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

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.