An Experimental Data Base and Modeling Effort to Describe the Atmospheric Gas-Aerosol Mass Transfer of Semi-Volatile Organics

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The purpose of this research is to improve our understanding of the factors that control the gas to aerosol phase distribution of semi-volatile organic compounds. Results from this program will provide modelers with tools to predict vaporization and condensation of semi-volatiles as gas and particle concentrations change with time. Most of the current efforts to predict organic partitioning have been based on equilibrium theory. Although these efforts have substantially advanced our understanding of the partitioning process under ideal conditions, there are strong indications that atmospheric equilibrium between the gas and particle phases may not be established for most compounds of interest over relatively short time scales (minutes to hours). To obtain experimental data to develop and test models of semi-volatile mass transfer between the gas and particle phases, combustion emissions will be aged in a large 190cm3 outdoor Teflon film environmental chamber. Polynuclear aromatic hydrocarbons (PAHs) will be used as example compounds, but the methodology will be applicable to other organic semi-volatile toxics. Net PAH loss rates from particles and the gas phase will be monitored. Rates of PAH mass transfer to the from the particles will be estimated using a new inner particle diffusion model which considers mass transfer in a liquid organic layer surrounding the particle. Three different types of particles will be used. They will be generated from wood, diesel and coal combustion to represent particle types which have different amounts of a "liquid phase" surrounding an inner carbon core. It is expected that the amount and type of organic material that forms this outer layer of the particle will strongly influence inner organic particle diffusion, enthalpies of desorption from the surface, and ultimately rates of particle semi- volatile uptake and loss. By changing the concentration of semi-volatile vapors and/or particles in the chamber, an d thereby perturbing the equilibrium between gas and particle phase organic, these researchers will be able to investigate rates of semi-volatile sorption (condensation) and evolution to and from particles. To radically alter equilibrium conditions, some experiments will be conducted by passing combustion emissions through a large gas stripper/denuder to remove the gas phase semi-volatile component before particles enter the chamber. Semi-volatile deuterated PAH will be added to the gas phase to observe rates of mass transfer to the particles.