Takashima and colleagues present IO and O₃ data from a number of ship cruises in the western Pacific spanning from the Arctic to Antarctic albeit with a gap in the southern tropics to subtropics. The work focuses on two empirical findings high IO observed in the western Pacific warm pool (WPWP) and a negative correlation between IO and O₃ observed at low O₃ mixing ratios. The authors demonstrate that a 0-D chemical box model using the dominant O₃-dependent iodine flux from the ocean surface cannot capture this anti-correlation then posit that O₃-independent pathways (missing global models) are necessary to reproduce the trend.

This latter point is well made, however, as it advocates for a rethinking of the representation of iodine fluxes from the ocean it demands a rather high level of scrutiny. The critical illustration is Fig. 3, in which three box-model cases: 1) only O₃-dependent iodine fluxes, 2) roughly half dependent half independent fluxes, 3) fully independent fluxes are compared with observations of O₃ and IO. While the overall correlation is negative, The Case-1 envelop guides the eye to see that the low O₃ observations appear to cluster as two populations which are displaced along the overall negative correlation but individually have positive correlation. All box model cases show consistent behavior for O₃ above ~13 ppbv, roughly parallel to the overall correlation, only those with an O₃-independent iodine source can reproduce the lowest O₃ mixing ratios. However, without offering specific evidence of the O₃-independent source other explanations bear consideration. I have the following suggestions for the authors to consider:

1. Is there any specific evidence to support an O₃-independent source of iodine?
   1. What measurements of organic iodine fluxes and concentrations are available in the study area, what are the modeled organic fluxes from e.g. Ordóñez et al., (2012)?
   2. For the photooxidation of I^- (Watanabe et al., 2019) is there a difference in solar illumination or some other photo-activity proxy between the different observations?
2. Since the points not captured by Case 1 are plausibly only vertically displaced from it, what is the effect of varying the initial O₃ mixing ratio? This could be caused by some upwind loss process or else reflect variable entrainment (Kanaya et al., 2019).
3. The subcases already illustrate the effect of varying the magnitude of the iodine flux, and by extension sea-surface I^-, but what about the speciation of the iodine flux (i.e. I₂ vs HOI), would changing this change the correlation? There is likely a pH dependence to speciation of the O₃-dependent fluxes (e.g. Macdonald et al., (2014); Moreno and Baeza-Romero, (2019)). In addition, since the photooxidation pathway emits I₂ (and not HOI) this could offer insight into that hypothesis also.
4. It seems that the authors have not included heterogeneous reactions which recent studies have suggested have been previously underestimated (Tham et al., 2021), could these impact the trend?

Furthermore, it would be helpful if the authors could be more specific in where they expect the posited O₃-independent source to be relevant. Is this a feature of the WPWP or relevant across latitudes? Is it possible that there is a less direct influence O₃ might play? In particular, studies of ice cores and tree rings (Cuevas et al., 2018; Legrand et al., 2018; Zhao et al., 2019) indicate a roughly threefold increase in iodine since c. 1950 at least ~50% attributed to anthropogenic O₃. If half of the inorganic flux were O₃-independent as suggested by Case 2, then either some other cause should be searched for, or the change in O₃-dependent fluxes to produce the observed change is even more dramatic than previously thought.

I have the following specific comments through the manuscript:

Line 160-161: Chlorophyll alone is not enough to exclude an organic iodine source on two counts. Firstly, organic iodine fluxes are not necessarily biotic in origin but might have an abiotic source. Secondly, the mesotrophic conditions characterized by MODIS correspond to those conditions observed to have the largest fluxes of organic iodine in some previous studies e.g. Jones et al., (2010).

Line 167: The authors state that there are insufficient data to document diurnal IO variations accurately, however, Fig. 2 indicates good temporal coverage was achieved for some days and it seems evident that there is wealth of IO data more generally. Is there some particular set of data which are missing or something else limiting the retrieval of diurnal variation?

The authors describe an “iodine fountain” in the WPWP which does appear to exist in Fig. 4, however, as the authors acknowledge Fig. 6 shows no clear correlation between SST and IO. The evidence for attributing the fluxes to SST seems at best mixed. For both the WPWP and the Maritime Continent it is clear that there is a lot of variability. Examining the temperature contours it doesn’t seem clear that SST would better explain the pattern than latitude. What distinguishes the “fountain” from being a tropical feature of unknown cause from specifically tying it to SST?

Relatedly, the authors have described a number of differences between the western Pacific and Atlantic, e.g. higher SST, lower O₃. Related to the point above about latitudes, the authors seem to suggest that the “iodine fountain” is a particularity of the WPWP and perhaps maritime continent but not of the Atlantic. But a clearer message on this point would be helpful.

