Rate coefficients and mechanisms of the reaction of cl-atoms with a series of unsaturated hydrocarbons under atmospheric conditions

Kinetic and Products Study of the Atmospheric Degradation of trans-2-Hexenal with Cl Atoms

The gas-phase reaction between trans-2-hexenal (T2H) and chlorine atoms (Cl) was studied using three complementary experimental setups at atmospheric pressure and room temperature. In this work, we studied the rate constant for the titled oxidation reaction as well as the formation of the gas-phase products and secondary organic aerosols (SOAs). The rate constant of the T2H + Cl reaction was determined using the relative method in a simulation chamber using proton-transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) to monitor the loss of T2H and the reference compound. An average reaction rate constant of (3.17 ± 0.72) × 10^-10 cm³ molecule^-1 s^-1 was obtained. From this, the atmospheric lifetime of T2H due to Cl reaction was estimated to be 9 h for coastal regions. HCl, CO, and butanal were identified as primary products using Fourier transform infrared spectroscopy (FTIR). The molar yield of butanal was (6.4 ± 0.3)%. Formic acid was identified as a secondary product by FTIR. In addition, butanal, 2-chlorohexenal, and 2-hexenoic acid were identified as products by gas chromatography coupled to mass spectrometry but not quantified. A reaction mechanism is proposed based on the observed products. SOA formation was observed by using a fast mobility particle sizer spectrometer. The measured SOA yields reached maximum values of about 38% at high particle mass concentrations. This work exhibits for the first time that T2H can be a source of SOA in coastal atmospheres, where Cl concentrations can be high at dawn, or in industrial areas, such as ceramic industries, where Cl precursors may be present.

Cl-Initiated oxidation of methacrolein under NOx-free conditions studied by VUV photoionization mass spectrometry

The Cl-initiated oxidation of methacrolein (MACR, C₄H₆O) under NO_x-free conditions has been investigated in a fast flow tube by using a home-made vacuum ultraviolet (VUV) photoionization mass spectrometer complemented by high-level theoretical calculations. The key species such as intermediates and radicals together with products involved in the oxidation are observed online and confirmed in photoionization mass spectra. The reaction potential energy surfaces of the transient C₄H₅O and C₄H₆OCl radicals, formed from the hydrogen-abstraction reaction and the addition reaction of MACR with Cl atoms, with oxygen have been theoretically calculated to illuminate the formation of the peroxy radicals of C₄H₅OO₂ and C₄H₆OClO₂. The photoionization processes of these peroxy radicals, whose cations are not stable, and their individual self-reactions as well as bimolecular reactions with HO₂ radical are studied and discussed. In addition, kinetic experiments are also performed to get the time evolution of specific products and compared with theoretical models, providing a detailed insight into the reaction mechanism of the Cl-initiated oxidation of MACR.

Isoprene-Chlorine Oxidation in the Presence of NOx and Implications for Urban Atmospheric Chemistry

Fine particulate matter (PM_2.5) is a key indicator of urban air quality. Secondary organic aerosol (SOA) contributes substantially to the PM_2.5 concentration. Discrepancies between modeling and field measurements of SOA indicate missing sources and formation mechanisms. Recent studies report elevated concentrations of reactive chlorine species in inland and urban regions, which increase the oxidative capacity of the atmosphere and serve as sources for SOA and particulate chlorides. Chlorine-initiated oxidation of isoprene, the most abundant nonmethane hydrocarbon, is known to produce SOA under pristine conditions, but the effects of anthropogenic influences in the form of nitrogen oxides (NO_x) remain unexplored. Here, we investigate chlorine-isoprene reactions under low- and high-NO_x conditions inside an environmental chamber. Organic chlorides including C₅H₁₁ClO₃, C₅H₉ClO₃, and C₅H₉ClO₄ are observed as major gas- and particle-phase products. Modeling and experimental results show that the secondary OH-isoprene chemistry is significantly enhanced under high-NO_x conditions, accounting for up to 40% of all isoprene oxidized and leading to the suppression of organic chloride formation. Chlorine-initiated oxidation of isoprene could serve as a source for multifunctional (chlorinated) organic oxidation products and SOA in both pristine and anthropogenically influenced environments.

