On the way towards understanding the effect of sea-water surfactants on gas transfer velocity

Gas flux across the sea surface is proportional to the difference of partial pressure between the sea-water and the overlying atmosphere and also to a parameter called gas transfer velocity k, a measure of the, the measure of efficiency of the gas exchange. Although it depends mostly in in-water and atmospheric turbulence, the usual way to parametrize it is by the wind speed, the source of the turbulence which has the advantage of being easily available from ship base measurements and reanalyses. Unfortunately, measured values of gas transfer velocity at a given wind speed have a large spread in values. It has been long suspected that the coverage of the sea surface with variable amounts of surface-active substances (or surfactants). It has been shown that surfactants may decrease the CO2 air-sea exchange by up to 50%. However the labour intensive methods used for surfactant study make it impossible to collect enough data to map the surfactant coverage or even create a gas transfer velocity parametrization involving a measure of surfactant activity. This is why we decided check the possibility of using optical fluorescence as a proxy of surfactant activity.

 

We are in the third year of a 4-year research grant funded by the Polish National Science Centre, NCN (grant number 2021/41/B/ST10/00946). Our group has previously showed that fluorescence parameters allow estimation the surfactant enrichment of the surface microlayer, as well as types and origin of fluorescent organic matter involved. In order to study their possible usefulness in improving the parametrization of the gas transfer velocity k, we measure from the research ship of the Institute, R/V Oceania, all the variables needed for its calculation, namely CO2 partial pressure both in water (PiCCARO G2101-i) and in air (Licor 7200, semiclose path with heated tube and Licor 7500, open path) as well as vertical flux of this trace gas (with the GiLL WindMaster and WindMaster Pro for 3D air movement needed for eddy correlation) as well as meteorological conditions. The data are used to calculate gas transfer velocity values which are compared to ones calculated literature from parametrization functions. The differences between the two, together with the surfactant fluorescence parameters are be used to test the hypothesis that surfactants are main reason for the “noisiness” of k measurement results and hopefully to improve the k parametrization by adding a surfactant related variable to the wind speed which at present is the sole independent variable of most parametrizations.

 

After 3 years of the project we have data from six Baltic cruises and to three Atlantic ones, of which 2/3 have been already analysed. The poster will present the early results of the project and show progress towards the main goal of the research: finding a reliable optical proxy for surfactant to be used in gas transfer velocity parametrization.