Theoretical Study On The Atmospheric Oxidation Mechanism Of Typical Aromatic Compounds

Volatile Organic Compounds(VOCs), as the key precursor for regional ozone formation and PM2.5 pollution, are of great importance on the quality of the atmospheric environment and human health. A large number of scientific studies have shown that the atmospheric oxidation of VOCs would lead to the O3 concentrations overload. The reaction intermediates and products of low volatility will also lead to the increase of organic constituents in particulate matter, contributing to the formation of the so-called secondary organic aerosols(SOA). Detailed mechanistic model of VOC oxidation in the atmosphere would help us in analyzing the ozone formation potentials and SOA yields of different VOCs. The reliability of the atmospheric oxidation model of VOCs depends on the accuracy of the oxidation mechanism of single VOC. In the past decades, experimental and theoretical researches have made great progress on the atmospheric oxidation mechanism of alkanes, alkenes, alkynes and oxygen-containing compounds. However, problems and uncertainties persisted in the atmospheric oxidation mechanism of aromatic compounds. For example, in the smog-chamber experiments of toluene, about 30% of the carbon loss cannot be recovered from the product analysis; on the other hand, the intermediates generated in the experiments are difficult to be observed directly. In this paper, we have investigated the atmospheric oxidation mechanism of toluene, 2-methylnapthalene, furan and 2-methylfuran by quantum chemistry calculation coupled with the RRKM theory, gaining the more reliable mechanism by comparing the experimental and theoretical results. The main conclusion for this dissertation is summarized below:1. The atmospheric oxidation mechanism of tolueneThe atmospheric oxidation mechanism of toluene initiated by OH radical addition is investigated by quantum chemistry calculations at M06-2X, G3MP2-RAD, and ROCBS-QB3 levels and by kinetics calculation using transition state theory and unimolecular reaction theory coupled with master equation(RRKM-ME). The predicted branching ratios are 0.15,0.59, 0.05, and 0.14 for OH additions to ipso, ortho, meta and para positions(forming R1~R4adducts), respectively. The fate of R2, R4, and R1 is investigated in detail. In the atmosphere,R2 reacts with O2 either by irreversible H-abstraction to form o-cresol(36%), or by reversible recombination to R2-1OO-syn and R2-3OO-syn, which subsequently cyclize to bicyclic radical R2-13OO-syn(64%). Similarly, R4 reacts with O2 with branching ratios of 61% for p-cresol and 39% for R4-35OO-syn, while reaction of R1 and O2 leads to R1-26OO-syn.RRKM-ME calculations show that the reactions of R2/R4 with O2 have reached their high-pressure-limits at 760 Torr and the formation of R2-16O-3O-s is only important at low pressure, i.e., 5.4% at 100 Torr. The bicyclic radicals(R2-13OO-syn, R4-35OO-syn, and R1-26OO-syn) will recombine with O2 to produce bicyclic alkoxy radicals after reacting with NO. The bicyclic alkoxy radicals would break the ring to form products methylglyoxal/glyoxal and their corresponding co-products butenedial/methyl-substituted butenedial as proposed in earlier studies. However, a new reaction pathway is found for the bicyclic alkoxy radicals, leading to products methylglyoxal/glyoxal and2,3-epoxy-butandial/2-methyl-2,3-epoxy-butandial. A new mechanism is proposed for the atmospheric oxidation mechanism of toluene based on current theoretical and previous theoretical and experimental results. The new mechanism predicts much lower yield of glyoxal and much higher yield of butenedial than other atmospheric models and recent experimental measurements. The new mechanism calls for detection of proposed products2,3-epoxy-butandial and 2-methyl-2,3-epoxy-butandial.2. The atmospheric oxidation mechanism of 2-methylnapthaleneThe atmospheric oxidation mechanism of 2-Methylnaphthalene(2-MN) initiated by OH radicals is investigated by using quantum chemistry at BH&HLYP/6-311++G(2df,2p) and ROCBS-QB3 levels and kinetic calculations by transient state theory and unimolecular reaction theory coupled with master equation(RRKM-ME). This reaction is mainly initiated by OH additions, forming adducts Rn(2-MN-n-OH, n = 1-8). The fates of R1 and R3,representing the a- and b-adducts, are examined. The fates of R1 and R3 are found to be different drastically. In the atmosphere, R1 reacts with O2 via O2 addition to C2 position to form R1-2OO-a/s, which will undergo bimolecular reaction with the atmospheric NO or unimolecular isomerization via intramolecular H-shifts, of which the latter is found to be dominant and accounts for the formation of dicarbonyl compounds observed in experimental studies. The role of the tricyclic radical intermediates formed from the ring-closure of R1-2OO is rather limited because their formation is endothermic and reversible, being contrary to the important role of the analogous bicyclie radical intermediates in the oxidation of benzenes. On the other hand, fate of R3 is similar to that of benzene-OH adduct, and the tricyclic intermediates will play an important role. An oxidation mechanism is proposed based on the theoretical predictions, and the routes for the experimentally observed products are suggested and compared.3. The atmospheric oxidation mechanism of furan and 2-methylfuranThe atmospheric oxidation mechanisms of furan and 2-methylfuran(2-MF) initiated by OH radicals are investigated by transient state theory and unimolecular reaction theory coupled with master equation(RRKM-ME) at M06-2X, G3MP2-RAD and ROCBS-QB3 levels. For furan, the oxidation is initiated by its reaction with OH radicals through addition to C2 position, forming energetic adduct R2*(with excess of ~ 140 k J/mol relative to deactivated R2 at both levels), which will isomerize promptly to R2-iso* radicals by fused-ring C-O bond cleavage((35)E0K≠~ 80 k J/mol at both levels). Addition of OH radicals to C3 position plays a negligible role at 298 K. Deactivated R2-iso radicals would react with O2 by additions to form R2-iso-OO peroxy radicals, which would undergo intramolecular concerted elimination of HO2 to form 1,4-butenedial at rates of ~ 105s-1at ROCBS-QB3 level.The yield of 1,4-butenedial for the reaction between furan and OH at 760 Torr and 298 K is determined as 80.4% by RRKM-ME calculations, which is independent of NO concentration in the range from 40 ppt to 40 ppm, but decreases with the elevated pressure and decreased temperature. Deactivated R2(~19.6%) would react with O2 almost irreversibly and exclusively((35)G298K≠~-40 k J/mol at ROCBS-QB3 level) to form R2-5OO-s radical, which would react with NO to form R2-5O-s radical and nitrate R2-5ONO2-s. Ultimately, R2-5O-s would react with O2 to form 2-hydroxy-5-furanone and HO2 exclusively. In conclusion, the reaction between furan and OH will form 1,4-butenedial(~80.4%), 2-hydroxy-5-furanone(~19.6% × 90%) and nitrate R2-5ONO2-s(~19.6% × 10%) at 760 Torr and 298 K. Being similar to furan oxidation, the oxidation for 2-MF starts with OH addition to C2 and C5 positions to form energetic adducts R2*and R5*, which would isomerize promptly to4-oxo-2-pentenal(~37.1%) after reactions with O2 at 760 Torr and 298 K. Thermalized R2 and R5 would combine with O2 to form R2-5O-s(18.6% × 90%) and R5-2O-s(44.3% × 90%)after reaction with NO. Finally, R2-5O-s will form 2-hydroxy-2-methyl-5-furanone by reaction with O2 and R5-2O-s will lead to the formation of 2-hydroxy-5-furanone by elimination of-CH3. The reason for the lower yield of 4-oxo-2-pentenal in 2-MF oxidation than that of 1,4-butenedial in furan oxidation is also discussed.