New mechanistic understanding for atmospheric oxidation of isoprene initiated by atomic chlorine

Isoprene is the most abundant non-methane VOC and a significant SOA contributor. The atmospheric oxidation initiated by atomic chlorine is an important sink for isoprene, especially in certain regions with high Cl concentration, while its detailed oxidation mechanism remains unclear. In this work, we comprehensively investigated the reaction mechanism of isoprene with Cl using quantum chemistry calculation, and first elaborated the specific reaction mechanisms of chloroalkenyl peroxy radicals with HO₂/NO and the formation of 2-methylbut-3-enal, highlighting their important roles in the SOA formation. For the initial reactions, Cl additions to terminal carbons and H abstraction from CH₃ moiety of isoprene are the predominant reactions, which is consistent with previous research. Following the initial reactions, their subsequent reactions with O₂ and HO₂ (or NO) under different atmospheric conditions could lead to the formation of 17 highly oxidized molecules (HOMs), of which P10, P12, P16, P17, P19 and P33 generated by the subsequent reactions of the major first-generation products (MVK, CMBO, CMBA and MBO) have been detected in the reaction process of isoprene with Cl in the chamber experiment. In addition to auto-oxidation process, the reaction of chloroalkenyl peroxy radicals with HO₂/NO and their subsequent reactions are all easy to occur under atmospheric conditions, which could be crucial contributors to the formation of HOMs and SOA arising from the Cl initiated oxidation of isoprene. This study would be conducive to clarifying the atmospheric oxidation process of isoprene initiated by Cl and providing a new understanding of its SOA formation.

An experimental study of the gas-phase reaction between Cl atoms and trans-2-pentenal: Kinetics, products and SOA formation

The gas-phase reaction of trans-2-pentenal (T2P) with Cl atoms was studied at atmospheric pressure and room temperature. A rate coefficient of (2.56 ± 0.83) × 10^-10 cm³ molecule^-1 s^-1 was obtained using the relative rate method and isoprene, cyclohexane and ethanol as reference compounds. The kinetic study was carried out using a 300-L Teflon bag simulation chamber (IMT Lille Douai-France) and a 16-L Pyrex cell (UCLM-Ciudad Real-Spain), both coupled to the Fourier transform infrared (FTIR) technique. Gas-phase products and secondary organic aerosol (SOA) formation were studied at UCLM using a 16-L Pyrex cell and a 264-L quartz simulation chamber coupled to the FTIR and gas-chromatography-mass spectrometry (GC-MS) techniques. HCl, CO, and propanal were identified as products formed from the studied reaction and quantified by FTIR, the molar yield of the latter being (5.2 ± 0.2)%. Formic acid was identified as a secondary product and was quantified by FTIR with a yield of (6.2 ± 0.4)%. In addition, 2-chlorobutanal and 2-pentenoic acid were identified, but not quantified, by GC-MS as products. The SOA formation was investigated using a fast mobility particle sizer spectrometer. The observed SOA yields reached maximum values of around 7% at high particle mass concentrations. This work provides the first study of the formation of gaseous and particulate products for the reaction of Cl with T2P. A reaction mechanism is suggested to explain the formation of the observed gaseous products. The results are discussed in terms of structure-reactivity relationship, and the atmospheric implications derived from this study are commented as well.

Enhanced wintertime oxidation of VOCs via sustained radical sources in the urban atmosphere

Daytime atmospheric oxidation chemistry is conventionally considered to be driven primarily by the OH radical, formed via photolytic sources. In this paper we examine how, during winter when photolytic processes are slow, chlorine chemistry can have a significant impact on oxidative processes in the urban boundary layer. Photolysis of nitryl chloride (ClNO₂) provides a significant source of chlorine atoms, which enhances the oxidation of volatile organic compounds (VOCs) and the production of atmospheric pollutants. We present a set of observations of ClNO₂ and HONO made at urban locations in central England in December 2014 and February 2016. While direct emissions and in-situ chemical formation of HONO continue throughout the day, ClNO₂ is only formed at night and is usually completely photolyzed by midday. Our data show that, during winter, ClNO₂ often persists through the daylight hours at mixing ratios above 10-20 ppt (on average). In addition, relatively high mixing ratios of daytime HONO (>65 ppt) provide a strong source of OH radicals throughout the day. The combined effects of ClNO₂ and HONO result in sustained sources of Cl and OH radicals from sunrise to sunset, which form additional ozone, PAN, oxygenated VOCs, and secondary organic aerosol. We show that radical sources such as ClNO₂ and HONO can lead to a surprisingly photoactive urban atmosphere during winter and should therefore be included in atmospheric chemical models.

Gas-Phase Reaction of trans-2-Methyl-2-butenal with Cl: Kinetics, Gaseous Products, and SOA Formation

The gas-phase reaction between trans-2-methyl-2-butenal and chlorine (Cl) atoms has been studied in a simulation chamber at 298 ± 2 K and 760 ± 5 Torr of air under free-NO_x conditions. The rate coefficient of this reaction was determined as k = (2.45 ± 0.32) × 10-¹⁰ cm³ molecule⁻¹ s⁻¹ by using a relative method and Fourier transform infrared spectroscopy. In addition to this technique, gas chromatography coupled to mass spectrometry and proton transfer time-of-flight mass spectrometry were used to detect and monitor the time evolution of the gas-phase reaction products. The major primary reaction product from the addition of Cl to the C-3 of trans-2-methyl-2-butenal was 3-chloro-2-butanone, with a molar yield (Y_Prod) of (52.5 ± 7.3)%. Acetaldehyde (Y = (40.8 ± 0.6)%) and HCl were also identified, indicating that the H-abstraction by Cl from the aldehyde group is a reaction pathway as well. Secondary organic aerosol (SOA) formation was investigated by using a fast mobility particle sizer spectrometer. The SOA yield in the Cl + trans-2-methyl-2-butenal reaction is reported to be lower than 2.4%, thus its impact can be considered negligible. The atmospheric importance of the titled reaction is similar to the corresponding OH reaction in areas with high Cl concentration.

