Review of Yang et al., Transport of volcanic aerosol from the Raikoke eruption in 2019 through the Northern Hemisphere

General comments 

The submitted manuscript uses aerosol profiles measurements made by COBALD sondes at Lhasa and POPS at Boulder to study the distribution of aerosol from the Raikoke volcanic eruption in June 2019, particularly within the Asian Summer Monsoon Anticyclone but also including transport to Boulder.

Firstly, back trajectories are used to study the transport pathways to Lhasa, and then in the second part, the CLaMS model is used to try to simulate the profiles after injection of an SO2 tracer. (Not including the upper peak seen in the COBALD and POPS profiles around 470-480 K).

The model shows a reasonable ability to reproduce the observed results and some interesting sensitivity studies are also performed to try to obtain the best possible agreement and assess model settings. 

Overall this is an interesting and worthwhile study about an important topic and well within the scope of ACP, and I would be happy to recommend publication after some minor improvements.

The presentation is generally very clear and well-written with a small number of exceptions listed below.

To me, the agreement of the model and observations seems only modestly good so I would suggest a small amount of additional text in the discussion section or conclusions to put these results into context and how happy you were with them. 

I also note that the CLaMS simulations did not attempt to model the upper peak seen in Figure 2 and Figure 8 which seems disappointing, so some comment about that would also be welcome.

Specific comments

Lines 19, 34 – this is a very minor comment, but I am not aware of much discussion of volcanic injection of water vapor prior to Hunga.

Lines 39-41 The wording implies there are other examples of this in the record apart from Hunga?

Lines 44-46 These sentences need some minor re-wording for clarity. It is hard for the reader to understand  "… the circulation acts as a … barrier, trapping those air masses … Simultaneously, the barrier is permeable … This dual role … ". This reads like a superposition of contradictory states.

Lines 96 You should add a sentence to explain how β_{_air} is distinguished from β_{_particles.}

Line 165 Please re-word "are empirically highlighted" – I think the criterion is really just that BSR is high without high RH.

Line 172 "Typical ATAL profiles … " – do you mean typical enhancements in the profiles?

Lines 172-175 It seems to me that the distinguishing feature of the 2019 profiles is the magnitude of the peak rather than the height. You say the ATAL profile can reach 420-440 K at times.

Figure 3 the thin gray lines are very hard to see – I couldn't see them at all on my screen until I zoomed to at least 300%.

Lines 184-185 Please re-word "we performed backward-trajectory analyses based on in-situ ballon-borne measurements …" This reads to me that the back trajectories are using data from the balloon measurements.

Lines 184-221 The back trajectories are run for periods from 1.5 to 5 months. Do you have any confidence that the results are meaningful over such a long period of time?

Figure 4 Are the labels on the x-axes date and month? (dd.mm ?)

Figure 4 It would be very helpful to mark the height of the tropopause on these plots.

Lines 203-206 I don't quite understand this – what is the denominator of the fraction exactly? You need to give more detail on how you initialized the starting positions and times of the back trajectories., and how many you ran for each date.

Lines 209-211 Looking at the top right panel of Figure 5 it looks like the air parcel travels directly from the area of Raikoke westwards to Lhasa. However the text says it circles the globe three times, while the red and blue colors of figure 4 (second panel) make it look to me as if the plume circled the globe once.

Could you clarify this point please?

Lines 219-221 The 'clockwise advection' within AMSA isn't very noticeable to me on the back trajectory plots in Figure 5.

Lines 262-265 It was disappointing to me to read that the upper peaks from 1 August and 3 August were not going to be simulated. Could you perhaps add another sentence to explain why these weren't included too.

Figure 7 The correlation coefficient seems a limited metric because in some cases the peak is dispersed over a wide altitude range (in other words, you're correlating one gentle curve with another gentle curve, in which situation correlation is not very helpful). Is the relation between tracer fraction and BRB linear across all the different profiles? It looks like it is. Would it be more meaningful to calculate the fit across all the different profiles?

Line 323 I think this is the first time you have discussed horizontal entrainment, the previous discussion was about upwelling air diluting the aerosol concentration.

Lines 335-337 How have you shown that?

