Investigation of Freezing-Induced Anionic Interplay in Acoustically Levitated Artificial Seawater Droplets

Freezing of aqueous droplets plays a pivotal role in atmospheric processes, which is often governed by the soluble earth-abundant salts, minerals and their corresponding ions. These droplet freezing events in turn control the physical state, reactivity, and lifetime of aerosols at subzero temperature in the Celsius scale. In this study, we investigated the freezing behavior of artificial seawater droplets containing chloride and polyatomic anions like sulfate and bicarbonate using a cryogenically cooled acoustic levitation setup coupled with in situ Raman spectroscopy, enabling contact-free measurements of evolving ionic speciation. Spectral comparisons reveal that sulfate undergoes symmetry lowering and associated spectral broadening upon freezing, while bicarbonate remains a spectator at the given below-detection limit concentration. Complementary density functional theory calculations provide independent evidence of sulfate's symmetry lowering upon freezing and the associated Raman-band broadening. Control freezing experiments with pure sulfate, bicarbonate and their mixture further demonstrate that sulfate is the pivotal component which solely initiates a distinct ionic transformation during ice formation, providing molecular-level insight into the structural evolution of seawater ice crystals. This study directs the possibility of emerging heterogeneous interactions between sulfate-rich ice surfaces and atmospheric gases, including pollutants and refrigerants, underscoring their potential role in multiphase atmospheric chemistry.