Total Pressure Of Synthetic Air Research Articles

Kinetic study of the atmospheric oxidation of a series of epoxy compounds by OH radicals

Abstract. In this work, we study the kinetics of the gas-phase reactions of hydroxyl radicals with cyclohexene oxide (CHO), 1,2-epoxyhexane (EHX), 1,2-epoxybutane (12EB), trans-2,3-epoxybutane (tEB) and cis-2,3-epoxybutane (cEB) using the relative rate technique. The experiments were conducted at (298 ± 3) K and (760 ± 10) Torr ((1.01 ± 0.01) × 105 Pa) total pressure of synthetic air using different reference compounds in a 1080 L Quartz Reactor (QUAREC) and a 480 L Duran glass chamber. The following room temperature rate coefficients (cm3 molecule−1 s−1) were obtained: k1(OH+CHO)=(5.93±1.13)×10-12, k2(OH+EHX)=(5.77±0.83)×10-12, k3(OH+12EB)=(1.98±0.29)×10-12, k4(OH+cEB)=(1.50±0.28)×10-12 and k5(OH+tEB)=(1.81±0.33)×10-12. Except for previous studies on 1,2-epoxybutane and cyclohexene oxide, this is, to the best of our knowledge, the first kinetic study of the reaction of these compounds with OH radicals. We discuss the discrepancies found between the values obtained from the present study with values estimated from the structure–activity relationship method (SAR). Our findings indicate that pseudo-ethylenic character in the epoxy ring is an important factor to be included in the improvement of the SAR estimation method. Atmospheric lifetimes, reactivity trends and atmospheric implications are discussed considering the epoxy compound rate coefficients obtained in the present study.

Rate Coefficients for the Gas-Phase Reactions of Hydroxyl Radicals with a Series of Methoxylated Aromatic Compounds.

Rate coefficients for the reactions of hydroxyl radicals (OH) with a series of oxygenated aromatics (two methoxybenzene and five methoxyphenol isomers) have been obtained using the relative kinetic method in 1080 and 480 L photoreactors at the University of Wuppertal, Germany. The experiments were realized at 295 ± 2 K and 1 bar total pressure of synthetic air using in situ Fourier transform infrared spectroscopy for the chemical analysis. The following rate coefficients (in units of cm(3) molecule(-1) s(-1)) were determined: methoxybenzene (anisole), (2.08 ± 0.21) × 10(-11); 1-methoxy-2-methylbenzene, (4.56 ± 0.50) × 10(-11); 2-methoxyphenol (guaiacol), (5.40 ± 0.72) × 10(-11); 3-methoxyphenol, (6.93 ± 0.67) × 10(-11); 4-methoxyphenol, (5.66 ± 0.55) × 10(-11); 2-methoxy-4-methylphenol, (7.51 ± 0.68) × 10(-11); 2,3-dimethoxyphenol, (7.49 ± 0.81) × 10(-11); and 2,6-dimethoxyphenol (syringol), (8.10 ± 0.98) × 10(-11). The rate coefficients for the reactions of OH with 2,3-dimethoxyphenol and 1-methoxy-2-methylbenzene are first time measurements. The rate coefficients determined in this work are compared with previous determinations reported in the literature and also with the values estimated using a structure-activity relationship method. A comparison is performed between the OH rate coefficients obtained for methoxylated aromatics with those of other substituted aromatics in order to understand the influence of the type, number, and position of the different substituents on the reactivity of aromatics toward OH. In addition, a comparison is made between the OH and Cl rate coefficients for the compounds. The principal atmospheric sink of these methoxylated aromatic compounds during daytime is their reaction with OH radicals. The corresponding lifetimes for reaction with OH radicals and Cl atoms are 2-8 and 11-50 h, respectively.

