Unexpected high yield of acrolein underlies the importance of the hydrogen-abstraction mechanism in photooxidation of allyl methyl sulfide (AMS).

This work explores theoretically the gas phase oxidation of allyl methyl sulfide (AMS, H2CCHCH2SCH3) initiated by •OH radicals, focusing on the H-abstraction pathway at the M06-2X-D3/aug-cc-pVTZ and MN15/aug-cc-pVTZ levels of theory (m06Tz and mn15Tz). The formation of a prereactive complex (PRC) is involved in H-abstraction processes with two potential directions of approach for the OH radical, denoted as "α" and "ß". The PRCs, demonstrate increased reactivity, primarily due to the interaction between the sulfur atoms and the hydroxyl hydrogen. A scheme for the H-abstraction mechanism that supports the experimentally identified products and predicts the formation of some S-containing low volatility products is proposed. The comparison of the potential energy surface (PES) between the double bond addition and H-abstraction paths in the AMS molecule shows that at the m06Tz level of theory, the H-abstraction on C3 and the addition to C1 have nearly the same profile of energy, while at the mn15Tz level, the minimum energy channel is the addition to C1. The theoretical rate coefficient for each reaction channel was calculated, considering the formation of a PRC prior to reaching the transition state of each channel and assuming thermal equilibrium between reactants and the PRC. The rate constants were calculated in a multi-TS/multi-conformer way at the SVECV-f12/m06Tz and SVECV-f12/mn15Tz levels of theory. The SVECV-f12 method is consistent in its predictions in both systems and exhibits only minor deviations from the experimental rate constants. Despite some specific differences due to the DFT method supporting the SVECV-f12 calculations, both methodologies predict a significant H-abstraction contribution in the AMS + OH gas phase reaction, which explains the high formation yield for acrolein determined experimentally.

Assessment of atmospheric levels of carbonyls in an urban environment of Argentina.

It is well-documented that carbonyl compounds have adverse effects on human health. On the other hand, these oxygenated volatile organic compounds (OVOCs) are precursors of secondary pollutants such as tropospheric ozone or peroxy acetyl nitrate (PAN). In particular, formaldehyde, the simplest carbonyl, is the most abundant carbonyl in the air generated from the degradation of most volatile organic compounds (VOCs). This work presents for the first time the characterization and determination of levels of carbonyl compounds by passive monitoring performed from April-December 2021 in the city of Córdoba, Argentina, the second most populated Mediterranean city located in the center of the country. Annual concentrations, considering the 11 carbonyls measured, were in the range of 0.13-8.75 µgm-3. Formaldehyde and acetaldehyde were the carbonyls detected in the highest annual average concentrations of 4.44 ± 1.75 µgm-3 and 3.85 ± 1.44 µgm-3, respectively. These carbonyls represent a contribution of around 40-57% on total carbonyls measured. Statistical analysis to determine significant differences and Pearson correlations with the meteorological parameters were performed. Spring and summer were found to be the seasons with the highest carbonyl concentration linked to forest fire episodes, especially in springtime. The values for the C1/C2 and C2/C3 ratios showed that sources of carbonyl formation are anthropogenic. In addition, the prop-Equiv concentration was determined, where formaldehyde and acetaldehyde were the main producers of tropospheric ozone. The ozone formation potential (OFP) showed that spring and summer are the seasons where carbonyls contribute to the formation of tropospheric ozone.This study represents a first approach of the carbonyl concentration in the city and of the influence of meteorological parameters on the behavior of carbonyls.

Hydroxy esters atmospheric degradation: OH and Cl reactivity, products distribution and fate of the alkoxy radicals formed.

