Aerosol Optical Depth over the Sichuan Basin: An Intercomparison of CALIPSO and MODIS Observations (2006&ndash;2022)

Hu, Chengyu; Lim, Hwee San

doi:10.21203/rs.3.rs-6661860/v2

Preprints

https://doi.org/10.21203/rs.3.rs-6661860/v2

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17 Dec 2025

| 17 Dec 2025

Aerosol Optical Depth over the Sichuan Basin: An Intercomparison of CALIPSO and MODIS Observations (2006–2022)

Chengyu Hu and Hwee San Lim

Abstract. As the first satellite equipped with lidar for vertical atmospheric profiling, the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) provides unique insights into aerosol vertical distribution. In contrast, the Moderate Resolution Imaging Spectroradiometer (MODIS), a representative passive remote sensing instrument, excels in broad horizontal coverage for aerosol detection. This study systematically compares Level 2 aerosol optical depth (AOD) products from CALIPSO and MODIS over the Sichuan Basin, China, from 2006 to 2022, after harmonizing their spatial resolution and temporal windows. Through comprehensive statistical analyses of annual, seasonal, monthly, and regional averages, key findings emerge. While both datasets capture a consistent long-term temporal trend in AOD over the region, significant systematic biases are observed. Differences are more pronounced in winter and spring than in summer and autumn. The absolute AOD difference between the two sensors remains relatively stable, with nearly 60 % of discrepancies falling within the 0–0.2 range. MODIS demonstrates greater sensitivity to both high and low AOD extremes compared to CALIPSO. Spatially, regional deviations are most apparent in areas of aerosol extremes: MODIS tends to overestimate in high-AOD regions, whereas CALIPSO often reports higher values in low-AOD areas. These findings highlight the distinct methodological characteristics of each sensor and underscore the need for careful dataset selection or synergistic use, depending on specific research objectives in this complex geographical region.

Received: 09 Dec 2025 – Discussion started: 17 Dec 2025

Chengyu Hu and Hwee San Lim

Status: final response (author comments only)

CC1:
'Comment on egusphere-2025-6149', Chakradhar Rao Tandule, 28 Dec 2025
General Comments
The manuscript presents a long-term intercomparison of MODIS and CALIPSO aerosol optical depth (AOD) over the Sichuan Basin. The topic is of general interest, and the authors have compiled an extensive multi-year dataset. However, the study primarily documents statistical differences between two existing satellite products without sufficiently advancing measurement methodology, retrieval validation, or uncertainty characterization.
While the results may be informative at a regional or descriptive level, several fundamental methodological limitations restrict the depth of insight that can be drawn regarding measurement performance. Some statements imply relative superiority of one dataset over the other; given the lack of external validation, such statements may be misleading the independent datasets. As a result, the study remains primarily descriptive rather than advancing measurement methodology or validation.
Specific Comments
1. Lack of Measurement Validation Framework
The intercomparison is conducted exclusively between MODIS and CALIPSO AOD products, without the use of any independent ground-based reference data (e.g., AERONET or ground-based lidar). Consequently, the analysis evaluates only the relative consistency between the two satellite products and does not allow assessment of their absolute accuracy. Statements regarding overestimation or underestimation by either sensor should therefore be interpreted cautiously. Inclusion of even limited ground-based validation, or clearer acknowledgement of this limitation, would strengthen the study.
2. Methodological Simplicity of the Collocation Approach
MODIS and CALIPSO AOD are collocated by resampling both datasets to a common 1° × 1° grid and comparing monthly mean values. While this approach facilitates long-term climatological analysis, it does not account for:
differences in overpass times,

pixel-level spatial matching,

or the sparse sampling of CALIPSO due to its narrow nadir-only track.

