GENERAL COMMENTS:

This paper analyzes HFC mole fractions and emissions in eastern Asia from 2008 to 2020 based on measurements at Gosan Station and modeling using two independent Bayesian inversion frameworks. The emissions are compared with bottom-up emissions from activity data and emission factors. The results show strong HFC emission increases especially from 2015 to 2018, decreases since 2019, and an increase of eastern Asia to the global fraction. The recent decreases in Japan may be related to new control policies. HFC-134a is a main contributor to CO₂-equivalent HFC emissions but its share has been decreasing relative to HFC-125.

Overall I thought this was a strong and well-presented paper. The new findings are clearly highlighted and the graphs are interesting and informative. The results show important changes of individual HFCs (HFC-134a, HFC-143a, HFC-32, HFC-125, HFC-152a) from 2008 to 2020 in eastern Asian (eastern China, Japan, South Korea, North Korea and Taiwan), with some surprising results in the top-down to bottom-up comparisons. Most of my comments are minor and include double-checking a few calculations and interpretations, highlighting the emissions decline since 2019 and the GWP shift from HFC-134a to HFC-125, and improving clarity in some places. I recommend publishing the paper in ACP subject to addressing these minor revisions.

SPECIFIC COMMENTS:

Title: The title focuses on the acceleration of HFC emissions, which was strongest from 2015 to 2018, but misses the subsequent decrease in emissions since 2019 which is also an important finding.

Abstract:

(1) One of the interesting results is the relative decrease in GWP contribution for HFC-134a and the relative increase for HFC-125 in most countries (Figure 11). Consider adding this to the abstract.

(2) L31: Check that 15% is correct. This seems to be the 2018 peak rather than the 2016-2020 average.

(3) L33: Provide context for the 76 Gg/yr higher emissions. What are the total emissions?

(4) L36: Reading about a decline in HFC emissions in 2019 is unexpected after reading about the sharp increase in emissions since 2016 (L30). It may help to change ‘since 2016’ to ‘from 2016 to 2018’.

Introduction:

(1) L46: Provide a reference for the GWP of HFCs.

(2) L54: State which of the 4 groups China, Japan, South Korea, North Korea and Taiwan are part of.

(3) L87: Change ‘in eastern Asia’ to ‘in these four countries’ since the dearth of recent research doesn’t seem to apply to China (L82-84).

Section 2.1:

(1) L106: Measurements are taken every 2 hours. How long is the sample duration?

(2) L107: State how many samples were collected from 2008-2020 and used here. Around 50,000?

Section 2.2:

(1) L154: Define J(p) within the text. Define T.

(2) L169: Describe the working tank.

(3) L183: Describe what’s meant by flattened.

(4) L195: Why observation sites (plural) if Gosan is the only station being used? Do you mean the same subset of data was used for both inversion models?

Section 3.2:

(1) L249: HFC-134a emissions from North Korea and Taiwan are up to 1-2 Gg/yr (Figure S2) or 10-20% of values from eastern China and Japan, which isn’t nearly negligible. The average Taiwan emission peak is actually fairly similar to South Korea (Table 3). Use different wording and add a brief discussion.

(2) L265: This is an interesting finding.

(3) L286: Add a sentence or two describing South Korea (Figure 5c). The inversions agree well but seem to greatly exceed the inventory? Please briefly discuss North Korea and Taiwan, as you do for HFC-125.

(4) L338: The statement that South Korea showed the most substantial HFC-152a emissions growth doesn’t seem correct. Japan’s emissions increased from 0.8 to 3.4 Gg/yr (2.6 Gg/yr increase, L333) while South Korea increased from 0.06 to 1.4 Gg/yr (1.3 Gg/yr increase).

Section 3.3:

(1) L354: Why is 2015 omitted? I wondered this throughout the paper.

Section 3.4:

(1) L376: Add error bars to the emissions, which are given to 0.1 Gg/yr.

(2) L376: The text states a peak of 116 CO₂-eq Tg/yr in 2018, but Figure 10 shows a peak of around 128 CO₂-eq Tg/yr in 2018. The 2020 value of 110 Tg/yr (L375) also seems low. Please check.

(3) L408: Add ‘except North Korea’ since it didn’t have the largest portion of HFC-134a (Figure 11).

(4) L414: The changes to HFC-134a and HFC-125 in eastern China are 9-10%, which is still substantial.

(5) L418: This can read as if HFC-134a and HFC-125 are the environmentally friendly alternatives rather than the gases to be reduced.