Kinetics of the gas-phase reaction between ozone and three unsaturated oxygenated compounds: Ethyl 3,3-dimethyl acrylate, 2-methyl-2-pentenal and 6-methyl-5-hepten-2-one at atmospheric pressure

Rate coefficients for the gas-phase reactions of O3 molecules with three unsaturated oxygenated compounds have been determined using the relative kinetic technique in an environmental chamber with FTIR detection of the reactants at (298 ± 2) K in 760 Torr total pressure of synthetic air. The following rate coefficients (in units of 10−17 cm3 molecule−1 s−1) were determined: ethyl 3,3-dimethyl acrylate (0.82 ± 0.19), 2-methyl-2-pentenal (0.71 ± 0.16) and 6-methyl-5-hepten-2-one (26 ± 7).The different reactivity of the unsaturated oxygenated compounds toward O3 is discussed in terms of their chemical structure. In addition, a correlation between the reactivity of structurally different unsaturated compounds (alkenes and unsaturated oxygenated VOCs, such as ethers, esters, aldehydes, ketones and alcohols) toward O3 molecules and the HOMO (Highest Occupied Molecular Orbital) of the compounds is presented.Using the kinetic parameters determined in this work, residence times of these unsaturated compounds in the atmosphere with respect to reaction with O3 have been calculated. In urban and rural areas the main sink of 6-methyl-5-hepten-2-one is reaction with O3 molecules with a residence time in the order of few minutes.

Rate Coefficients for the Gas-Phase Reaction of Chlorine Atoms with a Series of Methoxylated Aromatic Compounds

The reaction of a series of oxygenated aromatics (two methoxybenzene and six methoxyphenol isomers) with chlorine atoms has been studied in two simulation chambers with volumes of 1080 and 480 L at the University of Wuppertal. Experiments were performed at 295 ± 2 K and a total pressure of synthetic air of 1 bar using the relative kinetic method with in situ Fourier transform infrared spectroscopy for chemical analysis. The following rate coefficients (in units of cubic centimeter per molecule per second) were determined: (1.07 ± 0.24) × 10(-10) for methoxybenzene, (1.20 ± 0.24) × 10(-10) for 1-methoxy-2-methylbenzene, (2.97 ± 0.66) × 10(-10) for 2-methoxyphenol (guaiacol), (2.99 ± 0.62) × 10(-10) for 3-methoxyphenol, (2.86 ± 0.58) × 10(-10) for 4-methoxyphenol, (3.35 ± 0.68) × 10(-10) for 2-methoxy-4-methylphenol, (4.73 ± 1.06) × 10(-10) for 2,3-dimethoxyphenol, and (2.71 ± 0.61) × 10(-10) for 2,6-dimethoxyphenol (syringol). To the best of our knowledge, this work represents the first determination of the rate coefficients for the gas-phase reaction of the chlorine atoms with the methoxy-aromatic compounds investigated. The reactivity of the methoxylated aromatics toward Cl is compared with that of other substituted aromatic compounds, and the differences in the rate coefficients are interpreted in terms of the type, number, and position of the different substituents on the aromatic ring. The atmospheric implications of the studied reactions are also discussed.

Gas-phase reactivity study of a series of hydrofluoroolefins (HFOs) toward OH radicals and Cl atoms at atmospheric pressure and 298 K

In order to assess the environmental impact of possible candidates such as hydrofluoroolefins to replace the CFCs in the industry, it is necessary to perform kinetic and product studies of their degradation in the air initiated by the main tropospheric oxidants. In this sense, the relative-rate technique has been used to determine rate coefficients for the reactions of 2-fluoropropene (CH3CFCH2); 3,3,3-trifluoro-2-(trifluoromethyl)propene ((CF3)2CCH2) and (E/Z)-1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CFCHF) with OH radicals and Cl atoms at (298 ± 3) K and (760 ± 10) Torr total pressure of synthetic air using different reference compounds.The experiments were performed in an environmental chamber with “in situ” FTIR spectroscopy to monitor the concentration–time profiles of the reactants. The following room temperature rate coefficients (in cm3 molecule−1 s−1) were obtained: k1(OH + CH3CFCH2) = (1.55 ± 0.39) × 10−11, k2 (OH + (CF3)2CCH2) = (6.58 ± 2.25) × 10−13, k3 (OH + ((E/Z)-CF3CFCHF); 93% E isomer) = (2.62 ± 0.76) × 10−12, k4 (Cl + CH3CFCH2) = (1.64 ± 0.26) × 10−10, k5 (Cl + (CF3)2CCH2) = (3.50 ± 0.85) × 10−11 and k6 (Cl + ((E/Z)-CF3CFCHF); 93% E isomer) = (4.52 ± 0.98) × 10−11. To the best of our knowledge this work provides the first kinetic study for the reactions of CH3CFCH2 and (CF3)2CCH2 with OH radicals and Cl atoms.Free-energy relationships are presented, and the effect of F atom and CF3 group substitution in the hydrofluoroolefins is discussed in terms of reactivity toward both OH radicals and Cl atoms.On the basis of the kinetic measurements, tropospheric lifetimes of CH3CFCH2, (CF3)2CCH2 and (E/Z)-CF3CFCHF will primarily be determined by their reaction with hydroxyl radicals with estimated lifetimes of approximately 1, 18 and 4 days, respectively. For the reactions of the fluoroalkenes with chlorine atoms the corresponding values of the lifetimes are 14, 66 and 51, respectively.

