Study shows very minor transport of TFA via seasalt aerosol from the oceans to land
EFCTC commissioned a study “Assessment of Possible Transport of Trifluoroacetic Acid from the Oceans to the Atmosphere” by Professor Simon L Clegg, University of East Anglia, to understand if significant quantities of TFA are transported in sea salt aerosol from the oceans, with the potential of further transport over land, in the gas phase for TFA that has been expelled from the aerosol by acidification. The study evaluated the potential for TFA to partition from aged sea salt aerosol, in the same way that hydrochloric acide (HCl) is partitioned from aged sea salt aerosol to the troposphere. There are studies published for HCl enabling a comparison to be made. This study was prompted in part by the published papers showing significant transport of longer chain PFAS from the oceans to land. See explanatory note.
The results imply that acidified seasalt aerosols will be a very minor contributor to gas phase TFA, except under a very limited set of conditions (low relative humidity (RH), high temperature, polluted conditions, and for the small submicron fraction of the aerosol in which high acidities can be attained, thus limiting the total displacement of TFA). Under other conditions – especially high RH and low acidity – the aerosol appears likely to be a sink of TFA. The study 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.
For a surface seawater TFA concentration of 150 ng L-1 (a factor of 7.8×10-9 times that of Cl-), a literature model of global chlorine cycling suggests that there would be annual flux of TFA to the atmosphere in seasalt aerosol of about 14 tonnes annually. Acidification of seasalt aerosol by dissolution of HNO3 and from oxidation of dissolved dimethyl sulphide and SO2 to produce H2SO4 results in the expulsion of HCl to the gas phase through the Henry’s law equilibrium. Although TFA is a weaker acid than HCl, and its concentration in the aerosol very much lower than that of Cl-, the same mechanism can in principle apply.
The report reviews the current knowledge of the relevant thermodynamic properties of TFA – the acid dissociation constant and Henry’s law constant. Calculated equilibrium partial pressures of TFA above typical seasalt aerosols over a very wide range of pH are found to be lower than ambient gas phase concentrations (a few ng m-3) by a large margin, except at very low pH. The behaviour of TFA and HCl were also compared by calculating the equilibrium gas/liquid partitioning as a function of pH and for atmospheric liquid water contents ranging from aerosols in low relative humidity (RH) environments to those of cloud and fogwater. The patterns are similar: at atmospheric liquid water contents and pH typical of fog and cloud water the greatest fractions of both substances (approaching 100%) are expected to be in the aqueous phase. This is consistent with them both being removed by wet deposition. Conversely, at moderately acidic and low pH, and atmospheric liquid water contents typical of aerosols, the largest fractions of both total Cl- and total TFA per m3 of atmosphere will be found in the gas phase. Of the two acids, TFA has the greater tendency to partition into the gas phase. The predicted balance between gas and aerosol fractions of TFA is consistent with the limited number of atmospheric measurements that have been made.
Finally, 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.
The Report “Assessment of Possible Transport of Trifluoroacetic Acid from the Oceans to the Atmosphere” will soon be available on the EFCTC Website. The report was prepared for EFCTC by Professor Simon L. Clegg, School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ U.K. Professor Clegg’s key research interests are “Chemical thermodynamics of aqueous solutions; activity coefficient models and their application to problems in gas/aerosol partitioning in the atmosphere and air quality, brine chemistry, and solubility phenomena; predictive methods for physical properties of pure compounds (such as vapour pressures), and aqueous solutions; measurements of the properties of single aerosol particles using electrodynamic balances.”
Explanatory Note: Recent publications have reported experiments that have demonstrated effective enrichment of perfluoro alkyl acids (PFAAs) in nascent sea spray aerosols (SSA), suggesting that SSA are an important source of PFAAs to the atmosphere. The enrichment factor initially increases with increasing chain length from NPC=4 (NPC-number of perfluorinated carbons) with enrichment factors reported for NPCs from 4 to 11. The surfactant properties of these longer chain PFAAs are considered to contribute to the enhancement factors. However, the behaviour of surfactants can be influenced by many factors and could be more complex for surfactant mixtures. In contrast TFA is not a surfactant.
References: Influence of Water Concentrations of Perfluoroalkyl Acids (PFAAs) on Their Size-Resolved Enrichment in Nascent Sea Spray Aerosols, Bo Sha, Jana H. Johansson, Jonathan P. Benskin, Ian T. Cousins, and Matthew E. Salter, Environ. Sci. Technol. 2021, 55, 9489−9497, https://dx.doi.org/10.1021/acs.est.0c03804 ; and Sea Spray Aerosol (SSA) as a Source of Perfluoroalkyl Acids (PFAAs) to the Atmosphere: Field Evidence from Long-Term Air Monitoring Bo Sha, Jana H. Johansson, Peter Tunved, Pernilla Bohlin-Nizzetto, Ian T. Cousins, and Matthew E. Salter, Environ. Sci. Technol, https://doi.org/10.1021/acs.est.1c04277