Understanding the atmospheric transport and fate of fluorinated Persistent Organic Pollutants with global models

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Perfluorocarboxylic acids (PFCAs) are persistent pollutants found widely in the global environment where they have a range of adverse effects. Ecological risks from long-chain PFCAs (>7 carbon atoms), along with routes for human exposure, are well documented, leading to calls for more stringent regulation on their production. Short-chain PFCAs have received less attention but are of increasing environmental concern. Like long-chain species, short-chain PFCAs are detected in various remote regions (e.g. the Arctic) vast distances from industrial areas, indicating that they are mobile contaminants. Ultra-short chain PFCAs include trifluoroacetic acid (TFA), perfluoropropionic acid (PFPrA), and perfluorobutanoic acid (PFBA), with deposition from the atmosphere to the surface a major ecosystem source of these contaminants (e.g. Björnsdotter et al., 2020). In the atmosphere, secondary production of short-chain PFCAs may occur due to the oxidation of primary-emitted precursors, including fluorotelomer alcohols (e.g. Ellis et al., 2004) and gases that were introduced to replace ozone-depleting substances now banned by the Montreal Protocol (certain HCFCs, HFCs). Observational evidence shows deposition of short-chain PFCAs to the surface increased markedly in recent decades (Pickard et al., 2020; Garnett et al., 2022), but there are large uncertainties in their global budget. For example, a long purported significant natural source of TFA is now questioned (Joudan et al., 2021), while estimates of the amount of TFA deposited from the atmosphere are uncertain due to uncertainty in (1) the magnitude, distribution and trends of precursor emissions, (2) the mechanisms of precursor oxidation (and thus PFCA yields), and (3) the representation of atmospheric transport and other processes (e.g. wet/dry deposition) in global models. Moreover, the growing use of ultra-low GWP refrigerants (including hydrofluoroolefins, HFOs) as a source of TFA and other PFCAs requires urgent evaluation, bringing further policy relevance to this topic. To understand environmental risks posed by short-chain PFCAs there is a need to constrain their atmospheric budget and that of their precursors. To that end, key objectives of this project are to (1) develop the UCI 3-D chemical transport model to include major short-chain PFCAs and their precursors; (2) quantify global/regional deposition of short-chain PFCAs, including in remote polar regions; (3) explore variability in short-chain PFCA source-receptor pathways to uncertainties in model atmospheric transport and chemical/physical processes (e.g. tropospheric oxidants, treatment of dry/wet deposition); and (4) examine trends in short-chain PFCA deposition over recent decades and possible future trends based on precursor emission projections (e.g. explore the impact of the growing use of low-GWP HFO refrigerants).