the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jul 2023
Retrieval Algorithm for Aerosol Effective Height from the Geostationary Environment Monitoring Spectrometer (GEMS)
Abstract. An algorithm for aerosol effective height (AEH) was developed for operational use with observations from the Geostationary Environment Monitoring Spectrometer (GEMS). The retrieval technique uses the slant column density of the oxygen dimer (O2-O2) at 477 nm, which is converted into AEH after retrieval of aerosol and surface optical properties from GEMS operational algorithms. The AEH retrieval results show significant AEH values and continuously monitor aerosol vertical height information in severe dust plumes over East Asia, and the collection of plume height information for anthropogenic aerosol pollutants over India. Compared to the AEH retrieved from Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), the retrieval results show insignificant bias with a standard deviation of 1.4 km for the AEH difference over the GEMS observation domain from January to June 2021 due to uncertainty in input parameters for aerosol and surface. The AEH difference depends on aerosol optical properties and surface albedo. Compared to the aerosol layer height obtained from the tropospheric monitoring instrument (TROPOMI), differences of 0.78 ± 0.81 and 1.16 ± 0.92 km were obtained for pixels with single scattering albedo (SSA) < 0.90 and 0.90 < SSA < 0.95, respectively, with significant dependence on aerosol type.
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Status: final response (author comments only)
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RC1: 'Comment on amt-2023-136', Anonymous Referee #1, 29 Jul 2023
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AC1: 'Reply on RC1', Sang Seo Park, 16 Sep 2023
Thank you for the reviewer's comments. We attached our reply for the revised manuscript.
Citation: https://doi.org/10.5194/amt-2023-136-AC1
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AC1: 'Reply on RC1', Sang Seo Park, 16 Sep 2023
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RC2: 'Comment on amt-2023-136', Anonymous Referee #2, 01 Aug 2023
Review of "Retrieval Algorithm for Aerosol Effective Height from the Geostationary Environment Monitoring Spectrometer (GEMS)" by Sang Seo Park et al, 2023.
The paper introduces the newly developed GEMS aerosol effective height product. This is an interesting new product. Aerosol vertical profile and height products are scarse and satellite instruments that monitor aerosol height are needed to constrain models. Global products are few, notably the CALIOP instrument which resolves the aerosool vertical profile, albeit in a small swath, the TROPOMI instrument with global coverage which retrieves an effective height for global average aerosol properties, and the GOME-2 instruments, which retrieves an effective height for dense absorbing aerosol plumes. At present, no aerosol height product from a geostationary satellite is available, and the results from GEMS are very interesting scientifically.
General comments:
The paper presents some technical details of the retrieval algorithm and validation results from a limited but well-chosen intercomparison. However, at present the presentation quality of the paper and the scientific results are not sufficient to warrant publication. My main issue is that the level of detail is poor and seems rather arbitrary. From the manuscript, it cannot be deduced what methods were used, and even a definition and derivation of the AEH, the main product, is missing. On the other hand, some very detailed information of tests are presented, of which the results are not given and referred to in other papers.
The second major issue is that the presentation quality should be increased. At present, the manuscript is poorly structured, with results of tests in the algorithm section, paragraph describing different subjects than indicated, and numerous unjustified qualitative statements, like "good", and especially the overused word "significant". This should be improved by restructering the manuscript and quantifying statements. The use of English should be improved.
The scientific relevance is "significant", so I recommend that the manuscript be published eventually, but only after major revisions have been made. First, I recommend that the manuscript be completely restructured, following a normal setup including Introduction, Method and Data, Results, Conclusions and Discussion section, with all text strictly confined to their proper section, and with the specific comment detailed below included. Second, I recommend a full new review, because the manuscript in its present form cannot be reviewed for content.
Specific comments:Introduction:
l. 53-58. It is suggested here that UVAI is retrieved for atmospheric correction of gaseous species retrievals. This is not true. The UVAI has been retrieved as a ozone absorption residual, which turned out to be mainly due to absorbing aerosols. As such it has been useful for the detection of absorbing aerosols, but only ever in a qualitative way. These sentences should be changed to eitehr properly describe atmospheric correction or properly describe the function of the UVAI (and scattering AI).l. 55. cloud signals are not used to derive UVAI. In fact, aerosol signals are not used to derive UVAI as such, but aerosol (absorption) affects the reflectance in the UV measured by a satellite compared to a cloud-free and aerosol-free atmosphere, which can be computed. Clouds affect the reflectance in a different way, which can also be expressed in terms of the UVAI. Please, rephrase and be concise and be specific about the message here.
l 58-60. AOD and clouds are not specifically retrieved from the UV, more often in the visible (aerosols) and the near IR and IR for clouds. What is the purpose of this sentence about UV wavelengths here?
l. 78-80. and because of the generally much smaller signal from aerosols compared to clouds.
