María B. Blanco

Atmospheric Photooxidation of Fluoroacetates as a Source of Fluorocarboxylic Acids

A 1080 L environmental chamber with in situ FTIR spectroscopy detection was used to study the product distribution and the mechanism of the Cl-initiated photooxidation of a series of fluoroacetates. The gas-phase reactions of Cl atoms with ethyl trifluoroacetate (CF(3)C(O)OCH(2)CH(3)), methyl trifluoroacetate (CF(3)C(O)OCH(3)), and methyl difluoroacetate (CF(2)HC(O)OCH(3)) were investigated at 296 +/- 2 K and atmospheric pressure (approximately 760 Torr) of synthetic air. The fate of the fluoroalkoxy radicals formed in the reaction with Cl atoms mainly occurs through (i) an H-atom abstraction by reaction with O(2,) to produce the corresponding fluoroanhydride and (ii) an alpha-ester rearrangement via a five-membered ring intermediate to give the corresponding fluoroacetic acid. The yields of fluoroacids (CF(2)XC(O)OH, with X = H, F) obtained were as follows: 78 +/- 5, 23 +/- 2, and 30 +/- 5% for CF(3)C(O)OCH(2)CH(3), CF(3)C(O)OCH(3), and CF(2)HC(O)OCH(3,), respectively. Yields of <or=20, <or= 80, and <or=55% have been estimated for the anhydride formation from CF(3)C(O)OCH(2)CH(3), CF(3)C(O)OCH(3), and CF(2)HC(O)OCH(3), respectively. Formation of CF(2)O, with yield of 13 +/- 2% has been observed for the reaction of Cl with CF(2)HC(O)OCH(3). The measured yields are rationalized in terms of mechanisms consisting of competitive reaction channels for the radicals formed in the oxidation, that is, reaction with O(2), alpha-ester rearrangement and a decomposition pathway. The stability of the five-membered transition state of the alpha-ester rearrangement is correlated with the acid yields observed for the different fluoroacetates. Atmospheric implications, especially with regard to the fluorocarboxylic acid formation, are discussed.

OH-Initiated Degradation of Unsaturated Esters in the Atmosphere: Kinetics in the Temperature Range of 287−313 K

The kinetics of the gas-phase reactions of hydroxyl radicals (OH) with methyl methacrylate (k(1)), butyl methacrylate (k(2)), butyl acrylate (k(3)), and vinyl acetate (k(4)) have been investigated for the first time as a function of temperature using the relative technique. The experiments were performed in a 1080 L quartz glass photoreactor over the temperature range (T = 287-313 K) at a total pressure of 760 +/- 10 Torr synthetic air using in situ FTIR absorption spectroscopy to monitor the concentration-time behaviors of reactants. OH radicals were produced by the 254 nm photolysis of hydrogen peroxide (H(2)O(2)). The following Arrhenius expressions (in units of cm(3) molecule(-1) s(-1)) adequately describe the measured rate coefficients as a function of temperature: k(1) = (1.97 +/- 0.95) x 10(-12) exp[(921 +/- 52)/T], k(2) = (1.65 +/- 1.05) x 10(-11) exp[(413 +/- 34)/T], k(3) = (4.4 +/- 2.5) x 10(-13) exp[(1117 +/- 105)/T], and k(4) = (4.06 +/- 2.02) x 10(-12) exp[(540 +/- 49)/T]. All of the rate coefficients display a negative temperature dependence and low pre-exponential factor, which supports an addition mechanism for the reactions involving reversible OH-adduct formation. The rate coefficients (in units of cm(3) molecule(-1) s(-1)) determined at room temperature (298 K) were as follows: k(1) = (4.30 +/- 0.98) x 10(-11), k(2) = (6.63 +/- 1.42) x 10(-11), k(3) = (2.17 +/- 0.48) x 10(-11), and k(4) = (2.48 +/- 0.61) x 10(-11). The results are compared with previous values of the rate coefficients reported in the literature, which were mainly measured at room temperature. The reactivity of the various unsaturated esters toward the OH radical is discussed in terms of structure activity relationships and parallels are drawn with the OH-radical activities of structurally similar compounds. Using the kinetic parameters determined in this work, residence times of the esters in the atmosphere with respect to their reaction with OH have been determined and are compared with other possible degradation pathways. Possible atmospheric implications of the various degradation pathways studied are discussed.

