Please see my comments in the attached file.

Authors: Ewa M. Bednarz, Ryan Hossaini, Martyn P. Chipperfield

Title: Atmospheric impacts of chlorinated very short-lived substances over the recent past – Part 2: Impact on ozone

Scientific Significance: The scientific questions addressed in this paper are appropriate for ACP. This work addresses for the first time the recent (2010-2019) impact of chlorinated Very Short-Lived Substances (VSLS) on ozone.  The authors state they found “modest and non-negligible role of Cl-VSLS” to the stratospheric ozone budget. They also emphasized that continued Cl-VSLS emissions “could offset some gains by the Montreal Protocol”. They were the second group to “estimated” the ODP of dichloromethane. I highly recommend this work for publication. Below are a few comments that may add to the impact of this work.

Scientific Quality: This work is of high scientific quality. The authors use a state-of-the-art CCM (UM-UKCA) nudged to ERA-Interim and ERA-5 meteorology. They also show results from an ensemble mean from the UM-UKCA CCM. The Cl-VSLS chemistry is also represented in a detailed manner.

Presentation Quality: Generally, in good shape. However, I would increase the font size of Figures 3 and 4.

Specific Comment.

Abstract, line 16. Typo “for the first time. Using the”

Abstract, lines 19. I found it confusing in the abstract when the authors highlight the 2011, 2014, and 2020 years and state that up to 5-6DU monthly and zonal mean Arctic ozone reductions are simulated. Then in line 20 they highlight year 2020 with “~6DU ozone in total by the end of March”. They state 2020 was a recent cold winter. I would suggest reworking sentences here being more specific why you picked 2011 and 2014 relative to 2020?

Abstract, lines 21-23. The authors state that Cl-VSLS “do not considerably modify the magnitude of the recent ozone trends”. Please be more specific, is this is tropical, polar, global, everywhere, etc? Also, why would one expect the trend to be significant over a short period (i.e., 2010-2019), especially in the Arctic? I would suggest adding more detail in the abstract if you want to mention ozone trend results.

Abstract, line 22. The ODP of Cl-VSLS is quantified. The paper mentioned this was the second ODP derivation. What do you mean that it was “estimated” – is that a typical way to discuss the derivation of an ODP? This topic deserves a couple sentences to clarify why you feel it is important to put this discussion in the abstract. See my comment on lines146-147 below.

Lines 46-49. “We showed that the contribution from these Cl-VSLS to stratospheric chlorine had increased from 70 ppt Cl in 2000 to 130 ppt Cl in 2019, i.e. almost doubling over the first two decades of the 21st century.” One could make an argument that this information was taken from Bednarz et al., 2022, Part 1 – but it would be nice have this information brought to the abstract level when summarizing the trend results.

Lines 53-54. In addition to Chipperfield et al. 2018, Wargan et al., 2018, and Orbe et al., 2020, Stone et al. also came to this conclusion (that dynamical variability is driving the O₃ trend) using a chemistry climate model similar to UM-UKCA. Stone, K. A., Solomon, S., & Kinnison, D. E. (2018). On the identification of ozone recovery. Geophysical Research Letters, 45. https://doi.org/10.1029/2018GL077955.

Line 85-86. “Furthermore, no significant Cl- VSLS-induced Arctic ozone loss can be diagnosed from the model ERA-Interim nudged monthly and zonal mean data for the spring 2011; this might be related to the small size of the polar vortex in that year and thus difficulties in reproducing its dynamical properties in a nudged model setup.” This sentence is a bit disconcerting in that the reader is meant to figure why there are difference in the choice of reanalysis products. The main question in my mind is why even show ERA-Interim in this study? Presumably ERA-5 is the best ECMWF product to look at nudged ozone trends?

Figure 3 and 4. Please increase the font size of the titles and x-axis.

Lines 114-118. This is a very interesting discussion, i.e., “the impact of curbing emissions of long-lived ODSs achieved by the Montreal Protocol was estimated to reduce the magnitude of the Arctic ozone depletion in that spring by up to ~35 DU in mid-March compared to peak halogen levels in early 2000 (Feng et al., 2021).” It might be useful to bring this comparison of curbing the emissions of long-lived ODSs achieved by the Montreal Protocol up to the abstract level (i.e., versus 6 DU from Cl-VSLS)?

Lines 146-147. “The calculated stratospheric ODP of 0.0102 (confidence interval of 0.0062-0.0163) is similar to the whole atmosphere ODP metric, implying that CH₂Cl₂ has a relatively small effect on ozone below the tropopause in UM-UKCA.” This is an interesting result. Is there anything more you can say about this result? Is this due to where CH₂Cl₂ is emitted (e.g., China)?