Advancing ocean carbon monitoring: high-resolution subsurface retrieval of phytoplankton absorption and productivity via airborne dual-wavelength lidar over the South China Sea

Quantifying ocean primary productivity (OPP) in the South China Sea (SCS) is crucial for understanding regional carbon sequestration processes. In this study, an airborne dual-wavelength light detection and ranging (lidar) (ADWL: 532/486 nm) methodology is introduced to resolve vertical phytoplankton absorption and productivity dynamics. A full-chain inversion framework was developed to obtain OPP profiles from raw ADWL backscattering signals, and the lidar radiative transfer equation was integrated with spectral decoupling algorithms to isolate phytoplankton absorption, given interference from color dissolved organic matter (CDOM) and detritus. We establish a functional relationship between the total absorption coefficient at 486 nm and the chlorophyll a (Chla) concentration using in situ data and retrieve the vertical profiles of the Chla concentration along the ADWL flight track. The ADWL system effectively detected the subsurface maximum phytoplankton absorption and Chla concentration layer at depths ranging from 20 to 70 m and OPP profiles extending below the conventional euphotic zone depth. Validation at Stations C1 and C2 demonstrated high accuracy, with Chla concentration retrieval achieving R² = 0.89 (RMSE=0.13 mg m^-3) and OPP estimates yielding R² = 0.68 (RMSE=1.62 mg C m^-3 d^-1). Finally, we discuss the effects of the euphotic zone depth definition on the precision of OPP estimation, demonstrating that integration to the depth of zero productivity (PP∼0) is essential for reliable OPP quantification. Overall, the proposed methodology represents an important step toward future global-scale and high-precision ocean carbon monitoring using spaceborne ocean lidar system.