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CO 2 at 13.8 MPa and 40 °C, there was no detectable [BMIM][PF6] in the extract, indicating that the solubility is less than 10 ǁ5 mole fraction. In contrast, a mixture of CO 2 with a conventional organic liquid results in significant solubility of the liquid in the CO 2 -rich phase. The phase behaviour of the ionic liquid-CO 2 system resembles that of a cross-linked polymer-solvent system 9 , even though [BMIM][PF6] is a lowviscosity, low-molecular-weight liquid. The liquid phase increased in volume by only 10-20% when 8 MPa of CO 2 pressure was applied, perhaps because it is ionic; this corresponds to a more than twofold decrease in the molar volume.Naphthalene was chosen as our model non-volatile organic solute because it dissolves readily in [BMIM][PF6] (maximum solubility of 0.30 mole fraction at 40 °C) and in CO 2 (with a solubility of 0.013-0.017 mole fraction at 35 °C and pressures of 12.2-20.4 MPa; ref 10). A mixture of 0.12 mole fraction naphthalene in [BMIM][PF6] was extracted with CO 2 at 13.8 MPa and 40 °C with recoveries of 94-96% (Fig. 2). This near-quantitative recovery compares favourably with the dissolution of a similar amount of pure solid naphthalene with comparable amounts of CO 2 (Fig. 2). It is therefore possible to quantitatively extract a non-volatile organic solute from an ionic liquid using CO 2 without any contamination. Moreover, the dissolution of CO 2 in the ionic liquid is completely reversible: pure ionic liquid liquid does not dissolve in carbon dioxide, so pure product can be recovered.We synthesized 7 the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6], which is stable in the presence of either oxygen or water. Our primary objective was to show that CO 2 could be used to extract naphthalene, our lowvolatility model solute, from an ionic liquid, but it was important to show that the CO 2rich phase is not significantly contaminated by the ionic liquid, as would be expected during contact of CO 2 with any conventional organic solvent. We therefore investigated the phase behaviour of [BMIM][PF6] with CO 2 , as well as with naphthalene, and finally that of the [BMIM][PF6]-CO 2 -naphthalene ternary.CO 2 is highly soluble in [BMIM][PF6] (Fig. 1), reaching a mole fraction of 0.6 at 8 MPa, yet the two phases are not completely miscible: cloud points of mixtures ranging from 1.3 to 7.2 mole % of ionic liquid at 25°C could not be found at pressures up to 40 MPa, the highest pressure accessible with our equipment. The composition of the CO 2 -rich phase is essentially pure CO 2 . After extracting the ionic liquid with 55 g scientific correspondence NATURE | VOL 399 | 6 MAY 1999 | www.nature.com 29

Conifer needles from mountain areas of Alberta and British Columbia, Canada, were collected from sites that ranged in altitude from 770 to 2200 masl and were analyzed for polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCs) to determine if they are progressively concentrated in colder, more elevated mountain areas, where temperatures decrease as elevation increases. Concentrations of OCs in needles ranged from 43 to 2430 pg g(-1), 55-17500 pg g(-1), and 11-2930 pg g(-1) (dry weight), for total hexachlorocyclohexanes (HCHs), PCBs, and endosulfans, respectively. The more volatile OCs, with subcooled liquid vapor pressures (PL) > 0.1 Pa at 25 degrees C, increased at higher altitudes, whereas the less volatile OCs were either unrelated or inversely correlated with altitude. These spatial patterns were similar for species of spruce (Picea engelmannii and glauca) and pine (Pinus contorta and albicaulis). Back trajectories revealed that air masses arriving at these sites traveled over Asia and the Pacific Ocean before reaching the Rocky Mountains. Results from this study demonstrate that alpine ecosystems accumulate these chemicals to the same degree that is observed in polar environments that are known to receive contaminants by long-range transport.

Ammonia is released in the environment by many industries and other human activities. The major quantifiable sources of ammonia released to aquatic ecosystems across Canada are municipal wastewater treatment plants, at an estimated total quantity of 62,000 tonnes per year. Given the sources of ammonia releases in the environment and the properties of the substance, terrestrial plants and aquatic organisms are potential risk targets. A tiered assessment approach has been used to determine the ecological risk in the aquatic environment from ammonia released in municipal wastewater effluents. The results obtained for two case studies with the probabilistic risk analysis used in the highest tier support the conclusion that the conditions encountered in these two locations can lead to ammonia concentrations capable of producing an adverse ecological impact.

The fluxes of several organochlorine compounds (OCs) to and from a high-altitude lake were calculated to determine the relative contributions of rainfall, snowfall, runoff, gas absorption and volatilization, sedimentation, and outflow. Runoff inputs, particularly from a glaciated catchment, and gas absorption were major sources of many OCs. Losses by volatilization, outflow, and sedimentation were also important transport pathways for most OCs. We observed a net deposition of hexachlorocyclohexanes (HCHs) and DDT to Bow Lake by gas exchange, whereas hexachlorobenzene, chlordanes, and most polychlorinated biphenyls were seen to volatilize to the air. The insecticide lindane (␥HCH), widely applied across Canada as a seed dressing in early spring, was absorbed by Bow Lake in early summer but gradually equilibrated with water as the summer progressed. This was the result of lower concentrations of ␥HCH in air and rising air temperatures in late summer. Results indicate that glacially derived tributary inputs are dominant sources of persistent organic pollutants to these glacier-fed mountain lakes and that smaller contributions may derive from air-water gas exchange. This study highlights the importance of glacial sources of OCs to mountain lakes and corroborates recent evidence of OC contamination in glacially derived aquatic systems.Several organochlorine compounds (OCs) such as hexachlorocyclohexanes (HCHs), hexachlorobenzene (HCB), toxaphene, and polychlorinated biphenyls (PCBs) have contaminated alpine environments by atmospheric transport (e.g., Blais et al. 1998;Donald et al. 1998). Donald et al. (1998) showed that fish from alpine and subalpine lakes had concentrations of toxaphene that were two orders of magnitude higher than those in fish from nearby lakes at lower 1 Corresponding author. Present address: