Uncertainty and variability in atmospheric formation of PFCAs from fluorotelomer precursors

Perfluoroalkyl carboxylic acids (PFCAs) are environmental contaminants that are highly persistent, bio-accumulative, and have been detected along with their atmospheric precursors far from emissions sources. The importance of precursor emissions as an indirect source of PFCAs to the environment is uncertain. Modeling studies have used degradation mechanisms of differing complexities to estimate the atmospheric production of PFCAs, and these differing mechanisms lead to quantitatively different yields of PFCAs under differing atmospheric conditions. We evaluate PFCA formation with the most complete degradation mechanism to date, to our knowledge, using a box model analysis to simulate the atmospheric chemical fate of fluorotelomer precursors to long-chain PFCAs. In particular, we examine the variability in PFCA formation in different chemical environments, and estimate the uncertainty in PFCA formation due to reaction rate constants. We calculate long-chain PFCA formation theoretical maximum yields for the degradation of fluorotelomer precursor species at a representative sample of atmospheric conditions from a three-dimensional chemical transport model, and estimate uncertainties in such calculations for urban, ocean, and Arctic conditions using polynomial chaos methods. We find that atmospheric conditions farther from pollution sources have both higher capacities to form long-chain PFCAs and higher uncertainties in those capacities. Our calculations of theoretical maximum yields indicate that under typical Northern Hemisphere conditions, less than 10% of emitted precursor may reach long-chain PFCA end products. This results in a possible upper bound of 2-50 t year(-1) of long-chain PFCA (depending on quantity of emitted precursor) produced in the atmosphere via degradation of fluorotelomer products. However, transport to high-yield areas could result in higher yields. While the atmosphere is a potentially growing source of long-chain PFCAs in the Arctic, oceanic transport and interactions between the atmosphere and ocean may be relatively more important pathways to the Arctic for long-chain PFCAs.