| 10 Feb 2026
Highly Viscous Phase Behavior of Organic-Rich Urban PM2.5
Abstract. Atmospheric aerosol viscosity strongly influences particle phase state, internal mixing, and multiphase chemical processes, yet direct quantitative constraints for ambient urban PM2.5 remain limited. Here, we investigated the phase behavior and viscosity of organic-rich PM2.5 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 condition (≤ ~40 %) using the poke-and-flow technique coupled with fluid-dynamic simulations, yielding viscosities spanning from ~104 to > ~108 Pa·s. Compared with previously reported laboratory-based viscosity measurements, the inferred viscosities of ambient PM2.5 were comparable to or exceeded those reported for organic-rich ternary systems (i.e., sucrose–AS–H2O), that are commonly used as laboratory proxy systems in aerosol viscosity studies. These results indicate that organic-rich urban PM2.5 can exhibit highly viscous, semisolid to non-flowing behavior and provide quantitative, field-based, compositionally constrained viscosity data for urban aerosols.
This manuscript uses poke-flow measurements to determine the viscosity of organic-rich urban PM2.5 collected from Seoul and Beijing in the autumn. They find that the viscosities of the filter extract samples are comparable to or higher than laboratory model systems (sucrose/ammonium sulfate/water, and citric acid/ammonium sulfate/water). The two biggest drawbacks in this study are that the droplets created from filter extracts are not necessarily representative of the original aerosol morphology and the poke-flow technique was limited to an RH regime lower than during sample collection. However, these limitations are discussed and clear within the manuscript, and these measurements provide important context for the viscosities of laboratory model systems as compared to field-collected material. Overall, I believe this manuscript will be suitable for publication after the authors address the following comments.
Specific Comments:
Line 176, 178, 182: The language of single liquid and two-liquid I think can be confusing here since these are complex field samples, I would recommend changing to “single-phase liquid” and “two-phase liquid” or LLPS.
Figure 1: Are the scales for 60% and 20% the same as the scales for 85%? In addition, if feasible I think it would be helpful for the scale to remain consistent between samples for all of the 85% and below samples. It appears that the 95% samples are all at the same scale, so I think it would be helpful to visualize the relative changes in size across samples.
Line 219: The particles are semi-solid under the conditions of poke-flow, but during collection the RH was generally much higher. This is touched on at later points but please also include a discussion of what that could indicate for these samples here.
Line 235: Please add citations for why you are assigning 108 as the lower limit of viscosity for these particles and indicate if this is based on any physical or chemical properties.
Figure 3: Similar to Figure 1, please adjust these images if at all possible to make the scale bars consistent between samples.
Figure 4: Same thing as Figures 1 and 3, please adjust the scaling. Additionally, is there a reason why the brightness so different for Beijing 10/14 0 s vs after 1-2 hours? I think it is probably clear there is no change in morphology, but the difference in brightness makes it more challenging to tell.
Figure 5: Please indicate what experimental techniques were used to determine viscosities for the other studies included here.
Technical Corrections:
Line 166: “deducing” should be changed to deducting, subtracting, or another synonym. The same applies to Line 20 in the Supplementary Information.