Evaluation of the optical contamination of open path CO2 gas analyzers in measurements on a freshwater surface

Measurements of air-sea CO<sub>2</sub> fluxes with Eddy Covariance (EC) are generally affected by the optical contamination of open path gas analyzers. Since the conditions of saltwater environments may be similar to those of freshwater environments, in this work we evaluate whether this contamination is occurring in the LI-7500 open path CO<sub>2</sub> gas analyzer (''LI-COR Biosciences Ltd''), used in an experiment to measure greenhouse gas fluxes over the reservoir of the Itaipu Hydroelectric Power Plant. We evaluated the optical contamination indicators in the CO<sub>2</sub> measurements and two corrections for this: the numerical method called PKT method, proposed for CO<sub>2</sub> flux correction over ocean, and a multiple linear regression model. We find that the PKT method does not produce physically plausible results and that, contrary to what has been reported in ocean measurements, the measurements of the Itaipu experiment suffer less interference in more humid conditions. CO<sub>2</sub> correction by the regression model preserved the pattern of CO<sub>2</sub> flux daily variation, but it attenuated the magnitude of the fluxes: the average of the uncorrected fluxes was <math>-24.35</math>μgm<sup>-2</sup>s<sup>-1</sup>, while the average of the fluxes corrected by the statistical model was <math>-11.73</math>μgm<sup>-2</sup>s<sup>-1</sup>