Atmospheric Fate of a Series of Carbonyl Nitrates: Photolysis Frequencies and OH-Oxidation Rate Constants

Multifunctional organic nitrates are potential NO(x) reservoirs whose atmospheric chemistry is somewhat little known. They could play an important role in the spatial distribution of reactive nitrogen species and consequently in ozone formation and distribution in remote areas. In this work, the rate constants for the reaction with OH radical and the photolysis frequencies of α-nitrooxyacetone, 3-nitrooxy-2-butanone, and 3-methyl-3-nitrooxy-2-butanone have been determined at room temperature at 1000 mbar total pressure of synthetic air. The rate constants for the OH oxidation were measured using the relative rate technique, with methanol as reference compound. The following rate constants were obtained for the reaction with OH: k(OH) = (6.7 ± 2.5) × 10(-13) cm(3) molecule(-1) s(-1) for α-nitrooxyacetone, (10.6 ± 4.1) × 10(-13) cm(3) molecule(-1) s(-1) for 3-nitrooxy-2-butanone, and (2.6 ± 0.9) × 10(-13) cm(3) molecule(-1) s(-1) for 3-methyl-3-nitrooxy-2-butanone. The corresponding photolysis frequencies extrapolated to typical atmospheric conditions for July first at noon at 40° latitude North were (4.8 ± 0.3) × 10(-5) s(-1), (5.7 ± 0.3) × 10(-5) s(-1), and (7.4 ± 0.2) × 10(-5) s(-1), respectively. The data show that photolysis is a major atmospheric sink for these organic nitrates.

Gas-phase reactivity study of (E)-3-pentenenitrile and 4-pentenenitrile towards OH radicals and Cl atoms at atmospheric pressure

The relative-rate technique has been used to obtain rates coefficients for the reactions of (E)-3-pentenenitrile (CH3CHCHCHCN) and 4-pentenenitrile (CH2CHCH2CH2CN) with OH radicals and Cl atoms at 298 K and (760 ± 10) Torr total pressure of synthetic air using different reference compounds. The experiments were performed in an environmental chamber with “in situ” FTIR spectroscopy to monitor the reactants concentrations. The following room temperature rate coefficients (in cm3 molecule−1 s−1) were obtained: k1 (OH + CH3CHCHCHCN) = (5.05 ± 1.10) × 10−11, k2 (Cl + CH3CHCHCHCN) = (2.60 ± 0.26) × 10−10, k3 (OH + CH2CHCH2CH2CN) = (2.90 ± 0.64) × 10−11 and k4 (Cl + CH2CHCH2CH2CN) = (2.56 ± 0.26) × 10−10. This is the first kinetic study for all of these reactions. The effect of atom or group substituent on the overall rate coefficients is analyzed and the reactivity of nitriles is compared with those of other unsaturated compounds in the literature. In addition, a comparison between the experimentally determined rate coefficients from this work and the kOH and kCl predicted from k vs EHOMO correlations is presented.The atmospheric lifetimes at room temperature of these compounds has been estimated using the rate coefficients determined in this work.

