Natural variability in the efficiency of air sea gas transfer of carbon dioxide

Increasing atmospheric levels of the greenhouse gas carbon dioxide are contributing to the warming of the Earth’s atmosphere. The ocean has profound implications for the Earth’s climate by acting as a large sink of CO₂ - it has absorbed between one quarter and one third of all anthropogenic emissions of the gas since the industrial revolution.

Thus it is important that we understand and quantify the processes that influence this exchange for improving carbon budgets and future climate scenarios.

Wind is the primary controlling factor governing the transfer of CO₂ between the atmosphere and ocean due to its influence over the hydrodynamics of the surface ocean. Thus, the usual method for estimating the flux of CO₂ considers only the concentrations of the gas in the ocean and atmosphere and wind speed.

However, wind is not the only factor that affects air-sea gas transfer. Previous measurements in the Southern Ocean show that the dependency of gas transfer velocity on wind speed was found to very considerably from about 20% at moderate wind speeds to up to 100% at low and high wind speeds. Other factors thought to control this transfer include bubbles, waves and the role of surface active chemicals (surfactants) in the surface layer of the ocean. The impact of surfactants on air-sea CO₂ transfer is the focus of this study.

A multitude of different surfactants are found in the surface layer of the ocean, varying in composition (and so solubility and surface properties) and concentrations. They are most commonly produced by marine biota including phytoplankton.. Previous measurements show that natural surfactants can suppress air-water gas transfer in a tank, and artificially released synthetic surfactants can suppress air-water gas transfer in the field. However, evidence for the impact of natural surfactants over the ocean is generally lacking.

The authors devised a new technique to quantify the natural variability in air-sea CO₂ exchange and put it to the test on a 11,000km voyage in the Southern Ocean. This study location is important as the Southern Ocean is thought to be responsible for approximately half of the oceanic uptake of CO₂ due to vast areas of deep-water formation where carbon is sequestered.

The technique involves directly measuring the air-sea flux of CO₂ using eddy covariance and measuring the gas transfer efficiency (a function of surfactants) in parallel using a purpose-built system (Segmented Flow Coil Equilibrator). This enables the role of surfactants in supressing gas transfer to be assessed.

It was found that the sensitivity of air-sea flux to gas transfer efficiency (and so surfactants) is stronger at low wind speeds and weaker at high wind speeds. Neglecting these variations could result in biases in computed air-sea CO₂ fluxes.

Dr Mingxi Yang, lead author on the paper said:

"For decades, lab experiments have shown that naturally occurring surfactants (surface-active chemicals) can suppress the rate of air-sea gas exchange, but it was unclear to me how important this effect is over the real ocean. Meanwhile, people have released artificial surfactant patches over the ocean and observed suppression in gas exchange, but I wondered just how representative these artificial surfactants are? Here we developed a novel technique to measure the impact of natural surfactants on air-sea CO₂ transfer. By combining this measurement with state-of-the-art and direct measurement of air-sea CO₂ transfer, we show conclusively that natural surfactants can significantly reduce the rate of gas transfer, with the largest effect found at lower wind speeds. We need more of these observations along with measurements of seawater organic composition to fully understand the impact of surfactants on gas exchange and improve our estimates in the oceanic uptake of CO₂."

Further studies are needed which combine these measurements with observations of surfactant concentration and biological parameters to elucidate further the effect of natural organic compounds on gas transfer.


Cruise track colour-coded by the gas transfer efficiency. Blue areas indicate higher gas transfer efficiency, red areas lower efficiency.