Experimental study of atmospheric gas-phase TFA from sea spray aerosol

A preprint [1] ‘Newfound source of atmospheric gas-phase PFAS from sea spray aerosol’ has reported experimental studies on the transfer of TFA and other PFCAs (Perfluorinated carboxylic acids) from sea spray aerosol (SSA) particles to the gaseous phase. The preprint presents novel evidence that SSA also serves as a source of atmospheric gas phase PFAS, suggesting that their transport from the oceans to the atmosphere is much larger than currently assumed. The field observations and laboratory studies reveal that the amount of gaseous PFAS released may exceed particle-phase concentrations and depends primarily on the low pH of SSA particles, a result of the concentration of ocean water components in SSA particles upon formation and drying. According to the pre-print ‘to date, this mechanism has not been considered in estimates of oceanic PFAS emissions, indicating an underestimation of this source for PFAS deposition also in remote locations.’

The pre-print reported detection of four PFAS, including TFA, in the gas phase in ambient air and the sea spray simulation tank experiments tank headspace. The ambient atmospheric data was collected from June to mid-July 2022 at a remote marine site on Graciosa Island, Azores, Portugal and controlled SSA generation experiments used freshly collected Atlantic Ocean water in a simulation tank at the same field site and in a laboratory post-campaign. A mean ambient mixing ratio for gas-phase TFA of 73.10 ppt was observed, respectively, with increasing values at the end of the sampling period likely due to increased air temperature and sea surface temperature (SST). The observed values are significantly higher than previously reported measurements, which are in the range of 0.6-1.6 ppt for gas-phase TFA albeit from completely different environments. The authors comment that the ocean surrounding their measurement site on the Azores is influenced by the presence of oceanic gyres, which are known to concentrate plastics, thus making the sampling site a likely PFAS hotspot.

According to the authors, the observed high TFA concentrations at this remote location indicate a marine source of gas phase TFA and a possible underestimation of their gas-phase budget, especially in remote marine regions.

The proposed mechanism is that PFCAs (TFA) are present as sodium salts in SSA particles, and as the particles dry upon being released into the atmosphere, the pH drops and triggers redistribution of ions. Previous studies have shown that Na⁺ ions move to the particle surface while the other ions concentrate in the core when SSA particles are acidified or dried. The authors propose similar mobilization of PFCA^-Na⁺ complexes to the particle surface, where the simultaneous pH drop leads to protonation of anionic PFCAs (C_nF_2n+1COO⁻, including trifluoroacetate) into their neutral acid forms (C_nF_2n+1COOH, TFA). Neutral acids have higher vapor pressures and hence partition into the gas phase.

In tank experiments sea spray aerosol (SSA) generated with water samples from the Baltic Sea, North Sea, and the eastern North Atlantic Ocean (Azores) resulted in very different TFA mixing ratios, with much lower mixing ratios for the Baltic and North Sea (<10 ppt compared to ~ 200 ppt for the Azores). The tank experiment for the Azores seawater is also considerably higher than that measured in the Azore ambient air (73.1 ppt mean).

The pre-print does not estimate the quantity of TFA that might be transferred to the gas phase or discuss if TFA transport over land occurs, but comments on the critical role of SSA as a source and medium for the long-range transport of both the long- and short-chain PFCAs (including TFA).

Explanatory notes

The mean ambient mixing ratio for gas-phase TFA of 73.10 ppt at the location in the Azores is significantly higher than previously reported measurements (as the authors explain).  It is in the same range as observed for HCl, generated by the same mechanism as TFA from SSA [2]. The concentration of chloride in the Atlantic Ocean is about 100 million times higher than TFA (assuming 200 ng/l TFA).

One of the references in the preprint is a 2020 EFCTC paper ‘Transport of naturally occurring trifluoroacetic acid (TFA) by sea salt aerosol’ which discussed the potential transport of TFA via sea salt aerosol to the atmosphere [3].

However, a more recent report (2023) [4] for EFCTC “Assessment of Possible Transport of Trifluoroacetic Acid from the Oceans to the Atmosphere” by Professor Simon L Clegg, University of East Anglia, considered if TFA can be transported from the oceans to the troposphere, in the same way that HCl is partitioned from aged sea salt aerosol to the troposphere. Thus, for example, the US east coast, the Atlantic coast of Europe, and the Mediterranean all show strongly enhanced mixing ratios of HCl. Given that TFA is also acidic, it is possible at least in principle that TFA is expelled from acidified sea salt aerosols in the same manner as HCl and therefore constitutes an additional source of gas phase TFA in these and other regions. The report has similar conclusions to the preprint “Overall, these results suggest that displacement of TFA from seasalt aerosol may only occur where these conditions are met: at low ambient gas phase TFA concentrations (no more than a few ng TFA m-3), low RH, moderate to high temperatures, polluted conditions, and for the  submicron fraction of the seasalt aerosol (which is likely to be the only fraction that is sufficiently acid). Under other conditions – especially high RH and low acidity – the aerosol appears likely to be a sink of TFA.” The reports also concludes that only very minor quantities of TFA are transferred from the oceans to land via seal salt aerosol, i.e. consistent with the conclusion that the ocean is essentially a sink for TFA. Literature models of global chlorine cycling allow an estimate to be made of the flux of TFA that would be displaced from acidified seasalt aerosol if the behaviour is analogous to that of HCl but scaled according to the concentrations of TFA and Cl^- in surface seawater. This hypothetical value, although is unlikely to have practical relevance except possibly as an upper limit, is only 406 kg TFA/year globally. However, the ambient air results for the Azores suggest much higher transport of TFA to the atmosphere, at least in this location.