Photochemistry and Photophysics of n-Butanal, 3-Methylbutanal, and 3,3-Dimethylbutanal: Experimental and Theoretical Study

Dilute mixtures of n-butanal, 3-methylbutanal, and 3,3-dimethylbutanal in synthetic air, different N(2)/O(2) mixtures, and pure nitrogen (up to 100 ppm) were photolyzed with fluorescent UV lamps (275-380 nm) at 298 K. The main photooxidation products were ethene (n-butanal), propene (3-methylbutanal) or i-butene (3,3-dimethylbutanal), CO, vinylalcohol, and ethanal. The photolysis rates and the absolute quantum yields were found to be dependent on the total pressure of synthetic air but not of nitrogen. At 100 Torr, the total quantum yield Φ(100) = 0.45 ± 0.01 and 0.49 ± 0.07, whereas at 700 Torr, Φ(700) = 0.31 ± 0.01 and 0.36 ± 0.03 for 3-methylbutanal and 3,3-dimethybutanal, respectively. Quantum yield values for n-butanal were reported earlier by Tadić et al. (J. Photochem. Photobiol. A2001143, 169-179) to be Φ(100) = 0.48 ± 0.02 and Φ(700) = 0.32 ± 0.01. Two decomposition channels were identified: the radical channel RCHO → R + HCO (Norrish type I) and the molecular channel CH(3)CH(CH(3))CH(2)CHO → CH(2)CHCH(3) + CH(2)═CHOH or CH(3)C(CH(3))(2)CH(2)CHO → CHC(CH(3))CH(3) + CH(2)═CHOH, (Norrish type II) having the absolute quantum yields of 0.123 and 0.119 for 3-methybutanal and 0.071 and 0.199 for 3,3-dimethylbutanal at 700 Torr of synthetic air. The product ethenol CH(2)═CHOH tautomerizes to ethanal. We have performed ab initio and density functional quantum (DFT) chemical computations of both type I and type II processes starting from the singlet and triplet excited states. We conclude that the Norrish type I dissociation produces radicals from both singlet and triplet excited states, while Norrish type II dissociation is a two-step process starting from the triplet excited state, but is a concerted process from the singlet state.

Relative and absolute kinetic studies of 2-butanol and related alcohols with tropospheric Cl atoms

A newly constructed chamber/Fourier transform infrared system was used to determine the relative rate coefficient, k(i), for the gas-phase reaction of Cl atoms with 2-butanol (k(1)), 2-methyl-2-butanol (k(2)), 3-methyl-2-butanol (k(3)), 2,3-dimethyl-2-butanol (k(4)) and 2-pentanol (k(5)). Experiments were performed at (298 +/- 2) K, in 740 Torr total pressure of synthetic air, and the measured rate coefficients were, in cm(3) molecule(-1) s(-1) units (+/-2sigma): k(1)=(1.32 +/- 0.14) x 10(-10), k(2)=(7.0 +/- 2.2) x 10(-11), k(3)=(1.17 +/- 0.14) x 10(-10), k(4)=(1.03 +/- 0.17) x 10(-10) and k(5)=(2.18 +/- 0.36) x 10(-10), respectively. Also, all the above rate coefficients (except for 2-pentanol) were investigated as a function of temperature (267-384 K) by pulsed laser photolysis-resonance fluorescence (PLP-RF). The obtained kinetic data were used to derive the Arrhenius expressions: k(1)(T)=(6.16 +/- 0.58) x 10(-11)exp[(174 +/- 58)/T], k(2)(T)=(2.48 +/- 0.17) x 10(-11)exp[(328 +/- 42)/T], k(3)(T)=(6.29 +/- 0.57) x 10(-11)exp[(192 +/- 56)/T], and k(4)(T)=(4.80 +/- 0.43) x 10(-11)exp[(221 +/- 56)/T](in units of cm(3) molecule(-1) s(-1) and +/-sigma). Results and mechanism are discussed and compared with the reported reactivity with OH radicals. Some atmospheric implications derived from this study are also reported.

