Juan G. Navea, Department of Chemistry and Chemical and Biochemical Engineering, University of Iowa, Iowa City, IA 52242, Charles O. Stanier, Http://www.Cbe.Engineering.Uiowa.Edu/faculty/stanier/, University of Iowa, Iowa City, IA 52242, Mark A. Young, Chemistry, University of Iowa, Iowa City, IA 52242, Vicki H. Grassian, Chemistry and Chemical and Biomolecular Engineering, University of Iowa, Iowa City, IA 52242, and Shihe Xu, Health and Environmental Sciences, Dow Corning Corporation, Midland, MI 48686-0994.
We have carried out kinetic studies to determine the heterogeneous decay of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) in the presence of atmospheric particles at different relative humidity (RH). The heterogeneous decay of D4 and D5 mediated by atmospheric particles is simulated in an atmospheric chamber and monitored by means of FT-IR spectroscopy. The apparent heterogeneous uptake coefficient is analyzed for different aerosols including kaolinite, hematite, quartz, carbon black, and calcite. A box model is constructed to contrast the heterogeneous processes with the known gas phase degradation of these compounds through reaction with OH, which leads to atmospheric residence times of order days to weeks. Using the model, we quantify the required combination of thermodynamic and kinetic parameters needed for a reduction in the atmospheric lifetime. The model is constructed using three compartments (urban, semi-rural, and rural environments), and includes simulation of gas phase decay, thermodynamic partitioning of D4 and D5, diurnal release patterns, and heterogeneous reactions. The tuning of the box model to match observed concentrations of benzene (which reacts with OH on a similar timescale) is discussed.