Alissia M. Milani ^a, Kaitlyn J. Gruber ^b, Martin Shafer ^c, Christina K. Remucal <sup>a, b, d

a</sup> University of Wisconsin – Madison, Environmental Chemistry and Technology Program
^b University of Wisconsin – Madison, Department of Chemistry
^c University of Wisconsin – Madison, Wisconsin State Laboratory of Hygiene
^d University of Wisconsin – Madison, Department of Civil and Environmental Engineering

Atmospheric transport of per- and polyfluoroalkyl substances (PFAS) results in global distribution of these organic contaminants. Wet atmospheric deposition (rain and snow) may represent a significant input of PFAS to remote aquatic and terrestrial environments. Lake Superior is likely one such environment as it is primarily surrounded by forested land and a relatively low population density. Additionally, precipitation is the major water input for the northernmost of Laurentian Great Lakes, owing to its large lake surface area and relatively small watershed. However, the trends of PFAS levels from wet deposition are poorly characterized and constrained. From 2022 to 2023, wet deposition samples were collected weekly from nine sites around Lake Superior through collaboration with the National Atmospheric Deposition Program. Wet deposition samples were analyzed for 33 PFAS compounds in five major classes by isotope dilution, solid-phase extraction, liquid chromatography-tandem mass spectrometry (LC-MS/MS). Extractable organic fluorine (EOF) was analyzed via combustion ion chromatography to estimate the amount of PFAS not detected by targeted methods. The distribution of PFAS classes varied between sites, potentially indicating regional source impacts on the concentration of PFAS compounds in wet deposition. Perfluorocarboxylic acids (PFCAs) were the most frequently detected class and accounted for upwards of 75% of targeted PFAS by mass. The abundance of PFCAs is consistent with previous deposition studies and may be partially due to the atmospheric transformation volatile precursor PFAS. EOF measurements suggested the fraction of PFAS not detected in targeted methods varied spatially and seasonally across sampling sites. This further supports our hypothesis of complex and variable PFAS inputs to wet deposition. By studying the sources, transformations, and deposition of PFAS in precipitation, we can improve research strategies, influence policy-making, and minimize human exposure to these persistent
contaminants.