Kinetic Investigation of the OH Radical and Cl Atom Initiated Degradation of Unsaturated Ketones at Atmospheric Pressure and 298 K

Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with 4-hexen-3-one, 5-hexen-2-one, and 3-penten-2-one have been determined at 298 ± 2 K and atmospheric pressure of air. Rate coefficients for the compounds were determined using a relative kinetic technique with different reference compounds. The experiments were performed in a large photoreactor (480 L) using in situ FTIR spectroscopy to monitor the decay of reactants. From the different measurements the following rate coefficients (in units of cm(3) molecule(-1) s(-1)) have been determined: k(1)(OH + 4-hexen-3-one) = (9.04 ± 2.12) × 10(-11), k(2)(OH + 5-hexen-2-one) = (5.18 ± 1.27) × 10(-11), k(3)(OH + 3-penten-2-one) = (7.22 ± 1.74) × 10(-11), k(4)(Cl + 4-hexen-3-one) = (3.00 ± 0.58) × 10(-10), k(5)(Cl + 5-hexen-2-one) = (3.15 ± 0.50) × 10(-10) and k(6)(Cl + 3-penten-2-one) = (2.53 ± 0.54) × 10(-10). The reactivity of the double bond in alkenes and unsaturated ketones at 298 K toward addition of OH radicals and Cl atoms are compared and discussed. In addition, a correlation between the reactivity of the unsaturated ketones toward OH radicals and the HOMO of the compounds is presented. On the basis of the kinetic measurements, the tropospheric lifetimes of 4-hexen-3-one, 5-hexen-2-one, and 3-penten-2-one with respect to their reaction with hydroxyl radicals are estimated to be between 2 and 3 h.

Gas-phase oxidation of methyl crotonate and ethyl crotonate. kinetic study of their reactions toward OH radicals and Cl atoms.

Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with methyl crotonate and ethyl crotonate have been determined at 298 K and atmospheric pressure. The decay of the organics was monitored using gas chromatography with flame ionization detection (GC-FID), and the rate constants were determined using the relative rate method with different reference compounds. Room temperature rate coeficcients were found to be (in cm(3) molecule(-1) s(-1)): k(1)(OH + CH(3)CH═CHC(O)OCH(3)) = (4.65 ± 0.65) × 10(-11), k(2)(Cl + CH(3)CH═CHC(O)OCH(3)) = (2.20 ± 0.55) × 10(-10), k(3)(OH + CH(3)CH═CHC(O)OCH(2)CH(3)) = (4.96 ± 0.61) × 10(-11), and k(4)(Cl + CH(3)CH═CHC(O)OCH(2)CH(3)) = (2.52 ± 0.62) × 10(-10) with uncertainties representing ±2σ. This is the first determination of k(1), k(3), and k(4) under atmospheric pressure. The rate coefficients are compared with previous determinations for other unsaturated and oxygenated VOCs and reactivity trends are presented. In addition, a comparison between the experimentally determined k(OH) with k(OH) predicted from k vs E(HOMO) relationships is presented. On the other hand, product identification under atmospheric conditions has been performed for the first time for these unsaturated esters by the GC-MS technique in NO(x)-free conditions. 2-Hydroxypropanal, acetaldehyde, formaldehyde, and formic acid were positively observed as degradation products in agreement with the addition of OH to C2 and C3 of the double bond, followed by decomposition of the 2,3- or 3,2-hydroxyalkoxy radicals formed. Atmospheric lifetimes, based on of the homogeneous sinks of the unsaturated esters studied, are estimated from the kinetic data obtained in the present work.

Atmospheric Oxidation of Vinyl and Allyl Acetate: Product Distribution and Mechanisms of the OH-Initiated Degradation in the Presence and Absence of NOx

The products formed from the reactions of OH radicals with vinyl acetate and allyl acetate have been studied in a 1080 L quartz-glass chamber in the presence and absence of NO(x) using in situ FTIR spectroscopy to monitor the reactant decay and product formation. The yields of the primary products formed in the reaction of OH with vinyl acetate were: formic acetic anhydride (84 ± 11)%; acetic acid (18 ± 3)% and formaldehyde (99 ± 15)% in the presence of NO(x) and formic acetic anhydride (28 ± 5)%; acetic acid (87 ± 12)% and formaldehyde (52 ± 8)% in the absence of NO(x). For the reaction of OH with allyl acetate the yields of the identified products were: acetoxyacetaldehyde (96 ± 15)% and formaldehyde (90 ± 12)% in the presence of NO(x) and acetoxyacetaldehyde (26 ± 4)% and formaldehyde (12 ± 3)% in the absence of NO(x). The present results indicate that in the absence of NO(x) the main fate of the 1,2-hydroxyalkoxy radicals formed after addition of OH to the double bond in the compounds is, in the case of vinyl acetate, an α-ester rearrangement to produce acetic acid and CH(2)(OH)CO(•) radicals and in the case of allyl acetate reaction of the radical with O(2) to form acetic acid 3-hydroxy-2-oxo-propyl ester (CH(3)C(O)OCH(2)C(O)CH(2)OH). In contrast, in the presence of NO(x) the main reaction pathway for the 1,2-hydroxyalkoxy radicals is decomposition. The results are compared with the available literature data and implications for the atmospheric chemistry of vinyl and allyl acetate are assessed.

Products and mechanism of the reactions of OH radicals and Cl atoms with methyl methacrylate (CH2═C(CH3)C(O)OCH3) in the presence of NOx.

The OH radical and Cl atom initiated photodegradation of methyl methacrylate has been investigated in a 1080 L quartz-glass environmental chamber at 298 ± 2 K and atmospheric pressure of synthetic air using in situ FTIR spectroscopy to monitor the reactants and products. The major products observed in the OH reaction were methyl pyruvate (92 ± 16%) together with formaldehyde (87 ± 12%) as a coproduct from the C1-C2 bond cleavage channel of the intermediate 1,2-hydroxyalkoxy radical, formed by the addition of OH to the terminal carbon of the double bond which is designated C1. For the Cl atom reaction, the products identified were chloroacetone (41 ± 6%) together with its coproduct formaldehyde (35 ± 5%) and methyl pyruvate (24 ± 4%) together with its coproduct formylchloride (25 ± 4%). The results show that the fate of the intermediate 1,2-chloroalkoxy radical involves not only cleavage of the C1-C2 bond but also quite substantial cleavage of the C2-C3 bond. The present results are compared with previous studies of acrylates, showing different branching ratios for the OH and Cl addition reactions in the presence of NOx. Atmospheric implications are discussed.

FTIR Product Distribution Study of the Cl and OH Initiated Degradation of Methyl Acrylate at Atmospheric Pressure

A product study is reported on the gas-phase reactions of OH radicals and Cl atoms with methyl acrylate. The experiments were performed in a 1080-L quartz-glass chamber in synthetic air at 298 ± 2 K and 760 ± 10 Torr using long-path in situ FTIR spectroscopy for the analysis of the reactants and products. In the absence of NO(x) the major product observed in the OH reaction is methyl glyoxylate, with formaldehyde as a coproduct. For the reaction with Cl only formyl chloride (HC(O)Cl), CO, and HCl could be positively identified as products, however, the concentration-time behavior of these products show that they are secondary products and originate from the further oxidation of a major primary product. From this behavior and a comparison with simulated spectra unidentified bands in the residual product spectra are tentatively attributed to a compound of structure CH(2)ClC(O)C(O)OCH(3), i.e., formation of methyl 3-chloro-2-oxopropanoate from the reaction of Cl with methyl acrylate. The present results are compared with previous results where available and simple atmospheric degradation mechanisms are postulated to explain the formation of the observed products.

