the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Oct 2025
Transport of volcanic aerosol from the Raikoke eruption in 2019 through the Northern Hemisphere
Abstract. Volcanic injections into the upper troposphere–lower stratosphere (UTLS) affect climate by altering Earth's radiation budget and atmospheric chemistry. However, the pathways by which mid-latitude eruptions spread globally remain poorly understood. We combine nighttime Compact Optical Backscatter Aerosol Detector (COBALD) profiles over Lhasa with ERA5-driven Chemical Lagrangian Model of the Stratosphere (CLaMS) backward trajectories and global three-dimensional SO2-based tracer simulations. With this integrated framework, we track the Raikoke plume (21–22 June 2019; VEI 4) as it evolved within the mature Asian Summer Monsoon Anticyclone (ASMA). Balloon-borne measurements capture the plume’s arrival, vertical spreading, and dilution by ASMA-interior air. Trajectories reveal two principal pathways from distinct Raikoke plumes: (i) an upper-level branch within the summertime stratospheric easterly flow (~460–490 K) carrying the trailing filament of the vorticized volcanic plume (VVP), and (ii) a lower-level branch within the subtropical westerly jet (~390–430 K) carrying the main plume. Although the ASMA can act as a transport barrier at certain potential-temperature levels, it admits in-mixing along jet-aligned filaments and redistributes aerosols internally. SO2-based tracer simulations are sensitive to how parameterized small-scale mixing is represented in CLaMS, underscoring the need to adjust subgrid-scale mixing parameterizations when model resolution changes (here, from ERA-Interim to ERA5 reanalyses). Independent Portable Optical Particle Spectrometer (POPS) profiles over Boulder (USA) confirm the plume’s timing and altitude, providing out-of-region validation. Sensitivity to injection level indicates an additional ~4–5 km of uplift from aerosol-radiative lofting.
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Status: open (until 06 Jan 2026)
- RC1: 'Comment on egusphere-2025-4842', Anonymous Referee #1, 22 Dec 2025 reply
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- 1
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?