the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Sep 2025
Mechanistic Insights into I2O5 Heterogeneous Hydrolysis and Its Role in Iodine Aerosol Growth in Pristine and Polluted Atmospheres
Abstract. Higher iodine oxides are intricately linked to marine aerosol formation; however, the underlying physicochemical mechanisms remain poorly constrained, particularly for I2O5, which is stable yet conspicuously absent in the atmosphere. While reactivity with water has been implicated, the direct hydrolysis of I2O5 (I2O5 + H2O → 2HIO3) fails to account for this discrepancy due to its high activation barrier (21.8 kcal mol-1). Herein, we have probed heterogeneous hydrolysis of I2O5 mediated by prevalent chemicals over oceans through Born-Oppenheimer molecular dynamics and well-tempered metadynamics simulations. Our results demonstrate that self-catalyzed pathways involving I2O5 and its hydrolysis product HIO3 substantially reduce the reaction barrier, thereby accelerating the conversion of I2O5 to HIO3 in pristine marine environments. In polluted regions, interfacial hydrolysis of I2O5 mediated by acidic or basic pollutants (e.g., H2SO4 or amines) proceeds with even greater efficiency, characterized by remarkably low barriers (≤1.3 kcal mol-1). Collectively, these proposed heterogeneous reactions of I2O5 are highly effective, acting as a hitherto unrecognized sink for I2O5 and a source of HIO3—processes that facilitate marine aerosol growth and rationalize the high iodate abundances detected in aerosols. These findings provide mechanistic insight into the elusive I2O5-to-HIO3 conversion, offering a critical step toward improving the representation of iodine chemistry and marine aerosol formation in atmospheric models, with implications for climate prediction and environmental impact assessment.
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Status: open (until 13 Oct 2025)
- RC1: 'Comment on egusphere-2025-3770', Anonymous Referee #1, 10 Sep 2025 reply
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RC2: 'Comment on egusphere-2025-3770', Anonymous Referee #2, 16 Sep 2025
reply
The manuscript by Deng et al. adopted first-principles molecular dynamics to examine the heterogeneous hydrolysis of I₂O₅ and its role in aerosol growth. The study identifies interfacial mechanisms driven by iodic species in pristine conditions and by acid/base pollutants in polluted environments. These findings emphasize the importance of reactive atmospheric components in I₂O₅ hydrolysis and provide a mechanistic explanation for its sink and the observed I₂O₅-to-HIO₃ conversion. This topic is timely and relevant to Atmos. Chem. Phys., given its focus on aerosol formation process. The manuscript is overall well presented, with sound methodology and adequate supporting evidence. Nevertheless, I suggest that the authors consider my comments and perform the minor revisions before the manuscript can be recommended for publication.
- In conducting the metadynamics simulations, the authors appear to have employed SMS-MetaD rather than the more widely used MetaD approach. I am curious about the rationale behind this choice. The authors should explain the advantages of this method in the Methods section. Furthermore, to substantiate its feasibility, some successful case studies along with appropriate references should be provided, which would make the presented results more convincing and reliable.
- Beyond iodic acid, other iodine oxoacids such as HIO₂ and HOI also exist, along with various iodine oxides. In addition, there are many more pollutants, for instance, nitric acid and fluorinated carboxylic acids. Could these species also have an impact? Of course, I am not suggesting that additional calculations be included in this work, but at the very least, the manuscript should address the current limitations of the study and outline potential directions for future improvement.
- In Fig. 6, the process of a gas-phase DMA approaching the interfacial I2O5 is presented. However, conversely, would a gas-phase I2O5 approaching the interfacial DMA also lead to a reaction? Why was this scenario not considered?
- In the supporting information, some figure annotations or captions should be more detailed. For example, in Fig. S14, the arrows appear to indicate the ESP maxima or minima of the product molecule. Although I can make an educated guess, the authors should provide clearer labels or explanatory notes.
- For the ELF results shown in Fig. S15, the meaning of the different colored regions should be clarified, as many readers may not be specialists in theoretical studies. For the other figures and captions, I will not list further examples. However, I suggest that the authors carefully re-examine whether the information provided is sufficiently detailed, and consider it from the perspective of a non-specialist reader.
