Sea Salt Aerosol Identification Based on Multispectral Optical Properties and Its Impact on Radiative Forcing over the Ocean

Round 1

Reviewer 1 Report

The study uses spectral derivatives of satellite-derived aerosol optical depth (AOD) at multiple wavelengths (i.e., ??/?l and ?²?/?²l) to characterize aerosol types by their optical properties, with the goal of isolating sea salt aerosols to better understand their radiative forcing. The study uses six AERONET sites, cloud properties and aerosols from MODIS Terra, CERES shortwave fluxes, and aerosol type layer information from CALIPSO; it also uses the 6S model to simulate AOD across wavelengths. The authors find that different aerosol types have characteristic spectral derivative profiles that can be used to identify aerosol type in the MODIS AOD product, even in the absence of retrieved single scattering albedo. The manuscript is clearly written and flows well, but it could use further discussion of its motivation and the consequences of its results. I can recommend it for publication after minor revision. My specific comments are below.

 

Lines 44-51. Do we have reason to suspect a trend over time in the radiative forcing by sea salt aerosol?

 

Section 4.3. As this section says, “pure” type aerosols are rarely found in the real world; but in addition to mixing, CALIPSO frequently finds discrete layers of different types. As column measurements, MODIS and AERONET cannot distinguish multiple layers of aerosol. Does this matter for the purposes of this study?

 

Figure 6. Is there really so much mineral dust over the Southern Ocean? MODIS AOD retrievals can be less reliable at extreme latitudes, and it seems like DS and SS still have quite similar profiles even for the normalized second derivative.

 

Section 5. The discussion is short and a little anticlimactic—could you elaborate here on the motivation to focus on sea salt, beyond its having been dropped from the IPCC radiative forcing chart? What do the relationships in Figure 7 mean for the global climate? Do you recommend adding an aerosol type classification using spectral derivatives to the standard MODIS product?

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

In the manuscript the authors report the analysis of the first and second order spectral derivative of the aerosol optical depth (AOD), with the aim to determine the main aerosol components in aerosol mixtures, on the basis of their multispectral optical properties. The analysis is based on theoretical simulations (6S) and in situ (AERONET) and satellite (MODIS) measurements. Despite the method was still described and used in a previous paper of the same authors (Lin, T.-H.; Tsay, S.-C.; Lien, W.-H.; Lin, N.-H.; Hsiao, T.-C. Spectral Derivatives of Optical Depth for Partitioning Aerosol Type and Loading. Remote Sens. 2021, 13, 1544. https://doi.org/10.3390/rs13081544), in this manuscript the method is applied to quantify sea salt (SS) aerosol properties and their impact on radiative forcing over the ocean. 

In my opinion, the work should be published in the Remote Sensing journal because the identification of the main aerosol component in aerosol mixtures is still a challenge.

Nevertheless, the manuscript needs major revision mainly in comparison of the previous work. I suggest to highlight the differences of the conducted study compared to the previous work. In the manuscript are often reported the same observations already reported in the previous work and the same bibliographic references. I would suggest to refer to the previous work for the “Methodology”, only highlighting the differences. Moreover, the “simulation” and “spectral derivative” sections must be improved because they are better described in the previous work. Finally, the english should be revised.

Minor comment:

 1.     Lines 60-64: please add references

2.     Line 130: are typically used

3.     Line 132: 6S is a RTM consisting…(please delete the first “widely used”)

4.     Line 137:  model was used to reproduce spectral…

5.     Line 169-170: Eq. 2 depends on α related to the particle size distribution and the aerosol load (τ)

6.     Line 173: …the Normalized Derivative Aerosol Index

7.     Line 201-210: The authors must better explain which aerosol source the stations reported in the Table 1 refer.

8.     Line 345: “Figure count”…???

9.     Line 355: Figure 3c and 3d The axis labels are wrong.  

10. Line 415: Table 4 Please correct the number of digit of the errors

11. Some comment to the values reported in the tables could be useful.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

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