Summary

In their work on vertical profiles Bromine Monoxide (BrO) in the Arctic, the authors nicely motivate their investigation, put it in context of ongoing scientific discussion and – aside from presenting an excellent data set – clearly point out the scientific novelties of their work: By adapting the new flight pattern of “porpoising” to AMAX-DOAS measurements and performing the radiative transfer simulations on a finer grid, the vertical resolution of BrO profiles is improved. The higher resolved profiles are categorized into four clusters with each being investigated on chemical and meteorological effects. While the finding of high BrO concentrations close to the surface is often reported in literature, the authors used the higher vertical resolution to identify layers of increased BrO just above the surface layer. In general, the presented paper is of “outstanding” quality. In the following suggested minor revisions and technical corrections are listed:

Minor revisions

Line 238: The authors calculate box air mass factors for 4 forward viewing angles. However, these angles are not constant and depend on variations of the pitch angle on short time scales. This becomes visible in Figure S2 where the BrO DSCD peaks during the ascent when the flight altitude becomes less steep (just before 16:24), i.e. the pitch angle is smaller. The authors should include a small discussion on the variation of the pitch angle and how it can affect the retrieved BrO profiles.

Line 468: As the lofted BrO cluster is “clearly a large-scale event”, I wonder if it could be compared to satellite retrievals. As the authors speak about this work being the link between ground based and satellite-borne measurements, a small section on satellite comparison for this exceptional case on March 19^th would further prove the arguments made in this study.

Technical corrections

Line 31: “should be used as prior profiles” – “should be used as a priori profiles”

Line 130: “the Purdue ALAR aircraft and a University of Wyoming King-Air aircraft” – Maybe swap the description of both aircrafts to get a nice transition to the next sentence.

Line 144/145 Suggestion to rephrase: “NOx emissions in the area were dominated by two specific facilities during the campaign as observed with the HAIDI nadir spectrometer, and these facilities were located very close to each other (<1 km). For the purposes of this work, Prudhoe Bay will be shown as a point source centred between these two facilities.”

to

“NOx emissions in the area were dominated by two specific facilities during the campaign as observed with the HAIDI nadir spectrometer. As these facilities are in close proximity with less than 1km apart, Prudhoe Bay will be shown as a single point source centred between these facilities throughout this study.”

Line 145: “two specific facilities” – Is there a reason as to why the name of the facilities is not mentioned here?

Line 154: “field of view of 2.8°” – Is this the FWHM or was this value calculated from the optical properties of the lens?

Line 190/191: “since dSCDs are relative and SCDs depend both on stratospheric trace gas concentrations as well as solar/measurement geometry, which is observation-dependent.” – I don’t think this is a fair comparison of a column vs. a height resolved quantity. Also, the later introduced lower troposphere vertical column density (LT-VCD) has the same advantages as the mixing ratios. I don’t think the use of mixing ratios needs to be motivated here as it is a height resolved quantity and thereby conveys more information than a column quantity like SCD or LT-VCD.

Line 212: “function of particle extinction” – As measurements were conducted in a cloud-free atmosphere, I'd specify this to "a function of aerosol particle extinction". This sentence could be rephrased to “The observed O4 dSCDs are reliant on the vertical profile of O4 concentration and how light travels through the atmosphere. In a cloud free atmosphere this is mainly a function of aerosol particle extinction, so these dSCD observations can be used to retrieve the vertical profile of the aerosol extinction coefficient.”

Line 217: Why do you need to run the radiative transfer model with/without O4 absorption? Should this not be “with and without aerosol particles”?

Line 219: “200 observations” – How many observations were there in total?

Line 220/221: Maybe it’s helpful to include the information that on clear-sky days, a temperature inversion causes a stable layered atmosphere where the particle extinction profile is rather constant.

Figure 2 description: Did the authors perform a sensitivity test on how the results depend on different apriori assumptions? Since it is shown later that the BrO profile correlates with enhanced aerosol extinction, I wonder if there could be an auto-correlation for cases with elevated aerosol layers.

