Emissions, pathways and modelled impacts of climate change: Perflouroalkyl substances (PFAS)
Oceanic transport of perfluorooctanoic acid (PFOA) to and within the Arctic Ocean over the period 1950 to 2010 was modelled using NAOSIM and atmospheric transport and oceanic transport were modelled using MPI-MCTM. Modelled concentrations of PFOA in the Arctic Ocean show clear changes due to changes in ocean circulation that occurred during the mid-1990s and 2000s during a prolonged positive phase of the Arctic Oscillation. These model results for PFOA are very similar to the behaviour of 129I, a radionuclide tracer for which there is a considerable amount of empirical evidence consistent with model simulations.
Perfluoroalkylated substances of environmental concern include PFOA and PFOS, which are highly persistent. Both PFOA and PFOS are believed to be sufficiently acidic to exist mostly in the environment as anions that have very low vapour pressure, but are soluble in water. Their fate and transport in the world’s oceans under climate variability and climate change is therefore of paramount interest.
The modelled concentrations of PFOA in the surface ocean illustrate clear changes in ocean circulation that occurred during the mid-1990s and 2000s. The model results for PFOA are very similar to the behaviour of 129I, for which, in contrast to the situation for PFOA, there is a considerable amount of empirical observational evidence that is consistent with the model simulations. The model results for PFOA clearly show that the interannual to decadal variability of Arctic Ocean circulation under the present climatic conditions leads to exposure patterns that are spatially and temporally variable on basin-wide scales. These results have been confirmed by the observational and simulated dispersion of 129I (Karcher et al., 2012). The simulations also reveal the local surface stress curl over the Arctic Ocean basin as the key driver for the exposure patterns for a given release of a conservative tracer with low tendency to be bound by suspended matter. In conjunction with further experiments, they also show that the large-scale circulation changes affect regional exposure, residence time and lateral displacement of marine pollutants in the Arctic Ocean. This holds for Pacific, Atlantic, riverine and atmospheric sources. In light of a reducing sea-ice cover under future climate change scenarios, it is important to note that one of the consequences will be changed momentum transfer from the atmosphere to the ocean.
The contribution of PFAS via ocean currents into the Arctic Ocean was estimated to 14.8 +5.0 (8-23) ton/year, while the atmospheric PFOA transports into the Arctic were predicted to be about 1 t/yr. That is higher than previously anticipated (about 0.27 t/yr; Young et al., 2007). This flux is almost entirely by secondary PFOA (contributions from tropospheric chemistry; PFOA being a transformation product of fluorinated telomers), while primary PFOA contributes less than 5 percent. The current assessment of long-range atmospheric transport of PFOA has been that it would be insignificant as compared to oceanic transports unless the pKa was ≥ 3.5 (Armitage et al., 2009). The pKa is uncertain, that is, it is between −0.5 and 3.8 (Goss, 2008, in addition to others). In a recent study (Cheng et al., 2009), it was observed that PFOA behaved similarly to the highly acidic PFOS and a pKa less than 1 was concluded. No full coverage of the uncertainty in pKa of PFOA was accomplished in the ArcRisk model experiment. Instead, by adopting pKa = 2.8, a high estimate for the undissociated fraction was implied. However, even with this estimate, volatilization from the sea surface accounted for only 0.01 percent of the total PFOA flux from surfaces.