Airborne test of laser pump-and-probe technique for assessment of phytoplankton photochemical characteristics

Abstract

Initial results of the airborne LIDAR measurement of photochemical quantum yield, Φ_Po, and functional absorption cross-section, σ_{PS II}, of Photosystem II (PS II) are reported. NASA's AOL3 LIDAR was modified to implement short-pulse pump-and-probe (SP-P&P) LIDAR measurement protocol. The prototype system is capable of measuring a pump-induced increase in probe-stimulated chlorophyll fluorescence, ΔF/F_sat, along with the acquisition of `conventional' LIDAR-fluorosensor products from an operational altitude of 150 m. The use of a PS II sub-saturating probe pulse increases the response signal but also results in excessive energy quenching (EEQ) affecting the ΔF/F_sat magnitude. The airborne data indicated up to a 3-fold EEQ-caused decline in ΔF/F_sat, and 2-fold variability in the EEQ rate constant over a spatial scale a few hundred kilometers. Therefore, continuous monitoring of EEQ parameters must be incorporated in the operational SP-P&P protocol to provide data correction for the EEQ effect. Simultaneous airborne LIDAR measurements of Φ_Po and σ_{PS II} with EEQ correction were shown to be feasible and optimal laser excitation parameters were determined. Strong daytime ΔF/F_sat decline under ambient light was found in the near-surface water layer over large aquatic areas. An example of SP-P&P LIDAR measurement of phytoplankton photochemical and fluorescent characteristics in the Chesapeake Bay mouth is presented. Prospects for future SP-P&P development and related problems are discussed.