Figure 4: If the y-axis ticks would be at 0, 50, 100, 150, … it would be easier to identify which data points are mentioned in the text. If the result of the profile retrieval is a box-profile, maybe the box-whisker could be combined with a box-profile depiction, further enhancing the understanding of this plot. See sketched example below (green line added)

Figure 5: Is it possible to introduce LT-VCD and f200 before this figure appears?

Line 336: “from those surfaces” – “on those surfaces”

Line 370 to 374: A small table of plot depicting the distribution of values would help to follow the argumentation in these two sentences.

Figure 10: Maybe it is helpful to add the number of cases in each profile cluster – while these numbers should be clear at this point it is quite helpful for understanding the significance of each cluster.

Line 384: The authors should add how to identify the “two most stable potential temperature profiles” to make this sentence easier to understand.

Line 405: I don't find this part convincing - the lofted BrO cluster is difficult to compare here as it might just show completely different airmasses near ground and above. The Ozone could also just be depleted by other processes or on the particles itself (e.g. by speeding up the sink-term reactions) and thus not be linked to bromine chemistry.

Brockway and coauthors present a large set of AMAX-DOAS profiles of BrO leveraging a “porpoising” flight pattern to extensively sample the surface to 1 km altitude with high resolution. BrO profiles are divided into four clusters highlighting shallow near-surface BrO enhancements, more mixed boundary layers, and lofted layers of BrO associated with particles. There is a thorough and high-quality discussion of meteorological and chemical factors and the implications for satellite retrievals and modeling of Arctic BrO.  The main text and supplement together indicate that the underlying BrO are likely sound, however, a more quantitative approach to uncertainty is needed to contextualize the underlying data.

I have classified this as a major revision because it is necessary to ground the underlying data, and it is not yet clear that all key points are supported.

Major revision:

• More quantitative information is needed to demonstrate the validity of some retrieval choices taken by the authors. These are potential sources of uncertainty and should at least be bounded.

1. As the authors acknowledge in the supplement that the DOAS fitting of HCHO and BrO can have a tight anti-correlation arising from the similarity of their cross-sections (e.g. (Pinardi et al., 2013)). The authors note that HCHO is not significantly detected, and thereby infer that it can be omitted from the fits. However, this inference is not necessarily valid, without further information it is possible that the high BrO signal is limiting the ability to retrieve large HCHO columns. The critical criterion is the impact of including HCHO on the BrO dSCDs. The authors should report this and compare it to the fit uncertainty and/or the BrO columns themselves to bound the impact of this choice.
2. The authors choose to use a single constant stratospheric BrO profile for each flight. The authors note that flights are near local noon when stratospheric BrO is roughly constant. However, the springtime Arctic is expected to have high SZA (I estimate ~70° for most data from the information given but possibly more near takeoff) and therefore there might be a strong leverage on this. The authors should bound the possible impact of this choice or else provide more detail in the supplement about whether high SZA data were filtered, or what the range of SZA sampled to support their assertion.
3. The authors have employed a random sampling of the flight to retrieve a single aerosol profile for each flight. Statistically, this approach minimizes the bias of the average of the random sample. Some information to assess the success and/or validity of this approach for individual profiles is needed. Could the authors provide statistics or even better an example graph comparing measured and modeled O₄ dSCDs? As above, can the authors provide an estimate of the uncertainty arising from imperfect aerosol retrieval, or is this already propagated in the BrO optimal estimation? If it is the latter that is not clear. The recently published O₄ cross section (Finkenzeller and Volkamer, 2022) has revised the bands included for the fits here compared to the prior cross-section used (Thalman and Volkamer, 2013) would this impact the results?

• Uncertainty needs to be addressed when examining differences and variability. How does the uncertainty of individual profiles compare to the variability in the clusters, e.g. in Figs. 5 and 7? In the current manuscript, there is not a clear demonstration that the increase of BrO with altitude in cluster-4 data is significant – the best case being a comparison of 50-100 m to 200-250 m in Fig. 7. If there is not a significant increase, then what is the meaningful difference between cluster 3 and cluster 4? The variability of the retrieved profiles can obscure significant differences, but some assessment of the significance of the underlying profiles is needed. How much of the increase in BrO in lofted layers arises from a decrease in light path from aerosol, and how much is driven by constant or increasing BrO dSCDs? The significance of this result especially needs more context and support.