Section 3.5:

(1) L428: ‘on the other hand’ doesn’t apply if both have increased steadily (L427 and L429). Change the second increased steadily to ‘increased more slowly’.

Section 4:

(1) L472: The text here states an accelerating trend since 2015 while L30 states 2016.

(2) L472: Add that emissions have subsequently slowed or decreased since 2019.

(3) L482: Figure 12b shows a peak of ~15% in 2018 but smaller values at other times (L437), so using 15% as the post-2015 value seems too high both here and in the abstract.

Figures:

(1) Increase the font size in the Figures. Figure 9 is especially hard to read, even blown up.

(2) In each Figure label the panels (a, b, c…) and define them in the caption.

(3) Figure 10: Label the panels and delete ‘respectively’.

(4) Figure 12: Define all terms (Twn as Taiwan…). The caption for (b) only refers to half the legend (eastern Asia to global total) so make sure to also define ‘non-Annex-1…to unreported’.

Supplement:

(1) L49: State in the Figure S8 caption that the proportions are based on mass percentage.

(2) L51: Delete ‘except North Korea’ since HFC-134a also accounts for the largest percentage in North Korea (53%). (It seems like the interpretation of Figure 11 and Figure S8 was reversed.)

(3) L52: HFC-125 increased by 10% (from 15 to 25%) in eastern China and HFC-152a decreased by 12% (23 to 11%), which is more than ‘slight’.

(4) Figure S1: The scale is difficult to read.

(5) Table S1: State the year of the prior emissions (2008).

TECHNICAL CORRECTIONS:

Title and throughout: HFCs should be HFC (singular)

L55: Grammar - began

L103: Change ‘around’ to ‘surrounding’

L115: Correct the brackets

L120: Grammar - observations

L161: Grammar - matrices

L183: Fix the quotation marks

L196: Delete the comma

L208: Grammar

L271: Delete the comma. Change to ‘emissions for the whole of China’

L282: Extra bracket

L283: Add ‘…of HFC-32 in eastern Asia’

L293: Reference Figure 6a here, Figure 6b on L297, and Figure 6c on L302

L303: Reference Figure S4d for North Korea and Figure S4e for Taiwan

L315: Reference Figures 7 and S5

L319: Delete the comma

L328: Reference Figure 8a here, Figure 8b on L333, and so on for the other countries

L362: Grammar - near the

L377: Change to Figure 10a. On L383 change to Figure 10b

L437: Reference Figure 12b

L457: Grammar

L478: Grammar - steady

L498: Delete respectively

L508: Grammar - verb missing

Choi et al. presented a comprehensive analysis to estimate emissions of 5 most abundant HFCs from 2008 to 2020 in eastern Asia using ground-based observations at Gosan, South Korea. I think this paper contains important findings on ongoing usage and emissions of HFCs in eastern Asia.  The authors have done a wonderful job in data analysis and emission estimates.  The paper is overall well written and should be accepted for publication after my following comments are addressed.  My main critic of the paper is that the authors are buried too much in the details, but overlook the most important finding from this study - the cumulative emissions of five most abundant HFCs from East Asian countries made a turn-around point in 2019 (whether it is due to the Kigali Amendment influence or not) that increasing emissions trend from 2008-2018 is reversed to a decreasing trend in 2019-2020.

Major comments:

1. Section 3.2. Can I make a structure change for the authors to consider here? I see that discussions in this section is following a compound by compound flow, similar to Chapter 2 in WMO2022.  To me, this approach might be amiss in terms of the key conclusions you can draw from this study. The emission trends shown in Figure 3 for all HFCs, except 143a, point to two distinctive period 2016-2018 during which the two dominant emitting regions Eastern China and Japan show rapid increase in emissions and 2019-2020 during which emissions either decreased or plateaued.  Consider add a summary paragraph here right at the beginning of Section 3.2 (where Figure 3 sits), describing trends of all gases together.  And later, when you discuss each compound, you can refer to the corresponding discussion.  Even though reasons for this notable change in 2019 might be different for different countries and not necessary directly tied to F-gas regulation, but in my view such a trend is note-worthy and should be pointed out clearly at the beginning of discussion, instead of being buried among many other details in each compound section.
2. Figure 10 and Section 3.4 provide compelling evidence that emission trends for 2019-2020 are distinctively different from previous time period. This should be a key message from your paper! Currently the discussion in section 3.4 focuses on the details of emissions from each country, the relative contribution from each HFC, etc. In a way, you are look at each individual tree but forgot about the forest, that the cumulative emissions of five most abundant HFCs from East Asian countries made a turn-around point in 2019 (whether it is due to the Kigali Amendment influence or not).
3. I would think that Figure 10 will be a key take-away figure when this paper is published.  I have a few cosmetic suggestions to make this figure more appealing to eyes. The individual stacked-up bars for each year leave a very discrete view. You may want to make the bars wider (leaving only fine space between bars) so that they provide a more continued temporal flow from one year to the next. Second, the current choices of colors are not optimal. There is enough contrast, but colors are clashing to eyes.  Third, the legend for countries or HFCs are flipped in order when compared with the sequence they are stacked in the figure.
4. Section 4 - the major findings from this paper, I would suggest that “reverse of increasing emissions trend from 2008-2018 to a decreasing trend in 2019-2020” need to be added as one major finding. In a way, this is a more significant finding than all the three findings you listed.

Minor comments:

1. L20 & L96. It should be Kigali Amendment (upper case A).
2. I would suggest call them “most abundant HFCs” instead of “major HFCs”, here and after, to be consistent with WMO 2022. What exactly is a major HFC anyway?
3. “Our most important finding is that HFC emissions in eastern China and Japan have sharply increased since 2016.” Looking at figure 3, this is not an accurate statement as the increase for Japan and China (except HFC-143a) only lasts from 2016 to 2018.  Emissions for HFC-32, 125, 152a in both regions have decreased since 2019.
4. “HFCs are not ODSs since they are oxidised more readily in the troposphere and do not contain chlorine atoms.” Being oxidized in the troposphere is not the reason why they are not ODSs. A lot of the ODSs, e.g. CH3Br, CH3Cl, CH3CCl3, are primarily oxidized in the troposphere and have comparable or shorter lifetimes than many HFCs.  Suggest change to “HFCs do not contain ozone-depleting chlorine atoms”.
5. L51 & L95. Paris agreement  Paris Agreement
6. It should be Rigby et al. (2014). Parenthesis are missing.
7. Define AGAGE here.
8. May be rephrase to “Previous regional studies demonstrated that there is reasonable consistency between bottom-up and top-down emission estimates Europe (Graziosi et al., 2017) and the United States (Hu et al., 2017)”
9. “(mole fractions)”, shouldn’t you add the definition at L99, instead of here?
10. Can you explain briefly what is “a 2-year assimilation time window”? To me this is a bit jargon-ish, not necessary clear to readers what do you mean by “assimilation”.
11. Change to “presented in this study”.
12. L142 & L198. Missing parenthesis in references for “2021”, “2018”.
13. It is commonly referred to as “Asian Summer Monsoon”, not “summer Asian monsoon”. Consider revise.
14. Figure 2. Nice way of showing pollution magnitudes.
15. Figure 4 caption. Can you clarify what is “average values”? Average of the FLEXPART and NAME results?
16. L255-257. Here, are you commenting on the small differences between the UNFCC inventory vs. the EDGAR-v7 inventory?  I found this sentence to be somewhat out of place in this paragraph, as the remaining paragraph mostly discuss the gap between top-down and bottom-up inventories.  A good and punchy way of starting a paragraph is to write a key summary sentence that summarize of the main finding of the paragraph, e.g. “There are significant and growing gaps between the top-down emission estimates and bottom-up estimates in eastern China and Japan, etc.”. You can note the small differences between the UNFCC inventory vs. the EDGAR-v7 inventory somewhere in the paragraph instead of mentioning at the beginning of the paragraph.
17. L277-278. Consider revise to: HFC-32 abundance has been increasing substantially in recent years.
18. L279-282, why do you use eastern China sometimes and Eastern China other times? Please be consistent.
19. L304-306. This is an important finding about HFC-125.  I would suggest you move this sentence to the beginning, right after HFC-125 usages.
20. “HFC-152a, which has the lowest GWP (148) among HFCs” This is not a correct statement. There are many HFCs that have lower GWP. Did you mean among KA-regulated HFCs or the five most abundant HFCs in this study?
21. L353-354. Why is this sentence a paragraph of its own?  In fact, you don’t need this sentence at all. It is just a figure description. You can delete it and add “(Figure 9)” at the end of the sentence on L355.
22. Change to “HFCs are potent anthropogenic greenhouse gases”. You don’t need “,” here.
23. Change “,” after growth to “.” The second part is a sentence on its own.
24. You probably should add “South Korea” after Gosan to recapture things again for those who only read abstracts and conclusions.
25. “are” is missing after there.