Successful Derivation of Absorbing Aerosol Index from the Environmental Trace Gases Monitoring Instrument (EMI)
Round 1
Reviewer 1 Report
In this study, the authors retrieve the absorbing aerosol index (AAI) from the Environmental Trace Gases Monitoring Instrument (EMI) on board the Chinese GeoFen-5 and compare it with the AAI of TROPOMI in certain regions and dates. Additionally, the authors provide a correction of the AAI for striping reduction. A comparison with the AOD of AERONET against the AAI of both EMI and TROPOMI is also provided. This last point raises some doubts for me because AOD and AAI are variables that are related but not necessarily linearly dependent. In general, the methodology lack of clarity and the filters applied to the data are not explained in the manuscript. The topic of the paper could be of interest for the journal but major changes should be addressed before being published.
Major comments:
- The introduction is slightly difficult to follow. From the first paragraph to the second, the connection between both is not clear. The word “Furthermore” does not seem to fit in this context. I suggest to write clearer the Introduction.
- In the result section 4.1, there are high differences between the mean values of global EMI AAI and TROPOMI. Stating that they have similar pattern is not enough. Please, develop the possible reasons why there are such high differences.
- For the comparison of TROPOMI and EMI of Section 4.1, which was the size of the dataset compared between TROPOMI and EMI? Specifically, this is not clear for Figure 5. How many points were compared in the statistics of Table 2? Please, include this information in the manuscript.
- How did the authors handle the different spatial resolution of EMI and TROPOMI for the analysis? I could not find any reference to this topic in the manuscript.
- Section 4.2. is based on the comparison of AOD vs. AAI of EMI and TROPOMI. Perhaps this type of analysis does not make sense in the way it is explained in the manuscript, i.e., the daily trends of AOD do not have to be the same as the daily trends of AAI. For example, if there is dust, the AOD might be higher but, this is not always the case. Please, look at comment of line 297. This section should be rewritten and better explained: the AAI and AOD do not have to have a linear relationship. Therefore, comparing the correlations between AOD and AAI does not provide interesting information and can make sense only in certain cases. The correlations are very low between AOD and AAI because these variables do not fully depend on each other. Please, also correct the comments of the Conclusions like the one from lines 376-378.
- The data of EMI and TROPOMI seems to have been filtered for the comparison and the correlation analysis. This is not explained in the Methodology nor in the corresponding section. I believe it is important to explain the filters applied to the data in order to reproduce the work.
Minor comments:
Lines 27-28: I believe the correlation of the AOD is very low (0.03 for EMI in October). As I mentioned in the Major comments section, comparing the AOD against the AAI probably does not provide much information because those variables can be related but do not have to be related. Please, modify this sentence accordingly with the results of Section 4.2.
Line 20: Please, reformulate the mention to “the statistical method”. It is not formal for a scientific publication. Please, specify briefly which statistical method is used.
Line 40: The sentence is difficult to follow. I would replace the reference to the “lifetime”
Line 85: The reader might not be able to understand these sentences. Please, rewrite them.
Line 139: Here the authors mention that the atmosphere contains absorption. Which kind of absorption?
Equations 2-3: The explanation of the terms of Equation 2 and 3 are not properly written in the text. For example, in the Equation it is of the form: Asc but then in line 142 it is written “Asc”. Please, be rigorous with the mathematical equations. It occurs the same with Equation 5. Please, correct this.
Line 158: Could you please give more information about how you modeled AAI in the presence of clouds by using SCIATRAN?
Line 178: Please, develop this point. What do the authors mean with unsophisticated parametrization?
Line 237: To which box are the authors referring to?
Line 244: Why do you assume that the AAI>1 in August is due to biomass burning activities? It could be possible that it is also dust.
Table 2: It is not specified which data are being compared in Table 2. Is it referring to August, October or both? Please, write clearly all the information about the intercomparison and the size of the data compared.
Table 2: I would suggest to just write the abbreviations directly and explain them in the caption.
Figure 4: Please, provide a figure with bigger labels and titles. Currently it is difficult to read the labels. In addition, the size of the maps is different for the left and right figures. I suggest that all of them have the same size.
Line 275-277: What do the authors mean by stating that “the RMSE was somewhat unsatisfactory”? This should be explained clearly.
Line 283-284: How can EMI provide higher spatial detail in desert areas compared to TROPOMI if EMI has lower spatial resolution than TROPOMI? Please, explain this.
Figure 6: What is the vertical white line at 0º representing?
Figure 7: Why there are so few points in Figure7? Were the data filtered? If so, please explain in the manuscript.
Figure 7: In the legend it is written N=500 but there seems to be less than 200 points. Could you explain this? Additionally, to what corresponds the color bar?
Line 297: The AAI does not depend proportionally to the AOD. Please, correct this sentence. The AOD and AAI might be proportional in some cases but not in every case. Therefore, the comparison of the AAI with the AOD as if they were one linearly dependent of the other can lead to misunderstanding.
Line 300: The daily averages are shown, instead of the monthly averages in Figure 8. Correct the sentence.
Figure 8: Why in the left figure the points do not correspond with the daily labels? On the right figure the dots correspond with the ticks of the X axis.
Figure 9: With what specific data has the correlation been made? For example, with how many points or days of the month?
Figure 12: The plot of day 12.11.2019 has no correlation for the negative values of AAI. Do you have an idea why might this be happening?
Line 335: What does it mean that there were “suspected intense bushfire events”. Is there any source that can confirm this?
In general, please avoid the words like “somewhat” or “somehow” in the manuscript. It is imprecise for a scientific paper. Please, replace the words with a more appropriate term.
Author Response
We sincerely appreciate the referee for the valuable comments, which improves the quality of this manuscript a lot. Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Interesting research on aerosol tracking by remote sensing. I have no significant comments. Well, thank you.
Author Response
We sincerely appreciate the referee for the valuable comments.
Reviewer 3 Report
The paper presents for the first time retrieved the absorbing aerosol index based on the measurements of the environmental monitoring spectrometer EMI on board the Chinese GeoFen-5 satellite. The results from the satellite have been corrected statistically. The authors show consistency of the EMIAAI and data with the TROPOMI monitoring instrument. The consistencies between the two datasets have been discussed and have a high statistical significance. The successful retrieval has been compared with AERONET ground based data with coefficients highly significant. They also present a case study on the daily variability in a wildfire in Australia. The overall conclusion is that the AAI retrieval algorithm of the EMI instrument can be used for monitoring widely spread absorbing aerosol events. There is still work to be done which includes clouds and ozone absorption on AAI which needs to be mentioned in larger extent than its present description at the conclusions of the paper. Overall, the paper is of high caliber and could be published as it is.
Author Response
We sincerely appreciate the referee for the valuable comments.
Round 2
Reviewer 1 Report
I believe the authors have widely improved the manuscript with the changes they have made. I recommend to accept this manuscript with the new changes.