Kinetic studies of the reaction of ethyl glycolate HOCH2C(O)OCH2CH3 with OH radicals (kOH) and Cl atoms (kCl) have been conducted by the relative method using a glass atmospheric reactor by "in situ" Fourier Transform Infrared (FTIR) and Gas Chromatography equipped with flame ionization detection by Solid Phase Micro Extraction (GC-FID/SPME) at room temperature and atmospheric pressure. The following relative rate coefficients were determined using several reference compounds and two different techniques: kEG + OH-FTIR = (4.36 ± 1.21) × 10-12; kEG + OH-GC-FID= (3.90 ± 0.74) × 10-12; and kEG + Cl-GC-FID= (6.40 ± 0.72) × 10-11 all values in units of cm3.molecule-1.s-1. Complementary product studies were performed under comparable conditions to the kinetic tests, in order to identify the reaction products and to postulate their tropospheric oxidation mechanisms. The reaction of OH radicals and Cl atoms with ethyl glycolate initiates via H-atom abstraction from alkyl groups of the molecule. Formic acid was positively identified as a reaction product by FTIR. On the other hand, formaldehyde, acetaldehyde, glycolic acid; and formic acid were identified by the GC-MS technique. The Structure-Activity Relationship, (SAR) calculations were also implemented to estimate the more favorable reaction pathways and compare them with the products identified. Tropospheric lifetimes of τOH = 34 h and τCl = 5.5 days were estimated to determine how these investigated reactions might affect the air quality. In this sense, average ozone production of [O3] = 0.75 and a Photochemical Ozone Creation Potential, POCP, of 38 were calculated for the hydroxyl ester studied.

Atmospheric oxidation of furanones by •OH and •Cl radicals: In situ FTIR rate coefficient determinations, SAR and theoretical studies.

Gas-phase kinetics of the overall reactions of •OH and •Cl radicals with dihydrofuran-3(2H)-one (oxolan-3-one) and dihydro-2-methyl-3(2H)-furanone (2MTHF-3-one) were studied at 298 K and atmospheric pressure. The rate coefficients were determined using the relative method in a 480 L multipass glass reactor coupled to an FT-IR detection system. The rate coefficients found for oxolan 3-one and 2MTHF-one with •OH radicals (k1 and k2) and with •Cl atoms (k3 and k4) at 298 K and atmospheric pressure (in cm3 molecule-1 s-1) were: k1 = (1.86 ± 0.29) × 10-11, k2 = (2.64 ± 0.47) × 10-11, k3= (1.15 ± 0.28) × 10 -10, and k4 = (1.33 ± 0.32) × 10-10, respectively. Reactivity trends were developed by comparison with other similar structures and Fukui indices employed to determine the reactivity of different sites on the ring. The singularity of the reaction with •OH was assessed by computational studies which showed the formation of several stable hydrogen bonded complexes, explaining the difference with the reaction with the •Cl atom. SAR estimations of the rate coefficients were calculated and compared to the experimental values.

Product study of the reactions of γ-caprolactone and γ-heptalactone initiated by OH radicals at 298 K and atmospheric pressure: Formation of acyl peroxynitrates (APN).

A product study was performed for the reaction of γ-caprolactone (GCL) and γ-heptalactone (GHL) initiated by OH radicals at (298 ± 2) K and atmospheric pressure, in presence of NOx. The identification and quantification of the products were performed in a glass reactor coupled with in situ FT-IR spectroscopy. The following products were identified and quantified with the corresponding formation yields (in %) for the OH + GCL reaction: peroxy propionyl nitrate (PPN) (52 ± 3), peroxy acetyl nitrate (PAN) (25 ± 1), and succinic anhydride (48 ± 2). For the GHL + OH reaction, the products detected with their corresponding formation yields (in %) were the following: peroxy n-butyryl nitrate (PnBN) (56 ± 2), peroxy propionyl nitrate (PPN) (30 ± 1) and succinic anhydride and (35 ± 1). Upon these results, an oxidation mechanism is postulated for the title reactions. The positions with the highest H-abstraction probabilities for both lactones are analyzed. Specifically, the increased reactivity of the C5 site, as indicated by structure reactivity estimations (SAR), is suggested by the identified products. For both GCL and GHL degradation appears to follow degradation paths including ring preservation and opening. The atmospheric implications of the APN formation as a photochemical pollutant and as NOx reservoirs of species is assessed.

Experimental and Theoretical Kinetic Studies of the Ozonolysis of Selected Allyl Sulfides (H2C═CHCH2SR, R = CH3, CH3CH2): The Effect of Nascent OH Radicals.

