Review of the manuscript titled “Satellite Aerosol Composition Retrieval from a combination of three different Instruments: Information content analysis” by Stöffelmair et. al., 

This manuscript focuses on retrieval of AOD and its components from the valuable apace-borne instruments  such as SLSTR on Sentinel 3A/3B, IASI and GOME-2 on Metop A/B/C. Further, this study has used  SCIATRAN and uses MERRA-2 reanalysis data to study information content of the conducted retrieval and suggested that 6 to 15 degrees of freedom for the determination of aerosol components dependent on AOD and the underlying surface. 

This manuscript is well written and all the analysis are discussed in a very appropriate way. The information content analysis presented in this study will be meaningfully supporting the future AOD retrievals over various regions of the globe. I would recommend this manuscript for publication after these minor comments are addressed. 

Minor comments: 

1. Change the title, as in this study the satellite is being used to detect the AOD and its components, thus change the title as follows “Aerosol Composition Retrieval from a combination of three different space-borne Instruments: Information content analysis”. This title will be more appropriate.

2. Please correct the line 4 as “A simulation study has been carried out to analyse the information…”

3. The introduction is very well written and easy to follow.

4. The theory of information content and Optimal Estimation is also discussed in a very understandable way.

5. Please add a paragraph focusing only why SCIATRAN in particular is considered in this study and what are the advantages section 3.1 “ Radiative transfer forward model”.

6. Similarly, please add a paragraph focusing only why only these satellite instruments are used in this study and advantages of selecting these instruments in section 3.2 “Satellite Measurements and Observation Vector”.

7. Please improve the titles of all the figures, so that the readers can easily follow the figures.

8. Discuss why there is no data information available over Antarctica, may be add two to three sentences in the result section.

9. Separate the “Discussion and Conclusion” sections into two sections and also in discussion section section add a paragraph about how the information content analysis can be useful for the exiting and future satellite missions.

At the end, I would like to mention that this manuscript is short, concise, and the results are well discussed and contains valuable information for the enhancement of current and future retrieval process of AOD and associated components. I wish the authors in advance Merry Christmas and successful start to the new year!

1. The assumption of Lambertian surface reflectance removes one of the key sources of uncertainty when using visible and near-IR measurements for aerosol retrieval. Simulations made using this assumption will thus overestimate the information content of SLSTR measurements available for constraining aerosol properties.
2. The use of simple instrument noise estimates to set the covariance matrices used in the analysis ignores forward model error and correlations between the elements of the measurement vector, both of which will decrease the information available to constrain aerosol properties.
3. The authors also seem to neglect the vertical distribution of aerosol, which can have an impact on TOA radiance significantly exceeding aerosol composition in both the UV and TIR. Dealing with this in a real retrieval scheme will again reduce the information available for constraining aerosol composition.
4. The simplistic way that the measurements from the individual instruments is modelled (neglecting the problems of co-location and matching the very different viewing geometries and spatial sampling of the instruments cited, as well as not attempting to accurately model the spectral response of the real instruments) also neglects important sources of error which would be present in a real retrieval scheme and further abstracts the simulations performed from what might be expected from real measurements.
5. The authors never address another significant source of error and complexity in any aerosol retrieval scheme, which will be even more difficult to deal with when combining three instruments with completely different sensitivities and sampling - clouds!

• Are radiative-transfer calculations performed at the native resolution of each simulated instrument and then averaged onto the GOME-2 grid, or are all calculations performed on the GOME-2 grid from the start? (I assume the latter).
• For IASI, are you essentially simulating L1C data by performing the radiative transfer calculations assuming a flat instrument response function?
• In the case of GOME-2, are you simulating realistic GOME-2 spectra, or more simplified "GOME-2 like" spectra. Saying you use "a wavelength step of approximately 10 nm" is quite vague.
• I feel the description of the measurement errors/uncertainties should be included in the description of the observation vector.