“Chemicals have several ways to be present in the atmosphere,” explains Wania. Depending on temperature and weather conditions, as well as the size, shape, and the elements that make up the molecule, the same substance can be found dissolved in water, as a gas, or as a particle. The smaller the molecule, the more volatile it typically is, and therefore the more likely to be swept along with atmospheric currents as a gas. These gas phase molecules— the fliers—can move in several meters per second, making the trip from their points of origin to remote locations like the Arctic in days or weeks. At the opposite extreme, the waterborne swimmers can take years to reach the same destination.
The hoppers, intermediate-sized molecules that can move between gas, liquid, and particle phase, may take days, weeks, or even years to reach the Arctic after their initial release. These hoppers may be present in liquid water, but as temperatures warm they will evaporate to gas phase but then condense and return to join water when temperatures cool. They’ll repeat this cycle over and over again, rising and falling—or hopping—with daily and seasonal patterns of warming and cooling. It’s in this way that many persistent chemicals move with clouds and precipitation as storm systems and ocean currents circle the globe, and why temperatures so strongly influence how and where pollutants travel.
“Persistence and mobility is what makes something troublesome,” says Wania. “It’s a very difficult, laborious, and time-consuming process to prove toxicity, and by the time you have evidence it may be too late.” If a substance is “persistent, highly mobile, and can’t be contained, you have a problem you can’t rectify."
Another major influence on the movement and deposition of persistent pollutants is precipitation. Put simply, the more it rains or snows the more likely these contaminants are to wash out of clouds and be deposited on land, lakes, rivers, and oceans. In a recent paper Wania and colleague Torsten Meyer note, “Real substances affected by changes in rain rate include lindane, aldrin [both highly toxic and persistent pesticides], highly chlorinated PCBs, PBDEs, and some currently used pesticides.” When it’s warmer, more of these substances will tend to evaporate again and join the cloud layer, and from there the cycle of condensation and precipitation begins again. When present as aerosols, the contaminants may even accelerate precipitation as water droplets coalesce around the tiny solids.
Whatever affects atmospheric and ocean circulation clearly plays an important role in where environmentally roving persistent pollutants end up. The big hemispheric wind and ocean patterns known as gyres and oscillations all play a part—as do more localized storm systems and currents. “These routes all seem to force contaminants released in Europe to the Arctic,” explains Derek Muir, a senior scientist in aquatic ecosystems research with Environment Canada who specializes in contaminants. “Think about Chernobyl,” Muir says by way of illustration. “The radioactivity there ended up in western Scandinavia where a lot of reindeer were sacrificed as a result. Other contaminants follow the same pathway north from Russia.”
What happens once pollutants reach the far north is very much influenced by where there is ice. Ice typically stabilizes contaminants and holds them in place until they’re released again when temperatures rise high enough for melting to begin. Greenland, which Muir describes as “a big block of ice 3,000 meters or more thick,” appears to be acting as a source of contaminants in the Arctic as well as a sink.
On the east side of Greenland and across the Greenland Sea on the remote Norwegian islands of Svalbard that reach all the way up to 80 degrees north—in the path of air and water currents coming off of Greenland and the European mainland—levels of PCBs, PBDEs, and perfluorinated compounds have been found to be particularly high. Svalbard’s polar bears have contaminant levels higher than bears on the west side of Greenland or in the Canadian Arctic, says Muir, largely because of increased melting.