Atmospheric oxidation mechanism and kinetics of isoprene initiated by chlorine radicals: A computational study

The reaction with chlorine radicals (·Cl) has been considered to be one of indispensable sinks for isoprene. However, the mechanism of ·Cl initiated isoprene reaction was not fully understood. Herein, the reaction of isoprene with ·Cl, and ensuing reactions of the resulting isoprene relevant radicals were investigated by combined quantum chemistry calculations and kinetics modeling. The results indicate that ·Cl addition to two terminal C-atoms of two double bonds of isoprene, forming IM_1-1 and IM_1-4, are more favorable than H-abstractions from isoprene. Interestingly, the predicted reaction rate constant for the direct H-abstraction pathway is much lower than that of the indirect one, clarifying a direct H-abstraction mechanism for previously experimental observation. The IM_1-1 and IM_1-4 have distinct fate in their subsequent transformation. The reaction of IM_1-1 ends after the one-time O₂ addition. However, IM_1-4 can follow auto-oxidation mechanism with two times O₂ addition to finally form highly oxidized multi-functional molecules (HOMs), C₅H₇ClO₃ and ·OH. More importantly, the estimated contribution of ·Cl on HOMs (monomer only) formation from isoprene is lower than that of ·OH in addition pathway, implying overall HOMs yield from atmospheric isoprene oxidation could be overestimated if the role of ·Cl in transforming isoprene is ignored.

Anthropogenic control over wintertime oxidation of atmospheric pollutants

During winter in the mid-latitudes, photochemical oxidation is significantly slower than in summer and the main radical oxidants driving formation of secondary pollutants, such as fine particulate matter and ozone, remain uncertain, owing to a lack of observations in this season. Using airborne observations, we quantify the contribution of various oxidants on a regional basis during winter, enabling improved chemical descriptions of wintertime air pollution transformations. We show that 25-60% of NO_x is converted to N₂O₅ via multiphase reactions between gas-phase nitrogen oxide reservoirs and aerosol particles, with ~93% reacting in the marine boundary layer to form >2.5 ppbv ClNO₂. This results in >70% of the oxidizing capacity of polluted air during winter being controlled, not by typical photochemical reactions, but from these multiphase reactions and emissions of volatile organic compounds, such as HCHO, highlighting the control local anthropogenic emissions have on the oxidizing capacity of the polluted wintertime atmosphere.

Quantum mechanical study on the oxidation of ethyl vinyl ketone initiated by an OH radical

Oxidation of ethyl vinyl ketone (CH2CHCOCH2CH3) by an OH radical was carried out using the M06-2X/6-311++G(d,p) level of theory. For the OH-initiated oxidation of ethyl vinyl ketone (EVK), we have considered six H-atom abstractions and three addition reactions. From the energetic calculation of the species involved therein, the potential energy surface (PES) of all the reaction channels was constructed. From the energy profile, we found that the H-atom abstraction from the methylene group (-CH2-) of CH2CHCOCH2CH3 is energetically more favourable than the other H-abstraction channels. Moreover, we also observed that OH-addition to the α-carbon of the carbon-carbon double bond of the title molecule is energetically and thermodynamically more dominant than β-carbon and carbonyl carbon. The rate coefficients for all the reaction channels were calculated using the canonical transition state theory at the temperature range of 250-450 K and it reveals that among all the reaction channels, OH-addition to α-carbon is kinetically more dominant to the total rate constant. The total rate coefficient for the reaction at 298 K is found to be in good agreement with the reported experimental rate constant. Finally, we have determined the atmospheric lifetime of the title molecule.

Kinetics of atmospheric reactions of 4-chloro-1-butene

Chloroalkenes are among the important anthropogenic organic compounds emitted in the atmosphere as a result of their wide use in synthetic processes in industry. Despite their well-known adverse effects on human health and air quality, the chemistry of these chloroalkenes remains poorly explored. In this work, reactions of 4-chloro-1-butene (CBE), a representative example of chloroalkenes, with O₃, OH, NO₃, and Cl are investigated in a 100-L Teflon reaction chamber equipped with gas chromatography-flame ionization detector (GC-FID). The absolute rate method was used for the reaction with O₃ while the relative rate method was used for reactions with OH, NO₃, and Cl. The following rate constants were obtained at room temperature (298 ± 2) K and atmospheric pressure: (3.96 ± 0.43) × 10^-18, (2.63 ± 0.96) × 10^-11, (4.48 ± 1.23) × 10^-15, and (2.35 ± 0.90) × 10^-10 cm³ molecule^-1 s^-1, for reactions with O₃, OH, NO₃, and Cl, respectively. Atmospheric lifetimes of CBE calculated from rate constants of the different reactions obtained in this work showed that reaction with OH is the main loss process for CBE, while in coastal areas and in the marine boundary layer, the CBE loss by Cl reaction becomes important. Estimation of the value of the photochemical ozone creation potential (POCP) indicated that CBE has a large ozone formation potential. The present work underlines the need for further studies on the atmospheric chemistry of chlorinated VOCs.

