Theoretical Derivation of Aerosol Lidar Ratio using Mie Theory for CALIOP-CALIPSO and OPAC Aerosol Models

Chipade, Radhika; Pandya, Mehul

doi:https://doi.org/10.5194/amt-2023-104

Preprints

https://doi.org/10.5194/amt-2023-104

Preprints

21 Aug 2023

| 21 Aug 2023

Status: this preprint is currently under review for the journal AMT.

Theoretical Derivation of Aerosol Lidar Ratio using Mie Theory for CALIOP-CALIPSO and OPAC Aerosol Models

Radhika Chipade and Mehul Pandya

Abstract. The extinction-to-backscattering ratio, popularly known as lidar (light detection and ranging) ratio of atmospheric aerosols is an important optical property, which is essential to retrieve the extinction profiles of atmospheric aerosols. Lidar satellite observations can provide the global coverage of atmospheric aerosols along with their vertical extent. NASA’s Cloud-Aerosol Lidar with Orthogonal Polarisation (CALIOP) on-board Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) is the only space-based platform available so far, that provides the vertical profiles of extinction due to atmospheric aerosols. A physics-based theoretical approach is presented in the present paper that estimates lidar ratio values for CALIPSO aerosol models, which can be used as inputs to determine the extinction profiles of aerosols using CALIPSO data. The developed methodology was also qualified by comparing it with the lidar ratio values derived using AERONET datasets. Lidar ratio for CALIPSO aerosols models were estimated in the range of 38.72 sr to 85.98 sr at 532 nm whereas, at 1064 nm lidar ratio varied between 20.11 sr to 71.11 sr depending upon the aerosol type and their size distributions.

Aerosols are compositions of various particles and thus the presence of water vapour in the atmosphere can affect the optical properties of the aerosols. Thus, the effect of relative humidity on lidar ratio was studied using Optical Properties of Cloud and Aerosols software tool (OPAC) aerosol models, which are the standard aerosol models against the cluster classified AERONET and CALIPSO aerosol models. Water soluble particles contribute substantially in clean continental, clean marine, tropical marine and desert aerosol models and are hygroscopic in nature. Hygroscopic sulfate particles dominate the Antarctic aerosols during summertime. In presence of relative humidity between 0–80%, the lidar ratio values were observed to decrease from 53.59 sr to 47.13 sr, 53.66 sr to 47.15 sr, 53.70 sr to 47.16 sr and 55.32 sr to 48.78 sr at 532 nm for clean continental, clean marine, tropical marine and desert aerosols respectively, whereas lidar ratio gradually increased from 47.13 sr to 51 sr, 47.15 sr to 51 sr, 47.16 sr to 51 sr and 48.78 sr to 51.68 sr respectively for these aerosol models when relative humidity was between 80–99%; due to constituent hygroscopic particles. In case of Antarctic aerosols, the lidar ratio was observed to increase from 57.73 sr to 97.64 sr due to hygroscopic sulfate particles that backscattered heavily in presence of water vapour at 532 nm. The soot particles dominate the polluted continental and polluted marine particles causing an increase in lidar ratio over corresponding clean counterpart. Similar results were observed at 1064 nm for OPAC aerosol models.

Received: 22 May 2023 – Discussion started: 21 Aug 2023

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Radhika Chipade and Mehul Pandya

Status: final response (author comments only)

RC1: 'Comment on amt-2023-104', Anonymous Referee #1, 08 Sep 2023

I congratulate the authors for their high-quality work developing a theoretical approach to estimate the lidar ratio values for CALIPSO aerosol

models. The research results can be used to evaluate the extinction profiles of atmospheric aerosols by using CALIPSO data. The paper is well written with an excellent logical presentation sequence. The methodology is clearly outlined and based on valid assumptions. The authors describe the approach limitations and expected sources of uncertainties. The authors compared their results to ground observations and results from other consolidated and published results, demonstrating the research's accuracy and contribution to atmospheric science.
However, there are some issues that the authors should work on before the paper's publication:
a) There are a few typo errors that I found and highlighted in the attached file;
b) I'm afraid I have to disagree with the statement in line 300 when the authors discuss the increase of the backscattering coefficient with relative humidity. Figure 3 shows that the backscattering coefficient sensitivity to relative humidity is higher for clean continental aerosols than urban and polluted continental aerosols. Maybe the line patterns in the figure are hard to read in black and white, making it hard to read the information from the plot. I suggest preparing the figure using colors or a continuous line for the clean continental and marine continental for better identification;
c) I suggest that the authors modify the statement in line 308. According to Figure 6(e), both wavelengths show an increase in LIDAR ratio, but 532 nm has a more significant increase than 1064 nm. Also, the LIDAR ratio values are lower at 1064nm than at 532nm;
d) I recommend using (a), (b), and (c) instead of (c), (d), and (e) in Figures 4 and 6.
e) The Figures 1 to 6 look like the graphs are in low resolution. Their presentation quality can be improved.
Once again, congratulations for the good work.

Citation: https://doi.org/10.5194/amt-2023-104-RC1
RC2: 'Comment on amt-2023-104', Anonymous Referee #3, 10 Sep 2023

Please refer to the attached file.

