Field evaluation of an autonomous, low-power eddy covariance CO2 flux system for the marine environment

Eddy covariance (EC) air-sea CO2 flux measurements have been developed for large research vessels, but have yet to be demonstrated for smaller platforms. Our goal was to design and build a complete EC CO2 flux package suitable for unattended operation on a buoy. Published state-of-the-art techniques that have proven effective on research vessels, such as air stream drying and liquid water rejection, were adapted for a 2-m discus buoy with limited power. Fast-response atmospheric CO2 concentration was measured using both an off-the-shelf (“stock”) gas analyzer (EC155, Campbell Scientific, Inc.) and a prototype gas analyzer (“proto”) with reduced motion-induced error that was designed and built in collaboration with an instrument manufacturer. The system was tested on the University of New Hampshire (UNH) air-sea interaction buoy for 18 days in the Gulf of Maine in October 2020. The data demonstrate the overall robustness of the system. Empirical post-processing techniques previously used on ship-based measurements to address motion sensitivity of CO2 analyzers were generally not effective for the stock sensor. The proto analyzer markedly outperformed the stock unit and did not require ad hoc motion corrections, yet revealed some remaining artifacts to be addressed in future designs. Additional system refinements to further reduce power demands and increase unattended deployment duration are described.