I have the following technical comments through the manuscript:

Line 34: “006C” here is presumably “l”

Line 39: More recent papers on the O₃-dependent iodine source which should be mentioned for offering further consideration of physical and chemical drivers include Inamdar et al., (2020) and Carpenter et al., (2021).

Line 71: Inamdar et al., (2020) or else Mahajan et al., (2019) which includes the underlying measurements bear mentioning as more recent measurements of IO on the open ocean.

Line 88: Is this exposure time the same for all ELs or is this for a specific EL? If the latter the angle should be specified.

Line 96-101: The version of MMF described in Friedrich et al., (2019) uses Tikhonov regularization rather than optimal estimation for the aerosol retrieval. Was a more recent version used? Could the author provide the version numbers for MMF and VLIDORT?

Line 103-104: These a priori values are presumably the column integrals, this is should be more explicit by e.g. specifying the IO VCD

Line 104: While Sa is well understood by an expert audience to be the a priori covariance this should be defined for a non-expert audience.

Line 123: “they” here is presumably the fluxes, this is not clear. Line 32: Another recent paper with field evidence for iodine-derived aerosol particles is He et al., (2021)

Line 125: Hayase et al., (2010) and Hayase et al., (2012) predate Shaw and Carpenter, (2013) and show similar effects.

Line 141: Some more information on the O₃ data filtering would be useful, e.g. is the hourly average a running average or discrete average? What is the typical magnitude or relative magnitude of σ?

References:

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This paper presents measurements of iodine monoxide obtained using MAX-DOAS from 7 cruises from 2014-2018. The authors combine this work with box modelling and complementary measurements to suggest a larger role for iodine emissions under low ozone conditions. This work is based on an extensive dataset and the conclusions presented are interesting. There are some aspects of the work that should be further clarified prior to publication which are detailed below. 

Specific Comments:

Section 2.1: 

An explanation of the a priori choice is needed. Is it appropriate to use an identical a priori given the wide range of regions covered? Similarly, explanations for the choice of aerosol single scattering albedo, asymmetry parameter, and surface albedo should be given, preferably with references. 

The authors characterize the experimental uncertainty for the dSCDs, but then do not provide any comparable information for the retrieved IO VCDs or mixing ratios in the lowest 200m. The reader needs this information to interpret the level of support provided by the data for the conclusions. Similarly, given the importance placed on the near surface mixing ratios, the authors need to demonstrate the retrieval is sensitive to the near surface mixing ratio particularly given later comments suggesting that a priori selections play a large role in determining the retrieved surface mixing ratios. How large of an effect is this? If it is minimal compared to daily variations, that statement needs more support. This comment suggests the retrieval does not reflect the true atmospheric state, which raises questions about the ability of the authors to quantify the amount of IO present beyond a dSCD. These questions can be answered by showing averaging kernels that reflect the ability to retrieve the IO mixing ratio in the lowest layer (Peak near 1 near the surface with minimal values in other layers). To summarize over the entire data set the authors should provide statistics on the total DOFS for the retrieval as well as the DOFS in the near surface layer. This information will give the reader confidence that the IO values being presented are meaningful, and the papers conclusions are well supported. 

Section 2.4

A characterization of the uncertainties associated with the in situ ozone and CO measurements should be added to this section. 

Line 157. The authors mention insufficient data to show diurnal variations. This statement needs more clarification. You have a lot of data, more than most folks trying to measure IO, why do you not feel good about showing diurnal variations?

Line 207: Figure 6 doesn't support the statement of no correlation by itself. I just see timeseries of wind speed and SST, with no attempt to relate these quantities to IO or ozone. 

Section 4: Why is it important that IO was detected at low latitudes?

Figure 3: This figure needs error bars on the IO and ozone measurements to show the spread over the data set. I'm also unclear why the linear fit is calculated/shown. I didn't see a reference to it in the text, unless the goal is simply to show anti-correlation, in which case showing an R value makes more sense then the linear fit equation. If there is something important about the fit equation, it would be helpful to know what type of linear fit was done, particularly since the temporal variability of the IO  measurements and ozone measurements are not necessarily linked. 

Figure 4: Why plot dSCDs rather than VCDs or the surface mixing ratio? dSCDs don't really have much meaning to folks outside the DOAS community. While I find this figure very helpful for showing the cruise tracks and overall spatial extent of the data set, I find myself also wanting to be able to see each cruise plotted individually so I can examine the dataset for each cruise individually. Right now it seems like there are a lot of data points plotted on top of each other. Can you put plots for each cruise in the supplement for the curious reader?

Why are figures 5-7 only shown for 1 cruise? 

Data availability: I don't see a data availability statement showing where the data underlying this paper can be obtained, which I believe is a requirement for publication in ACP, and also a generally helpful thing to do for the broader scientific community. 

Technical Corrections:

Line 34: regiona006C to regional?