The atmospheric partitioning of decamethylcyclopentasiloxane(D5) and 1-hydroxynonamethylcyclopentasiloxane(D4TOH) on different types of atmospheric particles

A series of outdoor chamber experiments using different types of atmospheric particles were conducted to investigate the atmospheric gas-particle partitioning behavior of decamethylcyclopentasiloxane (D5) and its hydroxylated compound, 1-hydroxynonamethylcyclopentasiloxane (D4TOH). This was undertaken because the phase in which a compound exists directly influences its lifetime in the atmosphere, its deposition to the surface of the earth, and ultimately its potential impacts on human health. To treasure the phase distribution, or partitioning, between the gas and particle phases, aerosol systems were created in either the 190 m3 or 25 m3 outdoor Teflon film chambers at the UNC chamber facility in Pittsboro, NC. Diesel, wood, and coal soot were used as organic combustion particle sources, while Arizona fine dust was used as an inorganic particle source. The gas-particle partition coefficient, Kp, was used as a measure of the phase distribution. Kp can be expressed as Kp = (Cp/TSP)/Cg, where Cp(ng/m3) and Cg(ng/m3) are the concentrations in the particle and gas phases respectively, and TSP (μg/m3) is the total suspended particulate matter concentration. The effect of temperature is considered to be the key parameter that affects the partitioning coefficient. Chamber data showed for both D5 and D4TOH partitioning, that temperature was very important and could influence partitioning to the particle phase by two orders of magnitude in going from 25 to 0°C. It was also found that relative humidity (RH) was an important parameter affecting D4TOH partitioning especially with mineral dust particles.