A global atmospheric chemistry model for the fate and transport of PFCAs and their precursors

Abstract

Perfluorocarboxylic acids (PFCAs) are environmental contaminants that are highly persistent, and many are bio-accumulative and have been detected along with their atmospheric precursors far from emission sources. The overall importance of precursor emissions as an indirect source of PFCAs to the environment is uncertain. Previous studies have estimated the atmospheric source of PFCAs using models and degradation pathways of differing complexities, leading to quantitatively different results. We present results from simulations of atmospheric PFCA formation and fate using the chemical transport model GEOS-Chem. We simulate the most up-to-date chemistry available to our knowledge for the degradation of the precursors fluorotelomer alcohol (FTOH), fluorotelomer olefin (FTO), and fluorotelomer iodide (FTI), as well as the deposition and transport of the precursors, intermediates and end-products of the formation chemistry. We calculate yields of C3-C13 PFCAs formed from 4 : 2 to 12 : 2 fluorotelomer precursors and their deposition to the surface. We find that the ratio of long-chain to short-chain PFCAs increases strongly with distance from source regions. We compare our model results to remote deposition measurements and mid-latitude rainwater measurements. The model captures the observed relationship between rainwater abundance and PFCA chain length, as well as the average deposition rates at mid-latitude and Arctic sites, but underestimates the deposition of PFDoA, PFDA, and TFA at mid-latitudes and PFNA at the Devon Ice Cap. We provide estimates of cumulative PFCA deposition globally. We find that given the most recent emission inventory, the atmospheric source of PFCAs is 6-185 tonnes per year globally and 0.1-2.1 tonnes per year to the Arctic.

Fig. 1:

Measurement-model comparison. (a) PFCA abundance in rainwater across sites globally. (b) Mid-latitude wet deposition of C3–C12 PFCA across measurement locations. Observed geometric means and standard deviations across sites in blue, modeled geometric means and uncertainty in geometric means in orange. (c) Average deposition of C5–C11 PFCA to the Devon Ice Cap over the years 1977–2015. Observed mean deposition rates and their uncertainty are in blue. Modeled deposition rates and their uncertainty range are in red/orange, with color shading representing the fraction of the total uncertainty range contributed by uncertainty in yields (orange) and uncertainty in emissions magnitude (red). (d) Odd-even homolog deposition ratios over the Devon Ice Cap record. Observed ratios (blue), with modeled ratios using BASE (orange), NO_HYDR (green) and FTO_ONLY (red) scenarios.

Fig. 2:

Spatial distribution of modeled 2013–2015 annual average (a) PFOA deposition (ng/m2/yr), (b) deposition of all longer-chain PFCA (ng/m2/yr), (c) deposition of all shorter-chain PFCA, (d) Precursor-produced long-chain (>C7) to short-chain deposition ratio.