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Atmospheric Chemistry Projects
In order to fully understand atmospheric processes and its impact on both the climate and human health detailed information on major components is necessary. Air particulate matter (PM) is known for its adverse impact on heath, as well as on ever discussed climate changes. The building blocks of PM are metals, salts, organic species, and elemental carbon. While inorganics are fairly well identified and quantified, only 20-50% of organics are usually characterized, mainly due to difficulties in their analysis. The organic fraction within PM can be differentiated into polar and non-polar species. The majority of the non-polar and mildly polar compounds have been found to be directly related to primary emissions (e.g., incomplete combustion of fossil fuels), and are quite well characterized using various organic solvent extraction techniques and GC/MS analysis. Most of the more polar organic species are known to be formed through secondary processes (e.g., atmospheric oxidation) and have been found to partition more into particle phase, forming secondary organic aerosols (SOA). Our laboratory focuses on evaluating the polar organic species, focusing on determining the mechanism behind their formation through heterogeneous oxidation reactions and measuring their variability in the atmosphere.
Polycyclic aromatic hydrocarbons (PAHs) are found in atmospheric aerosols which are emitted to the atmosphere through the incomplete combustion of fossil fuels. PAHs and their by-products are known to have carcinogenic and mutagenic activities. Both the kinetics of these reactions and the pathways of the formation of products are important in environmental studies. While in the atmosphere PAHs partitioning between the gas and particle phase, with heavier species typically preferring the particle phase. Although many studies have determined the mechanisms behind their formation in the gas phase only limited knowledge is available on the products of the heterogeneous reactions of PAHs adsorbed on particles.
Identifying Reaction Products
The first phase of this project involves studying the reactions of phenanthrene, anthracene, fluoranthene, and pyrene with reactive gas species with a focus on the detailed identification of mainly oxygenated products. These PAHs are highly representative due to partitioning into the particulate phase their high relative abundance commonly observed in various forms of particulate matter. These reaction are simulated in a small-scale flow reactor with quartz filters used as the solid-phase support. Several oxidation products with multiple functional groups including aldehydes, carboxylic acids, hydroxy, ketone and nitro species were observed from reactions with gas-phase reactants such as NO2, O3, N2O5 and NO3. The identified reaction products help further evaluation of product accumulation kinetics and their significance in atmospheric processes. This work is being funded by NSF Career grant ATM-0747349 and additional REU support.
Heterogeneous oxidation of PAHs and other organic species involves many types of PM matrices. One major source of PM is the incomplete combustion of fossil fuels. To better understand the mechanism behind the heterogeneous oxidation of PAHs in the atmosphere our group has designed and constructed a large-scale reaction chamber (ca. 10 m3) in collaboration with Dr. Frank Bowman and David Hirschmann (Dept. of Chemical Engineering, UND). The large-scale reaction chamber is connected to a suite of instruments for online analysis and multiple sample collection system for offline analysis. Additionally a gas dilution system is directly connected to the chamber to provide various gas-phase oxidants such as NO2, NO and O3.
A wide range of gas and particle-phase reactions are simulated in the large-scale reaction chamber. Currently, systems are in place to inject model/control particles (e.g. ammonium sulfate and silica particles) as well as real world particles from a diesel engine. During these reactions the PM is injected into the chamber followed by gas-phase oxidants and finally the injection of PAHs. Kinetic information regarding the reactive uptake of PAHs is measured by the collection of the PM onto quartz filters (for direct GC/MS analysis) and pyrotube samplers (for thermal desorption/pyrolysis-GC/MS). Additionally the various products formed during the reactions will also be evaluated in terms of the kinetic rate of formation throughout the reactions. Here the knowledge gained from the heterogeneous oxidation of PAHs in the small-scale flow reactor is extrapolated to reactions performed within the large-scale simulation chamber.