Abstract
Wind, chemical enhancement, phytoplankton activity, and surfactants are potential factors driving the air-sea gas exchange of carbon dioxide (CO2). We investigated their effects on the gas transfer velocity of CO2 in a large annular wind-wave tank filled with natural seawater from the North Atlantic Ocean. Experiments were run under 11 different wind speed conditions (ranging from 1.5 m s-1 to 22.8 m s-1), and we increased the water pCO2 concentration twice by more than 1000 µatm for two of the seven experimental days. We develop a conceptual box model that incorporated the thermodynamics of the marine CO2 system. Surfactant concentrations in the sea surface microlayer (SML) ranged from 301 µg L-1 to 1015 µg L-1 (as Triton X-100 equivalents) with enrichments ranged from 1.0 to 5.7 in comparison to the samples from the underlying bulk water. With wind speeds up to 8.5 m s-1, surfactants in the SML can reduce the gas transfer velocity by 54%. Wind-wave tank experiments in combination with modelling are useful tools for obtaining a better understanding of the gas transfer velocities of CO2 across the air-sea boundary. The tank allowed for measuring the gas exchange velocity under extreme low and high wind speeds; in contrast, most previous parametrizations have fallen short because measurements of gas exchange velocities in the field are challenging, especially at low wind conditions. High variability in the CO2 transfer velocities suggests that gas exchange is a complex process not solely controlled by wind forces, especially in low wind conditions.
Highlights
The air-sea exchange of climate-relevant gases, such as carbon dioxide (CO2) and nitrous oxide (N2O), plays an important role in climate regulations (Liss and Duce, 2005)
We could visually observe the disintegration of the surfactants film and the build-up of waves at constant wind speed, and this likely distorted the correlation we found between the surfactant concentration and k(CO2) at the lower wind categories
It was difficult to calculate the individual physical parameters (u10, u∗, mss) for those conditions (Bopp, personal communication), which highlights the complexity of the processes and the fine threshold that determines the steady-state conditions. These results are in accordance with the findings reported by Asher (1997), who concluded that surfactants would increase the net CO2 flux because the effects are most pronounced in regions with low wind speeds where the direction of the CO2 flux is toward the atmosphere
Summary
The air-sea exchange of climate-relevant gases, such as carbon dioxide (CO2) and nitrous oxide (N2O), plays an important role in climate regulations (Liss and Duce, 2005). Air-sea exchange fluxes (F) are determined by the gas transfer velocity (k) and the difference between the gas concentrations (c) in the air and water (F = k [ca – cw]). Many of the gas transfer velocity parameterizations that are frequently used in oceanography are based on wind speed and temperature (Nightingale et al, 2000; Ho et al, 2006; Wanninkhof, 2014). The parameterizations describe the quadratic (Ho et al, 2006; Wanninkhof, 2014), cubic (McGillis et al, 2001, 2004), sub-linear (Krakauer et al, 2006), and multi-linear (Liss and Merlivat, 1986) relationships between wind speed and gas transfer velocity. Other parametrizations have been developed on the basis of surface water turbulence (Kitaigorodskii and Donelan, 1984), friction velocity (Deacon, 1977), or wave slope (Jähne et al, 1984); these parameters are not easy to measure in the field, and not available as global datasets, similar to wind speed (Kalnay et al, 1996)
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