2. AEH retrieval algorithm
Main comments:
* Please, introduce the AEH here. Only a reference is given to the paper by Park et al (2016). The reader should have some knowledge of the subject of the current paper, without having to search for other papers.* A GEMS instrumentation and data section is missing.
* A product description is missing. Please, provide some guidance with the presented flowchart.
l. 122: The AEH is apparently something with (1-1/e)* AOD. Please, give a definiition and/or derivation of the product.
l. 121: "(...) the aerosol vertical layer parameter, which represents the highest altitudes with existing effective aerosol extinction sensitivity." Please, rephrase. This sentence has no meaning. Which "highest altitudes"? What "existing sensitivity"?
l. 125: specify the GDF and the FWHM of the assumed aerosol layer profile.
l. 135.: "Because the spectral coverage is limited to 300-500 nm, .." Does this refer to GEMS? Why is it limited?
l. 136 -140: repetition of information, merge with introduction.
l. 142: ".. considering THE aerosol types .." What aerosol types? This is not clear at this point. Please, provide a methods section, in which the aerosol models are introduced which are used by the algorithm. Merge with the information provided later on aerosol properties/LUT calcalutions.
l 143-144: Repetition of information. Merge with an accuracy assessment section.
l. 151: "long-term GEMS (...) data." What do you mean with long-term? Be specific, if this is relevant at all.
l. 167: "From Nanda (...) retrieval." This is irrelevant information in this section. Please, restructure the manuscript to describe the AEH, ALH and the different tests and analyses seprately.
l. 174-176: Merge this with the actual description of the LUT on page 10.
l. 181: "Observed radiance fitting is affected by noise signals during radiance observation." I do not understand what this sentence means. Please, rephrase.
Table 2: It is unclear what is fitted. Please, describe the test and/or the polynomilas and their meaning
and the test results.page 9: Somehow during the description of the AEH algorithm we have entered some fitting testing. It is unclear what the reason and the outcome is. Please, provide an overview of this test, if relevant, and quantatative results.
l184-185: "Although the fitting quality was good overall, the setting with the smallest error was used in this study." I surely hope so! Howveer, as mentioned, the test and results are unclear. Pleasew, remove this statement and provide more useful information.
l. 192-197. This information is not about the LUT. Please, put it in a more appropriate section.
l. 201 and 203: Please rephrase the following:
"Thick aerosols"
"the effective scattering layer penetrates more deeply ..."
"dominant aerosols"l. 206: How significantly? This is useless information, please quantify.
Table 3: Please, provide the real part of the refractive index as well for the various aerosol models, and I suggest to add a separate section on aerosol models used.
l. 216: "...(AER_LH) retrieves vertically localized aerosol layers in free troposphere .." This is not true. Do you mean in cloud free scenes?
l. 218-219: "Spectral fit estimation of reflectance around the O2-A band is based on a neural network for the forward model calculation." This is not true. The spectral fit is the optimal estimation of the retrieval part, mentioned in l. 220. The forward model consists of a simulated spectrum provided by the neural network.
l. 221-223: "During the radiance fitting, AOD is also used as the main fitting parameter, but other aerosol parameters, such as SSA and scattering phase function, are assumed to be fixed values (Nanda et al., 2020)." ALH and AOD are (currently the only) fitted parameters, the aerosol model is assumed (SSA, g and aerosol layer thickness are fixed).
l. 241-242 "(..) layer height parameter is estimated by using the vertical profile of extinction coefficient at 532 nm. ". How is this done? Do you calculate an effective height? How? Or use the top and bottom aerosol layer products? Please, specify.l. 248. "Based on the retrieval sensitivity of AEH (...)". Which sensitivity? Please, show this in this paper, or repeat the conclusions from another.
l. 255: "Although the AOD and SSA for these plumes are differed significantly, their AEH results were similar. " Improper English, please rephrase. Why would you assume a correlation?
l. 256. " (...) the retrieved AEH values exhibited insignificant diurnal variation (..)"
What is insignificant? Can you show this in e.g. a line plot?
Figure 2: These small plots are not readable. Please, device a better way to show the case, it's diurnal variation and the correlations you try to show. 21 plots just won't do it. The caption is insufficientl. 266: "siginficant". What does this mean?
l. 267: Please rephrase the following:
"severe aerosol plumes"
"AEH was stably estimated"
and l. 279.
