Researches On The Oxidation Mechanisms Of Typical Organic Pollutants In The Atmosphere

Oxygenated volatile organic compounds (OVOCs) are very important trace constituents in the troposphere. They have extensive source and a wide variety of kinds and mainly including carbonyls, alcohols, ethers and esters etc. They have high reactivity and can react with oxidizing compounds in the troposphere (e.g., OH, Cl, NO3 and O3) or through direct photodegradation, producting activated radicals and second pollutants. Most of products in the atmosphere can contribute to the formation of secondary organic aerosol (SOA) through nucleation, hydration or absorption since their polarity and water-solubility are enhanced. Because of the small size, long average lifetime and adverse environment effects of SOA, they not only degrade air visibility, change global climate, but also are a severe threaten to human body. Therefore, the oxidation process investigation of volatile organic compounds in atmosphere is of great significance for exploring the regional and global atmospheric chemistry, climate change and environmental effects.Persistent organic pollutants (POPs) refers to persist in the environment, having serious hazards to human health and the environment, natural or synthetic oganic contaminants. They mainly include Dioxin, DDT, PCBs, etc. Polychlorinated biphenyls (PCBs) are recognized as a typical POPs. Due to the good thermal and chemical stability and low electrical conductivity, PCBs were used in a variety of industrial applications, such as insulating oil, heat carrier, lubricants, and so on. PCBs can be released as contaminants during multifarious processes, such as production of organic chlorinated pesticides, the open burning of chlorine-containing materials, or volatilization from contaminated soils, posing long hazard to humans and wild life. Therefore, it is of great significance to study the produce and transformation mechanism of these pollutants for pollution prevention and environment protection.Perfluorocarboxylates (CF3(CF2)nCOO-, PFCAs) possess excellent thermal stability and chemical stability, has been widely used in civil and industrial products. They have been detected globally in the environment and organisms, from the crowded urban area to the polar regions of the few human activities, its existence and pollution levels are beyond the scope of people’s thought. The source of the perfluorinated acids in the Arctic is remarkable because under environmental conditions they exist mainly as anions due to the low volatility and high water solubility, and are not susceptible to long-range atmospheric transport in the gas phase. One possible explanation is that precursor chemicals undergo atmospheric transport to remote locations and subsequent oxidation results in the production of PFCAs. N-ethylperfluorobutyramide, and fluorotelomer acrylate have been suggested to be important precursors. So studying detailed mechanisms and kinetic studies of atmospheric reactions of perfluorinated compounds to evaluate their contribution to PFCAs in remote environments has important theoretical value and practical significance.1 Atmospheric Oxidation of Unsaturated Esters(1) Atmospheric ozonolysis study of methyl acrylate and methyl 3-methyl acrylateDetailed reaction mechanisms of Methyl Acrylate (MA) and Methyl 3-Methyl Acrylate (M3MA) with ozone have been investigated using quantum chemistry calculations based on the CCSD(T)/6-31G(d)+CF//B3LYP/6-31+G(d,p) level of theory. Possible reaction channels and products have been presented and discussed. The temperature-dependent and pressure-independent overall and site-specific rate constants are calculated by employing multichannel RRKM theory. The obtained overall rate constants (in cm3 molecule-1 s-1) based on the CCSD(T)+CF energies can be described as: kMA+O3)=(3.74×10-13)exp (-3746.12/T) and k(M3MA + O3)=(5.1×10-13)exp(-3354.87/T) at 200～400 K and 760 Torr. Under atmospheric conditions, the dominant products of ozonolysis of MA and M3MA are methyl glyoxylate and formaldehyde, methyl glyoxylate and acetaldehyde, respectively. The results of theoretical study are in good agreement with the available experimental measurements. Researches on branching ratios and atmospheric lifetimes also have been obtained as complement to the experimental results.