The influence of vegetation on the cycling of persistent organic pollutants (POPs) assessed by a multi compartment box model

Because of its potential impact on human health, awareness of chemical pollution has been rising in the last decade. Especially the group of bioaccumulative organic xenobiotics is under suspicion to cause serious health problems. These substances usually have a long life cycle and the ability to biomagnify throughout the food chains. Due to their chemical properties, they are characterized by semivolatility, have a long-range transport potential (LRTP) and are also capable to distribute among different compartments in nature. Transport mechanisms of these substances include ocean currents, rivers and especially the atmosphere. In particular the colder higher latitudes are affected by pollution although the initial point of release is located typically thousands of kilometers away in temperate climate zones. Many of those compounds are released either from the industry or as agrochemicals. Among important mechanisms of atmospheric transport, vegetation plays a key role on the overall cycling and distribution in nature. The large canopy surface on the continent has an accumulation effect for chemicals in the atmosphere and is a preferred place for deposition. Many factors influence the overall deposition process at the canopy. Persistent organic pollutants (POPs) can settle either in the gas phase, attached to particles or by gas exchange reaction of leaves. To investigate different processes for the overall fate of these compounds, a multi media box model was developed and tested against increasing levels of vegetation complexity. This study concentrates on two different POP-chemicals (PCB-52 and DDT). Significant differences of overall contamination and compartment distribution are detected depending on the deposition process considered. Gas deposition of DDT and PCB-52 shows a strong dependence on the calculation method leading to a variation of chemical allocation and thus causing different contamination levels. Particle deposition causes further reduction of overall burden of DDT, but not for PCB-52. Large uncertainties of air-vegetation partitioning are found in quantifying the gas diffusion reactions. Both chemicals show a very similar behaviour for the tested partitioning factors although their compound properties are very different. Defoliation and the introduction of an additional vegetation soil causes reduction of the overall burden compared to a big leaf approach. It is obvious that the litter fall cycle leads to a faster reduction of POPs because a higher revolatilization with the atmosphere reduces significantly the overall contamination level. The accumulation effect of the canopy and the revolatilization from its soil induces an additional atmospheric cleaning. An overall POP filter effect of the vegetation depends to a large extend on the vegetation phenology. Simulations with a more detailed canopy wind profile indicate that big leaf canopies most likely underestimate the real overall contamination levels of DDT and PCB-52. A higher wind speed in the canopy top of a big leaf approach also means more turbulence, less deposition on the canopy and thus more atmospheric OH-radicals removal. Vegetation types are an essential factor in the overall fate of both chemicals. Differences between the vegetation types can have a significant impact on the overall