Direct Observation of Liquid–Liquid Phase Separation and Core–Shell Morphology of PM_2.5Collected from Three Northeast Asian Cities and Implications for N₂O₅Hydrolysis

The morphologies of aerosol particles significantly influence atmospheric processes, including heterogeneous chemistry. This study presents direct observations of liquid–liquid phase separation and the morphologies of PM_2.5 filter extracts collected from Seoul, Beijing, and Noto during autumn 2023. The PM_2.5 samples covered both polluted and clean environments and were predominantly organic-rich. Optical microscopy revealed that real-world samples from these cities underwent phase transitions with changing relative humidity and primarily existed in either two-liquid or three-phase states, while fully homogeneous or nonliquid states were rare. Moreover, a core–shell morphology, consisting of an organic shell and inorganic core, was frequently observed in the PM_2.5 samples. Calculations confirmed that the viscosity of the organic shell ranges from ∼10¹ Pa s in Seoul (liquid-like) to ∼10⁶ Pa s in Beijing (semisolid). As the shell becomes more viscous, the diffusivity of N₂O₅ decreases, thereby lowering the N₂O₅ uptake coefficient by 1–3 orders of magnitude and significantly restricting N₂O₅ uptake. These results highlight the crucial role of aerosol morphology in heterogeneous chemistry, as the organic-rich outer phase can significantly modify the reactive uptake coefficients and gas–particle interactions.