Given that the distance between consecutive CALIPSO ground tracks (~280–300 km at the study latitude) is comparable to or larger than the chosen grid size, the representativeness of monthly mean CALIPSO AOD within each grid cell is limited. This aspect should be more explicitly discussed, as it likely contributes to the weak correlations reported.
3. Incomplete Description of Retrieval Parameters and Processing Choices
The manuscript does not explicitly specify the exact scientific dataset variables used from the MODIS and CALIPSO products, nor does it clearly describe quality assurance thresholds or algorithm pathways (e.g., MODIS Dark Target vs. Deep Blue). Whether both TERRA and AQUA are considered for collocating with CALIOP. Such details are essential for reproducibility and for interpreting whether observed differences arise from measurement physics or processing choices.
4. Spectral and Physical Consistency Not Addressed
The direct comparison of CALIPSO AOD at 532 nm with MODIS AOD at 550 nm is performed without spectral adjustment or discussion of potential impacts. Although the wavelength difference is small, neglecting this aspect introduces an additional source of systematic uncertainty that remains unquantified. Similarly, the lack of supporting analysis using meteorological or aerosol-type information limits the physical interpretation of seasonal differences. A brief quantitative estimate or justification for neglecting this effect would improve methodological rigor.
5. Primarily Descriptive Nature of the Results
The study focuses on documenting spatiotemporal differences between two datasets, but does not introduce new retrieval methods, measurement concepts, or uncertainty analyses. The conclusions largely reiterate known characteristics of passive versus active aerosol observations, without substantially advancing methodological understanding.
Citation: https://doi.org/10.5194/egusphere-2025-6149-CC1
- AC1:
  'Reply on CC1', Chengyu Hu, 01 Jan 2026
  We sincerely thank you for their constructive and insightful comments on our manuscript titled “Aerosol Optical Depth over the Sichuan Basin: An Intercomparison of CALIPSO and MODIS Observations (2006-2022).” The feedback has helped us better recognize the limitations of our current work and identify areas for improvement. Below we provide a point-by-point response to the comments.
  
  General Comments
  Reviewer: The manuscript presents a long-term intercomparison of MODIS and CALIPSO aerosol optical depth (AOD) over the Sichuan Basin. The topic is of general interest, and the authors have compiled an extensive multi-year dataset. However, the study primarily documents statistical differences between two existing satellite products without sufficiently advancing measurement methodology, retrieval validation, or uncertainty characterization.
  Response: We appreciate your acknowledgment of the relevance of our study and the effort in compiling a long-term dataset. We agree that the current work is largely descriptive and focused on documenting inter-sensor differences. Our intention was to provide a regional climatological perspective on the consistency and discrepancies between two widely used satellite AOD products in a topographically complex region like the Sichuan Basin, and this study aims to provide a reference for scholars wishing to utilise these two datasets for regional AOD research.We acknowledge that the study does not introduce new retrieval methods or a comprehensive uncertainty framework. In the revised manuscript, we will more explicitly position the work as a regional intercomparison study and clarify its scope and limitations in the Introduction and Discussion sections.
  Specific Comments
  
  1. Lack of Measurement Validation Framework
  Reviewer: The intercomparison is conducted exclusively between MODIS and CALIPSO AOD products, without the use of any independent ground-based reference data (e.g., AERONET or ground-based lidar). Consequently, the analysis evaluates only the relative consistency between the two satellite products and does not allow assessment of their absolute accuracy. Statements regarding overestimation or underestimation by either sensor should therefore be interpreted cautiously. Inclusion of even limited ground-based validation, or clearer acknowledgement of this limitation, would strengthen the study.
  Response: We fully agree with this important point. In the original manuscript, we did not incorporate ground-based validation data such as AERONET, which limits our ability to assess absolute accuracy.As our original intention was to conduct a broad-scale data comparison, and Aeronet or other ground-based lidar systems can only provide individual point data, so we did not incorporate these data. In the revised version, we will:
  Explicitly acknowledge this limitation in the Methodology and Discussion sections.
  
  Clarify that statements regarding “overestimation” or “underestimation” are relative between the two sensors and not absolute.
  
  Include a brief discussion on the availability of AERONET sites in the Sichuan Basin and the challenges in collocating sparse ground measurements with satellite overpasses, suggesting this as a direction for future work.
  
