the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jun 2025
Hygroscopicity of Isoprene-Derived Secondary Organic Aerosol Mixture Proxies: The Importance of Solute Diffusion and Salting-In Effects
Abstract. Isoprene-derived secondary organic aerosol (SOA) components, such as the 2-methyltetrols (2-MT) and 2-methyltetrol sulfates (2-MTS), have been readily detected in atmospheric aerosols. SOA commonly exist in aerosol mixtures containing inorganic salts, such as ammonium sulfate (AS). Despite its prevalence in the atmosphere, the water uptake of 2-MT, 2-MTS, and their mixtures are not well understood. In this study, we determine the physicochemical properties of 2-MT, 2-MTS, and their mixtures with AS. 2-MT and 2-MTS are viscous and dynamic surface tension measurements were taken to determine organic diffusion coefficients. The droplet growth was measured and both subsaturated and supersaturated hygroscopicity are parameterized by the single hygroscopicity parameter κ. Furthermore, aerosol phase state and morphology were analysed using atomic force microscopy. Results show that solute diffusion and salting-in influence the water uptake of 2-MT and 2-MTS with AS. The diffusion for 2-MTS/AS becomes an order of magnitude greater than for the organic alone but 2-MT diffusivity remains unchanged in the presence of AS. 2-MT/AS aerosols present a plateau in sub- and supersaturated κ-values close to pure AS. 2-MTS/AS aerosols exhibit a similar behavior under subsaturated conditions. However, under supersaturated conditions, 2-MTS/AS behaves as an ideal well-mixed aerosol that can be described by traditional κ-Köhler theory. 2-MT and 2-MTS are abundant globally, and thus the impact from biogenic sources and non-ideal droplet activation properties must be considered in aerosol-cloud interactions.
Competing interests: Some authors are members of the editorial board of the ACP Journal.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2025-1935', Anonymous Referee #1, 03 Jul 2025
In this study, the authors demonstrates that viscosity could influence the diffusion within aqueous droplets, resulting in complex phase morphology and water uptake properties for the 2-MT, 2-MTS and their mixtures with AS under saturated and supersaturated conditions. This is a comprehensive laboratory work and provides new valuable data to better understand the water uptake of isoprene derived SOA. The paper is well written. The data are well presented and discussed. I have some questions related to the measurements and a few minor comments.
Comments on measurements
Line 150, “Droplet surface tension (σs/a) was measured using a pendant drop tensiometer with a modified profile analysis tensiometer (SINTERFACE Inc.); the experimental set up has been previously described in Fertil et al. (2025). Briefly described here, the pendant drop tensiometer generates a droplet of solution (< 10 μL) suspended from a 0.9-mm diameter needle (Beier et al., 2019; Fertil et al., 2025). Droplets remain suspended for 300 s to reach equilibrium”. What was the ambient relative humidity that the droplets exposed to in these measurements? When the droplets were at equilibrium with their surrounding, do the equilibrium composition of the droplets same as initial their stock solutions?
Line 455, “Consequently, current hygroscopicity measurements that occur at fast time scales may not capture the full water uptake process of the synthesized organics and their mixtures“. Can the authors comment if equilibrium hygroscopic measurements were achieved in all their investigated systems?
Other comments
For the water uptake measurements, would there be any volatility issue of 2-MT aerosols?
Line 290, “However, in comparison to previously studied organics, 2-MT and 2-MTS σs/a remains close to pure water in the dilute bulk regime (Fig. 1). Thus, 2-MT and 2-MTS surface activity is negligible for droplet activation.” Can the authors also comment if droplet size would affect the results?
Line 320, “This effect is more prominent in 2-MT than 2-MTS, as evident in its slower diffusion rates for concentrations >30 mM (Table S17).” Since the synthesized 2-MTS sample contain other species, will these species affect the diffusion rates?
Line 345, “The organic 2-MT molecules do not diffuse fast enough to fully accumulate at the surface and substantially lower surface tension.” As mentioned above, do the data collect at their equilibrium states?
Line 405, “Thus, it is believed that both 2-MT and 2-MTS organics slowly dissolve, and phase separate to form a relatively viscous phase under subsaturated conditions, corresponding to slow diffusion coefficients" When phase separation occurs, what is the morphology or thickness of the organic coating?
For the AFM, can the authors comment how well the AFM represents the morphology airborne aqueous droplets?
Citation: https://doi.org/10.5194/egusphere-2025-1935-RC1 - AC1: 'Reply on RC1', Akua Asa-Awuku, 16 Aug 2025
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RC2: 'Comment on egusphere-2025-1935', Anonymous Referee #2, 23 Jul 2025
In principle, this is a useful experimental contribution to aerosol research. The main issue, though, is the purity of the 2-MTS is reported to be only 73 wt% pure. This is around 63 mole% based on the listed impurities. Therefore, it is difficult to know what to do with any of the measurements for 2‑MTS, as they are only indicative of a mixed system and cannot be confidently assigned to the properties of pure 2‑MTS.