Minor revisions:

Line 246-248. Grid resolution and DoF do not interact linearly in this manner. The resolution can be partly inferred from the off-diagonal terms in averaging kernels which from the example in Fig. 3 show the resolution is roughly equally valid for the full altitude range. From the examples provided in kernels which peak at lower values are also broader and that the slight loss of resolution is in the middle of the profile. This is roughly as expected for porpoising maneuvers with a rigid telescope which will vary pitch away from horizontal in the middle of the profile.

If this is the basis of capping Fig. 4 and other figures at 850 m, they should be extended if there are sufficient statistics at higher altitudes.

Line 254 -257: It appears inconsistent that inhibited vertical mixing can explain high concentration below 50 m, but consistent mixing is invoked to explain near constant mixing ratios in the next 150 m. Can other hypotheses be offered, or else some discussion of variability in vertical mixing? From the cluster analysis it appears that this is a consistent feature of all clusters so I would suggest formulating a different hypothesis. Is it an effect from the 23% of data in clusters 3 and 4, or is it a result of an active source? This is partly addressed in Sects. 4.2 and 4.5, but the description should be consistent in different parts of the text. As discussed in Sect. 4.5 discussion is somewhat limited by filtering data from the lowest altitudes. Some more discussion of cluster 3 might be useful to addressing this.

Line 258: Above 250 m, the near-zero BrO omits some potentially important detail. Most of the retrieved DoF are above this altitude. How do the medians and ranges compare to the a priori?

Line 414: A recent publication (Wales et al., 2023) examines satellite and model surface BrO in the Arctic. Do the authors believe the latest findings address the limitations they outline?

Sect. 4.8: Some discussion of deeper AMAX-DOAS profiles such as those in (Volkamer et al., 2015) and (Koenig et al., 2017) is warranted. While it focuses on CHOCHO rather than BrO, (Volkamer et al., 2015) also includes a case study comparing AMAX-DOAS, surface MAX-DOAS (shipborne), and in situ (CE-DOAS) detection which is relevant to the discussion here.

Technical comments

Line 151: “dimer” should not be used to describe O₄ for the pressures and temperatures in Earth’s atmosphere. The other language used such as “associative collision” is more accurate, but it is best described as “collision induced absorption”.

Line 154: The mean elevation angles are not evenly spaced, as such it seems unlikely that the field of view is the same for all angles. Can the language be clarified?

Line 183-184: Does the horizontal distance estimate include the mean light path averaged over by the BrO measurement, the flight distance, or both?

Line 219: Can the authors provide more detail on the random sampling? What fraction of total measurements are the randomly selected 200? Are the 200 measurements selected independently or are later selections modified to ensure coverage?

Fig. 2: The Rayleigh extinction profiles should be shown for comparison.

Line 368: I believe e.g. should be i.e.

Fig. 11: I recommend showing ozone to zero in the middle panel.

Line 425: “prior profiles” here should be “a priori profiles”

Supplement Fig. S1: The authors invoke a cutoff effect from intercepting the surface to explain the positive O₄ optical density. How relevant is this effect over a high-albedo surface? Since BrO (and perhaps NO₂) also are typically maximum near the surface why is this effect relevant for O₄ but not BrO or NO₂?

References

Finkenzeller, H. and Volkamer, R.: O2–O2 CIA in the gas phase: Cross-section of weak bands, and continuum absorption between 297–500 nm, J. Quant. Spectrosc. Radiat. Transf., 279, 108063, https://doi.org/10.1016/J.JQSRT.2021.108063, 2022.

