Global distillation

OUR PLANET 8.6 - Chemicals



Global distillation



FRANK WANIA and DON MACKAY

describe how a recently-discovered phenomenon
is turning polar areas into chemical dumps





penguin

When sipping a cold beer on a muggy summer afternoon, it is hard not to notice water misting on the outside of the glass. The cold beer has cooled the glass, which in turn has cooled the surrounding air - which thus loses some of its capacity to retain water vapour. The excess water condenses like dew on the glass. A similar phenomenon causes spectacles to fog in winter when the wearer moves from the cold dry weather outside into a warm humid building. It is also responsible for frosting in refrigerators, icing on aircraft and morning dew on grass - but is not restricted to water.

A similar phenomenon is believed to occur on a global scale for certain environmental contaminants such as PCBs and pesticides such as DDT, lindane and toxaphene. These are persistent chemicals - which means that they are only very slowly degraded in the environment - and they are slightly volatile or 'semi-volatile': i.e. they evaporate, but slowly. They are usually applied or discharged in populated areas in temperate and tropical climates to soils, vegetation and water bodies from which they can volatilize. In some cases they are present in air emissions.

The compounds are transported in gaseous form by wind in the atmosphere until they meet cold temperatures. They then 'condense' either directly on to the Earth's surface, or on to solid particles contained in the atmosphere (aerosols) which are then deposited, often in rain and snow. The net effect is evaporation in warm low latitudes and deposition in cold higher latitudes. So there is a continuous transfer of contaminants in the global atmosphere towards the poles in a giant process resembling distillation.

This is not a one way process. In fact, the chemicals evaporate and deposit at both low and high temperatures. Transport of air towards the poles on average equals the transport of air towards the equator, and the atmospheric currents carry chemicals in both directions. But because low temperatures favour deposition there is a net transfer of contaminants from the surface to the atmosphere at low latitudes, and from the atmosphere to the surface at high latitudes (see overleaf).

Levels of contaminants are further increased in the cold and dark polar regions because the chemicals are less likely to be degraded than in warmer, more sunny regions. As a result these contaminants are present at surprisingly high concentrations in cold ecosystems in which they have never been used. The highest concentrations currently found of the pesticides *-HCH and toxaphene in sea water are in the Arctic Ocean. Once in these cold ecosystems the contaminants enter food chains and bioaccumulate in fish, birds, marine mammals and, of course, in humans.

It is believed that the chemicals' journey from the tropics to the polar regions can proceed in countless steps - the so-called 'grasshopper' effect - and it may involve convoluted detours and back-tracking. The journey may last for decades until the chemical is eventually degraded or permanently retained. Some persistent DDT molecules released in the 1950s may still be moving through the world's environment today. Observations of differences in summer and winter concentrations in the atmosphere suggest that the chemicals tend to evaporate during summer and deposit in the winter.

Some very involatile chemicals are also present in the atmosphere, but are entirely associated with aerosol particles. Once deposited, they can only re-enter the atmosphere under special circumstances, such as a dust storm. These can reach remote polar regions as well, but - because they have to make the long journey with a single 'hop' - the weather conditions have to favour poleward transport (fast meridional transport from mid-latitudes and no precipitation). Stable high-pressure systems over Siberia tend to favour efficient transfer of air masses from Eastern Europe into the Arctic, which has been documented by the occurrence of haze and particle-bound contaminants in the Arctic during the northern winter. This 'short-cut' transport occurs in days or, at most, in a few weeks.

The global distillation process has other subtle features. Apparently only a certain combination of properties renders a chemical susceptible to condensation in cold climate ecosystems. Some are too involatile to make the journey (other than by a single hop), others do migrate to the poles but are too volatile to condense there. There is a kind of global 'fractionation' at work - in the same sense that a petroleum refinery will fractionate crude oil into gasoline, diesel fuel, etc. Each chemical appears to have a specific temperature at which it starts to 'condense' significantly - and correspondingly a specific climate zone of preferential deposition and accumulation. Attempts to model the phenomenon by computer resulted in a hypothesis that there would be fractionation by latitude (i.e. chemicals differ in the distance they can journey towards the poles), an effect recently confirmed by analyzing PCBs in lake sediments.



Vulnerability and ethics

Clearly we should try to identify the properties which cause chemicals to accumulate in polar areas with a view to restricting their use. Various criteria based on vapour pressure, air-surface equilibrium partition coefficient and the temperature of vapour condensation on to aerosols have been suggested. Interestingly, metallic mercury satisfies these criteria, suggesting that it may be subject to enrichment in cold environments similar to that of the semi-volatile organic compounds.

Some may say that it is preferable to have these contaminants transfer and condense into high latitude regions where few people live, rather than stay and accumulate in the heavily populated areas where they are used, and where potentially many more people will be exposed. Yet can it be appropriate to use the remote polar areas as inadvertent dump sites for hazardous chemicals? Many cherish the existence of natural environments still largely unspoiled by the exploits of humankind. It is also difficult to accept that residents of polar regions should suffer the risk of chemical usage, while the benefits are enjoyed by those fortunate to live in warmer climes.

But this is not merely a question of environmental aesthetics and ethics. The chemicals may indeed pose a greater hazard in the Arctic than elsewhere because the people of the North rely on locally caught animals for food to a much larger extent. Even if fish and marine mammals in the North were to be no more contaminated than those in temperate and tropical zones, the people there would be more vulnerable. In fact the breast milk of Inuit women in Greenland and Arctic Canada has been found to contain several times the PCB content of milk from women living in industrial centres in mid-latitudes.

Ultimately, the controversy is not between North and South. The whole world community will benefit if these contaminants can be banned and replaced with more benign substitutes, which will degrade and will not be susceptible to distribution throughout our global environment

Dr. Frank Wania is an environmental chemistry consultant working out of Toronto, Canada, and Prof. Don Mackay is Professor of Environmental and Resource Studies at Trent University, Peterborough, Ontario, Canada.


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