Kinetics of the reaction of O3with selected benzenediols

AbstractThe kinetics of the reaction of O3with the aromatic vicinal diols 1,2‐benzenediol, 3‐methyl‐1,2‐benzenediol, and 4‐methyl‐1,2‐benzenediol have been investigated using a relative rate technique. The rate coefficients were determined in a 1080‐L smog chamber at 298 K and 1 atm total pressure of synthetic air using propene and 1,3‐butadiene as reference compounds. The following O3reaction rate coefficients (in units of cm3molecule−1s−1) have been obtained:k(1,2‐benzenediol) = (9.60 ± 1.12) × 10−18,k(3‐methyl‐1,2‐benzenediol) = (2.81 ± 0.23) × 10−17,k(4‐methyl‐1,2‐benzenediol) = (2.63 ± 0.34) × 10−17. Absolute measurements of the O3rate coefficient have also been carried out by measuring the decay of the dihydroxy compound in an excess of O3. The results from these experiments are in good agreement with the relative determinations. Atmospheric implications are discussed. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 223–230, 2003

FT–IR study of the ring-retaining products from the reaction of OH radicals with phenol, o-, m-, and p-cresol

The ring-retaining products of the OH-initiated degradation of phenol and o-, m-, and p-cresol in the presence of NO x have been investigated and their formation yields determined. The experiments were carried out in a large volume reactor at (298±2) K and 1000 mbar total pressure of synthetic air using FT–IR spectroscopy for the analysis of reactants and products. The products observed and their respective molar yields were: from phenol: 1,2-dihydroxybenzene (80.4±12.1)%, 1,4-benzoquinone (3.7±1.2)% and 2-nitrophenol (5.8±1.0)%; from o-cresol: 3-methyl-1,2-dihydroxybenzene (73.4±14.6)%, methyl-1,4-benzoquinone (6.8±1.0)% and 6-methyl-2-nitrophenol (6.8±1.5)%; from m-cresol: 3-methyl-1,2-dihydroxybenzene (68.6±13.4)%, 4-methyl-1,2-dihydroxybenzene (9.7±2.7)%, methyl-1,4-benzoquinone (11.3±2.5)%, 5-methyl-2-nitrophenol (4.4±1.5)% and 3-methyl-2-nitrophenol (4.3±1.6)% and from p-cresol: 4-methyl-1,2-dihydroxybenzene (64.1±11.3)% and 4-methyl-2-nitrophenol (7.6±2.2)%. Reaction pathways leading to the observed products are proposed and potential ramifications for the atmospheric reaction mechanisms of aromatic hydrocarbons are considered.

FT‐IR kinetic and product study of the OH radical and Cl‐atom–initiated oxidation of dibasic esters

AbstractUsing a relative kinetic technique, rate coefficients have been measured, at 296 ± 2 K and 740 Torr total pressure of synthetic air, for the gas‐phase reaction of OH radicals with the dibasic esters dimethyl succinate [CH3OC(O)CH2CH2C(O)OCH3], dimethyl glutarate [CH3OC(O)CH2CH2CH2C(O)OCH3], and dimethyl adipate [CH3OC(O)CH2CH2CH2CH2C(O)OCH3]. The rate coefficients obtained were (in units of cm3 molecule−1 s−1): dimethyl succinate (1.89 ± 0.26) × 10−12; dimethyl glutarate (2.13 ± 0.28) × 10−12; and dimethyl adipate (3.64 ± 0.66) × 10−12. Rate coefficients have been also measured for the reaction of chlorine atoms with the three dibasic esters; the rate coefficients obtained were (in units of cm3 molecule−1 s−1): dimethyl succinate (6.79 ± 0.93) × 10−12; dimethyl glutarate (1.90 ± 0.33) × 10−11; and dimethyl adipate (6.08 ± 0.86) × 10−11. Dibasic esters are industrial solvents, and their increased use will lead to their possible release into the atmosphere, where they may contribute to the formation of photochemical air pollution in urban and regional areas. Consequently, the products formed from the oxidation of dimethyl succinate have been investigated in a 405‐L Pyrex glass reactor using Cl‐atom–initiated oxidation as a surrogate for the OH radical. The products observed using in situ Fourier transform infrared (FT‐IR) absorption spectroscopy and their fractional molar yields were: succinic formic anhydride (0.341 ± 0.068), monomethyl succinate (0.447 ± 0.111), carbon monoxide (0.307 ± 0.061), dimethyl oxaloacetate (0.176 ± 0.044), and methoxy formylperoxynitrate (0.032–0.084). These products account for 82.4 ± 16.4% C of the total reaction products. Although there are large uncertainties in the quantification of monomethyl succinate and dimethyl oxaloacetate, the product study allows the elucidation of an oxidation mechanism for dimethyl succinate. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 431–439, 2001

