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Atmospheric freezing of water droplets suspended in air followed by cloud formation and precipitation represent fundamental steps of the terrestrial water cycle. These aqueous droplets exhibit distinct freezing mechanisms and thermodynamic requirements compared to bulk water often forming metastable supercooled water at subzero temperatures on the Celsius scale (<273 K) prior to crystallization. Here, we report on a real-time spectroscopic investigation combined with simultaneous visualizations of single aqueous droplet freezing events inside a cryogenically cooled ultrasonic levitation chamber with the ultimate goal of probing the molecular structure evolution and stages of ice formation. The observed droplet freezing follows a pseudoheterogeneous ice nucleation mechanism mimicking the process that occurs for atmospherically supercooled water droplets at the air–water interface. This proof-of-concept experimental setup allows future crystallization studies of homo- and heterogeneously doped aqueous droplets under simulated atmospheric environments—also in the presence of reactive trace gases, thus untangling dynamic molecular interactions and chemical reactions, which are of fundamental interest to low-temperature atmospheric chemistry delineating with ice nucleation mechanisms. 

This study explores an intriguing freezing chemistry of “forever chemical” – trifluoroacetic acid (TFA, CF₃COOH) in clouds, unravelling a missing link on ice nucleation.