Comment on acp-2020-1297

Three versions of the Carbon Bond 6 chemical mechanisms (CB6r1, CB6r2 and CB6r3) are implemented in a photochemical box model (KINAL) and compared in 7 day simulations without and with emissions, i.e., 6 simulations in total. The analysis focuses on concentrations of O3, NOx (NO + NO2) and HCHO averaged over the final 24 hours of each simulation (day 7). Sensitivity analysis of concentrations [c(i)] to the individual reaction rate constants [k(j)] of each mechanism is performed by computing lognormalized sensitivity coefficients S(ij) [= d ln c(i)/d ln k(j)] for day 7 of each box model simulation. The CB6r3 mechanism is currently available in the Community Multiscale Air Quality model (CMAQ; https://www.epa.gov/cmaq) and CB6r2 is currently available in the Comprehensive Air quality Model with extensions (CAMx; www.camx.com) although CAMx also includes CB6r4. CB6r1 was used very little because CB6r1 was quickly superseded by CB6r2. Sensitivity analysis of chemical mechanisms is useful to help modelers understand how the mechanisms influence their atmospheric simulations. The sensitivity analysis will be most useful when performed for conditions that are representative of many atmosphere simulations.

2) We have calculated the expected changes to surface clear-sky UV for several prominent northern cities for March 31, 2020 and April 30, 2020. We present these changes in Table 1 and have expanded the Discussion and Conclusions section to include more consideration of these effects.

Referee #1 Comments and Response
There is a great deal of interest in the extent of ozone depletion during the recent cold Arctic winter of 2020, with a number of papers appearing, especially in a JGR/GRL special issue. This paper adds to that work mainly by investigating how much worse the depletion would have been without the controls of the Montreal Protocol. This is done through a specified-dynamics 3-D model run with a scenario that assumes no protocol. Secondary aspects of the paper are presentation of Arctic ozone sonde observations (including 2020) and an investigation of different model denitrification schemes. I find that that paper has some interesting results would could make it suitable for publication in ACP. However, I do find that further analysis is needed in order to back-up some of the conclusions, along with clearer organization of the main points. I give my comments below.
General Comments 1) One main message of the paper is that the Arctic ozone depletion of 2020 would have been much worse without the Montreal Protocol. That is without doubt. However, it is not possible to know exactly how demand for CFCs and similar gases would have evolved. The 3.5%/year growth since 1985 is an assumption and that should be made clear. We agree that the exact level of ODS growth chosen is only one hypothetical pathway. We now make this fact clear and explain how we arrived at the 3.5% value in Lines 75-78: "The assumption of 3.5% per year growth matches that used in the Garcia et al. (2012) World Avoided study and is a good approximation of the grown rates seen in years immediately prior to emissions controls, thus representing an illustrative "business as usual" alternate trajectory." 2) It seems like more use can be made of the available sonde data for evaluation of the Real World run. There are ozone plots with model only (5, S6a), and plots with data only (2a, S3). Better use of the data could be made for evaluating the model.
We agree that more usage of sonde and other observational data strengthens the paper. We have added ozonesonde profiles to Figure 5, and now include a new figure (Figure 4) showing the direct comparison of different denitrification parameterizations to both ozonesonde data and MLS HNO3 profile data, which is described further in our response to General Comment (3). We have also added a new supplementary figure ( Figure S4) showing the comparison of the model to sonde data taken at Alert, Eureka, and Resolute for three days in the spring each to provide a more comprehensive review of how the new parameterization performs. The explanation of these new plots is presented in the Results section, Lines 207-225.
3) One message from the abstract is that the large Arctic ozone depletion of 2020 can be used to test the parameterization of PSC denitrification. Here the results presented do not go into enough detail. Maps of HNO3 are shown for 70 hPa. A number of questions come to mind. How well do the simulations do at other altitudes? How would this affect other winters? I realise that this are major questions but normally a study which aims to present an improved denitrification model would be based on more than just one altitude in one winter. Also, the ultimate choice of the denitrification scheme is based on the impact on ozone which is not shown. This is an indirect test and we need to see how large the sensitivity of ozone is.
We agree with the Reviewer that further explanation is needed to justify our choice of denitrification scheme. To that end, we have added in a figure ( Specific Comments 5) I think that the abstract needs a lot of work. It is short compared to what is possible in ACP and it lacks some details. Also, it seems to jump around in the topics covered. The abstract mentions 'record observed local lows'. It is not said where these numbers are from and if this paper is presenting the 2020 sonde data for the first time. The abstract also says that 'This provides an opportunity to test...' without stating how the parameterizations are being tested and what the results are. It is also not clear that it is the RW run which is used for the testing; the abstract makes it sound like the WA run allows the testing. After this, the abstract returns to the WA run so the summary of that is split. Overall, I think that the abstract needs a more logical flow to cover the results and more information to summarise what was found.
We have reformatted the abstract by adding more details about the model and instrument comparisons, expanding the summary of our parameterization changes, and grouping more of the WA results together. We agree that this improves the logic and flow of the abstract and gives the reader a better summary of what we present in our paper.

