Iustinian Bejan

The tropospheric degradation of isoprene : an updated module for the regional atmospheric chemistry mechanism

Abstract A highly condensed reaction scheme for the tropospheric oxidation of isoprene is presented. This mechanism was implemented into the regional atmospheric chemistry mechanism (RACM), which is an established chemical module for regional air quality modelling but contains an isoprene chemistry which is no longer state-of-the-art. The reaction scheme developed here is based on the recently published Mainz isoprene mechanism (MIM) that has been constructed for application in global chemistry transport models. The MIM code was reduced to a size suitable for use in regional atmospheric chemistry models. Redundant reactions were identified and removed from the reaction scheme by means of sensitivity analyses. The revised mechanism was successfully tested against the results of smog chamber experiments carried out in the European photoreactor EUPHORE. A model intercomparison between both the original and the updated RACM mechanism was performed for a number of well-defined scenarios employing conditions ranging from very clean to highly polluted air masses. The calculations revealed large deviations in the concentration–time profiles for key species of the isoprene degradation, particularly under “low-NOx” conditions. The new isoprene chemistry requires only a few additional reactants (7) and chemical reactions (7) and, therefore, offers the possibility for the successful application of the revised reaction scheme in chemistry-transport models (CTM) without an excessive increase in computational efforts.

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.

Investigations on the gas-phase photolysis and OH radical kinetics of methyl-2-nitrophenols

Methyl-2-nitrophenols can be emitted directly to the atmosphere or can be formed in situ via the oxidation of aromatic hydrocarbons. Nitrophenols possess phytotoxic properties and recent studies indicate their photooxidation is effective in producing secondary organic aerosols. Therefore, investigations on the major photooxidation pathways of these compounds with respect to assessing their environmental impacts and effects on human health are highly relevant. Presented here are determinations of the rate coefficients for the reactions of OH radicals with four methyl-2-nitrophenol isomers using a relative kinetic technique. The experiments were performed in a 1080 l photoreactor at (760 +/- 10) Torr total pressure of synthetic air at (296 +/- 3) K. The following rate coefficients (in units of cm(3) molecule(-1) s(-1)) have been obtained: 3-methyl-2-nitrophenol, (3.69 +/- 0.70) x 10(-12); 4-methyl-2-nitrophenol, (3.59 +/- 1.17) x 10(-12); 5-methyl-2-nitrophenol, (6.72 +/- 2.14) x 10(-12); 6-methyl-2-nitrophenol, (2.70 +/- 0.57) x 10(-12). Photolysis of the methyl-2-nitrophenols with the superactinic fluorescent lamps (320 < lambda < 480 nm, lambda(max) = 360 nm) used in the experiments was observed. Photolysis frequencies measured for the methyl-2-nitrophenols in the photoreactor have been determined and scaled to atmospheric conditions. The results suggest that photolysis rather than the reaction with OH radicals will be the dominant gas phase atmospheric loss process for methyl-2-nitrophenols.

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.

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.

Kinetics and Mechanisms of the Tropospheric Reactions of Menthol, Borneol, Fenchol, Camphor, and Fenchone with Hydroxyl Radicals (OH) and Chlorine Atoms (Cl)

Relative kinetic techniques have been used to measure the rate coefficients for the reactions of oxygenated terpenes (menthol, borneol, fenchol, camphor, and fenchone) and cyclohexanol with hydroxyl radicals (OH) and chlorine atoms (Cl) at 298 ± 2 K and atmospheric pressure. The rate coefficients obtained for the reactions of the title compounds with OH are the following (in units of 10(-11) cm(3) molecule(-1) s(-1)): (1.48 ± 0.31), (2.65 ± 0.32), (2.49 ± 0.30), (0.38 ± 0.08), (0.39 ± 0.09) for menthol, borneol, fenchol, camphor, and fenchone, respectively. For the corresponding reactions with Cl atoms the rate coefficients are as follows (in units of 10(-10) cm(3) molecule(-1) s(-1)): (3.21 ± 0.26), (3.40 ± 0.28), (2.72 ± 0.13), (2.93 ± 0.17), (1.59 ± 0.10), and (1.86 ± 0.29) for cyclohexanol, menthol, borneol, fenchol, camphor, and fenchone, respectively. The reported error is twice the standard deviation. Product studies of the reactions were performed using multipass in situ FTIR (Fourier transform infrared spectroscopy) and solid-phase microextraction (SPME) with analysis by GC-MS (gas chromatography-mass spectrometry). A detailed mechanism is proposed to justify the observed reaction products.

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.