PFAS in the Urban Atmosphere Detected with I‑CIMS and LC-ESI Orbitrap Mass Spectrometer

Per- and polyfluoroalkyl substances (PFAS) are persistent pollutants whose atmospheric fate in urban areas remains poorly characterized. We combined in-situ iodide CIMS with targeted offline LC-ESI Orbitrap MS to quantify 22 gas- and particle-phase PFAS in Houston, Texas. We calibrated I-CIMS for 18 PFAS and validated it against LC-ESI Orbitrap MS across 9 co-detected particle-phase compounds. Gas-phase ΣPFAS reached 2.2 pptv (17.7 ng m^-3) and particle-phase ΣPFAS reached 0.25 pptv (3.1 ng m^-3), with 4:2 FTOH, PFPrA, and TFA peaking at 2 to 3 pptv in the gas-phase, among the highest reported in urban atmospheres. Gas-phase PFAS correlated with temperature and ozone, while particle-phase PFAS correlated with humidity. The relationship between particle-associated fraction and subcooled liquid vapor pressure was shifted roughly two orders of magnitude relative to wintertime Toronto, and a further two orders relative to indoor environments, demonstrating that aqueous aerosol modulates PFAS gas-particle equilibrium under humid subtropical conditions. Comparison with Huntsville, Alabama, revealed an inverted PFCA chain-length distribution, demonstrating that local emissions can override regional and seasonal trends in PFAS composition. These results extend the atmospheric PFAS gas-particle partitioning framework to subtropical urban environments and demonstrate the value of combined online-offline measurements for characterizing urban PFAS loadings.