6) Line 23. Farman et al.
This has been corrected. 7) Line 24. 'PSCs were the primary culprit'. This is not really true. PSCs are an essential part of the chain of events which contains a few steps. CFCs (and other chlorine gases) could be seen as a culprit and the one that humans can control. Alternatively, PSCs could be described as the key step that was not understood and why the ozone hole was not predicted etc. 8) Line 39-40. 'near-complete recovery'. What does this mean? We don't expect a smooth return to e.g 1980 ozone levels everywhere. Models suggest the tropics may not get to those levels before column ozone starts to decrease due to circulation changes.

We have rephrased this and linked PSCs more explicitly to denitrification as follows in
The WMO 2018 Scientific Assessment of Ozone Depletion predicts a worldwide return to near 1980s values for ozone as a result of the Montreal Protocol specifically limiting ODSs. We recognize that there are other anthropogenic factors which could alter the background state in a different manner, but as these are highly dependent upon selected emissions trajectories and uncertain future policy decisions, we are restricting our analysis to the success of the Montreal Protocol itself. We have added a clarification to our statement of nearcomplete recovery, as well as the WMO 2018 reference, to Lines 48-50 that we believe more explicitly places it within this context: "the world appears to be on track for near-complete ozone recovery to near 1980s values as a result of decreasing ODSs by the second half of the 21st century (WMO 2018), and the Protocol has been ratified by every state represented at the United Nations" 9) Lines 68-69. You should make it clear that this scenario of 3.5% year growth since 1985 is an assumption and state why you choose these parameters.
We have clarified and expanded upon this choice as explained in our response to General Comment 1, with an emphasis on the fact that this is a scenario assumption only.
10) Line 87. Why do you use the smallest value for the WA runs? Please explain.
We agree that adding physical justification for our NAT parameter choice is necessary, and now include a sentence to that effect in Lines 204-205: "As a lower NAT particle density corresponds to larger individual particles, decreasing this parameter increases denitrification by increasing the settling velocity of the particles." Additionally, we have clarified that we chose the smallest value for both the final RW and WA runs based upon our testing of multiple NAT parameter choices. We now better differentiate between the prior value in SD-WACCM and our chosen value in both the Methods and Results sections, as explained in the response to General Comment 2. 11) Line 104. Waters et al seems to be a general MLS reference. Please also give one for the specific HNO3 product.
We have added a reference to Lambert et al., 2007, which is an HNO3-specific validation study.

