Measurement of water clarity is an important undertaking in oceanography, and the diffuse attenuation coefficient K_d is one of the most common metrics used to quantify turbidity. Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) is a lidar system with detection sensitivities at the photon level, which is capable of detecting the extremely weak energy of ocean subsurface. ICESat-2 has recently demonstrated its great ability in calculating K_d using the water column decay profiles. However, in the daytime or in nearshore areas, this ability is limited by the contamination of sparse data, background noise, afterpulses and seafloor reflections. In this study, we have proposed a novel algorithm to retrieve the diffuse attenuation coefficient using ICESat-2 bathymetric information (i.e., seafloor reflected signals). Because the seafloor signal level is much stronger than that from the water column, a significant advantage of this method is the greater noise immunity, enabling it to function effectively even under strong background noise and afterpulse interference. The ICESat-2 retrieved K_d results compare reasonably well with MODIS ocean color measurements with a Mean Relative Differences (MRD) of <32%.

ICESat-2/ATLAS,photon-counting lidar,diffuse attenuation coefficient