| 26 May 2026
Optical properties and global distribution of the Hunga aerosols 2022 observed by Aeolus and atmospheric lidars: new insights into the vertical sedimentation of stratospheric sulfate plumes
Abstract. Stratospheric aerosol plumes from the 2022 Hunga eruption were observed by the first-ever ultraviolet (UV) 355 nm Doppler and High Spectral Resolution Lidar (HSRL) on board the European Space Agency’s Aeolus satellite. Independent measurement of extinction and co-polarized backscatter coefficients for particles are shown, capturing the Hunga plumes up to ≈ 28 km in altitude. Global map of L2A product are produced for latitudes band up to [35° S–10° N]. They are analysed with sulphur dioxide (SO2) concentration and sulfate aerosol (SA) optical depth (OD). A plume composed of optically thick patches with high SA OD above 0.025 is captured above 26 km in altitude. It exhibits high UV signal extinction up to 350 Mm⁻¹, scattering ratio (SR) up to 40, local optical depth (LOD) above 0.2, and lidar ratio (LR) above 100 sr. These SA long-lived patches are observed drifting south and transported west. Two branches separate by mid February 2022: a southern tale at ≈ 25 to 27 km in altitude around latitudes [30° S–15° S] and a northern tale at ≈ 23 to 25 km in altitude around latitudes [15° S–10° N]. The LR and LOD measured by Aeolus for the ageing branches reveal lower values below 80 sr and 0.04 respectively. A short-lived plume with low SA OD and high SO2 concentration is observed at lower altitudes ≈ 18 to 22 km with less strong UV scattering properties, and appears to disaggregate quickly.
Review of Trapon et al., 2026 - Optical properties and global distribution of the Hunga aerosols 2022 observed by Aeolus and atmospheric lidars: new insights into the vertical sedimentation of stratospheric sulfate plumes
Summary
This well written and detailed study by Trapon et al., describes the stratospheric sulfate plumes generated by the Hunga-Tonga volcanic eruption in January 2022 as observed, on a global scale, by the Aladin lidar onboard the ESA Aeolus satellite. Based on the optical properties derived from the lidar measurements, the authors analyse the zonal and meridional evolution of the aerosol plumes over a 6-week period stating 10 days after the eruption, highlighting diffusion, sedimentation and disaggregation processes of the plumes. They also provide indications on the microphysical properties of the particles composing the plumes.
The authors follow a rigorous path in their study, what strongly supports their findings and conclusions. Tracking of the stratospheric aerosols with single overpasses is first demonstrated and validated by comparing the Aladin observations with those performed by ground-based lidars (in La Réunion) and another space-borne lidar (CALIOP). Weekly maps of the aerosol optical properties are then produced and compared to quantities yielded from passive instruments onboard the Metop satellite (sulphate aerosols and SO4). Each step of the analysis is precisely detailed.
This paper is of importance for several reasons. From lidar measurement perspective, it shows that the Aladin UV HSL lidar instrument could characterise particles in the stratosphere, in particular of sub-micron size, and thus even if the instrument was originally designed for wind measurements. It allowed developing and testing algorithms and data processing tools, which are certainly useful for the present EarthCARE mission with the ATLID lidar onboard. Still from sub-micron size particle measurement perspective, Aladin bridges a gap between CALIOP and ATLID, an advantage for the continuity of lidar measurements from space. This manuscript also confirms several hypotheses previously made in other studies (based on in-situ or passive instruments) on the Hunga plumes evolution or on the size and growth process of the sulfate particles at different ages.
The referee enjoyed reading the manuscript paper and following its argumentation (although if quite dense sometime) and strongly support its publication in this proposed journal, which is adapted to the subject.
Specific comments
1) The manuscript deals with optical properties of particles in the atmosphere. The main referee’s remark concerns the precision of the measurement products, or, said in other words, the uncertainties on the reported optical values.
The uncertainty aspect is tackled once, in lines 89-90 for the MLEsub products. The particle extinction coefficient αpart is said to by QC flagged valid if, among other, its standard deviations is below 1.0 × 10−2 m−1. Similarly, βpart is flagged valid if, among other, its standard deviations is below 1.0 × 10−3 m−1sr−1. Reported values for αpart and βpart in Table 1, are (order of magnitude) at the level of 100 Mm-1 and 5 Mm-1sr-1, respectively, that is, 1.0 × 10−4 m−1 and 5.0 × 10−6 m−1sr−1. It would say that the error (standard deviation) is in both cases much larger than the reported value. Can the authors explain the referee where his/her thinking is wrong ? Are the indicated MLEsub standard deviation upper values given for one single laser shot ? Should they be understood in another way ? Completing the text accordingly is recommended, to avoid other readers to be in the same situation as the referee.
2) Section 4 (line 271). Where, in Section 3 on the results, can a SR of 40 be read or found? In particular in Figure 6, the color scale goes up to 14, what prevents from evidencing such a value. Can the authors explain a bit more the source of this value ? (The referee apologies if he/she missed something.) This value is claimed to be a “characteristic” property of the early SA patch and is cited in the abstract - and therefore, prone to propagate further in the literature. Consequently, it should be well supported. (Related extinction coefficient and LR values can be read easily from Figure A3.) The same remark applies to the LOD value of 0.2.
Other specific comments
3) Line 80: It would be worth remembering to the reader that, due to instrument design, only co-polarized signals are measured with ALADIN - particularly in view of Section 3.1.2, where ALADIN and CALIPSO results are compared.
4) Line 140. “[...] signal accumulation was done between ≈ 02:00 and ≈ 15:00 UTC”. Not clear. Shall it be understood that the (night-time, according to Baron et al, 2023) ground-based lidar measurement started on 25 Jan. 15:00 UTC and finished on 26 Jan. 02:00 UTC ? Please, explain more clearly.
5) Line 269. As shown in Fig. A1b,d, a two-week Earth circumnavigation was observed. Are there evidences that such a phenomena was observed several times ? Shoudn’t the word “every” be replaced by “in” ?
Technical comments