12) Line 112. Spell out WOUDC.
This change has been made.
13) Lines 113. Give the location (latitudes at least) of all the stations here.
We have added locations of all stations as (latitude, longitude) in the text, and added the coordinates of the stations used in Figure 6 in the caption of that figure.
14) Lines 118-119. The paragraph ends on a confusing not because it is stated that Eureka is used for profile comparisons, but that leaves the reader wondering what the other sonde data is used for (time series at 50 mb, as it turns out).
We now specify in Line 129 that other station data is used for the time series at 50 mb so that the reader has this information earlier.
15) Lines 133-134. Here figure for the size of the 2011 Arctic ozone 'hole' is given -11 million km2. This is still significant (see statement on what is an ozone hole on line 59) but I think that the authors point is that it is smaller than that which has occurred in the Antarctic since the mid 1980s when the term 'ozone hole' was first used? These points need to be made clear here in the results section. This also relates to the title of the paper, which presents 2020 as the first time that historic meteorology would have led to an 'ozone hole'. A big factor in this change since 2011 is clearly the assumed increasing chlorine, but also the meteorology of 2020 had, I believe, anomalously low dynamical ozone replenishment. Can you comment on the importance of these factors? There are at least references for the low absolute column ozone in 2020 in the JGR/GRL special issues.
We now comment on the dynamical differences between 2011 and 2020, specifically the fact that 2020's polar vortex lasted longer than in 2011, with new references to two recent studies that compared these in great detail, as shown in Lines 147-150: "The difference is partly due to the increased chlorine loading in the WA, but we also note that,

17) Line 157. Caption for (b) should make it clear only model O3 is plotted.
We are plotting both the model ozone profile (dotted teal line) and the Eureka station ozonesonde ozone profile (solid teal line), along with the ozonesonde temperature profile (red line). We now reference each line explicitly in both the caption and the text to avoid confusion.
18) Line 158-159. Confusing as worded. The sonde, which shows the record low O3 values, is at Eureka, and then compared to the nearest model profile.
We have reworded this figure description and now state which line color corresponds to which profile, which we believe improves the readability of the figure.

19) Line 167. Better to say 'instrument' not 'satellite'.
This has been corrected.

We have reworded this caption to make what is being shown clearer. Specifically, we have corrected the dates, and now refer to all years with the term "markers" rather than "points," which we believe was needlessly confusing, given that some of the markers have the form of dots/points. We now explicitly differentiate what the open circles vs dots show and have changed the legend as suggested. We have also redone this panel with larger markers, which we believe improves the visual distinction of the different time series.
24) Line 180. 'bottom row, middle panel'. There is only 1 panel? I would suggest using '(e)' in any case.

This has been corrected, as detailed above in response to (20) and (22).
26) Line 182-183. There is a major leap in the argument here. Figure 3 is comparing model v MLS HNO3 to test the denitrification, which impacts HNO3 directly. Then, the argument jumps to using the O3 profile to test the model. There are many other proceses which would affect the model ozone profile (chemistry, dynamics) and so there needs to be much better justification for this.
As we describe in our response to General Comment 3, we agree and have added new plots (Figures 4 and S4) and expanded the denitrification discussion to address this point. 27) Line 184. 'insofar as they can be determined from reanalysis'. Better to say 'insofar as they are represented by the reanalyses'. This has been corrected.
29) Line 188-190. This is a long sentence and difficult to read. It is not clear when it mentions 'for the past decade ... compared to the preceding years'. Please revise.
We agree that this sentence needed clarification. The description of Figure 5 (previously Figure 4) has been reworked to present the important information more clearly, as detailed further in the response to Comment 30 below.