- It may be helpful to revise Scheme 1 to more clearly reflect the key ideas presented in the manuscript. For example, the ionic products that contribute to enhanced hygroscopicity are mentioned in the text but are not explicitly shown in the current scheme. In addition, specifying the molecular pairs responsible for hydrogen and halogen bonding would improve clarity; iodic acid, for instance, may engage in both hydrogen bonding and halogen bonding with water. The central label of “low volatility” could also be made more explicit by indicating the specific species it refers to.
- The mechanism mediated by DMA in Fig. 6 would be better presented in a manner consistent with the others, and I recommend that the corresponding reaction equation be included for completeness.
Suggested corrections
Line 40: Should “a typical higher I2O2-5” be “one of the highest iodine oxides”?
Line 48: “More recently, the experimental evidence” --> “A most recent experimental evidence”
Line 51: “found the direct” --> “found that the direct”
Line 63: “HIO3 abundant” --> “HIO3 is abundant”
Line 93: Check the word “consisting”
Page 11: Make sure Scheme 1 is clear enough.
Citation: https://doi.org/10.5194/egusphere-2025-3770-RC2 -
RC3: 'Comment on egusphere-2025-3770', Anonymous Referee #3, 17 Sep 2025
reply
This manuscript examines how gas–liquid interfacial reactions of higher iodine oxide (I2O5) influence the formation of marine iodine aerosols using molecular dynamics simulations. The authors elucidate the I2O5 heterogeneous hydrolysis mechanism, emphasizing the catalytic effects of atmospheric chemicals. These mechanisms are likely to provide evidence for the extensive presence of iodate in aqueous aerosols, offering guidance for refining atmospheric models of aerosol burden and radiative forcing. As a well-designed theoretical study with atmospheric implications, I recommend this work for publication, subject to my comments being addressed.
The authors suggest that iodine-mediated reactions are more likely to play an important role in pristine environments, whereas pollutant-mediated reactions dominate in polluted marine environments. These heterogeneous mechanisms may have significant atmospheric impacts and need to be evaluated by embedding them into atmospheric models. What challenges do the authors foresee in implementing this cross-scale simulation?
Line 11: I am not quite sure whether the expression ‘higher iodine oxides’ is widely used and easily understood, and the authors should check this.
Line 20: I suggest that the authors moderate some of their conclusions, as this study does not quantify the rates of the relevant chemical processes. Expressions such as “highly effective” should therefore be toned down. More generally, conclusions should avoid absolute or overly strong wording unless supported by quantitative data.
Line 31: The reference to Barnes et al. lacks bibliographic details—specifically the year of publication—and similar issues should be checked throughout the reference list. In addition, many studies have examined DMS-derived sulfur and its relation to aerosols; citing only two papers is insufficient. Please include more primary studies and relevant reviews to support the claim.
Line 38: In the section addressing the uncertain fate of IxOy, the authors should consider citing the experimental study by Finkenzeller et al. (Nat. Chem., 2023, 15, 129). They argued that stable I2O3 should be observable, but it was not, which also highlights the uncertainty in the fate of IxOy and supports the view of an unclear IxOy sink.
Line 44: “…particles through the reaction (R1: 2HIO3 → I2O5 + H2O)” should be revised to the more accurate wording “…particles through the dehydration reaction (R1: 2HIO3 → I2O5 + H2O).”
Line 64: Iodate, rather than HIO3, is likely abundant in the aerosol; therefore, the statement should be revised to ‘HIO3 (detected as IO3⁻)’.”
Line 262: In Scheme 1, according to the caption, the figure is supposed to illustrate the mechanism. However, it actually lacks many details and resembles more of a TOC-style figure. I suggest that the authors replace it with a figure that presents a more detailed depiction of the heterogeneous mechanism.
Citation: https://doi.org/10.5194/egusphere-2025-3770-RC3
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Deng et al. presented a theoretical study showing that, under the influence of atmospheric iodine species and pollutants, I2O5 hydrolysis can occur more readily at the surface of aqueous aerosols. These physicochemical processes are valuable, as I2O5, being a key chemical in the iodine cycling, has a significant impact on both iodine chemistry and the formation of iodine aerosols. Experimental investigation of gas–liquid interfacial reactions is challenging; therefore, the heterogeneous mechanisms revealed by the authors through Ab initio molecular dynamics simulations provide an important advancement of the previous understanding of the atmospheric fate of iodine oxides. This manuscript is thoughtfully prepared, with reliable methods and comprehensive data in both the main text and the supplementary material that support the conclusions. That said, certain aspects could benefit from minor revision, and I recommend publication after the authors have addressed my comments.
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Minor Comments: