Abstract
Abstract. In this study, we introduce new observations of sea–air fluxes of carbon dioxide using the eddy covariance method. The measurements took place at the Utö Atmospheric and Marine Research Station on the island of Utö in the Baltic Sea in July–October 2017. The flux measurement system is based on a closed-path infrared gas analyzer (LI-7000, LI-COR) requiring only occasional maintenance, making the station 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. 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 molar fraction 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. Nonstationary conditions caused unphysical results, which resulted in a high percentage (65 %) of discarded measurements. After removing the nonstationary cases, the direction of the sea–air CO2 fluxes was in good accordance with independently 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 nonstationarity of CO2 fluxes.
Highlights
Anthropogenic actions, such as combustion of fossil fuels and land use changes, have perturbed the global carbon cycle, resulting in climatic changes (e.g., Solomon et al, 2009)
At the beginning of this period, from July to mid-August, the atmospheric pressure of CO2 (pCO2) exceeded that in the sea, and the sea acted as a sink of atmospheric CO2
We presented new sea–air CO2 flux measurements, comparing two closed-path gas analyzer setups installed on a shore of an island in the Baltic Sea
Summary
Anthropogenic actions, such as combustion of fossil fuels and land use changes, have perturbed the global carbon cycle, resulting in climatic changes (e.g., Solomon et al, 2009). To better understand the global carbon cycle, measurements of the CO2 exchange between the atmosphere and marine ecosystems are required. These measurements are useful for developing parameterizations of gas exchange intensity, such as the gas transfer velocity, used in global carbon models (e.g., Takahashi et al, 2002). M. Honkanen et al.: Measuring turbulent CO2 fluxes in a marine environment rine ecosystems, since the direction and magnitude of these fluxes depend on the photosynthetic carbon assimilation in surface seawater. The continental margin seas cover only a small portion of the oceans, up to 15 % of the total ocean primary production takes place in these seas, which are responsible for over 40 % of the total oceanic carbon sequestration (Muller-Karger et al, 2005)
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