General Comments

This manuscript investigates the meteorological conditions spatial distributions of BrO in the Arctic leading to ozone depletion events as observed by ozone sondes and an in situ ozone monitor in Ny-Alesund, Spitsbergen. This is done by separating the ozone time series into ODE and non-ODE periods using a threshold value, and by calculating maps of the anomaly of meteorological parameters, sea ice conditions and BrO VCDs over the Arctic for both situations. Based on these anomaly maps, the impact of the spatial distribution of temperature, wind speed, boundary layer height and pressure as well as BrO and sea ice coverage has been investigated. The manuscript confirms many findings from previous studies, such as the impact of polar cyclones on ozone depletion and the occurrence ODEs during high wind speeds, which are probably due to heterogeneous release of reactive bromine from saline aerosols. It is found that certain distributions of polar low- and high-pressure systems lead to the southward transport of ozone depleted air towards Ny-Alesund. Furthermore, the seasonal and inter-annual variation of these anomalies is investigated and a case study on a particular ODE is presented.

The meteorological conditions leading to a release of reactive bromine and a subsequent ozone depletion are still not fully understood. Therefore, this manuscript provides a valuable contribution to this field of research and fits well into the scope of ACP.  The results of the study are described appropriately, but I feel that the description of the methods requires substantial revision. In particular, the “composite analysis” method presented in Sect. 2.7., which represents the key method of the study, should be re-written since it is lacking conciseness and is difficult to understand (see the specific comments below).

Sect. 3.2.1. describes in detail the impact of the ODE threshold values on the resulting anomaly maps, and concludes that there is only little impact on a qualitative basis. I therefore suggest to skip this section, and simply add the final sentence of this section (“No major differences in BrO and meteorological anomalies are observed when changing the ozone threshold value”) to the methods section.

Specific Comments

P2, L47: In addition to chlorine, I think it would be worth mentioning iodine as a potential booster for ozone depletion (Benavent et al, 2022).

P3, L68: Please add a reference for the lifetime of BrO and specify what exactly is meant with this value. While the photolytic lifetime of a BrO molecule is quite short, the lifetime of BrO in a certain air mass depends on various parameters, such as the presence of saline surfaces for recycling.

The detection limits of the ozone measurements, as well as the sensitivity of the MAX-DOAS vertical profile measurements, should be briefly discussed in Sections 2.1 and 2.2, respectively.

Section 2.1: It is not entirely clear how the number of ODEs is determined, in particular for the in-situ instrument. My understanding of a single ODE is a continuous period in time during which the ozone VMR remains below a certain threshold value. Here, it is not clear whether the number ozone depletion events are counted, or rather the number of hours during which ozone VMR remains below the threshold. This needs to be clearly defined. I think it would be inappropriate to count each hour of low ozone as a single ODE.

P5, L122: “The sensitivity to the choice of the threshold value”: Sensitivity of what?

Figure 1 is very hard to read. In the left panel, it is impossible to recognize individual non-ODE profiles due to the large number of overlapping profiles, and it is hard to see any patterns in the time series shown in the right panel since the x-axis covers a large time range of more than 10 years. It is therefore impossible to recognize any seasonality. I would therefore appreciate if some other way of presenting the data could be found. For example, the vertical profiles could be shown as box-whisker-plots, and the time series could be shown as a separate figure with a larger width. For the time series, it appears that only springtime values are shown. I suppose there are also measurements during the rest of the year, and it would be nice to show all data in order to give an idea about the complete seasonality of ozone.

P8, L174: I think it is not appropriate to call the (310-500) nm channel “visible” since light below 380 nm is not visible.

P8, L198: Can you be more specific with the location of the WRF domains, e.g. by providing coordinates of the centres of the domains?

The “composite analysis” method described in section 2.7 is difficult to understand and this section should be re-written (see also general comments). A very simple approach (anomaly = deviation from averages of maps of meteorological, chemical and sea ice parameters for ODE and non-ODE conditions) is described in a very complicated way:

• It does not become clear that the approach is applied to the spatial distribution of the observables over the entire Arctic region. During my first read, I thought this would refer to local parameters at the measurement site.
• Related to the ozone soundings, how do you define a “data point” (L216)? Is this the O3 VMR at a certain altitude (meaning that one profile consists of many data points), or a single ozone profile?
• It is not clear how you apply the threshold value to the ozone vertical profiles. Do you consider a measurement as ODE if the O3 VMR is below the threshold at some altitude, or does it need to be depleted over a certain altitude range?
• The number of ODEs stated in section 2.7 does not agree with the numbers in Table 2
• How do you define an “ODE day” (L216)? An ozone depletion at the time of the balloon sounding does not necessarily mean that ozone is depleted during the entire day.
• The calculation of the anomalies 24 h and 48 h before and after the time of O3 observation is explained in a quite cumbersome way and should be rewritten.

P9, L227: It is not mentioned that Y* is also calculated for the sea ice coverage parameter. It is not clear what you mean with “To obtain Y*(bar)_ODE, all selected Y* were averaged”. How is this selection performed?

P10, L260: It is not clear to me how you it can be concluded that “already ozone poor air is transported to the measurement site” if there are indications that recycling of Br_x on blowing snow took place. I think the opposite is likely as well, namely that saline particles are transported to the measurement site and ozone destruction took place all the time along the trajectory, and probably still takes place in situ.

P11, L262: Here you discuss an increase of the SIC on ODE days. It is hard to imagine that sea ice cover changes that rapidly, since sea ice formation is a very slow process, while ODEs occur over time scales of only a few hours.

Section 3.2.1: I suggest to skip this section as already detailed in the general comments

Sections 3.2.2 and 3.2.3 discuss seasonal and inter-annual variations of the anomalies. These are not sensitivity analyses, as the title of Section 3.2 suggests.

P17, L374: Please quantify the detection limit of the ozone measurements.

Section 3.3: Here vertical profiles of ozone from balloon soundings are compared to vertical profiles of BrO from MAX-DOAS. It is speculated that blowing snow plays a role in the release of reactive bromine. To further support this hypothesis, it would be important to also show and discuss vertical profiles of the aerosol extinction, which should be available from the MAX-DOAS measurements.

Figure 8: BrO does not seem to be present over the entire altitude range where ozone depletion is observed. Can you elaborate on the reasons for this discrepancy?

Technical Corrections

P2, R1-R6: Chemical formulas should not be in italic

P4, L103: On can either discuss a case study or observe a case, but observing a case study does not make much sense (this would at the very most be meta-science).

P4, L110: I suggest to rewrite this sentence as follows: “The vertical resolved ozone sonde profiles allow to study the altitude distribution of ODEs in the boundary layer”

P4, L113: Add “described below” to the end of the sentence since the threshold values are not defined yet.

P4, L116: Insert “does” before “not necessarily”.

P5, L118: Replace “enables” with “provides”.

P5, L122: It should be stated that the threshold value applies to the ozone VMR.

P5, L125: “The background level of ozone in the boundary layer is normally around 40 ppb”.

P5, L126: This sentence can be deleted since the application of the threshold value is already explained at the beginning of the paragraph.

P7, L136: Please explain the acronym/abbreviation “AWIPEV”

P7, L140: “sun light” -> “sunlight”

P8, L194: Replace “have” with “achieve”

Section 2.7: “Time point” is not a correct English term, I suppose you mean point in time or time of measurement.

P9, L220: “where” -> “when”

P21, L439: “the same pattern” -> “similar patterns”

P21, L452: “extend” -> “extent”

References

Benavent, N., Mahajan, A. S., Li, Q., Cuevas, C. A., Schmale, J., Angot, H., Jokinen, T., Quéléver, L. L. J., Blechschmidt, A.-M., 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, Nature Geoscience, 15, 770–773, https://doi.org/10.1038/s41561-022-01018-w, 2022.

The paper investigates the importance of polar cyclones and associated meteorological

condition on ozone depleting events (ODEs) observed in the proximity of Ny- ̊Alesund.

The authors apply complementary datasets of both ozone (O3) and BrO (e.g. in situ

observations and various remote sensing products) and combine these with Lagrangian

(FLEXPART) and Eulerian (WRF) modeling techniques. Though, no Chemistry Trans-

port Model (CTM) or process modeling where applied in this study. Overall, the paper

is

• well-structured,

• comprehensible in its application of the composite data analysis.

The language is

• comprehensive

• and for most parts only minor corrections may apply.

Please, see attachment for further details.