Estimation of aqueous solubility and hygroscopicity in poly- and perfluoroalkyl substances with COSMOtherm

This study predicts key atmospheric properties for 35 representative per- and polyfluoroalkyl substances (PFAS) using COSMOtherm and Köhler theory. Key parameters, including water solubility (log S_W), the air-water partition coefficient (log K_AW), the octanol-water partition coefficient (log K_OW), and the hygroscopicity parameter (κ), were predicted. Based on their environmental speciation, we provide guidance on the appropriate thermodynamic framework for each PFAS class. For neutral volatile PFAS, Henry's Law (via log K_AW) should be used to describe gas scavenging. For ionic PFAS, the κ-Köhler framework (derived from non-ideal Raoult's Law) is recommended to describe aerosol activation and cloud-mediated removal. This dual-parameter approach enables chemical transport models (CTMs) to apply the correct thermodynamic model for different PFAS classes. While atmospheric PFAS concentrations are typically too low to significantly influence cloud condensation nuclei (CCN) activity in polluted or continental air masses, their relative contribution could be more discernible in pristine locations with very low background CCN concentrations in the range of 10¹-10² cm^-3. Results show that increasing perfluoroalkyl chain length reduces aqueous solubility and increases partitioning from water to air (log K_AW) and from water to organic phases (log K_OW), with short-chain PFAS exhibiting higher κ values and greater wet scavenging potential. The predicted parameters enable CTMs to move beyond mass-based assessments, simulating hygroscopic growth and scavenging of PFAS-containing aerosols. This work bridges molecular-level prediction with atmospheric modeling, providing a theoretical foundation for integrating PFAS into CTMs to improve predictions of atmospheric fate, deposition, and long-range transport in clean atmospheric regimes.