"larger aerosol heights"l. 280-281: "In an ideal case, the AEH from GEMS was overestimated by around 0.5 km relative to the ALH from TROPOMI, (..). Why in an ideal case? What is overestimated? Do you mean the altitude was higher?
l. 278-284. Place in a discussion section and elaborate.l. 283-284. "(...) the GEMS AEH retrievals for the dust transport case study were good." Why? What is good? Please, quantify.
l. 285-286: "Furthermore, the retrieval area covered by GEMS is larger than by TROPOMI, as demonstrated by a comparison of Figures 2 and 3.". The shown area in Figs 2 and 3 are equal, so this is not demonstrated from comparing the figures. Rephrase.
l. 289. What does "greater aerosol height information" mean?
l. 289-291: "Although high height values were retrieved for clear-sky regions, in particular low latitude ocean regions, the AEH from GEMS was successfully retrieved over the area of interest for the case study.". Even after three tries, I fail to understand this sentence. Please, rephrase.
l. 311-321. This paragraph discusses Fig 4b., but it starts with the sentence on Fig 4a. Elaborate on Fig 4a, because it is not further described. Then describe Fig 4b in another paragraph.
l. 317-319. If surface reflectance is retrieved, why use a LER? Why mention this?
l. 319-321. What is the impact? If you mention this, then you should either explain why a LER is used and quantify the error/uncertainty, or use the surface reflectance.
l. 329. "To correct the inconsistency of definition, (...)". What correction has been applied?
l. 330. "After correction, ". What correction?
Figure 5. Same as Fig2: decrease the number of panels and device a cleverer way of presenting the results, 21 panels are not readable.
l. 351. "(...) the GEMS (..) AEH can be used in several application studies.". What studies? How? Please, remove a vague commentary and be explicit in what you mean.
l. 378-397. should go into a discussion section and be removed from the results section.
l. 381-382. "Although L2AERAOD accurately retrieved the optical and physical properties of aerosols (...)". Improper English, please rephrase.
l. 385. "has significant root mean square errors ". What does that mean? Please, rephrase.
l. 388. Quantify "significantly".
Figure 9. Caption is not complete. Specify the boxes, line and bars.
l. 439. "TROPOMI is limited to retrieving the ALH over scattering-dominant aerosols". This is difficult to understand and I think it is untrue (although I'm not sure what it mneans). The ALH v1 that was used in Nanda et al (2020) was retrieved only in cloud-free scenes with UVAI>1, so for absorbing aerosol only.
l. 440-441. "Griffin et al. (2020) reported that the small absorbing AI pixels are identified with small QA values in the offline product of ALH.". Please rephrase. There are no small pixels. You probably mean something like: "pixels with low UVAI values are identified by a QA value lower than .. (O.5?)". But this is not true, because low UVAI values are identified by low UVAi values. Please, explain what you actually mean.
l. 452-454. If this is true, then the reason for passive sensors to have a smaller variability needs to be explained, because I don't see it.
l. 557. "driving large deviations in some pixels". What does that mean?
Minor comments:l. 74. Oxygen-related absorption bands-> oxygen absoprtion bands
l. 78. add a reference to Xi et al, 2021, who combined O2-A and O2-B for the retrieval of ALH from TROPOMI.
l. 79-80. as the optical properties of aerosols in the atmosphere differ among aerosol types -> because of varying aerosol properties.
l. 87-90. ... and implemented the algorithm operationally.
l. 95-96. What is 'the standard product of aerosol'? Please, rephrase.
l. 153: "To consider the various retrieval products, the L2SFC retrieves .." Please, rephrase. L2FSC does not do something in consideration of products.
l. 158-159: "However, L2SFC accurately retrieved surface optical properties with high spatial resolution." Please, rephrase this vague statement. What is "high"? What is "accurate"?
l. 172-174. Remove this unnecessary sentence.
l. 213. The TROPOMI is spectrometer -> TROPOMI is a spectrometer
l. 215. crossing to equator -> crossing the equator
l. 230. lidar sensor -> lidar
l. 234-236. "This sensor is Sun synchronous orbit constellated to the A-train, and also cross to equator at 13:30 local time by ascending node." Improper English. Please, rephrase.
l. 240. "Because the aerosol profile product exists the vertical distribution of aerosol extinction coefficient, ... " Improper English. Please, rephrase.
Figure 3: What is the unit of the colorbar?
l. 276-277. Place this under the TROPOMI data section.
l. 360. ". most" -> ", most"
l. 362. "the AI value for UV" -> UVAI
l. 437. " scattering dominant aerosols". There is not such a thing as a dominant aerosol. The authors define "Scattering-dominant" as non-absorbing, but I suggest not to introduce these kind a strange wording. Just stick to absorbing or non-absorbing and better yet give the SSA as done here and leave it at that.
ReferencesXi Chen, et al., First retrieval of absorbing aerosol height over dark target using TROPOMI Oxygen B band: algorithm development and application for surface particulate matter estimates , Remote Sens. Env., doi: 10.1016/j.rse.2021.112674 2021.
Citation: https://doi.org/10.5194/amt-2023-136-RC2 - AC2: 'Reply on RC2', Sang Seo Park, 16 Sep 2023
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