Estimating the contribution of reentrainment to the atmospheric deposition of dioxin

Previous efforts to quantify known emission sources of polychlorinated dibenzodioxins (“dioxin”) generally have only been able to account for approximately 10 percent of the estimated deposition. The sources most often evaluated are direct emissions of dioxins from combustion sources. One of the unquantified sources often mentioned when explaining the discrepancies between the estimated emissions and deposition is reentrained dioxin-containing soil dust. Based on available data on typical levels of dioxins in soil and air, we estimate that “old” dioxin emitted into the atmosphere by crustal material reentrainment could account for only as much as 4 percent of the ambient dioxin concentrations. However, because the soil dust particles (dp > 2.5 Am) have much higher deposition velocities that the particles associated with fresh emissions from combustion sources (dp < 2.5 μm), this “old” dioxin could potentially account for as much as 70 to 90 percent of the total deposition of dioxin to environmental surfaces in urban areas, and 20 to 40 percent in rural regions.