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For more than two decades, fine aerosol work at UNC under the direction of Professor Kamens has focused on the chemical transformations that occur on atmospheric particles, which are associated with different aerosol systems. Much of this work has been performed in large outdoor Teflon film chambers located in Pittsboro, NC (shown right).
During the 1980s, we investigated potentially toxic compounds like polynuclear aromatic hydrocarbons (PAH) and halogenated dibenzodioxins and furans. We studied the extent to which O3, NO2, N2O5, and sunlight influence chemical and bacterial mutagenic changes (Figure 2) of organics on soot particles as these particles age in the atmosphere.
We reported that sunlight promotes the on soot particles as these particles age in the atmosphere. (Figure 3). From outdoor chamber experiments we developed rate constants for these reactions as a function of sunlight, water vapor, and temperature. These rate constants and PAH source signatures were then used in chemical mass balance receptor models to estimate particle source apportionment. This rate constant work led to the integration of gas phase smog kinetics with particle PAH and nitroPAH reactions and permitted the modeling of the daytime formation and decay of selected nitroPAH (like 2nitrofluoranthene) in both the gas and particle phases. (Figure 4) . We have employed these techniques to study the atmospheric stability of brominated and chlorinated dioxins and furans.
During the 1990s, our research group focused on semi-volatile organic (SOC) gas-particle partitioning. We developed an inner particle radial diffusion model and assembled an experimental approach for the generation of a data base for gas-particle mass transfer model testing. We also implemented an equilibrium-gas-particle partitioning technique which takes advantage of activity coefficient calculations. This provided one of the first approaches to estimating the partitioning of both polar and non-polar toxic semivolatiles. Current work addresses the impact of particle composition and water on gas-particle partitioning. We have also developed a more robust theoretical framework than currently exists for gas-solid aerosol surface partitioning.
For the past six years, we have been developing a kinetics model to predict secondary organic aerosol (SOA) formation from biogenic hydrocarbons (Figure 5). This integrates gas and particle phase chemistry and equilibrium partitioning thermodynamics. These models have been successfully used to predict secondary aerosol formation from a-pinene+O3 dark systems (top Figure 5, d-limonene) and NOx + light systems (bottom Figure 5). While this terpene work was initiated, we made the important discovery showing the potential for acid catalyzed reactions on SOA formation.
Following on this work, a collaboration with the Murry Johnson group at the University of Delaware, produced the cornerstone work of Tolocka et al., which was the first study to demonstrate the existence of organic particle phase oligomers from the reactions of gas phase ±-pinene with ozone. This was followed by the important discovery that diesel exhaust particles, which are significant contributors to urban atmospheres, are non-polar when emitted and experience a dramatic increase in polarity as they age in the atmosphere
. By tracking this polarity change, it is possible to model terpene SOA formation in the presence of dilute diesel exhaust a-pinene+NOx light systems.