|The authors have sufficiently addressed the reviewer and editor comments; therefore, this paper should be accepted after the authors address the following issues:|
Note – line and figure numbering refers to the track changes version of the paper. There is a problem with the figure numbering in this version as the first figure given is Figure 2.
- Page 1, Line 3 – In the abstract & elsewhere in the paper. The authors seem to think the main evidence for boundary layer bromine in the polar regions is from satellites. This is not the case. The main evidence for boundary layer bromine is from in-situ and ground based remote sensing data. In addition, aircraft data in the troposphere provides direct evidence of the presence of bromine (e.g. during POLARCAT, ARCTAS). Then, satellites have been used the provide the spatial scale of such enhancements, but are relatively insensitive to boundary layer enhancements of bromine. The authors should clarify their discussion of measurements to more accurately reflect the knowledge of bromine enhancements and why they use satellite data to validate their model. I am not asking the authors to use other data in the present study, but to recognize that other data exists and should be used in a 2nd step to evaluate and further improve the model.
- Page 1, Line 19 – bromine explosion events “may” play an important role in mercury deposition. It is not 100% clear this is the case, so insert the word may here. Additional up to date references are needed for this sentence.
Page 2, Line 3 – Custard et al is not a review paper.
Page 2, Lines 17-25 – This paragraph shows the authors have not thought about the difference between the stratosphere and knowledge of chemistry on ice surfaces at temperatures relevant to conditions in the lower troposphere. Specifically:
- Ice above -40 C has a liquid like layer, which does not behave as a gas-solid reactive surface.
- Multi-year ice has not been 100% discarded as a source of bromine and/or a surface on which bromine can be recycled once activated
- It is not true that acidity is not important on ice surfaces. The study used as an example here is for HOBr uptake onto frozen NaBr/NaCl solutions. However, other studies for other surface reactions have shown a dependence on the pH of the solution before freezing (e.g. Oldridge and Abbatt, JCP A, 2011, DOI: 10.1021/jp200074u). In addition, this reference should be included at the end of line 25.
- The sentence on lines 23-25 has no clear meaning.
Page 2, Lines 29-30 – A more accurate and clear sentence is needed here.
Page 3, Line 5 – annual cycle “in” both hemispheres. Fix on -> in
Page 3, line 21 – Should cite Toyota’s paper here, since this is where the mechanism came from
Page 5 – Note that in the track changes version, the first figure is Figure 2. This should be fixed in the final version.
Page 5, Line 21 – Standard PSC chemistry is likely not sufficient for aerosols in the troposphere. This does not need to be fixed for this study, but it should be noted and mentioned as an area to work on later during the future work/conclusions/perspectives.
Page 6, Lines 26-27 – Please spell out units of the deposition velocity so there is no confusion
Page 6, Line 32 – This is one of the areas where my first comment is important, see above.
Section 3.1 – the writing/figures in this section need some work. Here are some specific suggestions:
- Move Figure 1 from the response to reviewers into the paper, this is additional info that can be discussed in relationship to the ozone time series presented.
- I find the authors are discussing the figures in the supplement more than the figures in the paper, the authors should consider moving the more detailed supplement figures with BrO VCD from EMAC and GOME into the paper so the discussion is clearer. Both the VCD maps and the anomalies (shown now) should be in the paper.
- Figure 4 (anomalies of VCD for BrO – the months should be labeled on both the right and left panels.
Conclusions – a comparison/discussion of results from other bromine activation mechanisms is needed. Specifically, a discussion of mechanism from X. Yang and its prior evaluation compared to GOME-2 is needed (e.g. Theys et al., ACP, 2011, DOI: 10.5194/acp-11-1791-2011 and prior work). Where are the two different mechanisms working best, where are they failing? Is it necessary to use both mechanisms in a future study to evaluate them further?
General comment – the authors should re-edit the paper for clarity as some of the writing is still quite unclear as to the meaning of the sentences.