the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Oct 2025
Aerosol iodine recycling is a major control on tropospheric reactive iodine abundance
Abstract. Tropospheric reactive iodine influences the oxidizing capacity of the atmosphere and serves as an important source of ultra-fine particles. However, the paucity of observations of gas-phase and aerosol iodine, combined with incomplete understanding and representation of iodine chemistry in models, leads to substantial uncertainties in understanding iodine abundance, speciation, and impacts. Motivated by known gaps in previous modeling studies, we introduced speciated aerosol iodine and aerosol iodide recycling to the global chemical transport model, GEOS-Chem. Modeled aerosol iodine is speciated into fine and coarse mode soluble organic iodine (SOI), iodate, and iodide. Aerosol iodide is recycled into the gas phase via heterogeneous chemistry involving halogen nitrates and hypohalous acids to form I2, ICl, and IBr, which represents an additional source of gas-phase iodine to the atmosphere. Iodide dehalogenation doubles the tropospheric burden of reactive iodine (Iy) while reducing model-measurement bias for IO and aerosol iodine. The rate of aerosol iodine conversion to Iy is more than twice as fast as the combined rates of inorganic ocean emissions and the photolysis of organic iodine gases, suggesting that aerosols are important in mediating the abundance and lifetime of tropospheric Iy. The incorporation of SOI and iodate into the model prevents iodide dehalogenation by partitioning iodide into less reactive reservoirs, which has a stabilizing effect for reactive iodine chemistry. These findings have implications for reactive halogen abundances and global oxidant budgets in the troposphere.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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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Status: open (until 27 Nov 2025)
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RC1: 'Comment on egusphere-2025-4725', Men Xia, 08 Nov 2025
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AC1: 'Reply on RC1', Allison Moon, 08 Nov 2025
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Thank you for noticing the error in tables B1 and B2. The tables should indicate that the rate is khet[HOX/XNO3][halide], where khet is calculated with equation B1.
kchem represents the aqueous phase rate constant that is used to calculate the reactive uptake coefficient in equation B2.
This approach was introduced in Jacob (2000), and is the same as the calculation used in other GEOS-Chem halogen recycling papers including Wang et al. (2021) and Sherwen et al. (2016). Thank you again for your attention to detail.
We look forward to addressing your other comments!
Citation: https://doi.org/10.5194/egusphere-2025-4725-AC1
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AC1: 'Reply on RC1', Allison Moon, 08 Nov 2025
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Overall evaluation:
Moon et al. adds the aerosol iodine recycling processes into the GEOS-Chem model, simulates global iodine species distribution in 2022, and performs observation-simulation comparison. By doing so, the authors demonstrate that the chemical conversion from aerosol iodide back to gas-phase reactive iodine species is critical to global iodine chemistry. This work represents an important step in the development of GEOS-Chem, especially for studying halogen chemistry. The simulation results are acceptable, considering that this is the pioneering step in incorporating the aerosol iodine recycling module. However, there is a major concern about the treatment of heterogeneous reactions in this iodine module. It looks like the authors are merging the treatment of heterogeneous and aqueous reactions, which is unusual and confusing and may lead to unreasonable modeling results. This work has the potential to become a great paper if the above major comment could be properly settled. I also have some minor comments mainly about improving clarity and adding discussion. Overall, considering the existence of a major comment, I would suggest a major revision.
Major comment:
1. The expression of rate constants for heterogeneous reactions is very confusing. In Table B1, taking the reaction B1 as an example, k = kchem*[HOBr]*[Cl-]*[H+], where this k is noted as khet. However, in formula B1 (line 772), khet is calculated in another way where uptake coefficient is applied. Somehow these two calculations of khet are inconsistent. It seems that the calculation of khet in Table B1 and B2 is aqueous-phase reaction rates rather than heterogeneous reaction (meaning gas-particle interaction) rates.
I also have concern about the unit of kchem (M s^-1). Assuming that [HOBr], [Cl-], and [H+] all have a unit of M, then the unit of k in Table B1 becomes M s^-1 * M * M * M = M^4 s^-1, which is strange since the authors say the unit of khet is s^-1.
Table A3 also has similar issues. Within the Table, kchem has a unit of M s^-1, while in note c, the unit of kchem becomes M atm^-1, which is inconsistent. What’s more, since khet = kchem*[HOI]*[DOM]*[H+] and the value of kchem is given, it is not clear how Alpha, Dg, Henry’s K0 and Henry’s CR contribute to the calculation of khet. If the above parameters are indeed needed, why is the uptake coefficient of HOI not needed here?
Minor comments:
1. Line 69: although ocean emissions are undoubtedly the largest initial iodine source, the possible continental iodine source should not be ignored. Using nitrate-Tof-CIMS, the signal of HIO3 gas can be observed in continental urban areas, showing possible continental iodine source. Some brief discussion on continental iodine sources could be mentioned in the introduction part, although it is discussed in line 606-616 as remaining uncertainties.
2. Line 82: the concept of iodide dehalogenation is confusing to me. Since the authors mention that aerosol halides (which are supposed to refer to Cl-, Br-, and I-) are involved, it seems that this process should be generalized as dehalogenation rather than specifically iodide dehalogenation.
3. Line 86-87: the concepts of Bry and Cly, and dihalogen species are mentioned for the first time here. To avoid misunderstanding, please define them specifically. What is the composition of Bry and Cly, respectively? A definition can be given similar to that in line 93 for Iy.
4. Line 99-100: it is inspiring to know that there will be a follow-up paper that addresses the impact of this newly incorporated iodine chemistry on oxidants. It would be very interesting to know the impact on not only traditional oxidants, such as HOx, O3, and NO3 but also Cl and Br that can oxidize VOCs.
5. Line 105: does the HETP module also estimate sulfate?
6. Line 231: since some reaction rates are dependent on aerosol acidity, the authors should evaluate the accuracy or uncertainty regarding pH simulation.
7. Line 252-253: the authors mention tuning of aerosol iodine interconversion rates. This is an important statement, in which enough details should be given: (1) Exactly speaking, what reactions are tuned? (2) What kinds of observations are referred to?
8. Line 286: how shall we understand “out-of-the-box”?
9. Line 290-302: the simulation was performed for the year 2022, while observations used to compare with simulation were not necessarily performed in the same year. A brief discussion should be provided to state possible uncertainty caused.
10. Line 452-454: the authors mention a hypothesis that aerosol organics can combine with iodide and thus slow down iodide dehalogenation. Are there any laboratory experiments that support this hypothesis?
11. Line 692 Table A2: for note a, in principle the uptake should happen on the surface of all kinds of aerosols or surfaces, although with different uptake coefficients. For note b, the authors increase the uptake coefficient of I2Ox from 0.02 reported by Sherwen et al. to 0.10 shown in this table. Is there any fundamental evidence that supports this kind of tuning? At a minimum, some discussion should be provided. By the way, in line 695, “or 0.02” should be changed to “of 0.02”.
12. Line 774: A should be referred to as aerosol surface area density rather than aerosol surface area.
13. Line 860: In Table B3, khet only increases several percent when uptake coefficient is increased 100 times. Is this really true? If so, why is the khet so insensitive to changes in uptake coefficient?
14. The authors are suggested to briefly mention how well GEOS-Chem can simulate O3, since O3 + iodide reaction is very important for iodine activation.
15. How are iodine oxides (I2O2, I2O3, I2O4, and I2O5) produced in the model? Do they interconvert with each other?