Koenig, T. K., Volkamer, R., Baidar, S., Dix, B., Wang, S., Anderson, D. C., Salawitch, R. J., Wales, P. A., Cuevas, C. A., Fernandez, R. P., Saiz-Lopez, A., Evans, M. J., Sherwen, T., Jacob, D. J., Schmidt, J., Kinnison, D., Lamarque, J.-F., Apel, E. C., Bresch, J. C., Campos, T., Flocke, F. M., Hall, S. R., Honomichl, S. B., Hornbrook, R., Jensen, J. B., Lueb, R., Montzka, D. D., Pan, L. L., Reeves, J. M., Schauffler, S. M., Ullmann, K., Weinheimer, A. J., Atlas, E. L., Donets, V., Navarro, M. A., Riemer, D., Blake, N. J., Chen, D., Huey, L. G., Tanner, D. J., Hanisco, T. F., and Wolfe, G. M.: BrO and Br_y profiles over the Western Pacific: Relevance of Inorganic Bromine Sources and a Br_y Minimum in the Aged Tropical Tropopause Layer, Atmos. Chem. Phys., 17, 15245–15270, https://doi.org/10.5194/acp-2017-572, 2017.

Pinardi, G., Van Roozendael, M., Abuhassan, N., Adams, C., Cede, A., Clémer, K., Fayt, C., Frieß, U., Gil, M., Herman, J., Hermans, C., Hendrick, F., Irie, H., Merlaud, A., Navarro Comas, M., Peters, E., Piters, A. J. M., Puentedura, O., Richter, A., Schönhardt, A., Shaiganfar, R., Spinei, E., Strong, K., Takashima, H., Vrekoussis, M., Wagner, T., Wittrock, F., and Yilmaz, S.: MAX-DOAS formaldehyde slant column measurements during CINDI: intercomparison and analysis improvement, Atmos. Meas. Tech., 6, 167–185, https://doi.org/10.5194/AMT-6-167-2013, 2013.

Thalman, R. and Volkamer, R.: Temperature dependent absorption cross-sections of O₂-O₂ collision pairs between 340 and 630 nm and at atmospherically relevant pressure, Phys. Chem. Chem. Phys., 15, 15371–81, https://doi.org/10.1039/c3cp50968k, 2013.

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.

Wales, P. A., Keller, C. A., Knowland, K. E., Pawson, S., Choi, S., Hendrick, F., Van Roozendael, M., Salawitch, R. J., Sulieman, R., and Swanson, W. F.: Application of Satellite-Based Detections of Arctic Bromine Explosion Events Within GEOS-Chem, J. Adv. Model. Earth Syst., 15, e2022MS003465, https://doi.org/10.1029/2022MS003465, 2023.

Review of manuscript ID ‘egusphere-2023-1284’ titled ‘Tropospheric Bromine Monoxide Vertical Profiles Retrieved Across the Alaskan Arctic in Springtime’ by Brockway et al.

This manuscript offers new DOAS-measured BrO profiles from an aircraft, making observations at various altitudes to profile profiles at a high-altitude resolution. They observe different concentrations and profiles of BrO and report a lofted BrO profile. The paper is well written with a detailed discussion on the meteorological effect, although implications on chemistry are not explored in detail. Overall, the paper adds to the current literature but needs a few details before publication:

Comments:

Line 15: ‘at the Earth’s surface’ is not necessary.

Line 20: MAX-DOAS profile retrievals do not necessarily depend on prior BrO profiles. This depends on the method used for profile retrievals. Please remove this claim from the abstract and clarify this in the text.

Key point number 4: This is not a key point from the study but a future outlook – it does not belong in the key points.

Line 66: This is mainly driven by chlorine chemistry, with some contribution from bromine chemistry.

Line 80: Also mention how climate change is leading to increased iodine chemistry impacts (Benavent et al., 2022) along with bromine and chlorine.

Line 91: Add papers (Tuckermann et al., 1997; McElroy et al., 1999; Carlson et al., 2010; Liao et al., 2011; Benavent et al., 2022; Zilker et al., 2023)

Line 93: Please cite original papers that developed profile inversions rather than a later self-cited work.

Line 96: Please cite the original work that led to the inclusion of halogens in chemistry models instead of only citing your own works, e.g. (von Glasow et al., 2002).

Line 103: ‘the same instrument is used in this study.’