Gas-Phase Reactions of Isoprene and Its Major Oxidation Products

Isoprene carries approximately half of the flux of non-methane volatile organic carbon emitted to the atmosphere by the biosphere. Accurate representation of its oxidation rate and products is essential for quantifying its influence on the abundance of the hydroxyl radical (OH), nitrogen oxide free radicals (NO _x), ozone (O₃), and, via the formation of highly oxygenated compounds, aerosol. We present a review of recent laboratory and theoretical studies of the oxidation pathways of isoprene initiated by addition of OH, O₃, the nitrate radical (NO₃), and the chlorine atom. From this review, a recommendation for a nearly complete gas-phase oxidation mechanism of isoprene and its major products is developed. The mechanism is compiled with the aims of providing an accurate representation of the flow of carbon while allowing quantification of the impact of isoprene emissions on HO _x and NO _x free radical concentrations and of the yields of products known to be involved in condensed-phase processes. Finally, a simplified (reduced) mechanism is developed for use in chemical transport models that retains the essential chemistry required to accurately simulate isoprene oxidation under conditions where it occurs in the atmosphere-above forested regions remote from large NO _x emissions.

Gas-Phase Oxidation of Allyl Acetate by O3, OH, Cl, and NO3: Reaction Kinetics and Mechanism

Allyl acetate (AA) is widely used as monomer and intermediate in industrial chemicals synthesis. To evaluate the atmospheric outcome of AA, kinetics and mechanism of its gas-phase reaction with main atmospheric oxidants (O₃, OH, Cl, and NO₃) have been investigated in a Teflon reactor at 298 ± 3 K. Both absolute and relative rate methods were used to determine the rate constants for AA reactions with the four atmospheric oxidants. The obtained rate constants (in units of cm³ molecule^-1 s^-1) are (1.8 ± 0.3) × 10^-18, (3.1 ± 0.7) × 10^-11, (2.5 ± 0.5) × 10^-10, and (1.1 ± 0.4) × 10^-14, for reactions with O₃, OH, Cl, and NO_3, respectively. While results for reactions with O₃, OH and Cl are in good agreement with previous studies, the kinetics for the reaction with NO₃ is reported for the first time in this study. On the basis of determined rate constants, the tropospheric lifetimes of AA are τ_O3 = 9 days, τ_OH = 5 h, τ_Cl = 5 days, τ_NO3 = 2 days. On the basis of the products study, reaction mechanisms for these oxidations have been proposed and the reaction products were detected using thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) and Fourier transform infrared spectroscopy (FTIR). Results show that the main products formed in these reactions are carbonyl compounds. In particular, 2-oxoethyl acetate was detected in all four AA oxidation reactions. Compared to previous studies, several new products were determined for reactions with OH and Cl. These results form a set of comprehensive kinetic data for AA reactions with main atmospheric oxidants and provide a better understanding of the degradation and atmospheric outcome of unsaturated acetate esters in the troposphere, during both daytime and nighttime.

Theoretical investigations on the kinetics of Cl atom initiated reactions of series of 1-alkenes

The temperature-dependent rate coefficients were calculated for the reactions of Cl atoms with propene (R₁), 1-butene (R₂), 1-pentene (R₃), and 1-hexene (R₄) over the temperature range of 200-400 K. Canonical variational transition state theory (CVT) with small curvature tunneling (SCT) and conventional transition state theory (CTST) in combination with MP2/6-31G(d,p), MP2/6-31G+(d,p), and MP2/6-311 + G(d,p) level of theories were used to calculate the kinetic parameters. The obtained rate coefficients at 298 K for the reactions of Cl atoms with propene, 1-butene, 1-pentene, and 1-hexene are 1.36 × 10^-10 cm³ molecule^-1 s^-1, 1.53 × 10^-10 cm³ molecule^-1 s^-1, 4.61 × 10^-10 cm³ molecule^-1 s^-1, and 4.76 × 10^-10 cm³ molecule^-1 s^-1, respectively. In all these reactions, strong negative temperature dependence was observed over the studied temperature range. Cl atom addition across the double bond is the most dominant pathway. The contribution of abstraction channels towards their global rate coefficients was observed to be increasing from propene to 1-hexane. Atmospheric implications such as effective lifetimes and thermodynamic parameters of the test molecules were investigated in the present study.