FT-IR Kinetic and Product Study of the OH Radical-Initiated Oxidation of 1-Pentanol

A kinetic and product study has been performed on the reaction of OH radicals with 1-pentanol in a 480-L glass chamber and also in the EUPHORE outdoor photoreactor in Valencia, Spain. Analysis of reactants and products was made by long-path in situ FT-IR spectroscopy and gas chromatography with photoionization detection. Using a relative kinetic technique, a rate coefficient of k(OH + 1-pentanol) = (1.11 ± 0.11) × 10-11 cm3 molecule-1 s-1 has been measured at 298 ± 2 K and 740 Torr total pressure of synthetic air. The products observed in the reaction of OH with 1-pentanol and their fractional molar yields were as follows: pentanal (0.405 ± 0.082); butanal (0.161 ± 0.037); propanal (0.081 ± 0.019); ethanal (0.181 ± 0.042); and HCHO (0.251± 0.013). The results support the possible formation of 5-hydroxy-2-pentanone, and residual absorptions indicate the possible presence of compounds such as 3-hydroxypropanal. From the product studies, a detailed atmospheric degradation mechanism has been constructed for ...

Relative-rate study of the gas-phase reaction of hydroxy radicals with difunctional organic nitrates at 298 K and atmospheric pressure

Rate coefficients for the reactions of difunctional nitrates with atmospherically important OH radicals are not currently available in the literature. This study represents the first determination of rate coefficients for a number of C(3) and C(4) carbonyl nitrates and dinitrates with OH radicals in a 38 l glass reaction chamber at 1000 mbar total pressure of synthetic air by 298±2 K using a relative kinetic technique. The following rate coefficients (in units of 10-12 cm3 molecule-1 s-1) were obtained: 1,2-propandiol dinitrate, <0.31; 1,2-butandiol dinitrate, 1.70±0.32; 2,3-butandiol dinitrate, 1.07±0.26; α-nitrooxyacetone, <0.43; 1-nitrooxy-2-butanone, 0.91±0.16; 3-nitrooxy-2-butanone, 1.27±0.14; 1,4-dinitrooxy-2-butene, 15.10±1.45; 3,4-dinitrooxy-1-butene, 10.10±0.50. The possible importance of reaction of OH as an atmospheric sink for the compounds compared to other loss processes is considered.

Kinetics and products of the reactions of nitrate radical with monoalkenes, dialkenes, and monoterpenes

Rate constants for the reactions of NO{sub 3} with a number of aliphatic mono- and dialkenes and monoterpenes have been determined in a 420 1 reaction chamber at 1-bar total pressure of synthetic air by 298 K with a relative kinetic method. The products of these reactions have been investigated also at 1-bar total pressure of synthetic air with in situ FT-IR spectrometry and gas chromatography. In all cases, the initial formation of thermally unstable nitrooxy-peroxynitrate-type compounds containing the difunctional group -CH(OONO{sub 2})-CH(ONO{sub 2})- has been observed.

Fourier transform infrared study of the self reaction of C2H5O2 radicals in air at 295 K

AbstractFourier transform infrared spectroscopy was used to identify and quantify products of the self reaction of ethylperoxy radicals, C2H5O2, formed in the photolysis of Cl2/C2H6 mixtures in 700 torr total pressure of synthetic air at 295 K. From these measurements, branching ratios for the reaction channels equation image of k1a/(k1a + k1b) = 0.68 and k1c/(k1a + k1b + k1c) ⩽ 0.06 were established. Additionally, using the relative rate technique, the rate constant for the reaction of Cl atoms with C2H5OOH was determined to be (1.07 ± 0.07) × 10−10 × cm3 molecule−1 s−1. Results are discussed with respect to the previous kinetic and mechanistic studies of C2H5O2 radicals.