Line 105-115 –the text is dedicated to the BROMEX campaign that the authors participated in, but all the subsequent studies by other groups that have increased our understanding of bromine chemistry have been ignored.

Line 140: Few flights went much south of Atqasuk, Alaska, at which point the topography started to rise, so the ground elevation was often close to sea level – not clear how the ground level is close to sea level if the topography is rising.

DOAS settings – not including HCHO is not standard due to the substantial interference between BrO and HCHO. The authors mention that it did not have any effect, but no evidence for this is provided. Please demonstrate that the exclusion of HCHO did not affect the BrO fits and present a correlation plot between HCHO and BrO through the campaign in high and low HCHO regions.

How the authors deal with short-term variations of the aircraft pitch angle is unclear. It would be nice to see some sensitivity analysis or a discussion on the effect of short-term variations of the pitch angle.

Line 183: Is the horizontal distance just the flight path or includes the light path?

It is not clear where the reference spectra were collected from – were they collected for each flight individually – how was the area with ‘low’ trace gas concentration determined? If not, what is the effect of this?

It would also be nice to see the mean vertical ozone profiles in the lower 100 m, as bromine chemistry is highly active there. The authors have the data, why not show it?

Looking at the plots, it is unclear how the 4 clusters differ. The lofted BrO profile is indeed different, but aside from that, the other profiles are not very different when considering the variation.

The low-BrO day still has 20 pptv at the surface – does the ozone profile reflect this? The ozone profiles in the supplementary text show ozone mixing ratios only above 100 m.

The authors should include a comparison with satellite observations, especially for the lofted BrO day, which looks like a widespread event.

If inhibited vertical mixing explains values close to the surface, does that mean that the surface ozone was completely depleted?

Why is there a cutoff effect for O4 but not BrO or NO2?

References

Benavent N, Mahajan AS, Li Q, Cuevas CA, Schmale J, Angot H, Jokinen T, Quéléver LLJ, Blechschmidt A, Zilker B, et al. 2022. Substantial contribution of iodine to Arctic ozone destruction. Nat Geosci 15(10): 770–773. doi: 10.1038/s41561-022-01018-w

Carlson D, Donohoue D, Platt U, Simpson WR. 2010. A low-power automated MAX-DOAS instrument for the Arctic and other remote unmanned locations. Atmos Meas Tech 3(2): 429–439. doi: 10.5194/amt-3-429-2010

von Glasow Roland, Sander R, Bott A, Crutzen PJ, Glasow R, Sander R, Bott A, Crutzen PJ. 2002. Modelling halogen chemistry in the marine boundary layer 1. Cloud-free MBL. J Geophys Res 107(D17): 4341. doi: 10.1029/2001JD000942

Liao J, Sihler H, Huey LG, Neuman J a., Tanner DJ, Frieß U, Platt U, Flocke FM, Orlando JJ, Shepson PB, et al. 2011. A comparison of Arctic BrO measurements by chemical ionization mass spectrometry and long path-differential optical absorption spectroscopy. J Geophys Res 116(D00R02): 1–14. doi: 10.1029/2010JD014788

McElroy CT, McLinden CA, McConnell JC. 1999. Evidence for bromine monoxide in the free troposphere during the Arctic polar sunrise. Nature 397(6717): 338–341. doi: 10.1038/16904

Tuckermann M, Ackermann R, Golz C, LorenzenSchmidt H, Senne T, Stutz J, Trost B, Unold W, Platt U, Lorenzen-Schmidt H. 1997. DOAS-observation of halogen radical-catalysed arctic boundary layer ozone destruction during the ARCTOC-campaigns 1995 and 1996 in Ny-Alesund, Spitsbergen. Tellus B 49B(5): 533–555.

Zilker B, Richter A, Blechschmidt A, Gathen P Von Der, Bougoudis I, Seo S, Bösch T, Burrows JP. 2023. Investigation of meteorological conditions and BrO during Ozone Depletion Events in Ny-Ålesund between 2010 and 2021. (March). doi: doi.org/10.5194/egusphere-2023-522