Abstract. Sea-air fluxes of carbon dioxide (CO2) 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 CO2 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 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 (R2 = 0.99) sea-air CO2 fluxes when the latent heat flux was low, which proved that neither the drier nor the virtual impactor perturbed the CO2 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 CO2 were distributed within the expected frequency 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 CO2 fluxes was in good accordance with the measured CO2 partial pressure difference between the sea and the atmosphere. Atmospheric CO2 concentration changes larger than 2 ppm during a 30 min averaging period were found to be associated with the non-stationarity of CO2 fluxes. The Utö Atmospheric and Marine Research Station continues to monitor the regional CO2 exchange between the sea and the atmosphere, utilizing the results of this work.
Sensible and Latent Heat Flux Measurement during Rainfall by the Eddy Correlation Method Using a Fine Thermocouple Psychrometer.