  2. Methodological Simplicity of the Collocation Approach
  Reviewer: MODIS and CALIPSO AOD are collocated by resampling both datasets to a common 1° × 1° grid and comparing monthly mean values. While this approach facilitates long-term climatological analysis, it does not account for differences in overpass times, pixel-level spatial matching, or the sparse sampling of CALIPSO due to its narrow nadir-only track. Given that the distance between consecutive CALIPSO ground tracks (~280–300 km at the study latitude) is comparable to or larger than the chosen grid size, the representativeness of monthly mean CALIPSO AOD within each grid cell is limited. This aspect should be more explicitly discussed, as it likely contributes to the weak correlations reported.
  Response: This is a valid methodological concern. In the revised manuscript, we will:
  Expand the Methodology section to explicitly discuss the limitations of our collocation approach, including the sparse sampling of CALIPSO and its impact on representativeness.
  
  Acknowledge that the coarse grid (1°×1°) and monthly averaging likely smooth out fine-scale variability and contribute to the weak correlations observed.
  
  Note that this approach was chosen to facilitate basin-wide climatological analysis over a long period, but we will clarify that it is not suitable for pixel-level or diurnal comparison.
  
  3. Incomplete Description of Retrieval Parameters and Processing Choices
  Reviewer: The manuscript does not explicitly specify the exact scientific dataset variables used from the MODIS and CALIPSO products, nor does it clearly describe quality assurance thresholds or algorithm pathways (e.g., MODIS Dark Target vs. Deep Blue). Whether both TERRA and AQUA are considered for collocating with CALIOP. Such details are essential for reproducibility and for interpreting whether observed differences arise from measurement physics or processing choices.
  Response: We apologize for the lack of clarity. In the revised manuscript, we will provide the following details:
  Specify the exact datasets used: CALIPSO Level 2 5-km AOD at 532 nm, and MODIS MOD04/MYD04 3-km AOD at 550 nm from both Terra and Aqua.
  
  Clarify that we used the combined Dark Target and Deep Blue AOD product for MODIS over land, with standard quality flags applied.
  
  State that only daytime CALIPSO data were used to match MODIS daytime overpasses.
  
  Include a brief note on data filtering criteria (e.g., confidence level, cloud mask) to enhance reproducibility.
  
  4. Spectral and Physical Consistency Not Addressed
  Reviewer: The direct comparison of CALIPSO AOD at 532 nm with MODIS AOD at 550 nm is performed without spectral adjustment or discussion of potential impacts. Although the wavelength difference is small, neglecting this aspect introduces an additional source of systematic uncertainty that remains unquantified. Similarly, the lack of supporting analysis using meteorological or aerosol-type information limits the physical interpretation of seasonal differences. A brief quantitative estimate or justification for neglecting this effect would improve methodological rigor.
  Response: We acknowledge this oversight. In the revision, we will:
  Discuss the wavelength difference between 532 nm and 550 nm and note that, while small, it may introduce a minor systematic bias, especially under certain aerosol types.
  
  Cite relevant literature that supports the comparability of AOD at these wavelengths in the absence of strong spectral dependence.
  
  Briefly discuss the potential influence of aerosol type and meteorological conditions on seasonal differences, referencing related studies in the Sichuan Basin.
  
  5. Primarily Descriptive Nature of the Results
  Reviewer: The study focuses on documenting spatiotemporal differences between two datasets, but does not introduce new retrieval methods, measurement concepts, or uncertainty analyses. The conclusions largely reiterate known characteristics of passive versus active aerosol observations, without substantially advancing methodological understanding.
  Response: We agree that the study is descriptive in nature. Our primary goal was to provide a regional, long-term perspective on the consistency between MODIS and CALIPSO in a special geographical setting. In the revised Discussion and Conclusion, we will:
  More clearly frame the study as a regional intercomparison aimed at informing data selection for basin-scale aerosol research.
  
  Avoid overstating the methodological novelty and instead emphasize the practical implications for users of these datasets in topographically complex regions.
  
  Suggest specific follow-up studies that could build on our findings, such as multi-sensor fusion, uncertainty quantification, or integration with ground-based and model data.
  
