(gc1-2002)A Computational Approach to Understanding Aerosol Formation and Oxidant Chemistry in the Troposphere
EMSL Project ID
2393
Abstract
An understanding of the mechanisms and kinetics of aerosol formation and ozone production in the troposphere are currently a high priority because they are recognized as two major effects of energy-related air pollution. Atmospheric aerosols are of concern because of their affect on 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 volatile organic compounds (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. Tropospheric ozone is of concern primarily because of its impact on health. Ozone levels are controlled by NOx and by 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. The major oxidant for VOCs in the atmosphere is the OH radical. With the increase in VOC emissions, there is rising concern regarding the available abundance of HOx species needed to initiate oxidation. Over the last five years, there have been four field studies aimed at initial measurements of HOx species (OH and HO2 radicals). These measurements revealed HOx levels that are two to four times higher than expected from the commonly assumed primary sources. Such elevated abundances of HOx imply a more photochemically active troposphere than previously thought. This implies that rates of ozone formation in the lower region of the atmosphere and the oxidation of SO2 can be enhanced, thus promoting the formation of new aerosol properties. Central to unraveling this chemistry is being able to assess the photochemical product distributions resulting from the photodissociation of by-products of VOC oxidation. We propose to use state-of-the-art theoretical techniques to develop a detailed understanding of the mechanisms of aerosol formation in multicomponent (mixed chemical) systems and the photochemistry of atmospheric organic species. The aerosol studies involve an approach that determines homogeneous gas-to-particle nucleation rates from knowledge of the molecular interactions that are used to define properties of molecular clusters. Over the past few years the research team developed Dynamical Nucleation Theory, which is a significant new advance in the theoretical description of homogeneous gas-to-liquid nucleation, and applied it to gas-to-liquid nucleation of a single component system, water. The goal of the present research is to build upon these advances by extending the theory to multicomponent systems important in the atmosphere such as water-sulfuric acid. In addition, high level ab initio electronic structure calculations will be used to unravel the chemical reactivity of naturally occurring VOCs with the OH radical. Detailed mechanistic information regarding the degradation pathways and accurate reaction rate constants for key VOC reactions and will be obtained.
Project Details
Project type
Capability Research
Start Date
2002-01-04
End Date
2005-01-06
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Kathmann SM, GK Schenter, and BC Garrett. 2004. "Multicomponent Dynamical Nucleation Theory And Sensitivity Analysis ." Journal of Chemical Physics 120(19):9133-9141.
Kathmann SM, GK Schenter, and BC Garrett. 2005. "Ion-Induced Nucleation: The Importance of Chemistry." Physical Review Letters 94(11):116104.