Rate coefficients of the O3-initiated oxidation of allyl methyl sulfide (H2C═CHCH2SCH3, AMS) and allyl ethyl sulfide (H2C═CHCH2SCH2CH3, AES) were determined at atmospheric conditions by "in situ" FTIR. The relative kinetic experiments were performed using methylcyclohexane (McH) and carbon monoxide (CO) as nascent OH radical scavengers and in the absence of any scavenger, to determine the impact that the formation of OH radicals has on the rate coefficients. In the absence of scavengers, values of kAMS+O3 = (5.23 ± 3.57) × 10-18 and kAES+O3 = (5.76 ± 1.80) × 10-18 cm3 molecule-1 s-1 were obtained. In the presence of the scavengers, however, the rates decreased to kAMS+O3+McH = (3.92 ± 1.92) × 10-18 and kAMS+O3+CO = (2.63 ± 0.47) × 10-18 cm3 molecule-1 s-1 for AMS, and kAES+O3+McH = (4.78 ± 1.38) × 10-18 and kAES+O3+CO = (3.50 ± 0.27) × 10-18 cm3 molecule-1 s-1 for AES. On the basis of these results, we have decided to recommend the values obtained using CO as scavenger as those best representing the rate coefficient for the reactions of O3 with AMS and AES. The reaction mechanism was explored using DFT and post-Hartree-Fock computational methods. It is shown that the barrier for the common cyclization to primary ozonide (-3.7 ± 0.1 kcal mol-1) followed by other reactions, as well as that for the reaction with the sulfur atom (-5.1 ± 0.1 kcal mol-1), is small and quite close, meaning that both reaction paths should contribute significantly to the global reaction rate.

Volatile Organic Compounds Released During the Fast Pyrolysis of Peanut Shells and Environmental Implications.

The biomass conversion technologies, especially different types of pyrolysis, have been intensively studied to improve biomass energy transformation suggesting a low impact on the environment. In particular, fast pyrolysis of biomass is considered to be a thermal process in which the starting material is converted to bio-oil, char and gas products. In this work, volatile organic compounds (VOCs) of the gaseous fraction of peanut shells fast pyrolysis were collected and identified at atmospheric pressure. Aromatic compounds, hydrocarbons, furans and other oxygenated compounds were identified using solid phase microextraction (SPME) and gas chromatography coupled to mass spectrometry (CG-MS) as a detection system. The composition of volatiles was analyzed and compared with the constituents of liquid fraction for comparative purposes. Atmospheric implications of the main compounds identified in the gases fraction were assessed by determining tropospheric lifetimes of the VOCs identified and its impact on environment at the local, regional or global scale.

Degradation mechanism of 2-fluoropropene by Cl atoms: experimental and theoretical products distribution studies.

The gas-phase reaction products of 2-fluoropropene (2FP) with Cl atoms have been determined for the first time at 298 K and atmospheric pressure using a 1080 L quartz-glass photoreactor coupled with in situ FTIR spectroscopy to monitor reactants and products. Acetyl fluoride and formyl chloride were observed as the main products with yields of (106 ± 10)% and (100 ± 11)%, respectively. Electronic structure calculations of reactants, intermediates, products and transition states on a detailed mechanism of the reaction were performed by DFT procedures (BMK, M06, M062X/D3), as well as accurate composite methods on both the addition and abstraction reaction channels. From the joint experimental and theoretical studies, we concluded that the reaction occurs primarily via addition to the Cα carbon, with a smaller participation of the addition on the Cß carbon, which is not produced directly from the separated reactants but from the CH3CFCH2Cl intermediate radical through a submerged transition state. The abstraction channel occurs at larger energies than the addition ones, and also presents a submerged transition state, with a lower barrier. No products arising from this channel are expected. The proposed mechanism explains also why formaldehyde, predicted as a product by former theoretical studies, is not found among the experimental products. The atmospheric implications of the reaction products are assessed.

Kinetics, product distribution and atmospheric implications of the gas-phase oxidation of allyl sulfides by OH radicals.

Relative rate coefficients of the OH radical -initiated oxidation of allyl methyl sulfide (AMS, H2CCHCH2SCH3) and allyl ethyl sulfide (AES, H2CCHCH2SCH2CH3) have been measured at atmospheric pressure of synthetic air and 298 K: kAMS= (4.98 ± 1.42) and kAES= (6.88 ± 1.49) × 10-11 cm3 molecule-1 s-1 by means of in situ FTIR spectroscopy. In addition, the molar yields of the main reaction products of AMS with OH radicals formed in the absence and presence of nitric oxides (NOX) were determined to be the following: sulfur dioxide (95 ± 12) % and (51 ± 12) % for acrolein (50 ± 9) % and (41 ± 9) %. In the reaction of AES with OH radicals, the following molar yields were obtained: for sulfur dioxide (88 ± 13) % and (56 ± 12) % for acrolein (36 ± 9) % and (41 ± 9) %. The present results suggest that the abstraction at C3 plays an important role in the oxidation mechanism as the addition to the double bond. This work represents the first study of the OH radical interaction with AMS and AES carried out under atmospheric conditions. The atmospheric implications were discussed in terms of the atmospheric residence times of the sulfur-containing compounds studied and the products formed in the presence and absence of NOx. SO2 formation seems to be the main fate of the gas-phase allyl sulfides oxidation with significant acidifying potentials and short-chain aldehydes production like formaldehyde and acetaldehyde.