  Conclusion
  We are grateful for the thorough and thoughtful review, which has helped us improve the clarity, rigor, and positioning of our work. We believe the suggested revisions will strengthen the manuscript and provide a more balanced and transparent presentation of our intercomparison study. We are committed to addressing all points raised and will submit a revised version accordingly.
  Thank you for your time and comments.
  
  Happy new year!
  Sincerely,
  
  Chengyu Hu & Hwee San Lim
  
  Citation: https://doi.org/10.5194/egusphere-2025-6149-AC1
RC1:
'Comment on egusphere-2025-6149', Anonymous Referee #1, 10 Jan 2026
General Comments The manuscript addresses the important issue of aerosol loading in one of China's most polluted regions. The effort to compile 17 years of satellite data is appreciated. However, I cannot recommend this paper for publication in AMT. This paper applies existing, standard Level 2 products to a specific region without validating a new retrieval method or providing a deep technical analysis of instrument performance (e.g., signal-to-noise limitations, specific surface reflectance issues in the basin).
Major Issues
Sampling Bias & Seasonality Contradiction:The authors find that MODIS and CALIPSO show different seasonal peaks. This is a fundamental disagreement. The authors must quantify the "sampling effect". A standard approach is to collocate MODIS data only along the CALIPSO track to see if the discrepancy disappears. If MODIS matches CALIPSO, the issue is sampling. If they still disagree, it is a retrieval algorithm issue. The current analysis using monthly averages on mismatched grids is invalid.

Inappropriate Spatial Resolution:The authors regridded 3km (MODIS) and 5km (CALIPSO) data to 1°*1°. The Sichuan Basin has complex terrain while a 1° grid (approx. 100km) blurs the basin floor with the surrounding high mountains. This resolution renders the spatial analysis (Section 3.2) largely trivial. The analysis should be performed at a finer resolution (e.g., 0.25°).

Lack of Ground Validation:There are AERONET sites and ground meteorological/environmental monitor network in the region. Why were these not used? Comparing Satellite A to Satellite B without a ground reference makes it impossible to determine which sensor is correct when they disagree.
Citation: https://doi.org/10.5194/egusphere-2025-6149-RC1
- AC2:
  'Reply on RC1', Chengyu Hu, 13 Jan 2026
  Dear Reviewer,
  We sincerely thank you for your thorough and insightful review of our manuscript. We appreciate the recognition of our effort in compiling a 17-year dataset for the Sichuan Basin, a region of significant environmental concern. We have carefully considered your comments and would like to provide the following responses to address the major concerns raised.
  Major Issue 1: Sampling Bias & Seasonality Contradiction
  Reviewer Comment:
  The authors find that MODIS and CALIPSO show different seasonal peaks. This is a fundamental disagreement. The authors must quantify the "sampling effect". A standard approach is to collocate MODIS data only along the CALIPSO track to see if the discrepancy disappears. If MODIS matches CALIPSO, the issue is sampling. If they still disagree, it is a retrieval algorithm issue. The current analysis using monthly averages on mismatched grids is invalid.
  Our Response:
  We fully acknowledge the importance of distinguishing between sampling effects and retrieval algorithm differences. In our study, the primary objective was to conduct a climatological intercomparison over a long time period (2006–2022) and across the entire basin, rather than a pixel-level or track-aligned comparison.
  Given CALIPSO’s narrow swath and sparse ground track spacing, direct collocation along its track would result in extremely limited spatial coverage, making it difficult to represent the basin’s heterogeneous aerosol distribution. Our approach of aggregating data to a common grid was intended to maximize spatial representativeness over the 17-year period, albeit at the cost of fine-scale temporal matching.
  We agree that a track-aligned comparison could provide additional insight into retrieval differences under matched sampling conditions. In the revised manuscript, we will:
  Explicitly acknowledge that our analysis does not isolate sampling effects from retrieval differences.
  
  Include a discussion on how CALIPSO’s sparse sampling may influence seasonal statistics, especially in cloudy seasons.
  
  Suggest that future studies could perform strict collocation along CALIPSO tracks as a complementary analysis.
  