Atmospheric sink of β-ocimene and camphene initiated by Cl atoms: kinetics and products at NOxfree-air

Rate coefficients for the gas-phase reactions of Cl atoms with β-ocimene and camphene were determined to be (in units of 10⁻¹⁰cm³per molecule per s) 5.5 ± 0.7 and 3.3 ± 0.4, respectively.

An Exprimental and Computational Study on the Cl Atom Initiated Photo-Oxidization Reactions of Butenes in the Gas Phase

Temperature-dependent rate coefficients for the reactions of Cl atoms with trans-2-butene and isobutene were measured over the temperature range of 263-363 K using relative rate technique with reference to 1,3-butadiene, isoprene, and 1-pentene. The measured rate coefficients for the reactions of Cl atoms with isobutene and trans-2-butene are k_R1^298K= (3.43 ± 0.11) × 10^-10 and k_R2^298K = (3.20 ± 0.04) × 10^-10 cm³ molecule^-1 s^-1, respectively, at 298 K and 760 torr. Measured rate coefficients were used to fit the Arrhenius equations, which are obtained to be k_R1-Exp^269-363K = (4.99 ± 0.42) × 10^-11 exp[(584 ± 26)/T] and k_R2-Exp^269-363K = (1.11 ± 0.3) × 10^-10 exp[(291 ± 88)/T] cm³ molecule^-1 s^-1 for isobutene and trans-2-butene, respectively. To understand the reaction mechanism, estimate the contribution of each reaction site, and to complement our experimental results, computational studies were also performed. Canonical variational transition state theory with small curvature tunneling in combination with MP2/6-31G(d), MP2/6-31G(d,p), MP2/6-31+G(d,p), CCSD(T)/cc-pvdz, and QCISD(T)/cc-pvdz level of theories were used to calculate the temperature-dependent rate coefficients over the temperature range of 200-400 K. The effective lifetimes, thermodynamic parameters, and atmospheric implications of the test molecules were also estimated.

Mechanism and kinetics for the reactions of methacrolein and methyl vinyl ketone with HO2 radical

The mechanism and kinetics for the reactions of unsaturated aldehyde and ketone with HO₂ radical were investigated.

Atmospheric fate of hydrofluoroolefins, CxF2x+1CHCH2 (x = 1,2,3,4 and 6): Kinetics with Cl atoms and products

Rate coefficients for the gas-phase reactions of C_xF_2x+1CHCH₂ (x = 1, 2, 3, 4 and 6) with Cl atoms were determined at (298 ± 2) K and (710 ± 5) Torr of air using a relative rate technique. Two experimental setups with simulation chambers were employed with Fourier Transform Infrared (FTIR) spectroscopy and Gas Chromatography coupled to Mass Spectrometry (GC-MS) as detection techniques. The Cl-rate coefficients obtained were (in 10^-10 cm³ molecule^-1 s^-1): (0.85 ± 0.11) for CF₃CHCH₂, (1.11 ± 0.08) for C₂F₅CHCH₂, (1.12 ± 0.18) for C₃F₇CHCH₂, (0.97 ± 0.09) for C₄F₉CHCH₂, and (0.99 ± 0.08) for C₆F₁₃CHCH₂. Additionally, the gas-phase products were identified and quantified, when possible, by FTIR spectroscopy or GC-MS. The main reaction product was reported to be C_xF_2x+1C(O)CH₂Cl. The fluorinated species, C_xF_2x+1CHO and C_xF_2x+1C(O)CH₂Cl, were identified. CF₃C(O)CH₂Cl and CF₃CHO were found to be formed with molar yield of (69 ± 5)% and (9 ± 1)%, respectively. The global lifetime of the investigated C_xF_2x+1CHCH₂ due to their Cl-reaction is more than 100 days so this route does not compete with the removal by OH radicals. This lifetime is long enough for C_xF_2x+1CHCH₂ to be transported to remote areas where they can be degraded. However, at a local scale, in marine regions at dawn the removal of C_xF_2x+1CHCH₂ is expected to occur in ca. 1 day. The atmospheric degradation of these hydrofluoroolefins by Cl atoms is not expected to be a source of bioaccumulative perfluorinated carboxylic acids, C_xF_2x+1C(O)OH. Additionally, the UV absorption cross sections of CF₃C(O)CH₂Cl were determined together with the rate coefficient of the OH reaction by an absolute kinetic method at room temperature.

Is H Atom Abstraction Important in the Reaction of Cl with 1-Alkenes?

