Direct Observation of Liquid–Liquid Phase Separation and Core–Shell Morphology of PM_2.5 Collected from Three Northeast Asian Cities and Implications for N₂O₅ Hydrolysis

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 PM2.5 filter extracts collected from Seoul, Beijing, and Noto during autumn 2023. The PM2.5 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 PM2.5 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.