Enrichment of perfluoroalkyl substances in the sea-surface microlayer and sea-spray aerosols in the Southern Ocean

The accumulation of Perfluoroalkyl substances (PFAS) in the SML and their enrichment in SSA is consistent with marine aerosols as a vector for PFAS transport.Sea-spray (or sea-salt) aerosol (SSA) formation and their subsequent atmospheric transport and deposition have been suggested to play a prominent role in the occurrence of ionizable perfluoroalkyl substances (PFAS) in the maritime Antarctica and other remote regions. However, field studies on SSA's role as vector of transport of PFAS are lacking. Following a multiphase approach, seawater (SW), the sea-surface microlayer (SML) and SSA were sampled simultaneously at South Bay (Livingston Island, Antarctica). Average PFAS concentrations were 313 pg L, 447 pg L, and 0.67 pg m in SW, the SML and SSA, respectively. The enrichment factors of PFAS in the SML and SSA ranged between 1.2 and 5, and between 522 and 4690, respectively. This amplification of concentrations in the SML is consistent with the surfactant properties of PFAS, while the large enrichment of PFAS in atmospheric SSA may be facilitated by the large surface area of SSA and the sorption of PFAS to aerosol organic matter. This is the first field work assessing the simultaneous occurrence of PFAS in SW, the SML and SSA. The large measured amplification of concentrations in marine aerosols supports the role of SSA as a relevant vector for long-range atmospheric transport of PFAS.

the mass of PFAS and Naþin SSA are in line with reported mea-surements. Such large enrichment factors in SSA have also beendescribed for experiments simulating the formation of SSA underlaboratory conditions (Reth et al., 2011). Underfield conditions,Casal et al. (2017a,2017b)has shown that the profile of PFCA inAntarctic fresh snow is particularly enriched with long-chain PFCA,consistent with the results reported here of high EFSSAfor PFOA,PFNA and PFDA. As snowflakes scavenge aerosols during deposi-tion, the large concentrations of PFAS in SSA are consistent withsnow deposition being a relevant source of PFAS in cold environ-ments, and SSA are an important source of PFAS to snow.As far as we know, this is thefirstfield study reporting thesimultaneous measurements of PFAS in SW, the SML and SSA. Eventhough there is some enrichment of PFAS at the SML of the surfaceocean, most of the large concentrations of PFAS in marine aerosolare due to the amplification of concentration during the formationof SSA. Such amplification of concentrations by more than twoorders of magnitude can be facilitated by various processes relatedto the accumulation of PFAS at the surface of the sea and aerosols. Ithas been suggested that SSA can be inverse micelles (Ellison et al.,1999;Gerard et al., 2016;Tervahattu et al., 2002). The criticalmicelle concentration for PFOA is of 0.00885 M (3.7 109ng L1)(Reth et al., 2011), thus orders of magnitude higher than PFOAconcentrations in the SML and in SSA. The targeted PFAS aredissociated at marine pH, acting as amphiphilic surfactants, butPFAS alone cannot form micelles in SSA by their own. However,marine aerosols are often coated with an OM layer (Tervahattuet al., 2002). PFAS can be part of this layer of lipids (fatty acids,hydrocarbons) in SSA or being adsorbed onto it. PFAS could be partof micelles together with surfactant-like OM, abundant in the SMLand SSA, but the confirmation of this would require future work.SSA have a large surface area, much larger than the SML. Assuminga thickness of 100mm for the SML, 1 L of SML water corresponds to10 m2of sea surface. On the other hand, SSA have sizes rangingfrom 0.01mmto1mm(Johansson et al., 2019), which means that 1 Lof SSAs having these aerosol sizes have a surface area of 600,000 m2and 6000 m2, respectively. Thus, SSA has a surface area between600 and 60,000 that of the SML, consistent with EFSSAorders ofmagnitude larger than EFSML. The large variability of EFSSAmay bethe result to different sizes of SSA and different characteristics ofthe highly variable SML. These issues will require further researchin order to improve the models for the long-range transport of PFASto Antarctica and elsewhere.4. ConclusionsPFAS were found to be ubiquitous in SW, the SML and SSA inSouth Bay from Livingston Island (southern Shetlands), a repre-sentative environment of the maritime Antarctica. Nevertheless,there were some differences in the relative abundance of PFAS inthe different matrixes, with PFSA and PFCA with carbon number ofC4 and C6, or C6 and C8eC10, respectively, being dominant in theSML and the underlying waters. High concentrations of PFBA werefound in SSA. The enrichment factors in the SML and SSA, defined asthe ratio of concentrations in the SML or SSA and that of SW,respectively, ranged between 1.2 and 5, and between 522 and 4692.The enrichment of PFAS in the SML is consistent with the amphi-philic properties of PFAS and their moderate hydrophobicity. Thelarge amplification of concentrations in SSA may be due to the largesurface area of SSA, the formation of inverse micelles by the largeamounts of surfactant like OM present in SSA, which may includePFAS and other organic compounds present in the SML, or tosorption of PFAS to OM present in SSA. The efficient transfer of PFASfrom the ocean to the atmosphere through the formation of SSAmay be relevant for atmospheric long-range transport of PFAS toAntarctica and other remote environments.Author statementGemma Casas: Conceptualization, Methodology, Formal anal-ysis, Investigation, Writing Original draft, Writing review&editing.Alicia Martinez-Varela: Methodology, Investigation. Jose LuisRoscales: Methodology, Investigation. Maria Vila-Costa: Method-ology, Investigation. Jordi Dachs: Conceptualization, Formal anal-ysis, Investigation, Writing review&editing. Bego~na Jimenez:Conceptualization, Methodology, Investigation, writing review&editing.Declaration of competing interestThe authors declare that they have no known competingfinancial interests or personal relationships that could haveappeared to influence the work reported in this paper.AcknowledgmentsWe thank the staff of the Marine Technology Unit (UTM-CSIC)for their logistical support during the sampling campaign at Liv-ingston Island, and M. Pizarro for technical assistance. This workwas supported by Spanish Ministry of science to GC and AMVthrough predoctoral fellowships, and through projects SENTINEL(CTM 2015-70535-P) and ISOMICS (CTM 2015-65691-R). This research is part of POLARCSIC activities. The research group of Global Change and Genomic Biogeochemistry receives support from the Catalan Government (2017SGR800). Special thanks toTERNUA for their non profit collaboration by sponsoring with technical ecofriendly clothing and gear equipment for Antarctic campaigns