Highly viscous phase behavior of organic-rich urban PM 2.5

Atmospheric aerosol viscosity strongly influences particle phase state, internal mixing, and multiphase chemical processes, yet direct quantitative constraints for ambient urban PM 2.5 remain limited. Here, we investigated the phase behavior and viscosity of organic-rich PM 2.5 samples collected during autumn 2023 from the urban environments of Seoul and Beijing. Using filter extracts, relative humidity (RH)-dependent phase transitions and morphological evolution of the droplets were examined by optical microscopy, revealing frequent two-phase and three-phase morphologies during dehydration. Aerosol viscosity was quantitatively constrained at ∼ 290 K under experimentally accessible RH conditions (∼108 Pa s. Compared with previously reported laboratory-based viscosity measurements, the inferred viscosities of ambient PM 2.5 were comparable to or exceeded those reported for organic-rich ternary systems (i.e., sucrose–AS–H 2 O and citric acid–AS–H 2 O), which are commonly used as laboratory proxy systems in aerosol viscosity studies. These results indicate that organic-rich urban PM 2.5 can exhibit highly viscous, semisolid to solid phase states, and they provide quantitative, field-based viscosity estimates constrained by the bulk organic and inorganic mass fractions of urban aerosols. Although based on a limited number of filter samples and analyzed droplets, these findings offer a foundation for future, more extensive viscosity studies of urban aerosols.