years.