Citation: https://doi.org/10.5194/amt-2023-104-RC2
RC3: 'Comment on amt-2023-104', Anonymous Referee #5, 11 Sep 2023

(1) The authors claimed that they developed a physics-based theoretical approach to estimate lidar ratio values for CALIPSO aerosol models. In my point of view, this study employed the Mie theory to simulate lidar ratios using different aerosol models but did not develop any new methodology.
(2) The paper is not well written, especially for the poor punctuation.
For example, Line 99, ‘this study, attempts to’ -> ‘this study attempts to’; Line 163, ‘Bohren, and Huffman’ -> ‘Bohren and Huffman’.
(3) “backscattering coefficient” rather than “backscatter coefficient”, for example in Line 164, please change it throughout the whole paper.
(4) Line 121-150, The geometric standard deviation should be dimensionless.
(5) In Line 166 and 169, it was declared that n_r and n_i were used for real part and imaginary part of the refractive index, respectively. However, the symbols m_r and m_i were used instead in Table 1, 2, and 4.
(6) In Line 113 and 200, it pointed out that r_m represented median radius. But ‘mean radius’ was used instead throughout the paper.
(7) The unit of r_m was not specified in Table 1, 2, and 3.
(8) What do ‘Nr’ and ‘Dr’ mean in Figure 1 and 2.
(9) I disagree with the authors that the simulated lidar ratios were consistent to the in-situ values from AERONET. As can be seen from Table 7 to Table 11, the lidar ratios were underestimated compared to the in-situ values in most cases, especially for the cases of Category-1 and Category-3.
(10) Table 13, the simulated lidar ratio of dust at 1064 nm is much smaller than those in CALIPSO operational algorithm; the simulated lidar ratio of Clean Continental at 532 nm and Clean Marine at both wavelengths are much larger than those in CALIPSO operational algorithm, why?
(11) Why did the lidar ratios decrease when the relative humidity was between 0~80% while increase when the relative humidity was between 80~99% ?
(12) Significant progress has been made in the research community for improving the aerosol optics modeling. This manuscript is mostly relied on Hess (1998) that was published 25 year ago. We all know that lidar ratios are sensitive to the partilce nonsphericity, heterogeneity, and the absorption. Relevant discussions related to these issues and the weakness of the present study should be included.

Citation: https://doi.org/10.5194/amt-2023-104-RC3
RC4: 'Comment on amt-2023-104', Anonymous Referee #2, 12 Sep 2023

General Comments:
This manuscript applies Mie scattering theory to estimate the lidar ratio for different CALIPSO aerosol models. From a technical perspective, this study lacks significant innovation. Additionally, the analysis of the impact of relative humidity on the lidar ratio, based on the estimation results, holds some scientific value. Therefore, it is recommended to reconsider the acceptance of this study after the following issues have been well solved.
Specific Comments:
1) Many studies have already computed the Lidar ratio based on Mie scattering theory and analyzed the influence of relative humidity on the lidar ratio, such as Zhao et al. (2017). However, there are relatively few studies that apply these computed results to the retrieval of CALIPSO data. The author should, upon calculating the Lidar ratio, conduct an in-depth analysis by combining CALIPSO observational data to investigate the impact of relative humidity or aerosol hygroscopic growth on the retrieval of CALIPSO data.
2) Mie scattering theory is only applicable to spherical particles. In the CALIPSO aerosol models, several are predominantly non-spherical, and it's evident that Mie scattering theory cannot be applied to them. Some of the results in section 4.1 of this manuscript also confirm this. The author should, considering the results from section 4.1, distinguish between CALIPSO aerosol models for which Mie scattering calculations are applicable and focus the subsequent analysis only on those aerosol models where Mie scattering is applicable. It would be inappropriate to simply summarize the comparison results for various CALIPSO aerosol models in section 4.1 as "good agreement."
3) The calculation results regarding the impact of relative humidity on lidar ratio and backscattering coefficient offer many details for further exploration. For instance, lidar ratio appears to exhibit opposite trends with relative humidity variations at low and high relative humidity levels. Furthermore, if the vertical axis of Figures 3-6 could be presented in relative terms, it might allow for a better comparison of gradient differences between different curves.
Reference:
Zhao, G., Zhao, C., Kuang, Y., Tao, J., Tan, W., Bian, Y., Li, J., and Li, C.: Impact of aerosol hygroscopic growth on retrieving aerosol extinction coefficient profiles from elastic-backscatter lidar signals, Atmos. Chem. Phys., 17, 12133-12143, 10.5194/acp-17-12133-2017, 2017.

Citation: https://doi.org/10.5194/amt-2023-104-RC4

Radhika Chipade and Mehul Pandya

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Short summary

The extinction-to-backscattering ratio, popularly known as lidar ratio of atmospheric aerosols is an important optical property, which is essential to retrieve the extinction profiles of atmospheric aerosols. A physics-based theoretical approach is presented in the present paper that estimates lidar ratio values for CALIPSO and OPAC aerosol models, which can be used as inputs to determine the extinction profiles of aerosols using CALIPSO data.


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