General Comments:

This study presents comprehensive analyses of transport pathways of a rare volcanic eruption event and its impact on the stratospheric circulation by combining in-situ and satellite observations and global modeling. The results were reasonably structured throughout and well explained. However, most information in the main text seems to be focused rather on description of the results and lacks its significance or context. I think the authors should also emphasize the novelty of this work more clearly. A statement emphasizing how valuable in-situ measurements used in this study are and what can be achieved by using both the trajectory and global models can be included. I would also like to know if any studies that are similar to this work have ever done before. There are many places where the writings are vague. I would recommend improving clarity of writings to strengthen the paper’s scientific quality.

Specific Comments:

P1, L4: SO₂ -> sulfur dioxide (SO₂)

P1, L14: What does ‘out-of-region’ validation mean?

P1, L15: ‘Sensitivity…lofting’ – The meaning of this sentence is unclear. Does this mean ‘Sensitivity test using the CLaMS model indicates that the aerosols can be lifted additional ~4-5 km depending on the injection height due to aerosol-radiative lofting’?

P2, L18: It would be nice to add more recent work here, if there is any.

P2, L26: negative -> negative radiative

P2, L31: from an eruption -> from an eruption are

P2, L35: There should be something at the end of this sentence. ‘far longer than xxx’.

P2, L36: It needs a reference for the Brewer-Dobson circulation here.

P2, L42-44: Among…summer. Deep…circulation -> These need to be supported by references.

P3, L52: How was June 21-22 period determined to be during the anticyclone’s mature phase?

P3, L56-58: The…plumes. -> Are these also in Khaykin et al. (2022)?

P3, L59: when the site…levels. -> This sentence needs to be more specific. Was this based on the measurements of a specific variable, such as, geopotential height?

P3, L64: from the TROPOsperic Monitoring Instrument (TROPOMI)

P3, L65: from Lhasa and Boulder -> Are those measurement locations? Why was Boulder selected here?

P3, L67: I think the authors should state that why their work is new and how it is different from the previous studies. I think scarcity of in-situ measurements for the Raikoke or any volcanic eruption should be mentioned here.

P3, L75: Please include exact days of the SWOP campaign. Also, why were the ECC measurements not analyzed?

P3, L81: Please add information about location of the ASMA relative to Lhasa during July-August 2019. As an option, a contour of a geopotential height can be added in Fig. 1a.

P5, L103: A citation is needed for the iMet radiosonde.

P5, L110: Is ‘sizing’ referring to aerosol particles sizing?

P5, L121: …limit are excluded following Theys et al. (2024).

P6, L127: ‘applied’ can be substituted with ‘used’.

P6, L129: hybrid coordinate   -> hybrid  coordinate

P6, L132: main runs -> control runs

P6, L132: 0.3º x 0.3º (latitude x longitude)

P6, L134 (and elsewhere): 6 h -> 6 hours

L6, L147: what does ‘tracer injection mask’ mean? Is this term used in CLaMS modeling in general?

P7, L158: Briefly explain why this method is practical.

P7, L165: What does ‘empirically highlighted’ mean?

P7, L174: …differs markedly… -> Could this be ‘is significantly higher’?

P7, L178: Why was the particle number density over Boulder so high in August? I think the impact of large-scale global circulation transporting aerosols between Lhasa and Boulder should be explained.

P9, L187: Is the ‘eruption period’ same as 21-22 June 2019?

P10, Fig. 4: It is hard to understand the direction of transport in this figure. Is the air moving mostly to the East from the eruption location? Also, what is the unit of x-axis? It looks like x-axis has irregular intervals, which makes it hard to understand.

P11, L211: Which satellites were used in these studies?

P11, L215-217: This is somewhat vague. I would suggest revising it with more specific information for clarity.

P11, L223 (and elsewhere): What do ‘SO₂-based tracers’ mean?

P11, L225: global maps with -> global maps of?

P12, L231: the dynamical situation…-> the isentropic transport, showing outflow of the ASMA and in-mixing…

P12, L234: Which ‘observations’ specifically?

P12, L244: How does ‘Lyapunov exponent’ work in CLaMS?

P12, L252: driving data -> either change to ‘input’ or remove it

P14, L279-284: What is the significance of this result in terms of large-scale circulation?

P15, L286: How long did the initial injection last?

P15 (Figs. 7-10): The locations and dates are not easy to see on these figures. I recommend either using larger size fonts or placing them on them the title.

P16, L307-309: The relevance of these two sentences to this study is unclear.

P17, L311: A motivation for this study can be stated here as well.

P17, L337: What is the significance of the results in understanding role of volcanic eruption in our climate?