In this manuscript, Nicknish et al. describe their investigations on stratospheric decay timescales of sulfur dioxide (SO₂) following explosive volcanic eruptions, focusing on three specific events: Kasatochi in August 2008, Sarychev in June 2009, and Nabro in June 2011. The authors employ datasets from two space-borne limb-sounders (MLS and MIPAS), one nadir-sounder (OMI), as well as model simulations using WACCM. To my knowledge, this study represents the first comprehensive analysis integrating and comparing these datasets to assess the decay of SO₂ in the stratosphere. Given potential scenarios of climate engineering involving the deliberate injection of SO₂ into the stratosphere, understanding its conversion into sulfate aerosols and improving the fidelity of related models is of critical importance. The manuscript provides a clear and transparent description of the methodologies and procedures applied to analyze the datasets. The sensitivity analyses conducted to quantify uncertainties in the decay timescale calculations are robust and methodologically sound. The results are discussed in a manner that elucidates the strengths and limitations of each measurement technique, offering valuable insights to the reader. However, some conclusions would benefit from clearer articulation and some of a more nuanced presentation. These issues are detailed in the specific comments provided below. While the study does not yield definitive recommendations for specific model improvements, the work enhances our understanding of the specific characteristics of different stratospheric SO₂ measurements and provides a valuable framework for future research. I support the publication of this manuscript following revisions to address the comments and suggestions outlined below.

Specific comments:

l.22: ‘…is produced naturally in seawater’:

This is only one of the several sources of OCS. It is even not clear if the major source is direct emission from sea-water, or if OCS is mainly produced by oxidation of CS2 and DMS (which originate in sea-water) (e.g. Kremser et al., 2016).  Please be either more specific (or more general) here.

l.81: ‘…they potentially provide greater sensitivity to volcanic SO2’; l.86: ‘the greater sensitivity of limb sounders may be advantageous’; l.332: ‘This is likely to be a bias in the OMI data, perhaps due to the limited sensitivity of nadir instruments as the plume disperses…‘; l.399: ‘This may be a bias…’ 

I acknowledge the scientific caution when attributing biases in stratospheric SO₂ decay timescales to nadir-viewing instruments. However, the evidence presented in this study, as well as in previous works, strongly supports the existence of such a bias. This bias arises from the limited sensitivity of nadir-viewing instruments to diluted stratospheric SO₂ amounts compared to limb-viewing instruments, which benefit from significantly longer optical path lengths (several hundred times greater) through the dispersed plume. Given the robustness of this evidence, I recommend adopting clearer language to describe this phenomenon. Such clarity is important to avoid potential misinterpretation of nadir-viewing data, particularly in the future, when a substantial volume of nadir-derived data remains available while limb-measurements might no longer be conducted.

l.98: ‘high spectral resolution’

Should this not read ‘high spatial resolution’?

l.106: ‘are reported on pressure levels with an approximate spacing of…’

The reported retrieval-grid of remote sensing data is generally not  equal to its spatial resolution. Therefore, I would suggest to add the information on resolution from here: https://mls.jpl.nasa.gov/data/v5-0_data_quality_document.pdf, p. 157

l.146: ‘to the pressure coordinate of the MIPAS data’

Shouldn’t this read ‘to the altitude coordinate…’?

l.236: ‘The reason for this remains unclear, and no explanation or documentation for this difference was found in the literature.’

You might try to contact the MLS-team for a possible explanation(?)

Chapter 3.2: ‘Background seasonal cycle in the MLS data’

Is there any possibility to infer the disturbing seasonal cycle from comparing different years with less volcanic influence?

l.290, Table 1: 

I strongly recommend consolidating all the information from Table 1 and Table B1 into a single comprehensive table. Additionally, I suggest including the uncertainties reported in Höpfner et al. (2015) and the results from the WACCM simulations, including those for the 18–22 km layer if available. This enhancement would greatly improve the readability of the discussion in Chapter 3.3, allowing readers to follow the analysis more easily without needing to consult multiple tables.

l.290: ‘Their values show a clear increase of e-folding time with height, which is not as apparent in our results.’

However, the decay timescales provided here also show increasing values (with one exception of MLS in case of Kasatochi). Further, I suggest also to try to provide best-estimates from your analysis of the decay-timescales  at 18-22 km for MIPAS in case of Kasatochi and Sarychev – when looking at Fig. 2 and Fig. A1, there seems to be a clear signal.

l.295-314: comparison to WACCM

The discussion may give the impression that the uncertainties in the decay timescales derived from the measurements are too large to allow for meaningful comparisons with the model. However, upon examining Figures 5, C1, and C2, it appears that for the Sarychev and Nabro eruptions, the model significantly underestimates the decay timescales compared to the limb-sounding datasets, whereas this discrepancy is less pronounced for the Kasatochi eruption. Could you provide possible explanations for this observed difference?

l.355: ‘our main focus here is on the decay times of the stratospheric inputs of the indicated eruptions and not the total stratospheric mass.’

