In this paper, airborne measurements of upper tropospheric and lower stratospheric total bromine performed over North Atlantic, Norwegian Sea and north-western Europe regions in fall 2017 are reported. Total bromine is estimated from measured total organic bromine added to inorganic bromine derived from measured BrO and photochemical modelling. Combined with transport tracers observations, these bromine measurements suggest that total bromine enriched air masses persistently protruded into the lower stratosphere created a high total bromine region (HBrR) over the North Atlantic in fall 2017. Simulations using the CLaMS Lagrangian transport model show that the main sources of this high bromine region are former tropical upper tropospheric air transported by the Asian monsoon and hurricanes from Central America, and to a lesser extent transport across the extratropical tropopause. The impact of this total bromine content on lower stratospheric ozone is also evaluated based on simulations from the TOMCAT global 3-D model.

This paper is well written and clearly structured and presents very interesting results on the atmospheric bromine topic which fit well with the scope of ACP. I recommend the final publication of the manuscript after addressing the following specific comments:

Page 5, lines 146-147: Why only the limb viewing mode of the mini-DOAS instrument is used in this study? The nadir mode could also provide valuable information about the BrO column density above the varying altitude of the aircraft.

Page 6, lines 179-181: How is initialised the McArtim RT model for the calculation of NO2 and O3-vis reference SCDs? In a similar way as for the radiative transfer simulations of the limb measurement (see lines 186-190)? This information should be added.

Page 7, lines 201-202: Does the scaling method fully compensate for any cloud effect? Did you make any selection of the mini-DOAS limb measurements regarding the cloud conditions by removing the more problematic cloudy scenes in terms of RT modelling?

Page 9, lines 269-270: You should provide the list of Aura-MLS and ACE-FTS trace gases/tracers which are used to initialize the CLaMS simulations or at least a reference where this list can be found.

Pages 13-14, lines 400-427: the discussion is a bit difficult to follow here. Maybe the different bromine contributions (CH3Br, halons, VSLS, inorganic) corresponding to the selected typical delta_theta values (+/-5K, 78-88K) could be included in a table?

Section 4.3.1, Pages 17-19: the fractions of air values for various domains of the atmosphere are presented and discussed. Would it be possible to give an estimate of the uncertainties corresponding to these values? For instance, in the investigated high bromine region, the fraction of air from the tropical troposphere is 51.2% while the corresponding value in the lower stratosphere below and above the HBrR is 42.6%. How significant is this difference?

Page 24, line 770: Could simulations using the CLaMS model give indication about the seasonality of the bromine content in the high bromine region investigated in this study?

Technical corrections:

Page 6, line 176: ‘asent’ -> ‘ascent’

Page 10, line 292: ‘distince’ -> ‘distinct’

Rotermund et al. presented a comprehensive analysis of the contribution of VSLS-Br to stratospheric bromine budget combining recent airborne measurements and CTM simulation results.  This is definitely one of the top papers, likely the best paper, I have reviewed in the recent few years.  The paper is well written, with thorough analysis of aircraft measurements, great design of a set of helpful model simulations, and adequate discussion of the derived results in the context of previous literature on VSLS-Br. 

I recommend publication in ACP, and I only have some minor editorial comments:

1. Would it be better to say “aged air” instead of “former air”?
2. “one min time resolution”  one minute temporal resolution
3. Suggest changing to “nine distinct 3-D domains in the entire model atmosphere: the tropical troposphere, …
4. L300 & L599. Suggest change south-eastern Asia to Southeast Asia
5. Suggest change to “corresponding O3 loss”
6. Suggest replacing “off-line model” with TOMCAT model.
7. Here, you assume everyone reads this paper knows about equivalent latitude, which is likely not the case with most people.  I suggest you add a few sentences here explaining “what is equivalent latitude? What is the benefit of analysis in equivalent latitude coordinate?” for the benefits of general audience.
8. L391 & L393. I suggest you add here the potential temperature range you would define as UTLS and LS. (You did mention the range of LS in L407, but it makes more sense to move it here). It might also be helpful to add horizontal lines on Figure 4 for clear identification of these regimes. Figure 4, please make the black solid line thicker so that it is easy to see.
9. L423-424. Is it fair to conclude based on your WISE analysis of in situ measurements that Br(tot) remain approximately constant vertically, implying as altitude increases, there are little change (loss) in total Br, but rather a gradual conversion from organic Br to inorganic Br? To me, this is actually a very important message for the Bromine community.
10. I think “Impact on LMS O3” might be a better title. Consequences to me has a bit of negative tone.
11. You may consider redefine some of the acronyms here for those readers who come straight to the Conclusions section, e.g. NH, UTLS, LMS.