High concentrations of TFA have been measured widely in the Atlantic Ocean [5] leading to an estimated 40-80 million tonnes burden of TFA in the Atlantic Ocean suggesting that the Atlantic Ocean must contain a large natural burden of TFA [6]. However, in land areas adjacent to the Atlantic Ocean studies indicate that TFA is unlikely to have been present in the pre-industrial freshwater environment. Pre-industrial (>2000 year old) freshwater samples from Greenland and Denmark had no detectable TFA (<2 ng/l) [7]. The concentration of TFA in 113 Danish groundwater monitoring wells is strongly correlated to the groundwater recharge year, as derived from the tritium-helium dating method. TFA was not detected in tritium-free groundwater recharged before 1960 [8].  This is consistent with limited transport of TFA from the ocean to the land, as TFA occurring naturally must have been present in the Atlantic Ocean for a considerable period. TFA was also detected in the deep arctic ocean, with a reported ¹⁴C age of about 1000 years. [9].

References

1. Aggarwal, S., Salter, M., Garmash, O., Lopez-Hilfiker, F., Zinke, J., Thornton, J. A., Bertram, T. H., Wang, J., Zieger P. and C. Mohr Preprint 25 February 2026,  Newfound source of atmospheric gas-phase PFAS from sea spray aerosol | ChemRxiv
2. Erickson III, D. J., Seuzaret, C., Keene, W. C. and Gong, S. L., A general circulation model based calculation of HCI and CINO2 production from sea salt dechlorination: Reactive Chlorine Emissions Inventory, Journal of Geophysical Research,1999, 104, 8347 -8372.
3. EFCTC, Transport of naturally occurring trifluoroacetic acid (TFA) by sea salt aerosol, 2020, EFCTC-Paper-Transport-of-natural-TFA-by-Sea-Salt-Aerosol-02.04.2020-1-1-1.pdf
4. Clegg,S. L., Assessment of Possible Transport of Trifluoroacetic Acid from the Oceans to the Atmosphere, 2023 report for EFCTC, https://www.fluorocarbons.org/wp-content/uploads/2023/12/TFA-Report-ETCFC-Clegg-Sept2023.pdf
5. UBA (2024). Untersuchung von aktuellen Meerwasserproben auf Trifluoressigsäure  (Investigation of current seawater samples for trifluoroacetic acid). https://www.umweltbundesamt.de/publikationen/untersuchung-von-aktuellen-meerwasserproben-auf#:~:text=Der%20Bericht%20beschreibt%20die%20Entwicklung%20und%20Validierung%20einer,aus%20den%20Jahren%202022%20und%202023%20zu%20bestimmen
6. Lindley, A. A. An Updated Inventory of Fluorspar CaF2 Production, Industrial Use, and Emissions of Trifluoroacetic Acid (TFA) from 1930, Including the Period from 2000 to 2020, Journal of Geoscience and Environment Protection, 2025, 11 https://doi.org/10.4236/gep.2025.1311008 
7. Nielsen, O. J.; Scott, B. F.; Spencer, C.; Wallington, T. J.; Ball, J.C. Trifluoroacetic Acid in Ancient Freshwater. Atmos. Environ. 2001,35 (16), 2799−2801.
8. Christian N. Albers and Jürgen Sültenfuss, A 60-Year Increase in the Ultrashort-Chain PFAS Trifluoroacetate and Its Suitability as a Tracer for Groundwater Age, Environmental Science & Technology Letters 2024 11 (10), 1090-1095, DOI: 10.1021/acs.estlett.4c00525
9. Scott, B. F., Macdonald, R. W., Kannan, K., Fisk, A., Witter, A., Yamashita, N., Durham, L., Spencer C., & Muir, D. C. G. (2005). Trifluoroacetate profiles in the Arctic, Atlantic, and Pacific Oceans. Environ. Sci. Technol., 39, 6555-6560. https://doi.org/10.1021/es047975u