  Major Issue 2: Inappropriate Spatial Resolution
  Reviewer Comment:
  The authors regridded 3 km (MODIS) and 5 km (CALIPSO) data to 1°×1°. The Sichuan Basin has complex terrain while a 1° grid (approx. 100 km) blurs the basin floor with the surrounding high mountains. This resolution renders the spatial analysis largely trivial. The analysis should be performed at a finer resolution (e.g., 0.25°).
  Our Response:
  We understand the concern regarding spatial resolution. The choice of a 1°×1° grid was primarily driven by CALIPSO’s sampling limitations. At finer resolutions (e.g., 0.25°), CALIPSO’s sparse nadir tracks would result in extensive data gaps, making meaningful spatial averaging and long-term trend analysis unfeasible. The 1° grid was a compromise to ensure：
  Sufficient CALIPSO pixel counts within each grid cell over the monthly and seasonal period.
  
  Minimization of data voids, especially in earlier years when CALIPSO’s orbital coverage was less complete.
  
  A basin-scale perspective that remains relevant for regional climate and air quality assessments.
  
  We acknowledge that this resolution may smooth out some topographic and urban-rural gradients. In the revised manuscript, we will:
  Clarify the rationale for the 1° grid in the Methodology section.
  
  Discuss the implications of this choice for interpreting spatial patterns.
  
  Major Issue 3: Lack of Ground Validation
  Reviewer Comment:
  There are AERONET sites and ground meteorological/environmental monitor network in the region. Why were these not used? Comparing Satellite A to Satellite B without a ground reference makes it impossible to determine which sensor is correct when they disagree.
  Our Response:
  We recognize that ground-based validation is a cornerstone of satellite product evaluation. However, during our study period (2006–2022), there were no publicly available, continuously operating AERONET sites within the Sichuan Basin. While other ground monitoring networks exist for PM₂.₅ and other pollutants, they do not provide AOD measurements directly comparable to satellite retrievals. We have noted that some ground-based observation stations possess the capability to measure AOD, but the data is not publicly available.
  Our study was designed as a satellite-to-satellite intercomparison, not an absolute validation. Such intercomparisons are valuable for:
  Identifying systematic biases between two widely used global datasets.
  
  Highlighting regions and seasons where disagreements are most pronounced.
  
  Informing users about dataset consistency in topographically complex regions.
  
  We agree that incorporating ground truth would strengthen the conclusions. In the revised manuscript, we will:
  Explicitly state the absence of long-term AERONET data in the basin as a limitation.
  
  Cite relevant studies that have validated MODIS and CALIPSO over China using available but sparse ground measurements.
  
  Recommend that future work integrate emerging ground-based lidar or sun-photometer networks as they become available.
  
  Our Study’s Contribution and Request for Reconsideration
  We understand that our study does not introduce a new retrieval algorithm or perform high-resolution collocation. However, we believe it provides unique value as:
  The first long-term (17-year) basin-scale intercomparison of CALIPSO and MODIS AOD over the topographically complex Sichuan Basin.
  
  A practical reference for researchers choosing between or integrating these datasets for regional air quality and climate studies.
  
  An attempt to utilise CALIPSO data for large-scale spatiotemporal sequence studies, as it is well known that CALIPSO is typically applied to vertical distribution and point-based investigations.
  
  We are committed to revising the manuscript to:
  More clearly frame the study as a climatological intercomparison.
  
  Discuss methodological limitations transparently.
  
  Avoid overstating conclusions regarding sensor performance.
  
  We kindly request that the reviewer reconsider our manuscript for publication in AMT, given its regional relevance, long-term perspective, and utility for the aerosol remote sensing community.
  Thank you for your time and constructive feedback.
  Sincerely,
  Chengyu Hu & Hwee San Lim
  
  Citation: https://doi.org/10.5194/egusphere-2025-6149-AC2
RC2: 'Comment on egusphere-2025-6149', Anonymous Referee #2, 22 Jan 2026

This manuscript by Hu and Lim presents a study comparing the Aerosol Optical Depth (AOD) derived from the spaceborne lidar CALIOP (active remote sensing) and from MODIS (passive remote sensing) over the Sichuan Basin of China from 2006 to 2022. The authors analyze annual, seasonal, monthly, and regional AOD variations and identify significant differences between the two instruments during winter and spring. They also report that MODIS AODs exhibit greater sensitivity at high and low extreme values. Overall, however, this manuscript remains limited to reporting regional results of AODs and lacks in-depth analysis or discussions, for example, into the potential causes of the observed differences.

