This manuscript uses poke-flow measurements to determine the viscosity of organic-rich urban PM_2.5 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.