The entire chapter 5 of the manuscript (as well as section 2.5 ‘Calculation of total stratospheric SO2 burden’) is dedicated to the ‘Estimating the stratospheric SO2 burden’. Therefore, I don’t understand this statement. I would suggest to delete this sentence and extend chapter 5 a bit by extending Table 2 to include the estimations of stratospheric SO2 mass by the extrapolation methods used by Pumphrey et al. (2015) and Höpfner et al. (2015). I would also suggest to add the results (M(t0)) from the fits performed in the present work. It should be made clear that the method described here in section 2.5 is not adequate to calculate the total stratospheric SO2 burden in case of MIPAS and, to a less extend, also for MLS.

l.400: ‘…and should be considered when analyzing volcanic SO2 with OMI’

I would suggest to add here: ‘and other nadir-sounding instruments’.

l.413-417: eruptions with ash

The eruption of Puyehue in June 2011 was also rich in ash. Have you tried to inspect that one for any effects on SO2 lifetime? (e.g. Griessbach et al., 2016, doi:10.5194/amt-9-4399-2016)

l.424: ‘Our work suggests that the current SO2 data reported by available observational products are subject to significant uncertainty when examining the stratospheric lifetime of volcanic SO2 and suggests that more precise data is needed if chemical mechanisms and SO2 mass loading following an eruption are to be elucidated using observed decay times.’

On one hand, I support this statement, particularly considering the imminent loss of limb-sounding capabilities for stratospheric SO₂ observations, which will create a significant gap in our ability to monitor the stratosphere. On the other hand, I find the statement somewhat overly general. As noted in the manuscript, each observational technique has specific advantages and limitations in quantifying stratospheric SO₂. Therefore, to effectively evaluate and refine models, it may be more appropriate to tailor comparisons to align with the strengths of each dataset. For example, model results could be compared directly with nadir and MLS data closer to the eruption time, while comparisons with IR limb-sounding datasets might be more suitable for periods several weeks after the eruption.

Review of “Quantifying the decay rate of volcanic sulfur dioxide in the

Stratosphere” by Nicknish et al.

This is an interesting paper, but needs some additional explanations, justification of the approach and some data analysis. 

The basic idea is to quantify the decay rate for SO2 following volcanic eruptions.  The authors looked at MLS and MIPAS SO2 retrievals and did some additional comparisons with OMI SO2 observation. SO2 converts to H2SO4 which forms aerosols.  Stratospheric aerosols have been linked to surface cooling so understanding the conversion process is important.  

Major issues:

1. I was hoping to see a ‘back of the envelope’ check against the total aerosol eruption mass burden – the end product of SO2 oxidation. This would be a useful addition. (See Schulte et al., 2023,https://doi.org/10.5194/AMT-680 16-3531-2023 on computing the total mass). There are a number of stratospheric aerosol sources you can use – but probably GLOSSAC is the best.  This sort of ‘stupidity check’ would confirm that the SO2 estimates agree with aerosol production - which is why we care about this.
2. The MLS algorithm, as I understand it, generates negative mixing ratios for data on the edge of observability. The correct way to deal with these are to average the data over larger regions including both positive and negative mixing ratios. I was looking for a discussion of this and mention of MLS validation also found in Livesey et al. (2022) (found at https://mls.jpl.nasa.gov/eos-aura-mls/documentation.php). Discussion of how to use the data including quality flag screening that is appropriate for SO2 is also found there and should be mentioned in the data description (lines 65-75).  An equivalent MIPAS discussion is needed.
3. The division of the SO2 into three separate regions (10-14), (14-18), (18-22) made me uncomfortable. At high latitudes in winter these regions are all in the stratosphere – in the summer the 10-14km may include the troposphere. In the tropics (as with Nabro and many other eruptions), only 18-22km is entirely in the stratosphere. This distinction can play an important part since the water vapor content and OH concentration (eq. R1)of these layers can be quite different – upper troposphere vs lower stratosphere – and thus will affect the decay rate.  Since MLS and MIPAS also make water vapor measurements, the water vapor content can be added to the analysis.   It seems to me that the the authors should have used two layers - below the tropopause and above the tropopause - rather than what was done here. It is easy to get tropopause height information from reanalysis data sets (GFS, MERRA2, ERA5).
4. The authors neglect the transport between the layers. Exchange between layers needs to be discussed as possibly influencing the decay rate. 
5. The authors are using a zonal mean SO2 on a 10° latitude grid (ln 153). It seems like they could also construct a tighter latitude grid (say 5°) and a longitude grid and select high SO2 regions which might reduce the uncertainty (e.g. Fig. 2). I would like to see how this affects their decay rate and agreement between the two satellite instruments.
6. I don’t think adding OMI SO2 helps the paper at all. In fact, it just adds noise, not signal. This is because the OMI total column includes massive amounts of tropospheric SO2 (for most eruptions) which – as the authors note – probably explains the significant differences in total SO2 mass and decay rate.  If you add OMI you might as well take a look at SO2 measurements from AIRS (mentioned line 71) as well for completeness.  Anyway, I suggest you just drop this section – it really adds nothing.
7. The WACCM simulation is interesting but barely discussed (Fig. 5). Take a look at the water vapor in WACCM. Was it the same as MLS observations? This might explain the accelerated decrease.

Minor comments: 

Add layer labels to Fig. 2

Line 287 ‘less uncertain’  - how about ‘better’

Fig 4.  Why not connect the dots vertically. The figure – as is, is a little hard to read.

Line 418 “Honga-Tonga”  - the APARC group recommends using ‘Honga’ not HTHH or HT or other acronyms.  The Honga eruption is a good example where hydrolysis probably played a critical role in accelerating the decay of SO2 and conversion to aerosols as noted.  This is why I recommend the authors also take a look at H2O in other regions.