Moreover, AOD comparisons between CALIOP and MODIS have already extensively studied in prior literatures, as reflected in the references included. Thus, it remains unclear what novel data-processing approaches this work introduces to advance current understanding of the community. I recommend that the authors address this point explicitly in the introduction, clarifying the specific research gap and how their work aims to address it. Additionally, the manuscript requires considerable improvement in English language quality. Due to the substantial revisions needed in both content and presentation, I cannot recommend this work for publication in AMT at this stage. Below are specific comments that may assist the authors in improving their manuscript.
Specific comments:

Abstract: Why do authors focus on the AOD comparison over the Sichuan Basin? Please state the motivation more clearly. I also do not agree with the statements of “overestimate”, since there are no reference AOD values for your comparison.

L33, ‘comparative analyses’, authors do not compare the two systems, but the aerosol product AOD.

L34-40, the authors only list the results from some existing papers; however, it is hard to find the relationships and logic from this paragraph.

L55-58, it would be better to provide an overview introduction of the aerosol conditions over the Sichuan Basin, such as the seasonal patterns, aerosol types, sources, and so on. There should be many literatures regarding these.

L58, ‘far exceeding WHO limits’, provide the values.

L64, please replace the (Hu et al., 2024) by another paper from the CALIPSO team (i.e., Winker et al., 2007). It is important to give credit to the right team!

L67, ‘Leilei et al., 2023’, it is an error as you give the first name. Also, revise the format error in the corresponding paper in the reference list.

L68, ‘NASA’, there is no definition for the first appearance in the manuscript.

Section 2.2, lots of details regarding the data processing are missing. No uncertainty assessments and data quality control strategy for the two aerosol products are disclosed here.

L87, 1 degree is too rough for the grid since the authors will lose many details from the topography, which is a feature of the Sichuan Basin. Such a large grid can also be one of the reasons explaining the unexpectedly large differences in AODs from two instruments in the following Section.

Figure 2, it would be better to show the box plot, including more information such as median value, 25th and 75th percentiles, mean values, and so on.

L102, ‘the trend reversed’ is not a precise statement here.

L103, ‘greater than…’ provide the value.

L103, ‘three-year cycle’, cannot believe that this is a real situation of AOD variation.

L119, R2 values are unacceptably low, which may incorporate some issues from data processing rather than the actual performance of the two AOD products.

L124, ‘These errors likely arise from sensors and algorithms’, this is only a speculation from the authors without any evidence. Many scientists put their efforts into these instruments and algorithms; thus, it would be better to provide the reasons in detail rather than the use of the word “likely”. For example, CALIOP first determines the aerosol subtype so that an aerosol-type-specific lidar ratio can be determined before the retrieval of aerosol backscatter and extinction coefficient. That is to say, CALIOP AOD may not accurately depict the complex mixture of different aerosol types. Can this be one of the factors causing the AOD differences?

Figure 3, unexpected large difference in AODs during spring.

Figure 5, lidar is generally a more sensitive instrument compared with MODIS, making the results here somewhat odd.

L158-159, the conclusion here is just speculation?

Figures 8 and 9, and L170-177, the related text regarding these two figures is rather limited, as well as the corresponding analysis. What is authors’ intention to show these two figures?

Section 3.3, it would be helpful if the authors could move each paragraph of discussions to just follow their respective result text in section 3.2.

Citation: https://doi.org/10.5194/egusphere-2025-6149-RC2

Chengyu Hu and Hwee San Lim

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Short summary

This study compares how two key NASA satellites measure air pollution over China's Sichuan Basin. While both show similar long-term trends and spatial patterns, we found systematic differences in their measurements. Our research demonstrates that scientists should select satellite data based on their specific needs—whether for broad coverage or precision—to effectively monitor air quality in this geographically complex region.


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