the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Dec 2025
A novel technique for the humidity dependent calibration of hypoiodous acid (HOI) and iodine (I2)
Abstract. Hypoiodous acid (HOI) and molecular iodine (I2) are important precursors of reactive gaseous iodine, which plays an important role in the oxidative capacity of the atmosphere and in aerosol formation in the marine boundary layer. HOI and I2 are emitted from the ocean surface and recycled on atmospheric aerosol via heterogeneous chemistry. Measurements of these molecules, which are typically present in the marine boundary layer at the low-to-sub part per trillion (ppt) level, are sparse, in part due to difficulties in quantification with a lack of appropriate instrumentation and calibration techniques. A novel calibration technique is developed for HOI via generation from I2 hydrolysis and then 1:1 conversion of HOI back to I2 through a NaI trap, allowing the sensitivity of HOI to be calculated relative to I2, which is readily calibrated using a permeation tube system. Using this calibration method, we describe the use of a reduced pressure high resolution chemical ionisation mass spectrometer (CIMS) to characterise the sensitivities of HOI and I2 over a range of humidities representative of the marine boundary layer and to measure these molecules in the field. At humidities of over 50 % RH, the CIMS sensitivity of I2 is humidity independent whereas HOI exhibits a slight negative humidity dependence. The effect of inlet interactions on HOI and I2 signals is investigated, with HOI observed to convert to I2. The implications of these inlet interactions and humidity sensitivities for future ambient measurement configurations are discussed.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-5812', Anonymous Referee #1, 27 Dec 2025
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RC2: 'Comment on egusphere-2025-5812', Anonymous Referee #2, 01 Jan 2026
This manuscript describes the development, characterization, and application of an optimized quantitative method for the calibration of I₂ and hypoiodous acid (HOI) using bromide adduct chemical ionization mass spectrometry (CIMS). An indirect calibration method for HOI has been established where HOI is generated via the hydrolysis of I₂, and a NaI trap is used to achieve the 1:1 conversion of HOI back to I₂. This provides an alternative way for the calibration of HOI, which still remain very challenging to date. The manuscript is well written and the scope is suitable for Atmospheric Measurement Techniques. However, I have some concerns that need to be addressed prior to the consideration of publication.
(1) Section 2.3.1: The emission rate of the permeation tube is reported as 34.7±0.21 ng/min, quantified via the gravimetric method. However, critical experimental details for validating the reliability and robustness of the permeation tube as a calibration standard are not adequately specified in the text. (1) What is the time interval adopted for weight loss determination? E.g., weekly, monthly, or predefined intervals, rather than mentioning over several months; (2) Please mention the precise characterization of the environment during the gravimetric measurement, particularly the temperature fluctuation; (3) How to validate the stability or drifting of the emission rate over time? Is there any validation from other method like the n-hexane absorption method? This will be a solid approach to validate the permeation rate, which is critical for determining the I2 concentration; and (4) What is the range of the dilution flowrate being used in this study? It is not clear how using different pump speeds altered the concentration of I2?
(2) Section 2.3.2: Please state the range of humidity being used for the hydrolysis (Line 173). A 1:1 conversion of HOI to I2 was assumed here. Did the authors conduct further testing to check on the validity of this assumption? It is also not clear if the HOI absorption efficiency on NaI agglomeration or deliquescence will change under different environment, such as the changes of humidity (i.e., high humidity) or temperature?
(3) Figures 2: For the HOI peak fitting, how sure are the authors that the surrounding peaks will not interfere the HOI peak (i.e., in Fig 2b)? More details of the peak fittings are needed.
(4) Figure 3 and Figure 8: A major concern here is that the Bermuda observations reported I2 concentration of about less than 1 ppt, but the calibration for I2 used 1–3 x1010 molecules cm-3. Can the high concentration calibration represent the actual sensitivity to the ambient signal? How do the reagent ion (Br-) of CIMS reacts to the high I2 concentration? How much does it deplete and how much will the change of reagent ion affect the sensitivity of the CIMS? For CIMS, there is possible that the sensitivity to low concentration will response differently with the higher concentration.
(5) Figure 4: Provide the R2 value for Fig 4a as well. At high humidity levels, does there exist a critical humidity threshold (Water ratio = 0.7)? Is there a direct correlation between this threshold and the change in the signal proportion of Br⁻ hydration clusters (Br⁻, H₂OBr⁻, Br (H₂O)₂⁻)?
(6) Line 285: Increase in the signals of HI and IBr as observed by the NaI trap was noted. If the HI and IBr are produced from HOI, then the assumption of 1:1 is not absolutely true. The authors have noted that the impurities are negligible, but why not adjusting the ratio since they mentioned that the I2 which represented 96% of the change in signal? Or did the authors include this ‘negligible’ value in the uncertainty estimation?
(7) Figure 5: Why not presenting the I2 in concentration rather than signal? It will give a better view on the concentration of I2 conversion.
(8) Section 3.5.2: I am wondering whether the CIMS response linearly to the HOI concentrations? Suggest to add a figure for HOI calibration curve or to show how does the CIMS sensitivity response to various HOI concentrations. This is important information for the quantification of HOI in the ambient. Similar to my comments above, the calibration range should reflect the ambient levels.
(9) Line 393-394: This statement is not clear. Please justify what is the basis that ICl and IBr could be from the I- present from the HOI production step.
(10) In the appendix, Figure A1 and A2: The authors should verify if the peaks for HI, INO2, ICl and HIO3 are real and not interfered by the larger peaks. Will a longer averaged period for mass spectra will have better peaks for these iodine species, rather than using a 20-min averaged?
(11) Double check the formatting of references.
Citation: https://doi.org/10.5194/egusphere-2025-5812-RC2
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This manuscript presents a novel and well-designed calibration approach for quantifying hypoiodous acid (HOI) based on its conversion to molecular iodine (I2) using a sodium iodide trap, coupled with bromide chemical ionization mass spectrometry (Br-CIMS). The authors systematically investigate the humidity dependence of the instrument’s sensitivity to both HOI and I2, demonstrating that I2 sensitivity becomes essentially independent of humidity above ~50% RH, while HOI exhibits a slight negative dependence. The study also highlights significant inlet-mediated losses and interconversion between HOI and I2, especially in bent or salted inlet configurations mimicking field setups. The manuscript is well written and easy to read. I would recommend it to be published in this journal after the following comments being addressed and revised.
1. The 1:1 conversion of HOI to I2 via the NaI trap is assumed based on analogy with the HOBr–Br2 system. However, small signals of HI and IBr were observed upon trap insertion. Could side reactions or impurities in the NaI trap lead to non‑stoichiometric conversion? Is there additional experimental evidence (e.g., isotopic labeling, independent HOI quantification) to confirm the 1:1 conversion efficiency under varying humidity and concentration conditions?
2. The “water ratio” (H2OBr⁻/Br⁻) is used as a proxy for IMR humidity. While convenient, has this ratio been validated against direct humidity measurements (e.g., using a hygrometer) across the full range of conditions? Could temperature fluctuations in the IMR affect the relationship between the water ratio and actual H2O mixing ratio?
For I2, sensitivity becomes humidity‑independent above a water ratio of ~0.7. The explanation involves a balance between the stabilizing effect of H2O on the adduct and the lower formation enthalpy with H2OBr⁻. Is there quantitative kinetic or quantum‑chemical modeling to support this interpretation? Could the second water cluster Br(H2O)2⁻ play a non‑negligible role at very high humidity?
For HOI, sensitivity decreases with humidity, explained by QRRK theory. Are there computational or experimental data on the binding energies, vibrational frequencies, or number of effective oscillators for HOI·Br⁻ vs. I2·Br⁻ adducts to substantiate this argument?
3. Inlet loss experiments using a T‑piece and a salted PTFE guard show substantial HOI loss (65–75%) with concomitant I2 increase, attributed to reverse hydrolysis. How representative are these laboratory tests of real‑world marine boundary layer conditions, where aerosol composition, surface acidity, and humidity vary continuously? Have you considered performing similar tests with authentic sea‑salt aerosol or under varying RH/T conditions?
The HOI data are presented as an “upper limit” because background subtraction was not performed due to the lack of dry calibration. Could the use of isotope ratio filtering (as done for zeroing checks) or nighttime background estimation provide a way to constrain the background more rigorously? How might this affect the reported diurnal profile and peak mixing ratios?
4. The overall uncertainties for I2 and HOI are given as ~30%, but described as “lower limits” due to unquantified inlet effects. Could repeated loss experiments with different inlet configurations or flow rates help better constrain these uncertainties?
I2 mixing ratios show no clear diurnal cycle, contrary to expectations from photolysis. The authors suggest inlet or background effects, but could there be a daytime source of I2 (e.g., photochemical production or wind‑driven emission) compensating for photolytic loss? Is there corroborative data (e.g., from DOAS, other CIMS, or model simulations during BLEACH) to contextualize this observation?
5. The calibration and inlet loss corrections were developed for marine boundary layer conditions (high RH, sea‑salt influence). How transferable is this method to other environments (e.g., polluted coasts, polar regions, forests) where humidity, aerosol composition, and oxidant levels differ significantly?
For long‑term field deployments, how stable is the Br⁻ reagent ion source and the instrument sensitivity over weeks to months? What quality‑control measures (e.g., regular permeation tube checks, humidity‑adjustment protocols) are recommended for ongoing ambient monitoring of HOI and I2?