the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jul 2025
Aerosol Composition and Extinction of the 2022 Hunga Plume Using CALIOP
Abstract. We use the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) instrument to determine the microphysical properties of the stratospheric aerosol plume after the Hunga eruption in 2022, the largest so far after Pinatubo in 1991. In the early stages, low depolarization (<2 %) is found everywhere except in patches of high depolarization (up to 35 %) detected within the plumes of sulfur compounds up to 3 days after the eruption. As standard CALIOP L2 products are not operational in the case of the Hunga aerosol plume, we implement an iterative method of successive approximations to retrieve extinction profiles, by estimating the aerosol optical depth (AOD) and then the Lidar Ratio (LR). The AOD of the plume at 532 nm is between 0.5 and 1.25 on the first four days, then decreases rapidly and stabilizes at 0.047 ± 0.011 for March 2022. LR, initially above 70 sr, is estimated at 48 ± 6 sr between late January and late March 2022. Results are compared and validated with the solar occultation instrument SAGE III (Stratospheric Aerosol and Gas Experiment) on board the International Space Station (ISS) and Mie calculations. A comparison with limb-viewing instruments highlights significant quantitative disagreements in extinction and AOD estimates, which we attribute, in part, to the unusual size distribution of the aerosols within the Hunga plume.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3355', Anonymous Referee #1, 13 Oct 2025
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RC2: 'Comment on egusphere-2025-3355', Anonymous Referee #2, 03 Nov 2025
The manuscript "Aerosol Composition and Extinction of the 2022 Hunga Plume Using CALIOP" by Duchamp et al. presents a remote-sensing analysis of the stratospheric aerosol plume from the January 2022 Hunga Tonga–Hunga Ha’apai eruption. Using CALIOP measurements, the authors retrieve aerosol optical depth, lidar ratio, and extinction profiles through an iterative method adapted to the plume’s unique characteristics. Results are validated against SAGE III/ISS observations and Mie scattering calculations.
The authors provide a very useful literature overview and review of the Hunga eruption and recent findings. The paper is clearly written, methodologically robust, and offers valuable benchmarks for future volcanic aerosol monitoring.
I have no further remarks and suggest to accept the manuscript as is.
Citation: https://doi.org/10.5194/egusphere-2025-3355-RC2 -
RC3: 'Comment on egusphere-2025-3355', Anonymous Referee #3, 03 Nov 2025
This manuscript presents a study of the 2022 Hunga eruption using spaceborne lidar measurements CALIOP in combination with limb sounders. These instruments provide a comprehensive picture of the plume’s vertical evolution and the composition of ash and sulfates. The manuscript is well written, clearly structured, and easy to follow. The results are informative and valuable to the community. I have a few comments that may help further strengthen the paper:
1. Figure 2 presents 6 CALIOP subplots showing the plume structure with individual clouds (C1 and C2) clearly identified. This figure is very helpful and well described in the main text. However, as lidar provides only a cross-sectional profile of the plume, it is difficult for readers to visualize which part of the overall plume is being sampled. If feasible, I recommend adding an additional panel for each subplot that shows the corresponding CALIOP ground track overlaid on imagery from another instrument (e.g., geostationary satellites or IASI). This would give readers a better sense of the horizontal context and improve interpretability.
2. Section 3.2 discusses the evolution of AOD for individual plume features (e.g., C1 and C2). It should be clarified explicitly at the beginning of this section that the reported AOD values refer to plume-layer AOD rather than the total atmospheric column AOD. Since “AOD” typically denotes column-integrated optical depth, it would be helpful to define this clearly here and wherever the concept of plume-layer AOD is used to avoid ambiguity.
3. In Figure 1c, the horizontal section at 17.5 km suggests that the signal between approximately 22°S and 17°S latitude may be fully attenuated. Could the authors comment on how this attenuation might affect the extinction and plume-layer AOD retrievals for the plume near 30 km altitude?Citation: https://doi.org/10.5194/egusphere-2025-3355-RC3
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The paper by Duchamp et al. presents a detailed remote-sensing analysis of the stratospheric aerosol plume produced by the January 2022 Hunga Tonga–Hunga Ha’apai eruption. They are using CALIOP to calculate AOD, lidar ration and extinction profiles, which are there compared with other satellite data, as well as theoretical Mie scattering calculations. Overall, the paper provides a robust and methodologically transparent assessment of the Hunga stratospheric plume’s optical properties, offering valuable benchmarks for future volcanic aerosol monitoring.
The paper is well written, with clearly defined objectives and a well-described, rigorous methodology. They also provide an extensive literature review. I have only few minor comments.
L 67. “the most significant orbit of the day”. What is the sensitivity if you take into account e.g. two orbits and not the “most significant”?
L 305-310: The formula calculates the standard error not the standard deviation. So σ(x) should be the standard error.