teal markers) to observations from the OMI satellite (blue markers) and compare both with the WA (orange markers)."
Furthermore, we have rephrased the discussion about scatter, as it was initially meant to refer to the fact that pre-2020 WA points often were within the typical range of TCO seen in the RW and OMI measurements, with the additional chlorine not driving significant additional depletion. We believe the reworked text (Lines 226-230) now makes this clearer: "Prior to 2020, while the WA case is often lower than the other two, it is still within the range of TCO values seen in the RW and OMI time series. Furthermore, both the RW run and the OMI observations for 2020 spring display lower values than many WA springs, illustrating the key role of the unusually cold temperatures in addition to chlorine in driving the depletion in 2020. The WA spring of 2020 both displays levels of depletion previously unseen in the data or either simulation early in the spring and stays depleted longer than any other year." 31) Line 194. Figure 5 caption. The bold/non-bold text is reversed compared to other figures. The caption should state 'ozone' somewhere. Give the latitudes of the stations so that it is clear Eureka is north and Syowa is south. These are sonde stations so please show the data for comparison (or choose a close day when there is data).
We have fixed the caption text formatting and added ozonesonde data from the stations themselves to these plots, along with the locations of the station. In doing so, we had to change the day shown for the Antarctic from October 1, 2018 to October 7, 2018 due to data availability. 32) Line 196. I don't think you can use 'record' for a hypothetical model run? A larger assumed use of CFCs would give even lower O3! We have changed the phrasing for this sentence to clarify the context in which the WA depletion takes place and what we are comparing against in Lines 229-230: "The WA spring of 2020 both displays levels of depletion previously unseen in the data or either simulation early in the spring and stays depleted longer than any other year." 33) Figure S1. Please give a brief explanation of the 'total equivalent effective chlorine'. I assume this is the tropospheric equivalent chlorine loading? State the alpha factor used for the calculation of equivalent chlorine. What does 'effective' mean in this context?
We are showing the stratospheric equivalent chlorine loading, which is calculated as a linear combination of Cly and Bry, with Bry scaled by a factor of 60 in the midlatitudes and 65 in the polar regions. We have updated the figure caption of Figure S1 with this explanation.

Referee #2 Comments and Responses
The paper by Wilka et al. examines the significant Arctic ozone depletion that occurred during spring 2020 using the Specified Dynamics version of WACCM and observations from various satellite and ground based platforms. The study compares a simulation forced with observed "real world" boundary conditions of the major ozone depleting substances (ODS), with that forced with a "world avoided" scenario in which ODSs increase 3.5%/year after 1985. The paper demonstrates that under the extreme meteorological conditions of spring 2020, significantly greater Arctic ozone depletion would have occurred were it not for the Montreal Protocol. The paper also performs several sensitivity simulations to assess the model denitrification compared with MLS data, and refines the assumptions of the model NAT aerosol density. This is a well written paper that presents some important results concerning the stratospheric ozone impacts as a result of the Montreal Protocol. It follows other recent "world avoided" studies, focusing on the very cold Arctic conditions during spring 2020. I found the analysis to be mostly clear and the figures well presented. I have only a few minor comments and a suggestion (listed below) which should be mostly straightforward to address, but otherwise recommend publication of the manuscript.
Comments/corrections: 1) Figure 2a: This is a nice figure, but please try to enlarge the map in the lower left corner, or at least make the lettering significantly larger. The location names are unreadable as is (at least in the version I have). Figure 2a has been redone with darker lines, larger markers, and no text labels. We believe this makes reference to the markers in the legend of the whole figure easier, as the lettering is no longer competing with the geographic lines in the map.

The map in
2) L91-92: "... assumes a 3.5% per year increase ..." -Please provide a little more explanation on how this increase was determined.
We agree that this assumption needs further justification and have added the following explanation in Lines 75-78: "The assumption of 3.5% per year growth matches that used in the Garcia et al.
(2012) World Avoided study and is a good approximation of the grown rates seen in years immediately prior to emissions controls, thus representing an illustrative "business as usual" alternate history." 3) L162: "Although temperature histories can also be important ...." -"temperature histories" should be briefly defined/explained. I assume the authors are referring to how much the back trajectories of parcels encounter (or not) temperatures cold enough for PSC formation and ozone loss.
We agree that this term should be defined. We have elaborated on this point and reorganized the text as follows in Lines 182-187: "The figure shows that the largest depletion here tracks the lowest local temperatures of the profile (temperature shown in solid red). Although temperature histories can also be important, as activation can persist in air parcels which previously encountered cold air but are currently above the temperature threshold for PSC formation, this broadly supports the view that much of this year's ozone loss was related to widespread local cold temperatures This has been corrected. Figure 3 shows the progression of six increasingly denitrified ...." -only four model panels (a-d) are shown (not six). This is also referred to on L181-182.

5) L175: "
These references have been corrected and we have reworked both the explanation of Figure 3 and its figure caption to make it clearer which panel is being referred to when. 6) L188: "dailyminimum" -separate into two words.
This has been corrected.