The relative yields of products of the reaction of Cl atoms with 1-alkenes (C4-C9) were determined to see whether H atom abstraction is an important channel and if it is to identify the preferred position of abstraction. The presence of all the possible positional isomers of long chain alkenones and alkenols among the products, along with chloroketones and chloroalcohols, confirms the occurrence of H atom abstraction. A consistent pattern of distribution of abstraction products is observed with oxidation at C4 (next to allyl) being the lowest and that at CH2 groups away from the double bond being the highest. This contradicts with the higher stability of allyl (C3) radical. For a better understanding of the relative reactivity, ab initio calculations at MP2/6-311+G (d,p) level of theory are carried out in the case of 1-heptene. The total rate coefficient, calculated using conventional transition state theory, was found to be in good agreement with the experimental value at room temperature. The preferred position of Cl atom addition is predicted to be the terminal carbon atom, which matches with the experimental observation, whereas the rate coefficients calculated for individual channels of H atom abstraction do not explain the observed pattern of products. The distribution of abstraction products except at C4 is found to be better explained by reported structure activity relationship, developed from experimental rate coefficient data. This implies the reactions to be kinetically dictated and emphasizes the importance of secondary reactions.

Quantum Chemical Study on ·Cl-Initiated Atmospheric Degradation of Monoethanolamine

Recent findings on the formation of ·Cl in continental urban areas necessitate the consideration of ·Cl initiated degradation when assessing the fate of volatile organic pollutants. Monoethanolamine (MEA) is considered as a potential atmospheric pollutant since it is a benchmark and widely utilized solvent in a leading CO2 capture technology. Especially, ·Cl may have specific interactions with the N atom of MEA, which could make the MEA + ·Cl reaction have different pathways and products from those of the MEA + ·OH reaction. Hence, ·Cl initiated reactions with MEA were investigated by a quantum chemical method [CCSD(T)/aug-cc-pVTZ//MP2/6-31+G(3df,2p)] and kinetics modeling. Results show that the overall rate constant for ·Cl initiated H-abstraction of MEA is 5 times faster than that initiated by ·OH, and the tropospheric lifetimes of MEA will be overestimated by 6-46% when assuming that [·Cl]/[·OH] = 1-10% if the role of ·Cl is ignored. The MEA + ·Cl reaction exclusively produces MEA-N that finally transforms into several products including mutagenic nitramine and carcinogenic nitrosamine via further reactions with O2/NOx, and the contribution of ·Cl to their formation is about 25-250% of that of ·OH. Thus, it is necessary to consider ·Cl initiated tropospheric degradation of MEA for its risk assessment.

Gas-phase reaction of two unsaturated ketones with atomic Cl and O3: kinetics and products

The rate constants and products for the reactions of atomic Cl and O₃ molecule with 3-methyl-3-buten-2-one (MBO332) and 3-methyl-3-penten-2-one (MPO332) were determined in a 100 L Teflon chamber at 293 ± 1 K and atmospheric pressure.

FTIR gas-phase kinetic study on the reactions of some acrylate esters with OH radicals and Cl atoms

Acrylate esters are α,β-unsaturated esters that contain vinyl groups directly attached to the carbonyl carbon. These compounds are widely used in the production of plastics and resins. Atmospheric degradation processes of these compounds are currently not well understood. The kinetics of the gas phase reactions of OH radicals with methyl 3-methylacrylate and methyl 3,3-dimethylacrylate were determined using the relative rate technique in a 50 L Pyrex photoreactor using in situ FTIR spectroscopy at room temperature (298 ± 2 K) and atmospheric pressure (708 ± 8 Torr) with air as the bath gas. Rate coefficients obtained were (in units cm(3) molecule(-1) s(-1)): (3.27 ± 0.33) × 10(-11) and (4.43 ± 0.42) × 10(-11), for CH3CH═CHC(O)OCH3 and (CH3)2CH═CHC(O)OCH3, respectively. The same technique was used to study the gas phase reactions of hexyl acrylate and ethyl hexyl acrylate with OH radicals and Cl atoms. In the experiments with Cl, N2 and air were used as the bath gases. The following rate coefficients were obtained (in cm(3) molecule(-1) s(-1)): k3 (CH2═CHC(O)O(CH2)5CH3 + Cl) = (3.31 ± 0.31) × 10(-10), k4(CH2═CHC(O)OCH2CH(CH2CH3)(CH2)3CH3 + Cl) = (3.46 ± 0.31) × 10(-10), k5(CH2═CHC(O)O(CH2)5CH3 + OH) = (2.28 ± 0.23) × 10(-11), and k6(CH2═CHC(O)OCH2CH(CH2CH3)(CH2)3CH3 + OH) = (2.74 ± 0.26) × 10(-11). The reactivity increased with the number of methyl substituents on the double bond and with the chain length of the alkyl group in -C(O)OR. Estimations of the atmospheric lifetimes clearly indicate that the dominant atmospheric loss process for these compounds is their daytime reaction with the hydroxyl radical. In coastal areas and in some polluted environments, Cl atom-initiated degradation of these compounds can be significant, if not dominant. Maximum Incremental Reactivity (MIR) index and global warming potential (GWP) were also calculated, and it was concluded that these compounds have significant MIR values, but they do not influence global warming.

