Snow shows perfluorochemical source
Perfluorochemical concentrations in Arctic snow point to an atmospheric source.
3M Corp.’s voluntary phaseout of products based on perfluorooctane sulfonate (PFOS) chemistry has led to a rapid decline in PFOS levels in Arctic snow, according to new research published today on ES&T's Research ASAP website (DOI: 10.1021/es0626234). The general trend for PFOS in the ice cap is consistent with previously reported declines in PFOS levels in ringed seals (Environ. Sci. Technol. 2007, 41, 42–49). The new research could help put an end to the controversy within the scientific community over the source of PFOS and other perfluorochemicals in the Arctic.
Such a quick environmental response to the phaseout, which 3M announced in 2000, combined with new data on the relative concentrations of other perfluorochemicals in Arctic snow and ice, supports an atmospheric source, say the authors. The Arctic has no direct sources for nonvolatile perfluorochemicals, so their presence there has long been an environmental mystery.
University of Toronto chemist Cora Young and colleagues collected ice samples from the high Arctic in the spring of 2005 and 2006 and a series of samples from a 6.8-meter-deep pit on Devon Island in the Canadian province of Nunavut. A pit this deep dates back to 1996, the authors say. Young took large-volume samples and concentrated them 100-fold to find picogram-per-liter concentrations of several different perfluorochemicals. These levels are 2–3 orders of magnitude below those measured at lower latitudes.
“The drop in PFOS is dramatic—about 500% from the high in 1998 to the recent lower levels,” says Young. But Stockholm University chemist Ian Cousins has reservations. “I’m concerned about deriving time trends from the snow layers, because of the mobility of perfluorinated compounds, especially during melting of ice and snow in the spring and summer,” he says. “Ice melting could cause some chemical migration to lower depths, but not much,” Young says, noting that other ice-cap studies have reached similar conclusions.
Two different theories have been proposed to explain the presence of nonvolatile perfluorochemicals in the Arctic. According to the atmospheric theory, advanced by University of Toronto chemist Scott Mabury and colleagues, the most significant source is fluorotelomer alcohols. These are volatile precursors of commercial products such as stain-repellent coatings that escape from factories or consumer products into the atmosphere, where they are transported and oxidized to form perfluorooctanoic acid (PFOA) and other perfluorocarboxylic acids.
The competing, oceanic hypothesis, articulated most fully by Cousins and colleagues, is that historical emissions, relatively uncontrolled in the past, have their final resting place in the ocean, where currents transport them toward the poles. In the Arctic, these legacy pollutants are incorporated into the marine food web. They can also concentrate in the ocean surface layer, from which they can be taken up by marine aerosols and land on the ice.
Solving the mystery has important ramifications, says toxicologist Kurunthachalam Kannan of the New York State Department of Health’s Wadsworth Center. “If we can identify the major sources, effective regulatory decisions can be made to reduce future emissions,” he says.
A predominantly atmospheric source might suggest regulations designed to control emissions from current operations and products. A predominantly legacy source might prompt different action, Kannan says.
Canadian officials, concerned about the country’s Arctic regions, have taken an aggressive regulatory stance against new fluorotelomer products. The action is based on the atmospheric theory promoted by Mabury, according to John Arseneau, director general of Environment Canada’s risk-assessment directorate.
One way to search for a marine aerosol source is to use sodium as a tracer. Because perfluorinated chemicals are powerful surfactants likely to concentrate in the ocean surface layer, their abundance might be enriched relative to that of sodium. Young finds that on the ice cap, the ratio of PFOA to sodium is enriched 10,000 times relative to oceanic ratios. In addition, no correlation exists between levels of sodium and the perfluorochemicals, so she discounts marine aerosols.
Such dismissal is premature, says Cousins, who notes that enrichment factors for perfluorinated compounds are unknown. The extreme enrichments reported by Young most likely rule out marine aerosols as a source, argues marine chemist Robert Duce at Texas A&M University. In addition, “the complete lack of correlation with sodium also supports the contention that this material on the ice cap is not ocean-related,” he says.
