Degradation Mechanism And Kinetics Study Of Alkenyl Acetate In The Atmosphere

Alkenyl aetate compounds are important volatile organic compounds (VOCs) which have an extensive air emissions and a wide range of structural diversity. Once in the troposphere, they will undergo either photolysis or photochemical transformation by OH, NO3radicals, O3molecules and Cl atoms, to form important atmospheric species including ketones, aldehydes, anhydrides and the Criegee intermediates. These products make great impact on the atmospheric oxidation capacity, degrade air visibility and do harm to the life on Earth. Also they play a certain role in the formation of secondary pollutant and SOAs. Therefore, it is necessary to investigate degradation mechanism of acetate compounds in order to understand the complex environmental decomposition process.This paper aims to perform a systematic theoretical research for the gas-phase reaction mechanism of alkenyl aetate compounds by high-level Density Functional Theory (DFT) calculations. In this thesis, vinyl acetate (VAC), isopropenyl acetate (IPA), propenyl acetate (PA),3-hexenyl acetate (3-HA) and2-hexenyl acetate (3-HA) are investigated as representative acetate compounds.1. The ozonolysis of alkenyl aetate compoundsThe ozonolysis of alkenyl aetate compounds follows the so-called Criegee mechanism. The primary ozonides are formed readily via the addition of O3to the double bonds. The secondary reactions of energy-rich Criegee intermediate have been studied. From the results, we get the following important points:(1)The reaction mechanisms and kinetics for the reaction of VAC with O3For two decomposition channels of the primary ozonide (IM2), the cleavage process of C1-C2and04-05bonds to form formic acid anhydride (P1) and CH2OO is more favorable than that of C1-C2and03-04bonds break reaction. The reaction is dominant with a branching ratio of0.75at (293±3) K. HPMF, formic acid anhydride and formaldehyde are the main products of the reaction of VAC with O3. The formation mechanism of formic acid anhydride from CH3OCHOO is the NO abstract O atom in CH3OCHOO directly which is both thermodynamics and dynamics favorable. The rate constant is2.48×10-17cm-3molecule-1s-1at290K. The rate constants show independence on pressure, but positive dependence on temperature over the whole study range (0.1-10000Torr and200-1000K).(2)Mechanism and kinetics of the ozonolysis of isopropenyl and propenyl acetateFor the decomposition of the primary ozonides, the rupture of C2-C3and O4-O5bonds is more feasible than that of C2-C3and03-04bonds in thermodynamics and dynamics. The mechanism of formation of secondary organic aerosol (SOZ) is studied through two channels:anhydrides+biradicals and aldehydes+biradicals. Both of these two channels are preferred with moderate energy barriers and high exothermicity. SOZs are much more stable than The Primary Ozonides (POZs). The most favorable channel is NO directly abstraction of marginal O atom in the secondary reactions. H2O2and OH radicals can also be released via the reaction of biradicals with H2O. At298K and760Torr, the following rate coefficients (incm3molecule-1s-1) are obtained:k(IPA+O3)=1.30×10-18, k((E)-PA+O3)=3.02×10-18,K((z)-PA+O3)=9×10-19. The rate constants show dependence on temperature over the range of200-400K at760Torr.(3)Mechanistic and Kinetic study on the Ozonolysis of Isopropenyl Acetate and Propenyl AcetateThe ozonolysis of the four reactants(cis-3-HA, trans-3-HA, cis-2-HA, trans-2-HA)have same reaction mechanisms:POZs are produced through O3addition to the carbon-carbon double bonds and then two decomposition channels of POZs are followed. The reactions of O3with trans-3-HA and cis-2-HA are easier than that of O3with cis-3-HA and trans-2-HA. Among the further reactions of Criegee intermediates, the main products are1-hydroperoxypropan-l-ol+propionaldehyde for CH3CH2CH2OO and3-hydroperoxy-3-hydro-xyprpyl acetate+butyraldehyde for CH3CH2CH2CHOO. For further reactions of CH3C(O)C(CH2)2O2, the pathways leading to CH3COCH2CH2CH(OH)OOH along with3-oxopropyl acetate are more favorable than other channels. The calculated total rate constants are9.84×10-17,7.37×10-17,2.50×10-17and1.39×10-17cm3molecule-1s-1for cis/trans-3-hexenyl acetate and cis/trans-2-hexenyl acetate (298K and760Torr). The individual rate constants show independence on pressure and dependence on temperature.2. The mechanisms and kinetics of Cl-initiated oxidation of vinyl acetateTwo reaction types (Cl-addition and H-abstraction) and the subsequent reactions for the primary intermediates (IM1and IM2) have been proposed in the existence of NO and O2. The calculated results show that the Cl addition-elimination mechanism dominates the reaction between vinyl acetate and Cl. For the Cl addition reaction, the most energetically favorable isomer is that of Cl addition to the terminal carbon position (CH2Cl-CH-OC(O)CH3). Most H-abstraction pathways are not expected to play an important role in forming final products. The major possible degradation products have been identified, including CH2ClC(O)OC(O)CH3, CH3CO2CHO, CH2O, CO2, CH3COOH and CH3C(O)OCClO. The calculated total rate constant is2.67×10-10cm3molecule-1s-1(at298K and760Torr). The rate constants show dependence on pressure (0.01-10000Torr) and temperature over the range of200-1000K. The peak concentrations of Cl atoms could reach1×105molecule cm-3in the marine boundary layer and industrial area. Under the circumstances, atmospheric lifetime of VAC with Cl is only10.42hours. This suggests that C1-initiated oxidation of VAC could compete with the corresponding hydroxylation reaction (τOH=6hours) in determining the lifetimes of VAC during the early morning in coastal regions, the marine boundary layer and some industrial areas.