Overestimation of Downward Air-Sea Eddy CO₂ Flux Due to Optical Window Contamination of Open-Path Gas Analyzer

Abstract

Open-path eddy-covariance CO₂ flux over the ocean have been reported to be much larger in magnitude than the estimated bulk CO₂ flux, and optical window contamination of the open-path gas analyzers has been reported as the cause of the overestimations. During an on-board experiment over the ocean, we manually cleaned the optical window and directly compared CO₂ output signal before and after the window cleaning. It is found that both of the CO₂ fluctuation amplitude and correlation coefficient between vertical wind velocity and CO₂ mixing ratio have apparently increased, then resulting in the downward CO₂ flux overestimation when the optical window contamination accumulate. These apparent increases were reset after the manual window cleaning. Correlation coefficients between CO₂ and H₂O output signals, which represent cross-sensitivity between CO₂ and H₂O, can be useful indicators of optical window contamination of the open-path gas analyzer.