Diurnal photodegradation of fluorinated diketones (FDKs) by OH radicals using different atmospheric simulation chambers: Role of keto-enol tautomerization on reactivity.

Rate coefficients for the gas-phase reactions of OH radicals with a series of fluorinated diketones have been determined for the first time at (298 ± 3) K and atmospheric pressure using the relative method and FTIR spectroscopy and GC-FID to monitor both reactants and references. The following values, in 10-11 cm3 molecule-1 s-1, were obtained for 1,1,1-trifluoro-2,4-pentanedione (TFP), 1,1,1-trifluoro-2,4-hexanedione (TFH) and 1,1,1-trifluoro-5-methyl-2,4-hexanedione (TFMH), respectively: k1(TFP + OH) = (1.3 ± 0.4), k2(TFH + OH) = (2.2 ± 0.8), k3(TFMH + OH) = (3.3 ± 1.0). The results are discussed with respect to the keto-enolic tautomerization specific for ß-diketones. Based on the present results, the tropospheric lifetimes of TFP, TFH and TFMH upon degradation by OH radicals were calculated as 21, 13 and 8 h, respectively indicating that transport might play a role in the atmospheric fate of the studied compounds. Photochemical ozone creation potentials were estimated for TFP, TFH and TFMH to be: 23, 29 and 34, respectively.

OH-initiated degradation of methyl 2-chloroacetoacetate and ethyl 2-chloroacetoacetate: Kinetics, products and mechanisms at 298 K and atmospheric pressure.

Rate coefficients for the gas-phase reactions of OH radicals with CH3C(O)CHClC(O)OCH3 (k1) and CH3C(O)CHClC(O)OCH2CH3 (k2) were measured using the relative technique with different reference compounds. The experiments were performed at (298 ± 2) K and 750 Torr of nitrogen or synthetic air by in situ FTIR spectroscopy and GC-FID chromatography. The following rate coefficients (in units of cm3molecule-1 s-1) were obtained: k1FTIR= (2.70 ± 0.51) × 10-11; k1GC-FID= (2.30 ± 0.71) × 10-11 and k2FTIR= (3.37 ± 0.62) × 10-11; k2GC-FID= (3.26 ± 0.85) × 10-11. This work reports the first kinetic study for the reactions of OH radicals with the mentioned chloroacetoacetates. Additionally, product studies are reported in similar conditions of the kinetic experiments. Acetic acid, acetaldehyde, formyl chloride, and methyl 2-chloro-2-oxoacetate were positively identified and quantified as degradation products. According to the identified products, atmospheric chemical mechanisms were proposed. The environmental implications of these reactions were assessed by the tropospheric lifetimes calculations of the title chloroesters. Significant average ozone production of 4.16 ppm for CH3C(O)CHClC(O)OCH3 and 5.98 ppm for CH3C(O)CHClC(O)OCH2CH3, respectively were calculated.

FTIR product study of the Cl-initiated oxidation products of CFC replacements: (E/Z)-1,2,3,3,3-pentafluoropropene and hexafluoroisobutylene.

A product study of the reactions of (E/Z)-1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CF[double bond, length as m-dash]CHF) and hexafluoroisobutylene ((CF3)2C[double bond, length as m-dash]CH2) initiated by Cl atoms were developed at 298 ± 2 K and atmospheric pressure. The experiments were carried out in a 1080 L quartz-glass environmental chamber coupled via in situ FTIR spectroscopy to monitor the reactants and products. The main products observed and their yields were as follows: CF3C(O)F (106 ± 9)% with HC(O)F (100 ± 8)% as a co-product for (E/Z)-CF3CF[double bond, length as m-dash]CHF, and CF3C(O)CF3 (94 ± 5)% with HC(O)Cl (90 ± 7)% as a co-product for (CF3)2C[double bond, length as m-dash]CH2. Atmospheric implications of the end-product degradation are assessed in terms of their impact on ecosystems to help environmental policymakers consider HFOs as acceptable replacements.