Another issue is with the writing itself. There is an incredible overuse of the words traditionally (3 times) and traditional (16 times!). I would say those terms typically have no place in scientific writing. Especially if we're talking about kappa-Kohler theory and referencing a 2007 paper.
Also, overuse of scare quotes. E.g. salting-in is scare quoted two of three times in the text (but not in the title). Further, do "self-limiting" and "equilibrium" really need to be scare quoted? Both of those examples just confuse the reader. What are you even implying there?
Minor: show the structures for 2-MTS and MT in Figure 1 (you already show SOS and SDS).
Citation: https://doi.org/10.5194/egusphere-2025-1935-RC2 - AC2: 'Reply on RC2', Akua Asa-Awuku, 16 Aug 2025
Status: closed
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RC1: 'Comment on egusphere-2025-1935', Anonymous Referee #1, 03 Jul 2025
In this study, the authors demonstrates that viscosity could influence the diffusion within aqueous droplets, resulting in complex phase morphology and water uptake properties for the 2-MT, 2-MTS and their mixtures with AS under saturated and supersaturated conditions. This is a comprehensive laboratory work and provides new valuable data to better understand the water uptake of isoprene derived SOA. The paper is well written. The data are well presented and discussed. I have some questions related to the measurements and a few minor comments.
Comments on measurements
Line 150, “Droplet surface tension (σs/a) was measured using a pendant drop tensiometer with a modified profile analysis tensiometer (SINTERFACE Inc.); the experimental set up has been previously described in Fertil et al. (2025). Briefly described here, the pendant drop tensiometer generates a droplet of solution (< 10 μL) suspended from a 0.9-mm diameter needle (Beier et al., 2019; Fertil et al., 2025). Droplets remain suspended for 300 s to reach equilibrium”. What was the ambient relative humidity that the droplets exposed to in these measurements? When the droplets were at equilibrium with their surrounding, do the equilibrium composition of the droplets same as initial their stock solutions?
Line 455, “Consequently, current hygroscopicity measurements that occur at fast time scales may not capture the full water uptake process of the synthesized organics and their mixtures“. Can the authors comment if equilibrium hygroscopic measurements were achieved in all their investigated systems?
Other comments
For the water uptake measurements, would there be any volatility issue of 2-MT aerosols?
Line 290, “However, in comparison to previously studied organics, 2-MT and 2-MTS σs/a remains close to pure water in the dilute bulk regime (Fig. 1). Thus, 2-MT and 2-MTS surface activity is negligible for droplet activation.” Can the authors also comment if droplet size would affect the results?
Line 320, “This effect is more prominent in 2-MT than 2-MTS, as evident in its slower diffusion rates for concentrations >30 mM (Table S17).” Since the synthesized 2-MTS sample contain other species, will these species affect the diffusion rates?
Line 345, “The organic 2-MT molecules do not diffuse fast enough to fully accumulate at the surface and substantially lower surface tension.” As mentioned above, do the data collect at their equilibrium states?
Line 405, “Thus, it is believed that both 2-MT and 2-MTS organics slowly dissolve, and phase separate to form a relatively viscous phase under subsaturated conditions, corresponding to slow diffusion coefficients" When phase separation occurs, what is the morphology or thickness of the organic coating?
For the AFM, can the authors comment how well the AFM represents the morphology airborne aqueous droplets?
Citation: https://doi.org/10.5194/egusphere-2025-1935-RC1 - AC1: 'Reply on RC1', Akua Asa-Awuku, 16 Aug 2025
-
RC2: 'Comment on egusphere-2025-1935', Anonymous Referee #2, 23 Jul 2025
In principle, this is a useful experimental contribution to aerosol research. The main issue, though, is the purity of the 2-MTS is reported to be only 73 wt% pure. This is around 63 mole% based on the listed impurities. Therefore, it is difficult to know what to do with any of the measurements for 2‑MTS, as they are only indicative of a mixed system and cannot be confidently assigned to the properties of pure 2‑MTS.
Another issue is with the writing itself. There is an incredible overuse of the words traditionally (3 times) and traditional (16 times!). I would say those terms typically have no place in scientific writing. Especially if we're talking about kappa-Kohler theory and referencing a 2007 paper.
Also, overuse of scare quotes. E.g. salting-in is scare quoted two of three times in the text (but not in the title). Further, do "self-limiting" and "equilibrium" really need to be scare quoted? Both of those examples just confuse the reader. What are you even implying there?
Minor: show the structures for 2-MTS and MT in Figure 1 (you already show SOS and SDS).
Citation: https://doi.org/10.5194/egusphere-2025-1935-RC2 - AC2: 'Reply on RC2', Akua Asa-Awuku, 16 Aug 2025
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