In this manuscript, Pound et al. present a fairly comprehensive box model of the ocean surface microlayer (SML), coupling the deposition of ozone with the chemistry in the SML, with a major focus on oceanic emissions of inorganic iodine. This is great effort, providing a mechanistic representation of air-SML-ocean processes for chemistry-climate models, and the model can (in theory) be expanded for other compounds of interest. The manuscript is generally well prepared. I recommend this manuscript for publication in EGUsphere after the following comments and concerns are addressed.

My main concern is the model representation of SML itself. The thickness of the SML is operationally defined, with commonly quoted thickness ranging between 1 micron to up to 1 millimeter. The thickness and composition of SML may also depend on wind, although the dependency remains unclear. It remains unclear how the SML thickness affects the mass transfer as well as the subsequent ozone deposition and iodine emission. I acknowledge that there are many aspects remain poorly understood to include in the model but the impact of SML thickness looks like a lower hanging fruit to me.

What’s also unclear is the scalability of the SML model. Properties of SML have mostly been derived using coastal/offshore/nearshore observations, and observations obtained in the open ocean are not limited. Any robust measurements supporting the widespread existence of SML? I understand this is perhaps beyond the scope of this work. But it would be tremendously helpful for audience who are not entirely familiar with this topic if the authors can include a brief summary on this, which certainly puts this work into a broader perspective

Minor/technical comments:

Page 2 Line 38: “Recent work has also supports atmospheric iodine playing an important role in *partial* formation in the MBL…” Should this be *particle*?

Page 2 Line 40: I don’t think the in-text citation format is correct. 

Page 3 Lines 86-88: this is where the thickness of SML may play a role. Thickness and the reacto-diffuso-length together determine if the reaction occurs at the surface or the bulk. I would imagine the thickness also affects the depletion and replenishment of iodide in the SML.

Page 5 Lines 128-129: If the aqueous-phase chemistry of ozone (in SML) is explicitly represented in the model already, why would you need this r_c term in the resistance model? Ultimately the tendency of ozone in gas-phase consists of turbulent transport (r_a) to the surface and mass transport across the quasilaminar sublayer (r_b), as well as whatever chemistry occurs in the gas-phase and the aqueous-phase. For simple deposition models without explicit representation of aqueous-phase chemistry, r_c term summarizes the impacts of aqueous-phase chemistry in a simplified psuedu-1st order manner (a = k[I-]). But this work already has detained chemistry, would including r_c term be double-counting?

Page 5 Equations 4-6: please make sure all variables and parameters in those equations are clearly defined and explained in the text.

Page 5 Equation 9: shouldn’t this be the diffusivity in the SML? With all the organics and surfactants, would you expect the diffusivity be the same as (dilute) water?

Page 10 Section 7: please clarify how the modeled ranges are calculated. I understand it is extremely challenging to mimic the experimental conditions with a model like this, therefore the fact that the model captures the orders of magnitudes is impressive in my opinion. I do wonder to what degree would the thickness of the SML affect the modeling results.

Pages 9-10. The GEOS-Chem implementation is interesting. However, I can’t help but notice the modeling period (2020-2021) enters a strong/rare La Niña event (2020-2023). The model-measurement comparison presented in this work obviously ignores inter-annual variabilities. If the global iodide input (Sherwen et al) is a climatology that does not include inter-annual variability, then perhaps there is no point to extend the simulation period to match the measurement years. Please comment if these IO measurements (Figure 12) would be subject to considerable inter-annual variability. 

Review of manuscript ID ‘egusphere-2023-2447’ titled ‘An improved estimate of inorganic iodine emissions from the ocean using a coupled surface microlayer box model’ by Pound et al.

This manuscript offers a box model of the ocean surface microlayer (SML) where ozone deposition and chemistry lead to the emission of iodine from the sea surface. This model aims to provide a mechanistic representation of air-SML-ocean processes leading to emissions, which have traditionally been parameterised in models. The new emissions of inorganic iodine show a large spatial difference compared to the past emission inventories, but the global mean is similar. However, the comparison with observations does not show an obvious improvement either in terms of iodine or ozone observations.

Overall, the manuscript is well-written, and the model is well-detailed. This represents a step up from the current description in models, and hence, I recommend this manuscript for publication after addressing the following comments:

Comments:

While the study offers new parameterisations for inorganic iodine emissions, the comparison with observations does not show a large improvement. Even if one does not consider the polar regions where the mismatch is largest (attributed possibly to sea-ice emissions), the new parameterization does not significantly improve the comparison. Indeed, no statistical analysis is provided to quantify whether there is an improvement or not.

