This paper discussing CH2Cl2 measurements during a recent 2023 field campaign was well-written and informative. It is a great fit for ACP and should be published with only a few minor corrections. This reviewer appreciated seeing measurements supporting recent research that dichloromethane from ASM can travel further in the UTLS to other parts of the globe. This reviewer only has a few minor corrections/comments below.

Figures:

Figure 1: The 1:1 line is yellow not red in the Figure.

Overall, figures with light colored axes are hard to read. An example is the light blue for Figure S5, perhaps a slightly darker shade of the color at least for the axes and label.

In the supplement could you add figures showing the CH2Cl2 as a function of altitude or POT? It would be great to see the data visualized in this way as well to help readers comparing to recent literature work. 

Body of Text:

Line 67: It would be nice either here or the supplement to list the altitude or POT range covered by the flights.

Line 83: Would be nice to have the instrument names spelled out the first time it is used for the reader to know.

Line 321: 2023 and not 2024

Section 3.4: This analysis is really interesting. It would be great to see a rough estimate of the CH2Cl2 chlorine injection for PVU>4 to bring it back to the ozone assessment information.

Summary: This paper applies transport modeling to estimate the origin and future destination of airborne measurements taken in the subarctic upper troposphere during the 2023 PHILEAS campaign.  The analysis focuses on three flight segments where dichloromethane (CH2Cl2) is greatly enhanced above background levels.  Backward trajectory experiments are used to estimate the locations and transit times since each flight segment’s last PBL contact, as well as to explore the dynamical conditions over Asia which were responsible for the transport of the sampled air mass to the subarctic upper troposphere.  Forward trajectory experiments are used to estimate the impact of the observed CH2Cl2 enhancements on the composition of the lowermost stratosphere.  It is found that the analyzed CH2Cl2 enhancements were associated with convective transport over east and south Asia, and there is a minor contribution of the enhanced CH2Cl2 events to the stratosphere.    

Overall Thoughts: This paper is a nice presentation of results from the 2023 PHILEAS campaign, and will add valuable context to the literature for the downstream fate of ozone-depleting substances lofted by Asian summer monsoon convection.  The paper is well-written overall and the analysis is sound.  However, I have some concerns about the organizational flow and tone of the analysis that should be addressed before the paper is ready for publication. 

Recommendation: Major Revision

General Remarks:

• I think a little more work can be done to justify and explain the analysis road map throughout the paper. There is one remark in the introduction about how specific events will be analyzed, but it isn’t clear until lines 204-205 that the majority of this paper will only discuss three flight segments from the entire campaign.  Moreover, the analysis of Figure 2 reveals four flights of interest, but there doesn’t appear to be any justification for the subsequent elimination of two of them (F09 and F16) from further consideration.  I recommend providing a little more early clarity about the scope of this work, as well as further justification that the chosen three flight segments are enough to serve the overall goals of the study.
• I am not totally satisfied with the justifications provided for the relatively minor contribution of dichloromethane enhancements to the lowermost stratosphere. The authors clarify that the study spans a short time period, and that there could be a higher contribution after 12 days. However it is clear from Table 1 that most of the particles that reach 2-4 PVU do not eventually cross the 4 PVU threshold, and I would only expect the likelihood of that to decrease with additional time.  The fact that the sampling was primarily below the ASMA (at 330-350K, as stated in lines 385-386) could be a simple way to justify this – it’s not that the ASM doesn’t have impacts on stratospheric composition, it’s just that they are not as pronounced for air masses that reach the subarctic upper troposphere.  With all this in mind, I suggest changing the tone of the discussion and conclusion (including the final two sentences of the abstract) to emphasize that the enhanced levels of dichloromethane observed during these specific segments had only a minor impact on the composition of the lower stratosphere based on the applied modeling approach.
• The description about the trajectory experiment configuration is a little unclear to me. Are the trajectories released in a “rectangular prism” shape with dimensions equal to the total latitude, longitude, and altitude spanning by the aircraft during that 5-minute flight segment?  Or are they simply released at the exact location of the flight track?  If the former, what are the dimensions of the “rectangular prism” in number of initialized trajectories?
• In the opening paragraphs of Section 3.3, there are several latitude and longitude ranges that are printed in the text for the PBL contacts. I don’t think this is particularly insightful without being able to visualize it on a map as given later in Figures 6-8.  I will also add that the first sub-section (3.3.1) has “location” in the title, although some location analysis has already been discussed above it.  I would consider streamlining the location discussion in the text so the map figures can be introduced at the same time. 
• There are several spots where I believe the word “observed” is misused, in the context of transport or PBL contacts. These processes were not observed by the HALO aircraft, they were simulated by FLEXPART.  I suggest going through and changing these instances to be more accurate.  I found examples of this on lines 220 and 257, though there may be others.

Technical Remarks and Typos:

• Both “extratropical” and “subarctic” are used in the paper to describe the region that was sampled during PHILEAS. In the abstract (lines 6-7), consecutive sentences use different terms.  It might be worth standardizing this term throughout. 
• Line 9: Change “Asia” to “Asian”
• Line 14-15: The parenthetical remark seems out of place given general statements are being made.
• Line 37: I suggest saying the ASMA “confines pollutants”. The way the sentence is laid out, “transport barrier” might be harder to visualize for an unfamiliar reader.
• Lines 50 and 55: Would it be better to list the long names for ACCLIP and PHILEAS here rather than waiting for the next section?
• Section 2.1: The first paragraph is one long sentence. I suggest this section’s text just be made into a single paragraph. 
• Line 63: Is it appropriate to define the HALO acronym?
• Line 78: “temporarily” instead of “temporally”?
• Line 83: Should the instrument acronyms be defined?
• Line 95: “As for the GhOST” seems out of place
• Line 174: “Major” and “elevated” seem redundant in the section title
• Lines 193-196: This technical description about the figure might be better in the caption so it doesn’t distract from the analysis.
• Line 214: What about “The origin of elevated CH2Cl2 events” for this section title?
• Line 232: I suggest “within the prior 12 days” instead of “in the respective 5 minute intervals”
• The first two paragraphs of Section 3.3.2 appear to be mostly technical in nature. I recommend moving all the relevant details back into Section 2 so that Section 3.3.2 can open with science. 
• Throughout section 3.4, the term “regions” is used to describe different PVU thresholds. I would suggest using “layers” instead to emphasize that these are vertical ranges, whereas I feel “regions” sounds horizontal. 
• Line 337: add “with time” after the word “portion”
• Figure 1 caption: the 1-to-1 line is yellow, not red.
• Figures 6-8: The text discussions describe transport time in days, but the figure colorbars show transport time in hours. For continuity, I suggest remaking these figures with a unit of days.
• Figures 9 and 10: the geopotential height labels are too small to read (if important), and the word “geopotential” appears to be missing from the labeling between the panels. I also suggest “analysis” in the captions instead of “analyse”. 
• Figures S5 and S6 are harder to appreciate without the same red “highlights” on the enhanced CH2Cl2 periods like exist in Figures 3 and 4. The discussions between lines 229-237 and lines 252-253 are much harder to interpret without these.  
• It seems that Figures S13-S20 are not referenced in the main body of text. I’m not sure if this is technically an issue or not with the journal, but nonetheless it does make me wonder why they were included with the submission if they don’t directly contribute to the analysis.