Reactivity of Cl atom with triple-bonded molecules. An experimental and theoretical study with alcohols

The reactivities of the Cl atom with triple-bonded molecules were examined by determining the rate coefficients of reactions of four triple-bonded alcohols (TA), namely, 2-propyn-1-ol, 3-butyn-1-ol, 3-butyn-2-ol, and 2-methyl-3-butyn-2-ol, using the relative rate method, at 298 K. The rate coefficients (k) of reaction of the four alcohols with Cl vary in the range (3.5-4.3) × 10(-10) cm(3) molecule(-1) s(-1). These values imply significant contribution of the Cl reaction in the tropospheric degradation of TAs in the conditions of the marine boundary layer. A striking difference is observed in the reactivity trend of Cl from that of OH/O3. Although the reactivity of OH/O3 is lower with triple-bonded molecules, as compared to the double-bonded analogues, the reactivity of the Cl atom is similar for both. For a deeper insight, the reactions of Cl and OH with the simplest TA, 2-propyn-1-ol, are investigated theoretically. Conventional transition state theory is applied to compute the values of k, using the calculated energies at QCISD and QCISD(T) levels of theory of the optimized geometries of the reactants, transition states (TS), and the product radicals of all the possible reaction pathways at the MP2/6-311++G(d,p) level. The k values calculated at the QCISD level for Cl and the QCISD(T) level for OH reactions are found to be very close to the experimental values at 298 K. In the case of the Cl reaction, the abstraction of α-H atoms as well as the addition at the terminal and middle carbon atoms have submerged TS and the contribution of the abstraction reaction is found to be significant at room temperature, at all levels of calculations. Addition at the terminal carbon atom is prominent compared to that at the middle carbon. In contrast to the Cl reaction, only addition at the middle carbon is associated with such low lying TS in the case of OH. The individual rate coefficients of addition and abstraction of OH are lower than that of Cl. The negative temperature dependence of the computed rate coefficients in the temperature range 200-400 K shows that the difference in the TS energy of Cl and OH affects the pre-exponential factor more than the activation energy.

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.

Insights into mechanistic photodissociation of chloroacetone from a combination of electronic structure calculation and molecular dynamics simulation

The stationary and intersection structures on the S(0) and S(1) potential energy surfaces of CH(3)COCH(2)Cl have been determined by the CAS(10,8)/cc-pVDZ optimizations and their relative energies are refined by the CASPT2//CAS(10,8)/cc-pVDZ single-point calculations. Non-adiabatic molecular dynamics simulations were performed on the basis of the state-averaged CAS(10,8)/cc-pVDZ calculated energies, energy gradients, and Hessian matrix for the S(0) and S(1) states. It is found that the features of the S(1) potential energy surface and non-adiabatic effect control the selectivity of the two α-C-C bond fissions, which provides a reasonable explanation why one α-C-C bond was observed as a primary channel and the other is ruled out even if CH(3)COCH(2)Cl is excited at 193 nm. The β-C-Cl fission is determined to be a dominant channel once the CH(3)COCH(2)Cl molecule is excited to the S(1) state and the β-C-Cl:α-C-C branching ratio is estimated by the RRKM rate theory to be 15:1 at 193 nm, which is overestimated in comparison with the value of ~11:1 inferred experimentally. The present calculation reveals that the α-C-C fission might take place in the ground electronic state as a result of the S(1) → S(0) internal conversion upon photolysis at 308 nm. However, the measured kinetic energy distributions of the α-C-C fission products suggest that the fission does not involve internal conversion to the ground state. To solve this issue, we need to perform non-adiabatic quantum dynamics simulation on accurate S(0), S(1), and S(2) potential energy surfaces, which is still a challenging task currently.

The effects of active chlorine on photooxidation of 2-methyl-2-butene

Active chlorine comprising hypochlorite (OCl⁻), hypochlorous acid (HOCl) and chlorine (Cl₂) is the active constituent in bleach formulations for a variety of industrial and consumer applications. However, the strong oxidative reactivity of active chlorine can cause adverse effects on both human health and the environment. In this study, aerosolized Oxone® [2KHSO₅, KHSO₄, K₂SO₄] with saline solution has been utilized to produce active chlorine (HOCl and Cl₂). To investigate the impact of active chlorine on volatile organic compound (VOC) oxidation, 2-methyl-2-butene (MB) was photoirradiated in the presence of active chlorine using a 2-m³ Teflon film indoor chamber. The resulting carbonyl products produced from photooxidation of MB were derivatized with O-(2,3,4,5,6-pentafluorobenzyl) hydroxyamine hydrochloride (PFBHA) and analyzed using gas chromatograph-ion trap mass spectrometer (GC/ITMS). The photooxidation of MB in the presence of active chlorine was simulated with an explicit kinetic model using a chemical solver (Morpho) which included both Master Chemical Mechanism (MCM) and Cl radical reactions. The reaction rate constants of a Cl radical with MB and its oxidized products were estimated using a Structure-Reactivity Relationship method. Under dark conditions no effect of active chlorine on MB oxidation was apparent, whereas under simulated daylight conditions (UV irradiation) rapid MB oxidation was observed due to photo-dissociation of active chlorine. The model simulation agrees with chamber data showing rapid production of oxygenated products that are characterized using GC/ITMS. Ozone formation was enhanced when MB was oxidized in the presence of irradiated active chlorine and NO(x).

