the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Nov 2023
Multi-angle aerosol optical depth retrieval method based on improved surface reflectance
Abstract. Retrieval of terrestrial aerosol optical depth (AOD) has been a challenge for satellite Earth observations, mainly due to the difficulty of estimating surface reflectance caused by land-atmosphere coupling. Current satellite AOD retrieval products have low spatial resolution under complex surface processes. In this study, based on our previous studies of AOD retrieval, we further improved the estimation method of surface reflectance by establishing an error correction model and then obtained a more accurate AOD. A lookup table is constructed using the Second Simulation of Satellite Signal in the Solar Spectrum (6S) to obtain high-precision retrieval of AOD. The retrieval accuracy of the algorithm is verified by AERONET (Aerosol Robotic Network) observations. The results indicate that the retrieved AOD based on the improved method of this study has advantages in fewer missing AOD pixels and finer spatial resolution, as compared to the MODIS AOD product and our previous estimation method. Among the nine MISR angles, the optimal correlation coefficient (R) of retrieved AOD and observed AOD can reach 0.89. Root mean square error (RMSE) and relative mean bias (RMB) can reach a minimum values of 0.20 and 0.32, respectively. This study will help to further improve the accuracy of retrieving multi-angle AOD at large spatial scales and long time series.
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RC1: 'Comment on amt-2023-204', Anonymous Referee #3, 13 Dec 2023
General comments:
The authors improve upon their own prior work to estimate the surface reflectance from MISR measurements, and then use the improved estimate in conjunction with a look-up table-based approach to obtain a new aerosol optical depth product. They validate their product with AERONET measurements. They contend that the new product agrees well with AERONET data and has a higher spatial resolution and fewer missing pixels than the MODIS product.
There are a number of major flaws in the manuscript. The new methodology is not explained clearly. The description is convoluted and the novelty is not obvious. While the new product does have less missing pixels than the MODIS product, the improvement compared to existing products seems to be marginal. In my opinion, the authors need to do a significant amount of work to make this suitable for publication in AMT. For this reason, I would recommend rejection of the manuscript as currently written.
Main comments:
Sections 2.1 and 2.2: There is information overload here. The authorize should summarize in one or two paragraphs exactly what information is used and from what datasets.
Section 3.1: The description here is very confusing and hard to follow.
Section 3.2: On the other hand, there needs to be more detail here about how exactly the surface reflection is corrected to account for atmospheric effects as that is critical to the topic addressed by the authors. How exactly is the 6S model used for the atmospheric correction step a? More details need to be provided here, such as how atmospheric parameters relevant to the RT calculation (gas absorption? Aerosol optical depth? Aerosol layer height? Surface BRDF model?) are chosen.
In Equation (1), how are the MISR and MODIS BRDF obtained?
Conceptually, what is the difference between the “MODIS surface reflectance at the MISR angle” and the “MISR surface reflectance”?
Section 4.1: How do we know that the “improved” surface reflectance is more reflective of reality?
Figure 5 should be accompanied by a figure showing the results using the old surface reflectance data, so that the changes in the AOD results can be clearly seen.
Figure 7: There is a lot of scatter in the results. I am not sure the new method is doing as good a job as the authors claim.
Figure 8: These results are better but there is still a fair bit of systematic bias compared to AERONET data.
Again, the old results should be overplotted to show the improvement.
Figure 9: The comparison with MODIS is also not very good.
Table 1 is very confusing and there does not seem to be any explanatory description of the results therein.
Technical comments:
The paper is very poorly written and extremely hard to understand. There are grammatical and typo errors littered throughout the document. Here are some examples of awkwardly phrased sentences:
Lines 115-116: 36 channels of MISR data are included, all of which can be retrieved for AOD.
Lines 164-165: Therefore, it can provide aerosol characterization parameters with high accuracy and validate the aerosol parameters from satellite retrievals
Lines 198-199: In 6S model, a series of parameters related to the simulated imaging date atmospheric conditions need to be input
Lines 257-258: Compare the improved AOD with the previously retrieved AOD, and analyze the accuracy and spatial distribution trend of the improved AOD.
Line 124: Arcgis, ENVI not defined
Line 163: AOD values can be retrieval -> AOD values can be retrieved; an retrieval error -> a retrieval error
I strongly recommend that the paper be proofread before possible resubmission.
Citation: https://doi.org/10.5194/amt-2023-204-RC1 -
AC1: 'Reply on RC1', Lijuan Chen, 14 Jan 2024
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2023-204/amt-2023-204-AC1-supplement.pdf
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AC1: 'Reply on RC1', Lijuan Chen, 14 Jan 2024
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RC2: 'Comment on amt-2023-204', Anonymous Referee #1, 25 Dec 2023
General Comments:
In this manuscript, mainly based on the first author’s previous works (Chen et al., Tellus B: Chemical and Physical Meteorology, 2021, 73, 1940758; Chen et al., Advances in Space Research, 2021, 67, 858-867) and with the same MISR and AERONET datasets (MISR data with 9 camera angles on June 12, 2018 for AOD retrieval map show, Taihu and Xuzhou AERONET sites), an improved linear correction Equation (Eq. 3) is used to update the MISR surface reflectance. With the new updated MISR surface reflectance, the aerosol optical depths (AODs) of 9 camera angles are retrieved by the lookup table by 6S, respectively, and further systematically validated by the AERONET, as well as the MODIS AOD products.
Specific Comments:
However, this manuscript didn’t explain in what way to extract the semi-empirical relationship in Eq. (3), which plays a most import role in the completeness and logic of this manuscript. Besides, since the title of manuscript is about “multi-angle aerosol optical depth retrieval method”, it is a great pity that this manuscript didn’t discuss how to retrieve the AODs and other key aerosol optical parameters by taking the full advantage of 9 camera angles’ measurements together with the Eq. (3), which is also very important to improve the MISR’s aerosol retrieval accuracy. Moreover, the structure and content of this paper are very similar to previously published paper (Chen et al., Tellus B: Chemical and Physical Meteorology).
Technical corrections:
(1) Figs. 1, 3, 4: for the surface reflectance results shown in these figures, which wavelength band is used?
(2) Fig. 2: For figure 2, more detailed description needs to be added in section 3.3, such as what kind of aerosol type (model) was used for atmospheric correction and AOD retrieval? Have the authors considered the error transfer caused by the aerosol model and atmospheric correction?
Citation: https://doi.org/10.5194/amt-2023-204-RC2 -
AC2: 'Reply on RC2', Lijuan Chen, 14 Jan 2024
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2023-204/amt-2023-204-AC2-supplement.pdf
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AC2: 'Reply on RC2', Lijuan Chen, 14 Jan 2024
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