Atmospheric transmittance modeling for spaceborne oceanic lidar: Global assessment and distribution characteristics at the Fraunhofer line of 486.1 nm

Developing new operational wavelengths like the 486.1 nm Fraunhofer line is pivotal for advancing spaceborne lidar's oceanographic profiling. However, atmospheric attenuation during laser and signal passage critically limits subsurface detection depth and data product accuracy. A comprehensive assessment of the system's global performance therefore necessitates accurate quantification of the global atmospheric transmittance at 486.1 nm. This study presents a comprehensive methodology to address this need. Initially, the extinction Ångström exponent for 532/486.1 nm was computed for various spherical and non-spherical aerosols, using Mie scattering theory and the T-matrix method, respectively. Subsequently, aerosol optical parameters at 486.1 nm were retrieved through wavelength conversion of the global aerosol properties measured by CALIOP at 532 nm. The reliability of this conversion was validated against AERONET sun-photometer observations at 500 nm, demonstrating good agreement. Combined with the transmittance of atmospheric molecules and ozone derived from ERA5 reanalysis data, the global spatial distribution of the total atmospheric transmittance at 486.1 nm was comprehensively mapped. Results reveal a global mean of 0.610, with higher atmospheric transmittance values observed in polar regions and lower values within dust transport belts. Globally, over 90% of the transmittance values are concentrated within the range of 0.5 to 0.7, providing a critical quantitative basis for the design and performance prediction of future spaceborne ocean lidar systems.