Gas−Particle Partitioning of Atmospheric PCDD/Fs: Measurements and Observations on Modeling

The gas−particle partitioning of 2−8PCDD/Fs in ambient air was investigated at a semirural U.K. site, sampled with glass microfiber filters (GFFs) and polyurethane foam (PUF) plugs, and quantified by HRGC−HRMS. Nine samples, each ∼1000 m 3 of air, were collected over different periods of 7 days between May and Aug, when the average ambient temperatures varied between 12 and 17 °C for the different sampling periods. The proportion of the different homologues associated with the PUF plugs (i.e., operationally defined as the gas phase) varied greatly. 2CDFs were almost exclusively (>99.5%) in the gas phase, along with 82−96% of the 4CDD/Fs, 15−65% of the 6CDD/Fs, and 2−20% of the 8CDD/Fs. PCDFs tended to be slightly more associated with the gas phase than the equivalent homologue group of the PCDDs. Plots of the log partitioning coefficient ( K p ) versus the log subcooled liquid vapor pressure ( ) gave slopes in the range −0.64 to −0.79 ( x̄, −0.68) and intercepts of −6.49 to −7.32 ( x̄, −6.73) for the homologue groups. The data set was used to compare the Junge−Pankow adsorptive gas−particle distribution model and a parametrization of Pankow's absorption model by Harner and Bidleman. Although uncertainties remain over the physicochemical property data reported for PCDD/Fs, the former overpredicted the proportion of PCDD/Fs found associated with the particulate phase by between a factor of 1.1 and 5. The latter gave better agreement, to within a factor of 1.2−1.5, despite the requirement to use calculated rather than measured values for the octanol:air partition coefficient ( K oa ).