Measuring turbulent CO<sub>2</sub> fluxes with a closed-path gas analyzer in marine environment

Abstract

Abstract. Sea-air fluxes of carbon dioxide (CO 2 ) were measured using the eddy covariance method at a new station established on the Utö island in the Baltic Sea. The flux measurement system is based on a closed-path infrared gas analyzer (LI-7000, LI-COR) requiring only occasional maintenance, so the station is capable of continuous monitoring. However, such infrared gas analyzers are prone to significant water vapor interference in a marine environment, where CO 2 fluxes are small.In July-October 2017, two LI-7000 analyzers were run in parallel to test the effect of a sample air drier which dampens 5 water vapor fluctuations, and a virtual impactor, included to remove liquid sea spray, both of which were attached to the sample air tubing of one of the analyzers. The systems showed closely similar (R 2 = 0.99) sea-air CO 2 fluxes when the latent heat flux was low, which proved that neither the drier nor the virtual impactor perturbed the CO 2 flux measurement. However, the undried measurement had a positive bias that increased with increasing latent heat flux, suggesting water vapor interference.For both systems, cospectral densities between vertical wind speed and CO 2 were distributed within the expected frequency 10 range, with a moderate attenuation of high-frequency fluctuations. While the setup equipped with a drier and a virtual impactor generated a slightly higher flux loss, we opt for this alternative for its reduced water vapor cross-sensitivity and better protection against sea spray. The integral turbulence characteristics were found to agree with the universal stability dependence observed over land. Non-stationary flow conditions caused unphysical results, which resulted in a high percentage (up to 63 %) of discarded measurements. After removing the non-stationary cases, the direction of the sea-air CO 2 fluxes was in good accor-15 dance with the measured CO 2 partial pressure difference between the sea and the atmosphere. Atmospheric CO 2 concentration changes larger than 2 ppm during a 30 min averaging period were found to be associated with the non-stationarity of CO 2 fluxes.The Utö Atmospheric and Marine Research Station continues to monitor the regional CO 2 exchange between the sea and the atmosphere, utilizing the results of this work.