the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Dec 2025
Photochemical and ozone-induced aging significantly alter the viscosity of aqueous trans-aconitic acid aerosol particles
Abstract. Aging processes of organic aerosols, including reactions with gas phase oxidants, such as ozone (O3), as well as photochemical reactions, can significantly alter their physicochemical properties. While previous research has examined how photochemical aging and ozonolysis affect the physicochemical properties of organic aerosols, our study investigates the combined effect of photolysis and ozonolysis. We use aqueous trans-aconitic acid as a proxy for secondary organic aerosol particles (SOA), selected for its ability to absorb UV light and for containing a C=C double bond that is susceptible to ozonolysis. We observe significant mass loss in single particles levitated in an electrodynamic balance when exposed to either O3 or UV light (375 nm), as well as to both aging processes simultaneously, resulting from fragmentation reactions followed by the volatilization of some of the products. Viscosity measurements at 17 % relative humidity revealed an increase of nearly 4 orders of magnitude after both UV exposure and combined UV and O3 exposure at 60 % mass loss. Interestingly, continued UV-aging beyond 60 % mass loss resulted in a viscosity decrease, whereas combined UV and O₃ exposure led to a further viscosity increase. Hygroscopicity exhibited only a modest decline after 20 % mass loss during UV-aging and remained constant with further UV exposure; this reduction was less pronounced when UV-aging occurred in the presence of O₃. Overall, our results indicate that the mixing times within accumulation mode SOA particles may increase from 4 s to 4 h after aging under dry boundary layer conditions.
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Status: open (until 15 Jan 2026)
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RC1: 'Comment on egusphere-2025-5928', Anonymous Referee #1, 16 Dec 2025
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AC1: 'Reply on RC1', Cynthia Antossian, 18 Dec 2025
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Our reply is provided as a pdf file.
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AC1: 'Reply on RC1', Cynthia Antossian, 18 Dec 2025
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RC2: 'Comment on egusphere-2025-5928', Anonymous Referee #2, 20 Dec 2025
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This paper investigates the impact of photochemical and ozone-induced aging on the viscosity, hygroscopicity, and phase state of aqueous trans-aconitic acid (AA) aerosol particles using carefully designed electrodynamic balance experiments. The science is robust, the paper is clearly written, and the work represents a meaningful contribution to understanding how combined UV and O₃ aging influence organic aerosol properties. The clearly presented results on a model system provides potential implications for SOA processing and the resulting properties after atmospheric processing. The authors convincingly show substantial mass loss, significant viscosity enhancement, inhibition of efflorescence, and marked changes in mixing times.
The paper is well written, whilst also being full of methodological rigour. The sections are well organized and judicious use of appendices leads to an efficient read. Some minor comments to follow, which the authors should reflect upon. None of them should be viewed as criticism of the paper.
Efflorescence point of untreated AA - You report that untreated AA typically effloresced at RH 59.9%. Does this value fall within the known range of values in the literature, or at least beneath any reported deliquescence points? And if so, can reference(s) be provided?
Assumption regarding κ retrieval after aging - The statement that fitting at higher RH provides a better estimate of κ, as deviations from ideality become less important with dilution, is reasonable. However, this implicitly assumes that no further chemical processing occurs once UV and O₃ are switched off. It would be useful to state this assumption explicitly. It would also be good to discuss whether the possibility of slow processing post UV and/or ozonolysis is (un)likely.
Kinetic limitations - related to the previous point, the κ interpretation also appears to assume no kinetic limitations to hygroscopic mass transfer. It may be worth briefly commenting on whether slow diffusion / kinetic constraints could influence κ retrieval, particularly given the wider viscosity discussion.
Link to brown carbon potential - the results could be relevant to the developing brown carbon narrative. You already hint at the formation of potentially more photoreactive intermediates when describing the non-linear photolysis kinetics (“slow in the beginning, then speeds up…”). Some speculation on the possible implications for BrC formation or optical properties could strengthen the wider atmospheric relevance of the work.
I think in the text and references “O’meara” should be “O’Meara”
Citation: https://doi.org/10.5194/egusphere-2025-5928-RC2
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In this manuscript, Anotossian et al. present a study on the heterogeneous aging of trans-aconitic particles levitated in an EDB by ozone and UV illumination. They observe mass loss over the course of aging, indicating the fragmentation reactions that lead to volatile products. These changes coincide with an increase in particle viscosity, as determined by measuring water diffusion rates, and a decrease in hygroscopicity. The authors compare the differences due to aging by UV and ozone alone to aging by both methods.
The evolution of the physical properties of organic aerosol particles, such as viscosity, is an important and somewhat understudied area. The insights gained in this manuscript are interesting, although the scope of the analytical tools applied limit the depth of insight. The following points seek to clarify some of the observations and provoke further explanations.
1) In Figure 2, it is not described from where the cross-section measurement is obtained.
2) For context, how does the light absorption of trans-aconitic acid compare with typical brown carbon chromophores, such as 4-nitrocatechol?
3) Are reactive nitrogen species formed during the production of ozone from air? Are these accounted for? Typically, pure oxygen is used to avoid the formation of additional reactive species.
4) How exactly was the irradiance at the particle calculated? 0.16 W/cm2 seems low for a laser that has beam diameter of <1.5mm.
5) To account for the Stokes force, the speed of the gas flow over the particle is needed. What is the speed of the gas and how was this determined?
6) In calculating Dw, a large RH step change is used. Given that Dw changes significantly with RH, what RH does the inferred value of Dw correspond to?
7) The use of fractional Stokes-Einstein in single and multi-component particles has been explored by Sheldon et al. (https://doi.org/10.1039/D2EA00116K), with a focus on citric acid. For pure citric acid particle, the fSE works well. However, the addition of co-solutes changes the behavior significantly. The reacted mixtures studied here will likely not follow a straightforward fSE relationship either, although I think that without direct viscosity characterization, this is the only way to infer anything about viscosity. I suggest the authors include this reference with some discussion on the limitations of using fSE in multicomponent mixtures.
8) Is there any possibility that the particle charge changes over the course of oxidation due to the formation of charged species that are taken up or lost by the particles?
9) Corresponding to Figure 8, the decay with ozone is described as linear. However, it is not clear to me that this is not simply a slow exponential decay. The inference of a reaction order from these data is also not clear to me, due to the coupling of particle mass with the volatility of the products. The data does not show the change in the concentration of the reactant with time and, thus, may be of limited use when determining reaction kinetics.
10) How were diffusivity and viscosity estimated made on unreacted particles that exhibited efflorescence?
Other issues:
The general structure of the article is a little unusual, with results appear in the methods section. This creates a slightly awkward flow to the manuscript.