|Herrmann et al revised their manuscript describing WRF-Chem simulation of Arctic ODEs and comparison to observations in Utqiagvik, Alaska and Summit, Greenland. The major revision that was completed was the addition of comparison of model results with ground-based observations at Utqiagvik during the OASIS campaign. This was a major undertaking, and the authors are commended for doing this, as this addition significantly strengthened the manuscript, which now adds to the great body of literature stemming from the OASIS campaign. The added comparisons to this data and associated published literature further supports the authors’ results, elevating the impact of the results. My comments below focus on the newly added figures and text, with line numbers referring to the tracked changes version of the manuscript.|
Figure 4: In response to Reviewer #2 (top of page 14), the authors state that Figure 4 now includes observed BrO (which would be excellent), but I do not see that in either the tracked changes or manuscript file. Perhaps the figure was accidentally not updated?
The addition of Figures 7 and S5 and the associated new discussion of simulated production of BrNO2 from reaction of N2O5 with Br- during oil field influence is really interesting and a great addition to the paper. In particular, higher BrNO2 is predicted earlier during the OASIS campaign when NOx was elevated. This discussion would benefit significantly from integration of discussion of the results of Custard et al (2015, Atmos. Chem. Phys., “The NOx dependence of bromine chemistry in the Arctic atmospheric boundary layer”), who completed 0-D modeling of OASIS, examining the role of NOx in bromine chemistry and predicting BrNO2 production during the same time period as simulated in the current work.
Several places in the manuscript refer to an under-prediction of BrO over land, and it is stated that this is discussed in a later section (presumably the brief mention on page 28?). To support this, I suggest adding a sentence on Line 509 that refers to Pratt et al (2013, Nat. Geo.) and Peterson et al. (2018, ACS Earth & Space Chem), both of which report MAX-DOAS BrO observations over the tundra snowpack (up to >100 km inland). In fact, in the Utqiagvik region, Peterson et al (2018) observed higher BrO over the tundra than the FYI. Adding a sentence referring to previous measurement of BrO over inland tundra snowpack provides an explanation for the current study’s result (given the short lifetime of BrO) and will strengthen the manuscript as a result.
Line 2: In response to Reviewer #1, the authors changed “studied using the regional WRF-Chem” to “studied using the open-source software package WRF-Chem”. However, this didn’t address the reviewer’s comment. It is important for someone not familiar with WRF-Chem to understand that it is a regional model, and not another type of software.
Line 4: In response to Reviewer #1, the authors clarified elsewhere that Br2 is emitted from snow above sea ice rather than the sea ice itself, but this sentence still needs to be updated by deleting “ice and”. Also, Lines 577-578 needs to be revised as well, as it states “…oxidation of bromide by ozone directly from the sea ice.”
Line 25: I suggest changing “is most likely destroyed” to “is destroyed”.
Line 49: I suggest adding the following to this new sentence “…field-based experiments, and Wren et al. (2013, ACP) and Halfacre et al. (2019, ACP) through lab-based experiments.”
Lines 214-215: I suggest replacing Wang et al. (2019b) here with the more appropriate Halfacre et al. (2019, ACP).
Figure 7: The addition of this figure is quite valuable. However, it is very difficult to discern the black trace as currently plotted. Also, the time zone plotted needs to be stated, either in the figure or caption, in this figure and all other similar figures (Figures 2, 4, 10, 11, 16). I also suggest removing the “00:00” from the x axis labels and instead label more ticks to make it easier to interpret by having more dates labeled.
Figures 6, 8, & 9: Please provide the time zone that the vertical profile times correspond to.
Figure 9 bottom-right: If the BrO observations above 100 m are known to be inaccurate as stated in the caption, then they should not be plotted in the figure.
Lines 441-442: Reference to Moore et al. (2014, Nature) should accompany the added text here.
Line 508: Rather than citing Pratt et al. (2013) here, I suggest adding Jacobi et al. (2012, JGR, “Chemical composition of the snowpack during the OASIS spring campaign 2009 at Barrow, Alaska) since it describes the snow composition during the OASIS campaign, and Krnavek et al. (2012, Atmos. Environ., “The chemical composition of surface snow in the Arctic: Examining marine, terrestrial, and atmospheric influences”), since it compares [Br-] over tundra, FYI, and MYI, to the publications already listed. I encourage the authors to review these and other Arctic snow composition measurements studies, which refute their statement on page 18 of the response that “the assumption of no bromide content of snow on land or near coasts can be correct in many circumstances”, as this statement is not supported by measurements.
Response Page 16: Note that Wang and Pratt (2017) simply used jBr2 as a term to define the timing of radiation-dependent emission of Br2 from the snowpack as a representation of the observations by Pratt et al. (2013), which showed the Br2 was produced from Arctic snow in a chamber only upon irradiation and no addition of O3 or other gas-phase oxidant. This mechanism was replicated in the lab by Halfacre et al. (2019, ACP), who showed Br2 production upon irradiation of ice containing Br- and an OH precursor. Therefore, it is incorrect that Br2 is required for condensed-phase snowpack Br2 production.
Line 623-624: I encourage the authors to add acknowledgement of the individuals that conducted the OASIS measurements and produced the data used. These BrO measurements by CIMS and DOAS are not trivial whatsoever, and these individuals should at least be recognized here, as their data significantly contributed to the manuscript. While H. Sihler and U. Platt are listed under the author contributions, there were other individuals that conducted the CIMS measurements, in particular.