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 openpath 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.