(2) Mechanism and Kinetics for OH Radical-Initiated Atmospheric Oxidation of ethyl acrylateOH radical-initiated atmospheric oxidation of ethyl acrylate (ethyl 2-propenoate, EA) has been investigated by performing density functional theory (DFT) calculations. Optimizations of the reactants, intermediates, transition states and products were carried out at the MPWB1K/6-31+G(d,p) level. Single-point energy calculations were performed at the MPWB1K/6-311+G(3df,2p) level of theory. The detailed oxidation mechanism was presented and discussed. The results show that the OH addition is more energetically favorable than the H abstraction. Rice-Ramsperger-Kassel-Marcus (RRKM) theory was used to predict the rate constants over the possible atmospheric temperature range of 180～370 K. The Arrhenius expression adequately describes the total rate constant:k(EA+OH)=(1.71×10-12)exp(805.42/T) cm3 molecule-1 s-1. At 298 K, the atmospheric lifetime of ethyl acrylate determined by OH radicals is about 16.2 h. In order to find out the effect of alkyl substitution on the reaction activity, rate constants for the reactions of methyl acrylate, methyl methacrylate and butyl acrylate with OH radicals were also discussed. Calculation results show that the reaction activity may increase with the increased electron-donating substitution for electrophilic addition reaction.2 Atmospheric Oxidation of Poly chlorinated BiphenylsThe OH radical-initiated atmospheric oxidation degradation of 2,2’,4,4’-tetrachlorobiphenyl (PCB-47) and 2,4,4’-trichlorobiphenyl (PCB-28) was investigated by using quantum chemical calculations. All possible pathways involved in the oxidation process were discussed. Potential barriers and reaction heats have been obtained to assess the energetically favorable reaction pathways and the relatively stable products. The study shows that:(1) The OH radicals are more likely to attack the C3 and C5 atoms of the aromatic ring in the PCB-47 molecule to form PCB-OH adducts. Subsequent reactions are the addition of O2 or NO2 molecule to the PCB-OH adducts at the ortho position of the OH group.(2) Water molecule plays an important role during the whole degradation process.(3) The individual and overall rate constants were calculated by using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory over the temperature range of 180～370 K. At 298 K, the atmospheric lifetime of PCB-47 determined by OH radicals is about 9.1 days.(4) The OH radicals are more likely to attack the C5 and C12 atoms of the aromatic ring in the PCB-28 molecule to form PCB-OH adducts. Subsequent reactions are mainly the reactions of O2 molecule with the PCB-OH adducts.(5) The dominant reaction products for PCB-28 with OH radicals are glyoxal, hydroxylated PCBs and several unsaturated aldehyde or ketone compounds.3 Atmospheric Oxidation of Perfluorinated compoundsThe OH radical-initiated oxidation of N-ethylperfluorobutyramide (EtFBA) and 4:2 fluorotelomer acrylate (4:2 FTAc) in the presence of O2/NOx was investigated theoretically by using density functional theory (DFT). All possible pathways involved with potential barriers and reaction heats in the oxidation process were discussed. Calculations show that:(1) The H abstraction from the C2-H2 bond in EtFB A is the most energetically favorable because of the lowest barrier and highest exothermicity.(2) The atmospheric lifetime of EtFB A determined by OH radicals is short, about 4.6 days at 296 K. However, the atmospheric lifetime of its primary oxidation products, C3F7C(O)N(H)C(O)CH3, C3F7C(O)N(H)CH2CHO and C3F7C(O)NH2, is much longer, about 30～50 days. It demonstrates the possibility that the atmospheric oxidation degradation of PFAMs contributes to the burden of observed perfluorinated pollutants in the Arctic region.(3) This study reveals for the first time that the water molecule plays an important catalytic effect on several key elementary steps and promotes the degrada-tion potential of EtFBA.(4) For the reaction of OH radical with 4:2 FTAc, OH addition to the terminal carbon C1 in C=C bond is the most energetically favorable path. The main oxidation products of 4:2 FTAc are P1 and formaldehyde.