OH- and O3-initiated atmospheric degradation of camphene: temperature dependent rate coefficients, product yields and mechanisms

Gas-phase rate coefficients for the reactions of OH and O3 with camphene have been measured over the temperature range 288–311 K using the relative rate method. The experiments were carried out in an environmental chamber using long-path FTIR spectroscopy to monitor the reactants. Room temperature rate coefficients (in cm3 per molecule per s) of k(camphene+OH) = (5.1 ± 1.1) × 10−11 and k(camphene+O3) = (5.1 ± 1.1) × 10−19 were obtained for the OH and O3 reactions, respectively. The temperature dependence of the reactions are best fit by the Arrhenius expressions (in cm3 per molecule per s) k(camphene+OH) = (4.1 ± 1.2) × 10−12 exp[(754 ± 44)/T] for the OH reaction and k(O3+camphene) = (7.6 ± 1.2) × 10−18 exp[−(805 ± 51)/T] for the O3 reaction. To the best of our knowledge, this is the first report of the temperature dependencies for the reactions of OH and O3 with camphene. In addition, product studies have been performed at (298 ± 2) K and 760 Torr of synthetic air for the reaction of OH + camphene in the absence and presence of NOx, and for O3 molecules + camphene at (298 ± 2) K and 750 Torr of synthetic air. For the OH reaction the following molar product yields were obtained: acetone (10 ± 2)% and (33 ± 6)%, and formaldehyde (3.6 ± 0.7)% and (10 ± 2)% in the absence and presence of NOx, respectively. Formaldehyde with a molar yield of (29 ± 6)% was the only product uniquely identified and quantified for the O3 reaction.

Ozonolysis of a series of C7–C9 unsaturated biogenic aldehydes: reactivity study at atmospheric pressure

Rate coefficients for the reactions of O3 with trans-2-heptenal, trans-2-octenal and trans-2-nonenal have been determined at 298 K and (990 ± 10) mbar in an environmental chamber with in situ FTIR spectroscopy. The following rate coefficients in units of kO3 × 1018 (cm3 molecule−1 s−1) were obtained: (2.47 ± 0.73) for trans-2-heptenal, (2.37 ± 0.68) for trans-2-octenal and (2.05 ± 0.20) for trans-2-nonenal. It is shown that rate coefficients for the addition of O3 molecules and OH radicals to the double bond of alkenes and unsaturated and oxygenated volatile organic compounds (OVOCs) at 298 K are related to a good approximation by the expression: log kOH = 0.16 log kO3 − 7.55. Furthermore, a correlation between the reactivity of unsaturated VOCs toward O3 molecules and the energies of the Highest Occupied Molecular Orbit (HOMO) of the unsaturated VOCs is presented and potential atmospheric implications of the results are discussed.

Tropospheric chemical degradation of vinyl and allyl acetate initiated by Cl atoms under high and low NOx conditions

The products of the reactions of Cl atoms with vinyl acetate (VA) and allyl acetate (AA) have been investigated in a 1080 L chamber using in situ FTIR. The experiments were performed at 296 K and atmospheric pressure of synthetic air in the presence and in the absence of NOx. For the reaction of Cl with VA in the presence of NOx formic acetic anhydride, acetic acid and formyl chloride are the major reaction products. In the absence of NOx, the yields of these products are significantly reduced and formation of the carbon-chain-retaining compound CH3C(O)OC(O)CH2Cl is observed. For the reaction of Cl with AA in the presence of NOx acetoxyacetaldehyde and formaldehyde were observed as the main products. In contrast, without NOx, the observations support that the major reaction pathway is the formation of the carbon-chain-retaining compound CH3C(O)OCH2C(O)CH2Cl. The reaction mechanisms leading to the products are discussed. The formation of the high yields of formyl chloride and formaldehyde in the reactions of Cl with VA and AA, respectively, are at odds with currently accepted mechanistic pathways.