Spatial Aspects Of The Large-Scale Fate And Transport Of Semivolatile Organic Chemicals
Creators: name: Von Waldow, Harald Bernd Kurt
Publisher: name: Zenodo
Year: 2010

description: Chemical contamination of the environment is recognised as a major problem which has to be managed on local, national and international levels to ameliorate and avoid irreparable damage to ecosystems and human health. Of particular concern are persistent and toxic chemicals that may bioaccumulate, travel large distances and contaminate ecosystems far away from emission sources. Substances known to possess these properties are called persistent organic pollutants (POPs) and most of them are halogenated hydrocarbons. As a consequence of their phase partitioning behaviour, they can occur in the gas phase to be available for efficient atmospheric long-range transport, but also partition into surface compartments and biota. POPs are used as pesticides, as technical chemicals with a wide range of applications, and might be produced unintentionally as by-products during the production of other chemicals or during other anthropogenic processes such as waste incineration.

International agreements such as the POP Protocol of the UNECE Convention on Long-Range Transboundary Air Pollution and the the Stockholm Convention on POPs that ban and/or regulate the production and use of POPs require the assessment of chemicals with respect to their toxicity and bioaccumulativity, their persistence in the environment and their long-range transport behaviour.

Mathematical fate- and transport models are indispensable tools to evaluate chemicals with respect to persistence and long-range transport potential. A target variable calculated with these models is the environmental exposure to a chemical. It indicates the part of the potential hazard of a chemical that is caused solely by the chemical's fate and transport behaviour, regardless of toxicological substance properties. Metrics for both, persistence and long-range transport potential are based on exposure. Usually, pulse emissions are assumed to calculate exposure. Here, it is show that exposure does not depend on the shape of the emission function. Exposure calculated for the pulse release of a particular mass of contaminant is equal to exposure calculated for any arbitrary dynamic release of the same mass. This result allows for a more general interpretation of exposure-based metrics for persistence and long-range transport potential of a chemical. It further extends the meaning of model evaluations at steady-state. Steady-state models yield valid results for certain indicators, without the need to assume that chemical concentrations in the environment have reached or will reach steady-state.

The long-range transport potential of a chemical is the least well defined screening criterium for POPs. Measured levels of a chemical at locations distant from sources can be used as evidence of long range environmental transport, but until now, there has been no quantitative measure of the distance of a location from likely source areas of chemicals. Here an Eulerian atmospheric tracer transport model is employed to calculate a measure of remoteness which describes the effective distance of a location from distributed emission sources, taking into account geographical distance and atmospheric transport pathways. Maps of remoteness for two generic emissions scenarios that represent areas for emissions of industrial and technical chemicals and of pesticides are presented. The results can be used to better interpret spatial patterns of measured and modelled concentrations of chemicals in the global environment, to derive long-range transport potential metrics for specific substances, and to plan future measurement campaigns and extend monitoring networks.

The global distillation hypothesis is the dominant paradigm to explain large scale fractionation patterns of semivolatile organic compounds and therefore related to the conceptual understanding of the mechanisms of POP long-range transport. It states that observed fractionation patterns are a result of global patterns of environmental temperatures interacting with the temperature dependencies of the chemicals' phase partition coefficients. Here, an alternative hypothesis is presented, the differential removal hypothesis . It proposes that the differences of average atmospheric loss rates, acting along a gradient of remoteness from emission sources, cause observed patterns of global fractionation. A model experiment and a thorough analysis of datasets of concentrations of polychlorinated biphenyls (PCB) in European air are used to compare both hypotheses. The results indicate that the variation of temperature across Europe and into the Arctic has no effect on observed fractionation patterns of PCB congeners in air, but that the differential removal hypothesis explains these patterns very well.

Finally, the relationship between measured concentrations and effective distance from emission sources is used to calculate effective residence times in air for a set of PCB congeners. The effective residence times compare well with values calculated by a multimedia mass balance model and can serve as an empirically derived metric for the long-range transport potential of semivolatile organic compounds.