3-D modeling supports perfluorinated theory
New 3-D modeling provides further backing to the theory that the PFOA and other fluorochemicals found in the Arctic comes from fluorotelomer alcohols used in stain and grease repellants.
An emerging theory that explains how PFOA (perfluorooctanoic acid ) and other PFCAs (perfluorocarboxylic acids) have contaminated the Arctic has received a boost from a new modeling study posted today on ES&T’s Research ASAP website (10.1021/es051858x). The theory contends that Arctic contamination is due to atmospheric transport and breakdown of fluorotelomer alcohols, chemicals that are used in products that include stain protectors, microwave-popcorn bags, fast-food wrappers, polishes, and paints.
View larger image
The theory proposed by University of Toronto chemist Scott Mabury, Ford Motor Co. atmospheric chemist Tim Wallington, and colleagues is that air currents transport fluorotelomer alcohols from densely populated industrialized regions such as the northeastern U.S. to remote regions, including the Arctic. Along the way, chemical reactions in the atmosphere transform the fluorotelomer alcohols into PFOA and other PFCAs. Ford and Mabury are among the authors of the new study.
Regulators are investigating PFOA, which is found at low levels in human blood, and other PFCAs because these compounds are ubiquitous in the environment, extremely persistent, and bioaccumulative to varying extents. Last summer, the U.S. EPA’s science advisory board recommended that the agency classify PFOA as a likely human carcinogen. In June 2004, Canada issued a temporary ban on some fluorotelomer compounds that could break down in the environment to PFOA and other PFCAs.
Fluorotelomer alcohols are volatile chainlike compounds made up primarily of carbon, fluorine, and hydrogen atoms. They are manufactured by companies that include DuPont (U.S.), Atofina (France), Clariant (Germany), Asahi Glass (Japan), and Daikin (Japan). Two numbers are often used to refer to the fluorotelomer alcohols. The first gives the number of carbons that are bonded to fluorine. The second gives the number of carbons that are bonded to hydrogens, so that C8F17CH2CH2OH = 8:2 FTOH, which is the most important commercially. DuPont estimates global production at 12 million kilograms, and sales of the fluorotelomer alcohols generate about $700 million annually, according to Dupont’s Global PFOA Strategy Update, which the company presented to EPA’s Office of Pollution Prevention and Toxics in January 2005.
“This work makes the argument very well that fluorotelomer alcohols are a potential source of PFCAs,” says Roger Atkinson, an atmospheric chemist at the University of California, Riverside. He notes that similar atmospheric reactions are known to convert some of the chemicals that have replaced CFCs (chlorofluorocarbons ).
In their study, which used a sophisticated 3-D atmospheric chemistry model called IMPACT, the scientists released 1000 metric tons of 8:2 FTOH globally. 8:2 FTOH has an atmospheric lifetime of about a month; this gives it ample time to disperse widely throughout the northern hemisphere, including the Arctic. The model predicts that the concentrations of 8:2 FTOH in the atmosphere will only be 5 times higher in source regions such as the eastern U.S. than they are in the Arctic, according to Sanford Sillman, a University of Michigan atmospheric modeler and coauthor of the paper.
The distinctive feature of the fluorotelomer alcohol’s atmospheric chemistry is that the 8:2 FTOH is most likely to be transformed into PFCAs in rural and remote locations, Sillman says. This is because the cascade of reactions that transform telomer alcohols into PFCAs involves hydroxyl and peroxy radicals. In polluted urban areas, NOx uses up the peroxy radicals. However, in rural and remote locations, where NOx is scarce, the 8:2 FTOH embarks on a cascade of reactions with hydroxyl and peroxy radicals that eventually yields PFCAs.
The model finds that roughly 5% of the fluorotelomer alcohols in the atmosphere are converted to PFCAs. The resulting PFCAs are distributed throughout the northern hemisphere, with highest levels in the Arctic and the mid-Atlantic Ocean. The model predicts that the levels peak in the Arctic during the summer, when they are roughly twice as high as the concentrations in the eastern U.S.
But in winter, the highest concentrations are found in the eastern U.S. To test the validity of these predictions, the scientists are now measuring atmospheric concentrations. On an annual average basis, the model predicts that concentrations of PFCAs in the Arctic and U.S atmosphere will be similar.
The amount of PFOA produced from the 8:2 FTOH depends on the location (see figure) and the time of year, but ranges from 1 to 10%. This is the correct order of magnitude to explain the levels of PFOA that have been observed in Arctic wildlife, the authors note.
The model used in this study is one of a handful of state-of-the-art 3-D simulation programs for global atmospheric chemistry, according to Sillman. It “is a mature model that’s consistent with how the atmosphere operates and accounts for atmospheric reactions that include photolysis, reactions involving the hydroxyl and peroxy radicals, and NOx species,” Mabury adds. “We’ve taken the smog-chamber results on atmospheric chemistry together with the environmental observations [and] put them together in a realistic model and obtained a result that is consistent with the theory,” he says.