This begs the question, what else is affecting the emissions? It would be helpful if there were a section in the discussions on the possible effects of surfactants or other mechanisms that can inhibit emissions (in most cases, the model overestimates the observations). This is mentioned in the conclusions, but no discussion on these effects and studies that have shown them is presented.

Indeed, looking at the ozone comparison, the match seems to be worse for most stations. The model was run from 2019 to 2021, but the observations are from 2014. Why was this period used for the model run? What are the possible reasons for no improvement? How much is inter-annual meteorological variability expected to affect the comparison? Why not use a climatology for the GAW stations?

Please include IO observations from the Indian Ocean (Mahajan et al., 2019a, b; Inamdar et al., 2020), the Pacific (Takashima et al., 2022) and the Malaspina circumnavigation cruise (Prados-Roman et al., 2015) for model comparison and validation. Observations from the Arctic Ocean are also missing in this comparison (Benavent et al., 2022). The authors also do not include coastal observations, but this is most likely to avoid macroalgae emissions – please state this clearly. In addition, ground-based observations in the free troposphere should also be compared with those of (Dix et al., 2013) (Volkamer et al., 2015), considering that the model has organic emissions.

Initial work on the effect of iodine on ozone has been ignored. For example, the first large-scale modelling efforts by (Saiz-Lopez et al., 2012) have not been cited, and nor has the work that identified the atmospheric chemistry of iodine through its impact on ozone loss and HOx changes (Vogt et al., 1999; Alicke et al., 1999; Allan et al., 2000; Saiz-Lopez and Plane, 2004; Bloss et al., 2005). Even the recent publications showing the large-scale impacts of short-lived halogens on ozone, aerosols, HOx and aerosols have not been cited (Saiz-Lopez et al., 2023).

Line 38: ‘particle’ instead of ‘partial’

Line 43: The contribution of inorganic iodine emissions to stratospheric iodine loading is very small, considering its short lifetime. Please make it clear that this is due to organic iodine emissions.

Line 61: There is field proof of this – parameterised inorganic iodine emissions had to be reduced to 40% to match observations in the Indian Ocean (Mahajan et al., 2021).

Line 66: and decrease in sea-ice extent leading to more exposed seawater in the Arctic.

Line 195: The authors mention that the other processes for the depletion of I^- are likely to be minor – however, considering significant emissions of iodocarbons from the ocean surface, would I^- contribute towards this, and if so, how can this be considered?

Line 224: ‘by ∼170% when using the rate coefficient from Brown et al. (2023)’ – not sure how this calculation has been made. From the figure, the change is approximately 0.7/0.3 (an increase of a factor of 2.3).

Figure 6: The x-axis is wind speed, but the caption mentions SST.

Figure 11: Caption – state what this is change relative to.

Figure 7: The x-axis is wind speed, but the caption mentions SST. Add space between the O₃ concentration number and the unit.

Figure 13: Please make clear whether this is the difference in ozone due the parameterisations or due to iodine chemistry.

References:

Alicke, B., Hebestreit, K., Stutz, J., and Platt, U.: Iodine oxide in the marine boundary layer, Nature, 397, 572–573, https://doi.org/10.1038/17508, 1999.

Allan, B. J., McFiggans, G., Plane, J. M. C., and Coe, H.: Observations of iodine monoxide in the remote marine boundary layer, J. Geophys. …, 105, 14363–14369, 2000.

Benavent, N., Mahajan, A. S., Li, Q., Cuevas, C. A., Schmale, J., Angot, H., Jokinen, T., Quéléver, L. L. J., Blechschmidt, A., Zilker, B., Richter, A., Serna, J. A., Garcia-Nieto, D., Fernandez, R. P., Skov, H., Dumitrascu, A., Simões Pereira, P., Abrahamsson, K., Bucci, S., Duetsch, M., Stohl, A., Beck, I., Laurila, T., Blomquist, B., Howard, D., Archer, S. D., Bariteau, L., Helmig, D., Hueber, J., Jacobi, H.-W., Posman, K., Dada, L., Daellenbach, K. R., and Saiz-Lopez, A.: Substantial contribution of iodine to Arctic ozone destruction, Nat. Geosci., 15, 770–773, https://doi.org/10.1038/s41561-022-01018-w, 2022.

Bloss, W. J., Lee, J. D., Johnson, G. P., Sommariva, R., Heard, D. E., Saiz-Lopez, A., Plane, J. M. C., McFiggans, G., Coe, H., Flynn, M., Williams, P., Rickard, A. R., and Fleming, Z. L.: Impact of halogen monoxide chemistry upon boundary layer OH and HO2 concentrations at a coastal site, Geophys. Res. Lett., 32, 1–4, https://doi.org/10.1029/2004GL022084, 2005.

