This is the review of manuscript entitled “Temperature–RH Dependent Viscosity of Organic Aerosols from 273 to 303 K: Implications for Global N₂O₅ Uptake” by Ullah et al.

This work measures the viscosity of sucrose–H₂O droplets serving as organic aerosol surrogates using bead mobility and poke-and-flow experiments for temperatures between 273 and 300 K and 20-90% RH. The viscosity values which span 9 orders of magnitude are then applied to estimate the fragility of aqueous sucrose droplets using a fit to the Vogel–Fulcher–Tammann formulation. The experimentally derived viscosity and fitted fragility are used to extend viscosity predictions to 230 K and corresponding characteristic mixing time scales, and N2O5 uptake for typical tropospheric temperatures and humidity.

The topic of this study fits well within the scope of the journal Atmospheric Chemistry and Physics. In places, more information needs to be provided to better understand the implications. Measurements are performed over 30 K but then the data is extrapolated over 43 K to cover typical tropospheric conditions. The interpretation of N2O5 uptakes under tropospheric conditions could be ambiguous.

An ACP conclusion section is missing. The current atmospheric implication section seems not to qualify.

General comments:

Sample preparation: The droplets are 40-100 µm in diameter. A conditioning and mixing time scale are given. The former seems to be experimentally set? The latter is derived. A ratio of greater than 1 is taken as evidence that the droplets are in equilibrium with RH. Typically, the indication that solution droplets are in equilibrium with surrounding RH is when they stopped growing. Since water uptake is proportional to 1/r, growth for such large particles can take long (several minutes, 10s of minutes). A mixing time scale might not be sufficient to assure equilibrium. Conditioning must be longer than the mixing time scale but also longer than the growth process to take up sufficient water. Estimates of these numbers (see textbooks, like Seinfeld and Pandis) would be beneficial to have. Given the conditioning times, I feel that the prerequisite is fulfilled, however, providing a number for the largest droplets would be beneficial to the reader.

Viscosity was measured across 30 K but then extrapolated by 43 K to 230 K. This implies a large caveat. Especially considering that at lower temperatures a very small change in humidity will have a substantial effect on viscosity. Hence such a large extrapolation can yield large uncertainties. There is uncertainty in RH from measurements, from the VFT fit, and from averaging the reanalysis data. This should be clearer articulated in manuscript abstract and conclusions. For these reasons, one could argue that the application to tropospheric conditions may not be similarly emphasized compared to the laboratory viscosity measurements.

More information has to be provided how the altitude–latitude profiles of zonal-mean temperature and RH from Copernicus Climate Data Store were derived. Which ERA5 model was used? The supplement does not provide any information. What is the meaning of Eq. S3? A hypsometric equation with a lapse rate? Did you use this to convert pressure/temperature fields into heights? Why not show pressure levels instead of altitude? Anyway, it is not clear which data sets are used, how they are averaged (value range/uncertainty), etc. This may impact some of the interpretations of the particle phase state, leading to inconsistency. Could published climatological means be used instead?

RH impacts the phase state as calculated but only if the particle is in equilibrium with RH. Only then condensed-phase water activity equals RH. At cold temperatures, when diffusion slows down and the particle viscosity is likely higher, this may not be the case. Has this been considered when looking at zonal plots? I assume, even at colder temperatures the authors assume equilibrium between condensed phase and gas phase? This would be another large caveat. We know that organic particles can be in disequilibrium with the gas phase, e.g., (Berkemeier et al., 2014).

The greater extent of liquid and semisolid particles at higher altitudes over the southern polar region compared to the extratropics and tropics seems to be counterintuitive. At surface levels maybe, but at heights above 1-2 km, one would expect the particle phase to be on the solid side due to extreme low temperatures. The resulting greater N2O5 uptake in this region is not discussed.

Concluding section is missing. See guidelines for authors.

Specific comments:

Line 32-33: Please update with more recent reviews or articles on these topics.

Line 39-40, and 57: The following works who studied uptake kinetics for RH variation and temperatures of the upper troposphere could be cited as well: (Li and Knopf, 2021; Li et al., 2020; Slade and Knopf, 2014).

Line 44 and following: Study by (Lienhard et al., 2015) is missing. They also parameterized water diffusion in sucrose for different water activities.

Line 50-51: The article by (Knopf et al., 2024) would be beneficial in this list. They also discuss uptake at low temperatures.

Line 59: Surfactants and phase separation can also reduce the uptake of N2O5. See, e.g., (McNeill et al., 2006; Cosman et al., 2008; Gaston et al., 2014).

Line 123-124: Please elaborate what is meant by infinite viscosity? The viscosity at which glass transition manifests?

Line 128: How do you derive kappa from previous equations? No relationship with kappa is given.