Product distribution and mechanism of the OH- initiated tropospheric degradation of three CFC replacement candidates: CH3CF[double bond, length as m-dash]CH2, (CF3)2C[double bond, length as m-dash]CH2 and (E/Z)-CF3CF[double bond, length as m-dash]CHF.

The OH radical initiated photodegradation of 2-fluoropropene (CH3CF[double bond, length as m-dash]CH2), 3,3,3-trifluoro-2-(tri-fluoromethyl)propene ((CF3)2C[double bond, length as m-dash]CH2) and (E/Z)-1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CF[double bond, length as m-dash]CHF) has been investigated for the first time using a 1080 L quartz-glass environmental chamber at 298 ± 2 K and atmospheric pressure of synthetic air coupled with in situ FTIR spectroscopy to monitor reactants and products. The major products observed in the OH reaction were CH3C(O)F (98 ± 5)% together with HC(O)H (89 ± 7)% as a co-product, CF3C(O)F (103 ± 8)% together with HC(O)F (96 ± 7)% as a co-product and CF3C(O)CF3 (91 ± 8)% together with HC(O)H (98 ± 12)% as a co-product from the C1-C2 bond cleavage channel of the intermediate hydroxyalkoxy radical, formed by addition of OH to the terminal carbon of the double bond which is designated C1 of 2-fluoropropene, (E/Z)-1,2,3,3,3-pentafluoropropene and 3,3,3-trifluoro-2-(tri-fluoromethyl)propene, respectively. The present results are compared with previous studies for the reaction of OH with the separate isomers (E) and (Z) of 1,2,3,3,3-pentafluoropropene. In addition, atmospheric implications of the reactions studied are discussed.

Gas-phase degradation of 2-butanethiol initiated by OH radicals and Cl atoms: kinetics, product yields and mechanism at 298 K and atmospheric pressure.

Relative rate coefficients and product distribution of the reaction of 2-butanethiol (2butSH) with OH radicals and Cl atoms were obtained at atmospheric pressure and 298 K. The experiments were performed in a 480 L borosilicate glass photoreactor in synthetic air coupled to a long path "in situ" FTIR spectrometer. The rate coefficients obtained by averaging the values from different experiments were: k OH = (2.58 ± 0.21) × 10-11 cm3 per molecule per s and k Cl = (2.49 ± 0.19) × 10-10 cm3 per molecule per s. The kinetic values were compared with related alkyl thiols and homologous alkyl alcohols, where it was found that thiols react faster with both oxidants, OH radicals and Cl atoms. SO2 and 2-butanone were the major products identified for the reactions of 2-butanethiol with OH radicals and Cl atoms. The product yield of the reaction of 2-butanethiol and OH radicals were (81 ± 2)%, and (42 ± 1)% for SO2 and 2-butanone, respectively. For the reactions of 2-butanethiol with Cl atom, yields of SO2 and 2-butanone were (59 ± 2)% and (39 ± 2)%, respectively. A degradation mechanism was proposed for the pathways that leads to formation of identified products. The product distribution observed indicated that the H-atom of the S-H group abstraction channel is the main pathway for the reaction of OH radicals and Cl atoms with 2-butanethiol.

Atmospheric sink of ß-ocimene and camphene initiated by Cl atoms: kinetics and products at NO x free-air.

Rate coefficients for the gas-phase reactions of Cl atoms with ß-ocimene and camphene were determined to be (in units of 10-10 cm3 per molecule per s) 5.5 ± 0.7 and 3.3 ± 0.4, respectively. The experiments were performed by the relative technique in an environmental chamber with FTIR detection of the reactants at 298 K and 760 torr. Product identification experiments were carried out by gas chromatography with mass spectrometry detection (GC-MS) using the solid-phase microextraction (SPME) method employing on-fiber carbonyl compound derivatization with o-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine hydrochloride. An analysis of the available rates of addition of Cl atoms and OH radicals to the double bond of alkenes and cyclic and acyclic terpenes with a conjugated double bond at 298 K is presented. The atmospheric persistence of these compounds was calculated taking into account the measured rate coefficients. In addition, tropospheric chemical mechanisms for the title reactions are postulated.

Mechanism and Product Distribution of the O3-Initiated Degradation of (E)-2-Heptenal, (E)-2-Octenal, and (E)-2-Nonenal.