Inamdar, S., Tinel, L., Chance, R., Carpenter, L. J., Sabu, P., Chacko, R., Tripathy, S. C., Kerkar, A. U., Sinha, A. K., Bhaskar, P. V., Sarkar, A., Roy, R., Sherwen, T. T., Cuevas, C., Saiz-Lopez, A., Ram, K., and Mahajan, A. S.: Estimation of Reactive Inorganic Iodine Fluxes in the Indian and Southern Ocean Marine Boundary Layer, Atmos. Chem. Phys., 20, 12093–12114, https://doi.org/10.5194/acp-20-12093-2020, 2020.

Mahajan, A. S., Tinel, L., Hulswar, S., Cuevas, C. A., Wang, S., Ghude, S., Naik, R. K., Mishra, R. K., Sabu, P., Sarkar, A., Anilkumar, N., and Saiz-Lopez, A.: Observations of iodine oxide in the Indian Ocean marine boundary layer: A transect from the tropics to the high latitudes, Atmos. Environ. X, 1, 100016, https://doi.org/10.1016/j.aeaoa.2019.100016, 2019a.

Mahajan, A. S., Tinel, L., Sarkar, A., Chance, R., Carpenter, L. J., Hulswar, S., Mali, P., Prakash, S., and Vinayachandran, P. N.: Understanding Iodine Chemistry over the Northern and Equatorial Indian Ocean, J. Geophys. Res.  Atmos., 8104–8118, https://doi.org/10.1029/2018JD029063, 2019b.

Mahajan, A. S., Li, Q., Inamdar, S., Ram, K., Badia, A., and Saiz-Lopez, A.: Modelling the Impacts of Iodine Chemistry on the Northern Indian Ocean Marine Boundary Layer, Atmos. Chem. Phys., 8437–8454, https://doi.org/10.5194/acp-2020-1219, 2021.

Prados-Roman, C., Cuevas, C. a., Hay, T., Fernandez, R. P., Mahajan, A. S., Royer, S.-J., Galí, M., Simó, R., Dachs, J., Großmann, K., Kinnison, D. E., Lamarque, J.-F., and Saiz-Lopez, A.: Iodine oxide in the global marine boundary layer, Atmos. Chem. Phys., 15, 583–593, https://doi.org/10.5194/acp-15-583-2015, 2015.

Saiz-Lopez, A. and Plane, J. M. C.: Novel iodine chemistry in the marine boundary layer, Geophys. Res. Lett., 31, L04112, https://doi.org/10.1029/2003GL019215, 2004.

Saiz-Lopez, A., Fernandez, R. P., Li, Q., Cuevas, C. A., Fu, X., Kinnison, D. E., Tilmes, S., Mahajan, A. S., Gómez Martín, J. C., Iglesias-Suarez, F., Hossaini, R., Plane, J. M. C., Myhre, G., and Lamarque, J.-F.: Natural short-lived halogens exert an indirect cooling effect on climate, Nature, 618, 967–973, https://doi.org/10.1038/s41586-023-06119-z, 2023.

Takashima, H., Kanaya, Y., Kato, S., Friedrich, M. M., Van Roozendael, M., Taketani, F., Miyakawa, T., Komazaki, Y., Cuevas, C. A., Saiz-Lopez, A., and Sekiya, T.: Full latitudinal marine atmospheric measurements of iodine monoxide, Atmos. Chem. Phys., 22, 4005–4018, https://doi.org/10.5194/acp-22-4005-2022, 2022.

Vogt, R., Sander, R., von Glasow, R., Crutzen, P. J., Glasow, R. V, and Crutzen, P. J.: Iodine chemistry and its role in halogen activation and ozone loss in the marine boundary layer: a model study, J Atmos Chem, 32, 375–395, 1999.

Volkamer, R., Baidar, S., Campos, T. L., Coburn, S., DiGangi, J. P., Dix, B., Eloranta, E. W., Koenig, T. K., Morley, B., Ortega, I., Pierce, B. R., Reeves, M., Sinreich, R., Wang, S., Zondlo, M. A., and Romashkin, P. A.: Aircraft measurements of BrO, IO, glyoxal, NO2, H2O, O2-O2 and aerosol extinction profiles in the tropics: Comparison with aircraft-/ship-based in situ and lidar measurements, Atmos. Meas. Tech., 8, 2121–2148, https://doi.org/10.5194/amt-8-2121-2015, 2015.