Line 147-149: For the reacto-diffusive length, did you account for the temperature and humidity dependency for the N2O5 diffusion coefficient and for the rate constant? Maybe a supplemental plot of this length as a function of temperature and humidity could be beneficial for the reader.

Line 175, figure 1: In caption, please state the component investigated.

Line 183: Viscosity was measured across 30 K but then extrapolated by 43 K. This needs to be stronger pointed out in abstract and throughout the manuscript. As it will be later (line 198) where it is stated that at lower temperatures a very small change in humidity will have a substantial effect on viscosity. Hence such a large extrapolation can yield large uncertainties.

Line 194-197: This reads like a repetition of previous paragraph?

Line 208-210: How are the Copernicus Climate Data applied to get altitude resolved temperature and humidity levels?

Line 214-221: This interpretation is based on RH in polar regions. However, polar regions are much colder than the tropics. Maybe this has validity at surface levels, though in winter at 230 K at the surface, particles are likely solid independently of RH. At higher altitudes, it is even less likely that particles are liquid. Is condensed-phase water activity in equilibrium with RH?

Line 246, section 4.1: You do not discuss the high N2O5 uptake at greater altitudes in the southern polar region. But state on lines 252-254 that cold conditions reduce uptake. Somehow, there is some inconsistency.

Technical corrections:

Throughout document and supplement: Variables in equation are typically italic font and subscript is in normal font.

Line 128 and other instances: Hygroscopicity parameter is typically a Greek kappa.

Line 162: Erroneous hyphen between “magnitude” and “from”?

Line 251: What do you mean with “several shorts”?

Line 260: You mean the “uptake coefficient levels at ….”.

Supplement:

Line 36: Erroneous period, missing space?

Line 37: Missing space before ”(Haynes”.

Line 137: Superfluous space.

References

Berkemeier, T., Shiraiwa, M., Pöschl, U., and Koop, T.: Competition between water uptake and ice nucleation by glassy organic aerosol particles, Atmos. Chem. Phys., 14, 12513-12531, 10.5194/acp-14-12513-2014, 2014.

Cosman, L. M., Knopf, D. A., and Bertram, A. K.: N2O5 reactive uptake on aqueous sulfuric acid solutions coated with branched and straight-chain insoluble organic surfactants, J. Phys. Chem. A, 112, 2386-2396, 10.1021/jp710685r, 2008.

Gaston, C. J., Thornton, J. A., and Ng, N. L.: Reactive uptake of N2O5 to internally mixed inorganic and organic particles: the role of organic carbon oxidation state and inferred organic phase separations, Atmos. Chem. Phys., 14, 5693-5707, 10.5194/acp-14-5693-2014, 2014.

Knopf, D. A., Ammann, M., Berkemeier, T., Pöschl, U., and Shiraiwa, M.: Desorption lifetimes and activation energies influencing gas-surface interactions and multiphase chemical kinetics, Atmos. Chem. Phys., 24, 3445-3528, 10.5194/acp-24-3445-2024, 2024.

Li, J. and Knopf, D. A.: Representation of Multiphase OH Oxidation of Amorphous Organic Aerosol for Tropospheric Conditions, Environ. Sci. Technol., 55, 7266-7275, 10.1021/acs.est.0c07668, 2021.

Li, J. N., Forrester, S. M., and Knopf, D. A.: Heterogeneous oxidation of amorphous organic aerosol surrogates by O3, NO3, and OH at typical tropospheric temperatures, Atmos. Chem. Phys., 20, 6055-6080, 10.5194/acp-20-6055-2020, 2020.

Lienhard, D. M., Huisman, A. J., Krieger, U. K., Rudich, Y., Marcolli, C., Luo, B. P., Bones, D. L., Reid, J. P., Lambe, A. T., Canagaratna, M. R., Davidovits, P., Onasch, T. B., Worsnop, D. R., Steimer, S. S., Koop, T., and Peter, T.: Viscous organic aerosol particles in the upper troposphere: diffusivity-controlled water uptake and ice nucleation?, Atmos. Chem. Phys., 15, 13599-13613, 10.5194/acp-15-13599-2015, 2015.

McNeill, V. F., Patterson, J., Wolfe, G. M., and Thornton, J. A.: The effect of varying levels of surfactant on the reactive uptake of N2O5 to aqueous aerosol, Atmos. Chem. Phys., 6, 1635-1644, 10.5194/acp-6-1635-2006, 2006.

Slade, J. H. and Knopf, D. A.: Multiphase OH oxidation kinetics of organic aerosol: The role of particle phase state and relative humidity, Geophys. Res. Lett., 41, 5297-5306, 10.1002/2014gl060582, 2014.