SOURCES OF DIOXINS TO BALTIC AIR Volatilization and Resuspension As Potential Secondary Sources of Dioxins to Air

Sammanfattning: Persistent organic pollutants (POPs) are ubiquitous contaminants characterized by semi-volatility, low water solubility, high lipophilicity and inherent toxicity. A combination of these properties results in long-rang transport, bioaccumulation and biomagnification through food webs. Elimination of the production, use and emissions of these POPs has been ongoing since the 1970s. However, the levels of some POPs are still unacceptably high in some parts of the environment and due to their high persistence levels only decline very slowly over a long period of time. This is especially true for POPs in the Baltic Sea due to long water residence time of approximately 40 years. Numerous studies have been carried out to explore the behavior and fate of the POPs in Baltic regions using analytical methods or modeling approaches. Air-soil exchange plays an important role in controlling the environmental fate of POPs in surface media. Air is a transport medium, which spreads chemicals far away from sources. Soils have received an input of POPs from the atmosphere over a long time period. These chemicals have accumulated in soil solids and, as primary emissions are released, can potentially be rereleased to other environmental media. Therefore, soil could become a significant "secondary" source of some POPs to the air. In this study, the aim was to determine if volatilization and/or resuspension are potential sources of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/Fs) ("dioxins") to Baltic air. Sources of these compounds to Baltic air are particularly interesting because levels of dioxins in fatty fish in the Baltic exceed the levels that are considered fit for human consumption in the European Union guideline. The fugacity quotient approach has been previously shown to be a useful method for exploring the equilibrium status of two connected environmental compartments. Fugacity quotients between the atmosphere and soil are calculated for seventeenth toxic 2, 3, 7, 8,-substituted dioxin congeners . A multimedia mass balance model designed for the Baltic Sea region (POPCYCLING-Baltic) is also employed to study the long-term exchange between air and soil. Estimated fugacity ratios from model simulations are compared with calculated fugacity quotients. Moreover, sensitive analysis is undertaken in order to evaluate the relative effect of background concentration, resuspension and bioturbation transport to the transfer flux from soil to air. Master’s Thesis 2011 II Fugacities of dioxins in soil are additionally measured directly using equilibrium passive sampling devices. Among available passive samplers, polyoxymethylene 17 μm (POM-17) are chosen to absorb freely dissolved PCDD/Fs molecules in soil. Total soil concentrations are measured to provide input data for the POPCYCLING-Baltic multimedia fate and transport model. Estimated fugacities of dioxins will be compared with directly "measured" fugacities in soil. The predictive ability of the model is assesses by comparing estimated and "measured" fugacity. Calculated fugacity quotients showed that lower chlorinated dibenzofuran are close to equilibrium between soil and air while other congeners show disequilibrium. Estimated soil/air fugacity ratios are higher than one but soil still accumulates dioxins because transport process is very slow and non-equilibrium can be maintained for a long period of time. Due to the seasonal variation in concentration, volatilization is higher in summer than in winter. Therefore, net gaseous flux between soil and air can be observed in summer. Sensitivity analysis revealed that volatilization flux is proportional to background soil concentration. High background soil concentration results in high volatilization fluxes and vice versa. The simulation showed that the contribution of resuspension flux to air pollution levels is relatively small in comparison to the influence of variation in background soil concentration. If relatively high and unrealistic resuspension velocities are used as inputs in the model, resuspension is a significant source to the atmosphere. In contrast to background soil concentration and resuspension, bioturbation has no effect on volatilization flux even though high bioturbation rates are used as model inputs. In conclusion, except for light congeners, soil is still a sink of PCDD/Fs present in Baltic air. However, the increase in soil/air fugacity ratios suggest an increasing important of soil-to-air transport in the near future. Equilibrium passive samplers using POM strips are considered as a very simple, reproducible, and inexpensive partitioning method. However, the largest disadvantage of using passive samplers for dioxins is the long time to reach equilibrium. It takes 6 months for PCDD/Fs to obtain equilibrium between soil and POM strips, which exceeded the time for doing a 45 credit thesis. The analytical phase of the experiment is still on-going, and thus it was not possible to include the experimental results in this study.