The O3-molecule initiated degradation of three 2-alkenals (E)-2-heptenal, (E)-2-octenal, and (E)-2-nonenal 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 experiments were performed in the absence of an OH scavenger. The molar yields of the primary products formed were glyoxal (49 ± 4) % and pentanal (34 ± 3) % from the reaction of (E)-2-heptenal with O3, glyoxal (41 ± 3) % and hexanal (39 ± 3) % from the reaction of (E)-2-octenal with O3, and glyoxal (45 ± 3) % and heptanal (46 ± 3) % from the reaction of (E)-2-nonenal with O3. The residual bands in the infrared product spectra for each of the studied reactions are attributed to 2-oxoaldehyde compounds. Based on the observed products, a general mechanism for the ozonolysis reaction of long chain unsaturated aldehydes is proposed, and the results are compared with the available literature data.

Atmospheric Degradation of CH2═C(CH3)C(O)OCH3 Initiated by OH Radicals: Mechanistic Study and Quantification of CH3C(O)C(O)OCH3 in NOx Free Air.

The product distribution of the gas-phase reaction of OH radicals with methyl methacrylate (CH2═C(CH3)C(O)OCH3, MMA) in the absence of NOx was studied at 298 K and atmospheric pressure of air. The experiments were performed in a Teflon chamber using solid-phase microextraction (SPME) with GC-MS and GC-FID for product identification and quantification, respectively. In the absence of NOx, methyl pyruvate (CH3C(O)C(O)OCH3) was identified with a yield of 76 ± 13% in accordance with the decomposition of the 1,2-hydroxyalkoxy radicals formed. In addition, a detailed quantum chemical study of the degradation of MMA was performed by density functional theory (DFT) methods using the MPWB1K functional. This calculation suggests that formation of methyl pyruvate, from C1-C2 scission of 1,2-hydroxyalkoxy radical, is kinetically and thermodynamically the most favorable reaction path taking into account the electronic properties of reaction intermediates and transition states. The difference observed on the degradation mechanism of MMA in the presence and absence of NOx was explained in terms of the associated thermochemistry. Furthermore, this study propose that reaction between peroxy radical (RO2(•)) and hydroxyl radical (OH) became relevant at NOx-free environments. This statement is in agreement with recent studies concerning small peroxy radicals such as CH3OO(•).

Atmospheric Sink of (E)-3-Hexen-1-ol, (Z)-3-Hepten-1-ol, and (Z)-3-Octen-1-ol: Rate Coefficients and Mechanisms of the OH-Radical Initiated Degradation.

A kinetic study of the gas-phase reactions of OH radicals with three unsaturated biogenic alcohols, (E)-3-hexen-1-ol, (Z)-3-hepten-1-ol, and (Z)-3-octen-1-ol, has been performed. The rate coefficients obtained are (in units of 10(-10) cm(3) molecule(-1) s(-1)) k1 (OH + (E)-CH2(OH)CH2CH═CHCH2CH3) = (1.14 ± 0.14), k2 (OH + (Z)-CH2(OH)CH2CH═CHCH2CH2CH3) = (1.28 ± 0.23), and k3 (OH + (Z)-CH2(OH)CH2CH═CHCH2CH2CH2CH3) = (1.49 ± 0.35). In addition, a product study on the reactions of OH with (E)-3-hexen-1-ol and (Z)-3-hepten-1-ol is reported. All the experiments were performed at (298 ± 2) K and 1 atm of NOx-free air in a 1080 L photoreactor with in situ FTIR detection of organics. This work constitutes the first kinetic study of the reactions of OH radicals with (Z)-3-hepten-1-ol and (Z)-3-octen-1-ol as well as the first determination of the fate of the hydroxy alkoxy radicals formed in the title reactions. An analysis of the available rates of addition of OH and Cl to the double bond of different unsaturated alcohols at 298 K has shown that they can be related by the expression log kOH = (0.29 ± 0.04) log kCl - 10.8. The atmospheric lifetimes of the alcohols studies were estimated to be around 1 h for reaction with OH radicals. The products formed in the title reactions are mainly carbonylic compounds that can contribute to the formation of ozone and PANs-type compounds in the troposphere.

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.

Atmospheric oxidation of vinyl and allyl acetate: product distribution and mechanisms of the OH-initiated degradation in the presence and absence of NO(x).

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.