Photochemistry of aromatic hydrocarbons: implications for ozone and secondary organic aerosol formation

Abstract

Aromatic hydrocarbons constitute an important fraction (~20%) of total volatile organic compounds (VOCs) in the urban atmosphere. A better understanding of the aromatic oxidation and its association in urban and regional ozone and organic aerosol formation is essential to assess the urban air pollution. This dissertation consists of two parts: (1) theoretical investigation of the toluene oxidation initiated by OH radical using quantum chemical and kinetic calculations to understand the mechanism of O3 and SOA precursors and (2) experimental investigation of atmospheric new particle formation from aromatic acids. Density functional theory (DFT) and ab initio multiconfigurational calculations have been performed to investigate the OH-toluene reaction. The branching ratios of OH addition to ortho, para, meta, and ipso positions are predicted to be 0.52, 0.34, 0.11, and 0.03, respectively, significantly different from a recent theoretical study of the same reaction system. Aromatic peroxy radicals arising from initial OH and subsequent O2 additions to the toluene ring are shown to cyclize to form bicyclic radicals rather than undergoing reaction with NO under atmospheric conditions.Isomerization of bicyclic radicals to more stable epoxide radicals possesses significantly higher barriers and hence has slower rates than O2 addition to form bicyclic peroxy radicals. At each OH attachment site, only one isomeric pathway via the bicyclic peroxy radical is accessible to lead to ring cleavage. Decomposition of the bicyclic alkoxy radicals leads primarily to formation of glyoxal and methyl glyoxal along with other dicarbonyl compounds. Atmospheric aerosols often contain a considerable fraction of organic matter, but the role of organic compounds in new nanometer-sized particle formation is highly uncertain. Laboratory experiments show that nucleation of sulfuric acid is considerably enhanced in the presence of aromatic acids. Theoretical calculations identify the formation of an unusually stable aromatic acid-sulfuric acid complex, which likely leads to a reduced nucleation barrier. The results imply that the interaction between organic and sulfuric acids promotes efficient formation of organic and sulfate aerosols in the polluted atmosphere because of emissions from burning of fossil fuels, which strongly impact human health and global climate.