Global distillation

ENDS (1996) Evidence accumulates on global distillation of POPS , The ENDS Report, no 261, pp9-l1... 

Many processes are operative in the environment that contribute to the regional elimination of a contaminant by altering its distribution. Contaminants with sufficiently high vapor pressure can evaporate from contaminated terrestrial or aquatic compartments and be transferred through the atmosphere to new locations. Such processes of global distillation are considered largely responsible for the worldwide distribution of relatively volatile organochlorine pesticides such as lindane and hexachlorobenzene. Entrainment by wind and upper atmospheric currents of contaminant particles or dust onto which the contaminants are sorbed also contribute to contaminant redistribution. Sorption of contaminant to suspended solids in an aquatic environment with commensurate sedimentation can result with the removal of contaminants from the water... 

Considerable attention has been given to the investigation of organochlorine residues in the marine polar ecosystems where the two effects of global distillation and bioaccumulation combine to produce high residues in predatory species such as marine mammals, fish and sea birds. As these compounds preferentially reside in fatty tissues, the high fat content of many of the northern marine species, and the inherited contamination through lactation, indicate that such ecosystems may be some of the most vulnerable. 

The Global Distillation Model, also called the Grasshopper Effect and the Cold-Condensation Effect, predicts a global fractionation of these chemicals will occur whereby more... 

Figure 6. The global distillation effect. Semi-volatile chemicals tend to move toward cold climates as a result of their lower vapor pressures at colder temperatures. is the temperature of conden.salion ( C) defined as the temperature at which half the total air concentration of that compound is in the vapor phase, and half is in the aerosol phase assuming that constant aerosol concentrations and other standard conditions prevail. We expect chemicals with higher volatilities and lower Tc values to be transported the furthest. Based on Wania Mackay (1993, 1996),...

This distillation effect has been shown to influence global distributions of many semivolatile organic contaminants. Simonich Hites (1995) showed that concentrations of the most volatile POPs in tree bark (HCH, HCB, PCA) increased significantly at higher latitudes whereas less volatile compounds (DDT, endosulfan) were not correlated with latitude, consistent with the predictions of the global distillation model (Fig. 6). 

Figure 7. Model prediction of a chemical distribution following an emission pulse at the equator. The distribution peaks are shown a.s a progression in time with Iq = time 0 and hypothetical times following in the order// ti /t and 14. The global distillation model predicts that relatively mobile, persistent semi-volatile chemicals will gradually migrate toward cold environments. Data are from Wania Mackay (19%).

The effects of climate change on the speciation and fate of mercury in Polar ecosystems is particularly important. Not only is mercury increasing in the atmosphere but atmospheric deposition will be favored in colder climates due to changes in atmospheric redox chemistry. This means that mercury released in equatorial areas will undergo a global distillation via a process similar to the grasshopper effect observed with semi-volatUe organic pollutants. 

---