Gas phase reactions of unsaturated esters with Cl atoms

BACKGROUND, AIM, AND SCOPE

Acrylate and methacrylate esters are alpha,beta-unsaturated esters that contain vinyl groups directly attached to the carbonyl carbon (CH(2)=CHCOO- and CH(2)=CCH(3)COO-, respectively) and are widely used in the polymer plastic and resin production. Rate coefficients for Cl reactions for most of the unsaturated esters have not been previously determined, and a good understanding is needed of all the atmospheric oxidation processes of these compounds in order to determine lifetimes in the atmosphere and to evaluate the impact of these reactions on the formation of photo-oxidants and therefore on health and environment.

MATERIALS AND METHODS

The relative rate technique has been used to obtain rate coefficients for the reactions between the Cl atom and a series of unsaturated esters. The experiments have been carried out in a static Teflon reactor at room temperature and atmospheric pressure (N(2) as bath gas) using gas chromatography with flame ionization detection as detection system.

RESULTS

The following rate coefficients are obtained (in cubic meter per molecule per second): methyl acrylate + Cl = 1.71 +/- 0.13 x 10(-10); methyl methacrylate + Cl = 2.30 +/- 0.18 x 10(-10); ethyl acrylate + Cl = 1.82 +/- 0.13 x 10(-10); ethyl methacrylate + Cl = 2.71 +/- 0.21 x 10(-10); butyl acrylate + Cl = 2.94 +/- 0.23 x 10(-10); butyl methacrylate + Cl = 3.83 +/- 0.30 x 10(-10); methyl 3-methyl acrylate + Cl = 2.21 +/- 0.17 x 10(-10); and methyl 3,3-dimethyl acrylate + Cl = 3.58 +/- 0.28 x 10(-10).

DISCUSSION

Rate coefficients calculated for Cl reactions are around one order of magnitude higher than OH ones. The effect in the reactivity of increased substitution at the carbon-carbon double bond is analyzed and also the effect of the identity of the alkyl group R in the -C(O)OR. Atmospheric lifetimes of the compounds against the attack by the major oxidants are estimated and the atmospheric implications are discussed.

CONCLUSIONS

The dominant atmospheric loss process for acrylate esters is clearly their daytime reaction with the hydroxyl radical. However, in coastal areas and in the marine boundary layer and in some industrial zones, Cl-atom-initiated degradation of the unsaturated esters considered here can be a significant if not dominant homogeneous loss process.

RECOMMENDATIONS AND PERSPECTIVES

Product analysis should be necessary in order to evaluate the real environmental impact of these reactions. OH and ozone reactions of most of the considered compounds have already been studied and products determined, but kinetic and products information for NO(3) radical reactions is especially scarce.

Decomposition of substituted alkoxy radicals--part I: a generalized structure-activity relationship for reaction barrier heights

An update and expansion of our readily applicable structure-activity relationship (SAR) for predicting the barrier height E(b) to decomposition by beta C-C scission of (substituted) alkoxy radicals is presented. Such alkoxy radicals are key intermediates in the atmospheric oxidation of volatile organic compounds, and a correct description of their chemistry is vital to the understanding of atmospheric chemistry; nevertheless, experimental data on these reactions remain scarce. The SAR is based on quantum chemical characterizations of a large set of alkoxy radicals, and accommodates alkoxy radicals with alkyl- (-R), oxo- (=O), hydroxy- (-OH), hydroperoxy (-OOH), alkoxy (-OR), alkylperoxy- (-OOR), nitroso- (-NO), nitro- (-NO2), nitrosooxy- (-ONO), and nitroxy- (-ONO2) functionalities, as well as 3- to 6-membered rings and some unsaturated side chains. The SAR expresses the barrier height to decomposition, E(b) = 17.9 kcal mol(-1) + Sigma N(s) x F(s), as a linear function of the number N(s) of these substituents on the relevant carbons, and the substituent-specific activities F(s) derived from the quantum chemical calculations, allowing facile predictions based solely on the molecular structure. For low barriers, < or = 7 kcal mol(-1), a simple curvature correction is required. The SAR-predicted barrier height E(b) can be used to predict the high-pressure rate coefficient for alkoxy decomposition k(diss) at or around 298 K.