Please see the attachment.

General Comments

In this work the Hyperspectral Infrared Atmospheric Sounder Type II (HIRAS-II), aboard the Fengyun 3E (FY-3E) satellite, is used to investigate the possibility to detect and retrieve the SO₂ emitted from volcanic eruptions. To do that, a methodology is described in order to select the most sensitive channels to SO₂ from the large number of hyperspectral channels recorded by the sensor. To minimize the influence of atmospheric water vapor and temperature to SO₂, the procedure proposes to select SO₂-sensitive channels that contain similar information on variations in atmospheric temperature and water vapor themself. Finally, to test the procedure, the 29 April 2024 eruption of Mount Ruang in Indonesia has been considered.

Here, the possibility of using FY-3E - HIRAS-II for monitoring eruptive volcanic clouds is shown. This polar sensor is part of the set of polar and geostationary satellites working at different wavelengths used for the SO2 monitoring, whose synergic use can significantly improve the monitoring of these natural phenomena. The proposed procedure is interesting but needs to be clarified in several parts. The considered test case shows that there is a qualitative analogy between the SO2 cloud detected by HIRAS-II and that detected by TROPOMI on board Sentinel 5p.

Specific Comments (legend: “r” = row nuber)

- r23: in this work only qualitative information are extracted

- r94: clarify the reference, Li et al., 1994 doesn't contain the equation inserted. Check also the sign of the different terms and define theta.

- r96: T is not present in the formula. You could explicit the dipendence from T in the planck function (by written Bs(Ts) in the first term and B(T) into the integral)

- r109-110: clarify if LBLRTM allows the possibility to insert a user defined atmospheric PTH profiles

- r116 Paragraph 2.2: are the HIRAS-II data freely available? Where they ca be downloaded? This information could be inserted in the text.

- r125-r127: what about the NEdT for the short-wave bands?

- Table 1: as written in the paper "Its measurements span a continuous spectrum range of 648.75 to 2551.25 cm⁻¹ at a resolution of 0.625 cm⁻¹". In the table seems that the different spectral intervals are not in continuity. For example: the Long spectral range ended at 1136 cm-1 and the Mid spectral range start at 1210 cm-1 (lack of 74 cm-1). Why some channels have been not considered?

- r139: citation not present in the bibliography

- r141-r142: Please insert the references:

       - Theys, N.; De Smedt, I.; Yu, H.; Danckaert, T.; Van Gent, J.; Hörmann, C.; Wagner, T.; Hedelt, P.; Bauer, H.; Romahn, F.; et al. Sulfur dioxide retrievals from TROPOMI onboard Sentinel-5 Precursor: Algorithm theoretical basis. Atmos. Meas. Tech. 2017, 10, 119–153.

       - Theys, N.; Hedelt, P.; De Smedt, I.; Lerot, C.; Yu, H.; Vlietinck, J.; Pedergnana, M.; Arellano, S.; Galle, B.; Fernandez, D.; et al. Global monitoring of volcanic SO 2 degassing with unprecedented resolution from TROPOMI onboard Sentinel-5 Precursor. Sci. Rep. 2019, 9, 1–10.

- Figure 3: enlarge the x and y number labels (as in plot (b)). You should use ppbv instead of ppmv (in both plots). Here the brightness temperature is indicated as Tb, while in the text with BT, please standardize.

- r217-r219: explain better why the similarity in the Jacobians in the different spectral ranges is important to minimize the influence of water vapour and temperature to SO2.

- Figure 4: This scheme it is not so clear to me:

is it correct that the spectral range selected for the water vapor absorption region is the same as for the SO2 absorption region? In this case the water vapor Jacobian marix (computed for a specific wavenumber, by varying the water vapour content) should be the same. The water vapor selection is in this case carried out by considering the maximum M and dP? In the scheme seems that only the cross-comparison between the water vapour Jacobians lead to the selection of the SO2 channels. Is it correct? Moreover, it is not also clear to me why only the 1155-1430 interval is considered for the SO2 Jacobian computation. SO2 presents two wide absorption bands around 1163 and 1370 cm-1, and until 1100 cm-1 the SO2 absorption is still meaningful. Why the whole 1100-1430 cm-1 spectral range it is not considered? I'm surprise to see that no one channels around 1163 cm-1 is selected. This SO2 absorption is inside of one of the TIR atmospheric window and generally used for the SO2 tropospheric retrievals.

- r226: the SO2 perturbation is generated by varying a default profile of 5%. But, during volcanic emission, the SO2 content is much higher and also confined at specific layers. How the SO2 Jacobian computed can be considered representative of a real case?

- r228-r231: except for the wavenumbers around 1360 cm-1, Figure 5 doesn't clearly emphasizes where are placed the other wavenumbers significant. In any case the left orange oval (that should emphasize the higher jacobian variability) is placed around 1210 cm-1 and not 1225 cm-1.

- r306: it is not clear which channels have been selected, please clarify.

- r317: why the 1165.125 cm-1 channel has been considered? The Paragraph 3.2.1 (SO₂ channel selection) indicates only the channels around 1360 cm-1.