Atmospheric concentrations, deposition and modeling of polychlorinated dibenzo-p-dioxins and dibenzofurans

Polychlorinated dibenzo-p-dioxins and dibenzofurans are two groups of chlorinated aromatic toxics that are highly persistent and widespread in the environment. These compounds have been the subject of much concern and debate due to their proven carcinogenic and bioaccumulative properties even at ultra-trace levels. In this research, ambient air concentrations and deposition rates of dioxins were monitored at a number of locations in the Houston area. Ambient concentrations were measured using high-volume air samplers while deposition rates were quantified using modified automated B1C samplers. Results indicated that all the 17 congeners are present in the ambient air of Houston. The most toxic congener, 2,3,7,8-TODD, was detected at concentrations between 0 and 2 fg/m3. While OCDD was the most prevalent congener in terms of dioxin mass in ambient air, 2,3,4,7,8-PeCDF was the main constituent of the total toxic concentration, I-TEQ. Statistical analysis confirmed temporal and spatial trends in the gathered ambient data. Higher ambient air concentrations were measured during the colder months. The collected data at the five monitoring sites also suggested a potential contribution from traffic. The particle size distribution data collected in this research showed that the dioxin congeners preferentially associated with particles with an aerodynamic diameter of <0.95 mum. The distribution between the vapor/particle phases in the atmosphere indicated a correlation with the level of chlorination and ambient temperature. Properties such as the subcooled liquid vapor pressure (PL°) and octanol-air partitioning coefficient (Koa) were used to describe the vapor-particle partitioning of the dioxins. Ambient air data were correlated to routinely measured air quality pollutants.; NOx was found positively and consistently correlated to ambient air concentrations of dioxins. The deposition results showed that while dry deposition flux was governed by the higher chlorinated congeners (hepta- and octa-), wet deposition flux was found to be a combination of congeners of different level of chlorination (tetra- through octa-). This difference shows that precipitation, while intermittent, is an efficient mechanism for removing dioxins from the air in the Houston area. From the deposition samples, empirical scavenging ratios (W) on the order of 104 and dry deposition velocities (nud) of <1 cm/s were observed for the dioxin/furan congeners. Finally, an empirical/statistical model that integrates NOx correlations, empirical W and nud values, and partitioning was developed to describe the spatial deposition distribution of dioxins in the Houston area and to indirectly estimate the dioxin loadings.