Uncertainty and variability in atmospheric formation of PFCAs from fluorotelomer precursors

Colin P Thackray,Noelle E Selin

doi:10.5194/acp-17-4585-2017

Abstract

Abstract. 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.

Highlights

We find that atmospheric conditions farther from pollution sources have both higher capacities to form long-chain Perfluoroalkyl carboxylic acids (PFCAs) and higher uncertainties in those capacities
We calculate the variability in PFOA and PFNA theoretical maximum yields for summer and winter Northern Hemisphere conditions, and quantify the parametric uncertainty in these theoretical yields for three representative test cases
We find a wide variety of theoretical maximum yields for both PFOA and PFNA across the Northern Hemisphere’s photochemical environments

Summary

Introduction

Perfluoroalkyl carboxylic acids (PFCAs) are environmental contaminants that are highly persistent, bio-accumulative (Martin et al, 2003a, b; Conder et al, 2008), and have been detected along with their atmospheric precursors far from emissions sources (Young et al, 2007; Shoeib et al, 2006; Stock et al, 2007) in snow (Xie et al, 2015), precipitation (Scott et al, 2006), and biota (Houde et al, 2006). Emissions of atmospheric precursors of PFCAs are rising (Wang et al, 2014a), leading to an increasing indirect source of PFCAs to the environment. These precursors, including fluorotelomer alcohols (FTOHs), react with atmospheric photochemical species (Ellis et al, 2003) in a multi-stage process to form PFCAs (Young and Mabury, 2010). We use a box model analysis to quantitatively estimate potential upper-limit atmospheric yields of PFCAs, incorporating uncertainty in the precursor degradation mechanism and variability of atmospheric PFCA formation due to photochemical background conditions

Methods

Results

Discussion

Conclusion