the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Feb 2026
Predicting Organic–Inorganic Aerosol Efflorescence Using Thermodynamically Modeled Viscosity
Abstract. Atmospheric aerosols, especially internally mixed organic-inorganic aerosols, exhibit complex phase behaviors that affect their size evolution, optical properties, and chemical reactivity, ultimately impacting climate and human health. Although parameterizations for secondary organic aerosol phase state exist, predictive models based on primary predictors for efflorescence in organic-inorganic aerosols remain underdeveloped. In this study, we evaluated several chemical parameters, including equivalent O:C ratio, organic mass fractions, glass transition temperature (Tg), and viscosity (η), and identified aerosol viscosity as the primary predictor of efflorescence relative humidity (ERH) in internally mixed organic-inorganic aerosols. We developed a linear viscosity-ERH model based on ERH and log10(η), which defines the boundary conditions for aerosol efflorescence when η< 4.76 × 10² Pa·s. Additionally, we showed that efflorescence is inhibited when η> 4.76 × 10² Pa·s. Validation using an independent dataset showed strong agreement between predicted and experimentally measured ERH (R² = 0.95). A multivariate regression model incorporating η and Tg improved prediction accuracy but was limited by Tg parameterization for complex organic-inorganic mixtures. Our findings highlight the role of aerosol viscosity in controlling efflorescence and emphasize the need to develop improved aerosol viscosity measurement techniques to better constrain aerosol phase transitions in atmospheric models.
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Status: open (until 31 Mar 2026)
- RC1: 'Comment on egusphere-2026-117', Anonymous Referee #1, 23 Feb 2026 reply
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CC1: 'Incorrect Tg values in Table S4', Thomas Koop, 26 Feb 2026
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-117/egusphere-2026-117-CC1-supplement.pdf
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AC1: 'Reply on CC1', Qishen Huang, 06 Mar 2026
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Dear Dr. Armeli, Dr. Peters, and Dr. Koop,
We sincerely appreciate your detailed comments, which helped us identify errors and improve our manuscript. We have carefully gone through your comments and made the corresponding corrections and revisions, which are included in this document along with our explanations and responses. All revisions mentioned in this response will appear in future versions of both the main text and the SI. We hope that our manuscript has now become stronger, and we would greatly appreciate any further suggestions for improvement. Please find our detailed responses and revisions in the attached supplement file.
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AC1: 'Reply on CC1', Qishen Huang, 06 Mar 2026
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RC2: 'Comment on egusphere-2026-117', Anonymous Referee #2, 09 Mar 2026
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This manuscript seeks to examine the physicochemical factors that affect the efflorescence relative humidity in mixed organic/inorganic aerosols, finding viscosity to be the most closely correlated. This paper is well written, presented clearly, and is an important topic well within the scope of ACP. However, I have in particular one major concern with the methodology that in my mind necessitates further analysis, as described below:
Major comment:
1. Line 107: The viscosity that is being correlated with the ERH is the viscosity at that ERH. Viscosity is a very strong function of RH, therefore, it is not surprising that there is a strong correlation between viscosity at the ERH and ERH. It may still be true that there is a deeper connection between viscosity and ERH, but that has not been proven without further analyses. Some analyses that could help answer this question and that I suggest include:
A. For a "representative" aerosol particle chemical system, how much does the viscosity change from say 60% RH to say 20% RH? If the change in viscosity for a fixed chemistry is comparable to the slope in Figure 2, that would suggest that most of the previously observed correlation is simply because viscosity is changing as a function of RH, rather than a deeper connection between viscosity and ERH. If the change in viscosity for a fixed chemistry is less than the slope in Figure 2, then that would suggest that viscosity being a function RH is part, but not all of the story, and that should be quantitatively discussed.
B. At a fixed RH, calculate the viscosity for each chemical system. If the correlation between viscosity and ERH is greatly reduced, this would also suggest that most of the previously observed correlation was due to the general correlation between viscosity and RH. If the correlation largely remains, that would suggest that there is a deep connection between viscosity and ERH, which would be very exciting! If the result is somewhere in the middle, as seems perhaps most likely, then again there should be some quantitative discussion of how much of the correlation is attributable to each factor.
Other comments:
2. For Figure 3b, the fit is indeed close to the x=y line, but it seems like that may be partially driven by the cluster of (0,0) points. What does the fit in Figure 3b look like without those points? It seems like some of the lower viscosity points are slightly underpredicted, which is not a major concern but I think warrants at least a comment.
3. If the major finding holds that viscosity controls ERH, and that high viscosity particles do not effloresce, could that mean that experimentalists are just waiting long enough to observe the efflorescence? It is worth commenting on what the typical timescale of these laboratory experimental observations are, what the typical time scales in the atmosphere are, and how long based on some simple physical principles efflorescence might be delayed for some of these higher viscosity compounds.
Citation: https://doi.org/10.5194/egusphere-2026-117-RC2 -
RC3: 'Comment on egusphere-2026-117', Anonymous Referee #3, 09 Mar 2026
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This manuscript studies the relationships controlling ERH in aerosol systems with different chemical compositions. I have following comments.
1. Page 6 line 155: Why kGT is assumed to be 2.5 for all organic water mixtures. There are references provided, however a sentence or two justifying this assumption is good.
2. What is the pH of these aerosol systems? pH impacts on aerosol properties greatly, and thus authors may want to comment on that.
3. Page 3, 10: some typos found. Please do an additional round of copy-editing.
4. Page 4, lines 100-105: Is it possible to add these equations as well to the equation lists for easy following?
5. Please expand the caption 3 to identify all abbreviations in the figure.
6. Is the change in viscosity due to differences in RH (e.g., 20% vs 75%) different between chemical systems studied? If so, do they follow any relationship with the structure/ functional groups present?
Citation: https://doi.org/10.5194/egusphere-2026-117-RC3
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This manuscript examines the key factors controlling the efflorescence relative humidity of internally mixed organic–inorganic aerosols. The authors identify aerosol viscosity as the primary predictor of efflorescence and develop a practical parameterization, including a viscosity threshold above which efflorescence is suppressed. The conclusions are well supported by the dataset and consistent with nucleation kinetics. Overall, the study addresses an interesting and important topic within the scope of Atmospheric Chemistry and Physics and is clearly written and well structured. I recommend publication after the authors address the comments below.
Specific Comments: