Probing the freezing chemistry of singly levitated aqueous trifluoroacetic acid droplets in a cryogenically cooled simulation chamber relevant to Earth's upper troposphere

Abstract

Trifluoroacetic acid (CF₃COOH, TFA), the primary upper terrestrial atmospheric degradation product of several fluorinated hydrocarbons primarily used as refrigerants, poses a significant environmental challenge due to its growing atmospheric accumulation and extremely low reactivity. This combined experimental and theoretical study of TFA-doped water droplets, conducted inside a cryogenically cooled ultrasonic levitator simulation chamber utilizing time-dependent Raman spectroscopy and optical visualization techniques, addresses the dynamic chemical changes during the freezing event for the first time. The low-temperature experimental approach mimics TFA's interactions within water droplets in the upper troposphere and Arctic regions, particularly at subzero temperatures. Key findings reveal structural transformation towards the formation of undissociated neutral TFA in hexagonal ice environments compared to the anionic form, providing fundamental insight into the role of TFA in ice nucleation. Furthermore, state-of-the-art electronic structure calculations provide insights into the stability of this undissociated TFA within the hexagonal ice-encapsulated environment, wherein structural distortions of the regular hexagonal ice crystal and secondary F–H interactions mostly between anionic TFA forms and the ice lattice are evidenced at the molecular level. This research untangles the chemical insight into the TFA's role in ice nucleation leading to cloud glaciation, hence providing a plausible reason behind its unexpected presence in remote Arctic regions through long-range transport.