Review:

Hedelt, P. at al. “Analysis of the long-range transport of the volcanic plume from the 2021 Tajogaite/Cumbre Vieja eruption to Europe using TROPOMI and ground-based measurements”

Volcanic eruptions have a significant impact on the climate and negatively affect air traffic and public health. The article is dedicated to the analysis of the 2021 Tajogaite volcano eruption in the Canary Islands. It was written by a team of renowned specialists, and the article reflects their high level of expertise. It is well-written and illustrated, making it of interest to a wide range of atmospheric physics specialists. Narrow-field experts will be particularly interested because the article uses a comprehensive set of the most modern data and tools – TROPOMI data, ground-based measurements, and trajectory models. One of the most challenging questions in the study of volcanic clouds is determining their altitude, and this issue is given considerable attention in the article. As shown in Figure 3, the accuracy of altitude determination using TROPOMI data is still not very high, and this issue clearly requires further research.

A minor remark: the top part of Figure 4 considers altitude, while the bottom two images show flight time and the number of trajectories. However, the same color palette is used for all images, which is misleading, especially considering the approximately equal intensity of the colors. It would be better to use a different palette for the two lower images. A similar approach is used in Figure 7, but it does not cause significant objections there because the images differ noticeably from each other.

The article can be published.

The manuscript “Analysis of the long-range transport of the volcanic plume from the 2021 Tajogaite/Cumbre Vieja eruption to Europe using TROPOMI and ground-based measurements” by P. Hedelt et al, presents a thorough characterization study following the 2021 eruption of the volcano Tajogaite/Cumbre Vieja.

The analyses shows the classification of both fresh aerosol near the source and aged aerosol at Lindenberg . Data are acquired with different instrumentation, mainly remote sensing, both from the ground and from satellite and they are supported by robust trajectory modeling that allows the two measurement sites to be unquestionably connected.

Non-spheroidal and irregular shape modeling for the retrieval of microphysical aerosol properties it’s certainly an interesting and innovative point of this paper even if applied in a monomodal case, an approximation which is however well legitimate in the context of this work.

The manuscript is clearly written and easy to read, even considering the different types of data and methodologies used.

Before publication, there are a few technical corrections that I suggest to implement and which I report below:

• Line 86: 2.600 km: 2.6 km or 2.600 m?
• Line 87 (and elsewhere): I recommend a non-breaking space between units of measurement and numbers and also between symbols and numbers as in lines 178, 448/449 and elsewhere
• Line 179: 50-m
• Line 209: It would be interesting to quantify how dry the air was
• Figures 8 and 9: the graphs are very dense with information and the presence of the scale only on the left graphs complicates the reading. I would suggest adding it also to the right graphs.
• Linea 359: please indicate which panel of the figure the variables refer to or alternatively indicate the abbreviated name of the variable in the graph title.

The authors are presenting a thorough characterization study of the 2021 Tajogaite/Cumbre Vieja volcanic eruption. The study is presenting both fresh (near vent) and aged (at Lindenberg) aerosol properties, obtained by ground-based and space-borne remote sensing instrumentation, and supported by trajectory modeling.

In my opinion the manuscript is well written, easy to follow, and holds scientific merit. However, in order to be improved I would kindly ask the authors to address the comment and suggestions below. 

Major comments:

1) The authors should stress a bit more in the Introduction what is the added value of this work, to the scientific community. Which scientific question will be answered here that was hazy for so long? In my opinion, the application of non-spheroidal and irregular shape modeling for retrieving the aerosol microphysical properties (even if applied in a monomodal distribution), is quite innovative. 

2) To my understanding it is difficult to discriminate volcanic ash from SO2 with a lidar instrument. The term "volcanic ash" should be change to "volcanic cloud" when referring to lidar aerosol classification. 

Minor comments:

1) line 17: I think "extinction-to-mass conversion factors" is more suitable here.

2) line 18: "volcanic secondary sulfate": Thus, the observations of volcanic ash from the lidar measurements stated above (line 10), is probably misleading. Please rephrase if needed. When the lidar-probed aerosol layer cannot be classified (i.e. primary or secondary sulfates or ash etc) consider using the term "volcanic cloud" instead of "volcanic ash". 

3) Section 2.2: Which products are used from each instrument (and with what uncertainty) ? Can you summarize them in table 1 ? 

4) Section 2.3, line 140: "extinction": Not clear if the aerosol extinction coefficient was obtained from elastic or inelastic scattering. Apart from inelastic extinction coefficient also elastic is used here? Please mention briefly here, the uncertainty of the products provided by RAMSES .

6) line 146: "Only measurements are considered that were taken close to the volcanic vent." -> "Only measurements taken near the volcanic vent are considered."

7) line 159: "differing"-> "different"

8) Figure 2: There are some negative values of SO2 VCD (in DU) as taken by the Brewer. What is their physical meaning? 

9) Figure 3: 

a) Please consider providing the points on the graphs with their uncertainty (or variation).

b) Seems like the number of dots in panel c) are bit more in number compared to the number of x in panel a). Please cross-check if this is the case. 

c) Are the cloud fractions and height difference correlated ?

d) If the TROPOMI SO2 layer height is taken within coarser spatial resolution, how would this affect the correlation with the volcanic cloud retrieval from the ground-based lidars? 

8) line 262: Yes, volcanic ash may play a role in this underestimation. Is this within the limits reported in the scientific literature?

But on top of that, here the authors are comparing SO2 retrieval from a passive sensor with aerosol (volcanic cloud) retrieval from active sensors. I wonder if this also plays a role. 

9) line 273: "lower"-> "less" and "lower than"->"below" 

10) line 281: "the day of the start"-> "the starting day" 

11) line 319: Is this due to the atmospheric conditions (e.g. wind speed etc), aerodynamics (e.g. particle size, etc)? Please elaborate more. 

12) line 364: "It can therefore be assumed... were not of spherical shape". Is this the the outcome of the droplet solidification? 

13) line 365: "the low δpar values near 2%": But these values indicate rather spherical of regular shape particles right? Provide also the relevant reference at the end of this sentence.

14) line 412: "...Vieja lava has a low SiO2 content, it is therefore .... ". Please provide the relevant reference for this statement.