Zhang et al present a measurement report of reactive halogen species in the arctic using MAX-DOAS to measure BrO and IO. The authors present a clear report of measurements and methods along with satellite validation. I recommend publication after correction of one broader issue with the paper and some minor corrections.

Broad issues: The article has “Measurement Report” in the title, but the paper reads more like a research article. The challenge here is that it doesn’t present any new findings, just more data verifying findings and publications of 20 years ago with new data. As a measurement report, this is good. As the conclusions read, it feels like the authors are overstating the novelty of the work. All of the trends and correlations and explanations are the same as previous studies. I recommend that the authors modify language in the abstract and conclusions to more consistently match the “Measurement Report” nature of the article consistent with the title. The authors allude to chemistry model optimization but don’t provide any justification as to which models or how these data would be applied.

Minor Corrections:

Pagination is inconsistent and starts over at the various sections.

Line 37: “provides critical in-situ validation”. Here we are missing how this data is critical.

Line 75: “marine boundary layer are severely scarce” Remove severely, scarce is sufficient unless you quantify how much the data is available compared to how much is necessary, severe sounds like an overstatement.

Lines 81-82, 88, 99: Phrases are in quotation marks. Are these direct quotes from an un-cited source? Where are these phrases coming from? In general, direct quotations are not appropriate, and if given, the reference must follow immediately from the single source.

Lines 93 – 100: Here the authors refer extensively to a figure found in the Supplement. If the article spends this much space on the figure, it needs to be in the regular paper and not the supplement.

Line 174, Fig 2: The various fit windows only show the fit of the species of interest for the specific fit window. NO2 for instance, is fitted in all of those windows but only shown in one. Other fitted species help to understand fit interference and structured residuals. Are there major differences in the NO2 retrieved values, even within overlapping windows in the UV? Please provide some commentary (or reference to previous work) as to why these fitting windows were chosen? The cited references span a variety of groups and instruments, it would be good to know why these were chosen (for instance, why does the BrO fit not use the Ring parameter?).

Line 199 and 203: References such as this should be formatted as Wagner et al. (2010).

Line 284: Refers to chapters. Is this a carryover from a dissertation format? Please update to the proper terminology.

Line 296: This line needs a break after ‘excluded’. Something is missing in the structure of the sentence, please revise.

Zhang et al. present ship-based MAX-DOAS observations of NO₂, HCHO, BrO, and IO conducted during a Shanghai–Arctic cruise. The study draws three main conclusions: (1) measured trace gas columns show good agreement with satellite products (TROPOMI, GEMS, and GOME-2); (2) elevated BrO levels are associated with air masses that experienced prolonged contact with sea ice; and (3) enhanced IO levels are linked to increased biological activity. Overall, the dataset is valuable and the study addresses timely topics, but the manuscript would benefit from deeper engagement with existing literature and more detailed descriptions of the retrieval methods and associated uncertainties.

Major comments

There is extensive literature spanning several decades on Arctic halogen activation, including so-called “bromine explosion” events and their underlying mechanisms. Given that a substantial portion of this manuscript focuses on enhanced bromine levels in the Arctic, it would strengthen the paper to more thoroughly situate the results within this established context, either in the introduction or in the discussion section.

Relevant examples include:

• Pratt et al. (2013), Photochemical production of molecular bromine in Arctic surface snowpack
• Swanson et al. (2020), Arctic reactive bromine events occur in two distinct sets of environmental conditions: A statistical analysis of six years of observations
• Peterson et al. (2017), Observations of bromine monoxide transport in the Arctic sustained on aerosol particles
• Brockway et al. (2024), Tropospheric BrO vertical profiles retrieved across the Alaskan Arctic in springtime

In particular, a comparison with previous MAX-DOAS observations of BrO in the Arctic would be useful.

Throughout the discussion (e.g., Lines 276, 418, and 515), the manuscript attributes enhanced BrO observed by MAX-DOAS to “bromine explosion” or bromine activation events. Traditionally, these events refer to pronounced enhancements of reactive bromine species during Arctic spring, often associated with significant ozone depletion, with BrO mixing ratios on the order of ~20–40 ppt. By late spring and into summer, when melting begins, gas-phase bromine levels typically decrease to background values, with only occasional enhancements associated with fresh snow (see Jeong et al., Multiphase reactive bromine chemistry during late spring in the Arctic). The mechanisms responsible for bromine activation are known to be strongly seasonally dependent.

The measurements presented here were made during summer, a period when previous studies generally report background BrO levels in the Arctic. In addition, the reported BrO vertical column densities from MAX-DOAS appear higher than those from previous Arctic DOAS measurements and are approximately a factor of 50 larger than GOME satellite observations. These discrepancies warrant further discussion. For example, do the authors expect the retrieved BrO to be primarily near the surface or distributed aloft? If the signal is dominated by near-surface BrO, what mixing ratios would be implied by the reported VCDs (0.2–0.5 × 10¹⁵ molecules cm⁻²), and are these values consistent with previous summertime observations?

Previous studies of springtime bromine activation also indicate that sea ice contact duration alone is insufficient to explain observed variability. Other controlling factors include meteorological conditions (e.g., wind speed and boundary layer height), sea ice age, the presence of fresh snow or frost flowers, and particulate bromine. Incorporating these factors into the discussion, alongside sea ice contact time, would provide a more balanced and mechanistic interpretation of the results.

The manuscript would also benefit from explicitly stating the uncertainties and detection limits of the MAX-DOAS retrievals.

Minor comments

• It should be clearly stated whether the MAX-DOAS and satellite products represent tropospheric columns only or total columns (troposphere + stratosphere).
• At several points (Lines 37, 71, 79, 110, 118, 520, 537), MAX-DOAS is referred to as an in situ measurement. MAX-DOAS is a remote sensing technique and should be described as such.
• Line 128: Is 169.18°W intended to be 169.18°E?
• Section 2.2.1: It would be useful to discuss uncertainties associated with separating tropospheric and stratospheric contributions using DSCDs, particularly in the presence of sharp stratospheric gradients.
• Lines 201–203: A surface albedo of 0.06 is reasonable for open ocean but likely too low for sea ice. Please justify the parameter choices and indicate whether sensitivity tests were performed to demonstrate that these assumptions do not significantly affect the results.
• Several figures are missing labels for color scales, and overall font sizes are too small. Increasing font size would improve readability.
• Throughout the manuscript, BrO and IO are referred to as “pollutants” or “pollution.” As reactive halogens are not pollutants in the traditional sense, alternative terminology is recommended.
• Line 424: PSCF should be spelled out at first mention. A brief description of the PSCF method should also be added to the methods section, including whether weighting was applied based on air mass residence frequency within each grid cell.