Radical chemistry in atmospheric ice analogs forms persistent trifluoroacetic acid.

Trifluoroacetic acid (TFA, CF3COOH), the most prevalent per- and polyfluoroalkyl substance (PFAS), is accumulating globally in precipitation, soils, and drinking water, yet its formation remains unresolved. Atmospheric models emphasize gas-phase oxidation of hydrofluorocarbons and hydrofluoroolefins, but measured abundances often exceed predictions, implying missing chemistry. Here, we identify a condensed-phase radical pathway to TFA in cryogenic trifluoromethane/carbon dioxide ice analogs. Energetic processing generates trifluoromethyl (CF3) and hydroxycarbonyl (HOCO) radicals that undergo barrierless recombination to form TFA. Fourier transform infrared spectroscopy tracks reactive intermediates and carboxylic acid functional groups, while synchrotron vacuum-ultraviolet photoionization reflectron time-of-flight mass spectrometry provides isomer-selective detection of TFA and diagnostic dimer fragments. These results support CF3-HOCO recombination as a potential condensed-phase route linking fluorocarbon degradation to persistent PFAS formation, highlighting the potential role of ice crystals and cloud particles as chemical reactors in atmospheric fluorine chemistry.