A Computational Approach to Understanding Oxidant Chemistry and Aerosol Formation in the Troposphere (original) (raw)

Ozone production and aerosol formation in the troposphere are recognized as two major effects of energy-related air pollutants. Tropospheric ozone is of concern primarily because of its impact on health. Ozone levels are controlled by NO x and by volatile organic compounds (VOCs) in the lower troposphere. The VOCs can either be from natural emissions from such sources as vegetation and phytoplankton or from anthropogenic sources such as automobiles and oil-fueled power production plants. It is of critical importance to the Department of Energy (DOE) in developing national energy use policies to understand the role of VOCs in determining air quality and how VOC emission or NO x emission control strategies should be designed. Atmospheric aerosols are of concern because of their affect visibility, climate, and human health. Equally important, aerosols can change the chemistry of the atmosphere, in dramatic fashion, by providing new chemical pathways (in the condensed phase) that are not available in the gas phase. The oxidation of VOCs and organic sulfur compounds can form precursor molecules that nucleate aerosols. DOE's Atmospheric Chemistry Program has identified the need to evaluate the causes of variations in tropospheric aerosol chemical composition and concentrations, including determining the sources of aerosol particles and the fraction that are of primary and secondary origin. A fundamental understanding of mechanisms for production of oxidants and aerosols in the troposphere is currently not available. We propose the use of advanced theoretical techniques to model the molecular processes that control ozone and aerosol formation. Accurate electronic structure methods will be used to unravel the chemical reactivity of the important naturally occurring VOCs, isoprene, and the organosulfur compound, dimethylsulfide (DMS), that is important in marine environments. Thermochemical data for isoprene, DMS, and their degradation products, detailed mechanistic information about the degradation pathways, and reaction rate constants for key elementary reactions and will be obtained. Molecular simulations will be used to understand the factors controlling aerosol formation by nucleation in binary and ternary systems of sulfuric acid, water, and ammonia. High level ab initio electronic structure calculations will be used to calculate interaction energies and reaction energetics, including reaction profiles, that are crucial to obtaining accurate potential energy functions, the potential energy functions will be used in molecular simulations of the kinetics of cluster formation and degradation, and the kinetic data will be used to model the process of nucleation.