Study On The Atmospheric Oxidation Of Selected Important Volatile Organic Compounds

Volatile organic compounds are important atmospheric pollutants, of which the aromatic compounds are one of the main constituents. Experimental studies have found that the primary and secondary products from the photo-oxidation of aromatic compounds contribute significantly to the secondary air pollution, e.g., glyoxal, methylglyoxal, and phenols play an important role in the formation of secondary organic aerosols. However, the photochemical mechanism of the aromatics in the atmosphere remains ambiguous due to the experimental difficulties and deficiencies in detecting the highly reactive products and radical intermediates, indentifying and quantifying multifunctional and unstable products, etc. For example, experimental studies on OH-initiated photo-oxidation of 1,2,4-trimethylbenzene only detected a ~65% total carbon balance from the product analysis. It is also unclear whether the ozonolysis of unsaturated 1,4-dicarbonyls from the oxidation of aromatic compounds will yield OH radical and contribute to the oxidizing capacity. Alternatively, theoretical studies based on quantum chemistry calculations for potential energy surfaces, classical transition state theory and quantum-mechanical unimolecular theory(RRKM) is becoming increasingly important in elucidating the reaction mechanism. Especially, theoretical investigation on the radical intermediates is beyond the capability of most of the experimental studies. Based on the rate constants predicted for the intermediate reaction steps, theoretical studies can suggest unforeseeable reaction steps, branching ratios of various products, and their dependence on temperature and pressure. When combined with the experimental studies, the theoretical study is of great help to elucidate the atmospheric fate of organic pollutants. This dissertation investigates the oxidation mechanism and reaction kinetics of 1,2,4-trimentylbenzene(TMB) and unsaturated 1,4-dicarbonyls in the troposphere.The main conclusion for this dissertation is summarized below:1. The atmospheric oxidation mechanism initiated by OH radicalCalculations based on M06-2X and ROCBS-QB3 indicate that the addition of OH to 1,2,4-trimethylbenzen occurs mainly to C1, C3 and C5, forming three TMB-OH adducts. In the atmosphere, then the reaction proceeds through O2 addition to the aromatic ring of the TMB-OH adducts to from peroxy radicals or hydrogen-abstraction from OH substituted Ci. The peroxy radicals exclusively cyclize to bicyclic radicals, which will combine with O2 again to produce bicyclic peroxy radicals. When reacting with atmospheric NO, the bicyclic peroxy radicals are converted to alkoxyl radical, which could undergo ring-breakage to form 1,2-dicarbonyls and unsaturated 1,4-dicarbonyls, or cyclize and form eventaully 1,2-dicarbonyls and unsaturated epoxy-1,4-dicarbonyls. Theoretical calculations suggest that epoxy carbonyls from the cyclization of alkoxyl radical may account for the carbon loss in experimental study.2. Ozonolysis of unsaturated 1,4-dicarbonylsThe ozonolysis reactions of three main unsaturated 1,4-dicarbonyls from the atmospheric oxidation of aromatic compounds are studied at M06-2X and ROCBS-QB3 levels. The fate of the Criegee Intermediates(CIs) is also investigated. The predicted rate constants for the ozonolysis reaction of butenedial, 2-methylbutenedial and 4-oxo-2-pentenal are 1.78 × 10-19, 6.82 × 10-17 and 9.08 × 10-18 cm3 molecule-1 s-1, respectively, at 298 K. Ozonolysis of these unsaturated 1,4-dicarbonyls will form three CIs as HC(O)CHOO(CI), HC(O)C(CH3)OO(MCI) and CH3C(O)CHOO(MOCI). Our study suggests that the intramolecular H-shift is possible for MCI only, and an utmost yield of 0.16 is suggested for the OH radical yield. The other two CIs would more likely cyclize to dioxirane.