Phase Behavior of Polluted and Clean Urban Winter PM_2.5from Optical Observations and Thermodynamic Equilibrium Modeling

The phase state of fine particles (PM_2.5) critically governs gas–particle partitioning and multiphase chemical reactivity, yet remains poorly constrained under real-world conditions. In this study, optical microscopy, poke-and-flow experiments, and thermodynamic modeling were combined to examine the phase behavior of wintertime urban PM_2.5 collected in Ansan, South Korea. Morphological analyses revealed humidity- and composition-dependent transitions, with liquid–liquid and liquid–liquid–solid states frequently observed in individual particles. Polluted, nitrate-rich aerosols predominantly exhibited liquid-like morphology and followed near-equilibrium partitioning of nitrate and ammonium. In contrast, cleaner, organic-rich particles likely exhibited higher viscosity and nonliquid characteristics, which may have led to partial deviation from equilibrium predictions. These results provide direct experimental evidence of phase complexity in ambient PM_2.5 and demonstrate that the particle phase state, modulated by relative humidity and composition, plays a critical role in determining thermodynamic behavior and atmospheric reactivity of urban aerosols.