Using a High Frequency Fluorescent Oxygen Probe in Atmospheric Eddy Covariance Applications

One major uncertainty of the ocean oxygen dynamics is the air-sea gas exchange, why measurements of oxygen fluxes across air-sea surface are fundamental, for the understanding of how the anthropogenic emissions of greenhouse gases e.g. CO2, CH4 and NO2 affect the global climate dynamics. The magnitude and direction of a gas flux at the air-sea interface is determined by the air-sea difference in partial pressure of the gas and by the efficiency of the transfer process. However, there is still a lack of understanding what mechanism controlling the efficiency of the air-sea gas exchange why comparing gases of different solubility could fundamentally increase this knowledge. Based on five field campaigns during the years 2010-2013 we here evaluate the potential use of the Microx TX3 oxygen sensor for eddy covariance applications, investigating if it meets the hard criteria to be used for eddy covariance measurements in atmospheric marine environments. The fast response optode Microx TX3 was used with two different types of tapered sensors in an EC-system. The sensor without optical isolation was found to attain sufficient response time and precision to be used in air-sea applications, however, the short sensor lifetime typically 1 to 4 days, depending on atmospheric conditions such as solar radiation and precipitation, limits the use for long term measurements. Spectral and co-spectral analysis shows oxygen to follow the same shape as for CO2 and water vapour when normalized. The sampling rate of the Microx TX3 is 2 Hz, however, the sensor was found to have a limited response and resolution yielding a flux loss in the frequency range f > 0.3 Hz. This can be corrected for by applying cospectral similarity using simultaneously measurements of latent heat as the reference signal. In average the magnitude of the cospectral correction added 20% to the uncorrected oxygen flux during neutral atmospheric stratification. The density and cospectral corrected fluxes was then used to determine the transfer velocity via the flux bulk formula. The partial pressure gradient was determined from the difference between the 30 minute average partial pressure of O2 measured with the Microx TX3 and the Seabird 37smp-ido, mounted at 4 meters depth 1 km upwind from the EC-tower. The transfer velocity of O2, shows a wind speed dependence similar to earlier studies made for the transfer velocity of CO2. For high wind speeds, however, the transfer velocity of oxygen displays a larger increase with increasing wind speeds than what is found for the transfer velocity of CO2.