Collaborative Research: Influence of Surfactants on Air-Sea Gas Exchange: 3He/SF6 Experiments in the Baltic Sea

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Gas exchange, the movement of gases between the atmosphere and the surface ocean (in both directions), is an important process to understand in global biogeochemistry. A particular area of interest is the rate of exchange of gases such as carbon dioxide and dimethyl sulfide, which play important roles in Earth's climate. Understanding and modeling air-sea exchange on a global scale requires understanding of how gas exchange rates vary in response to a number of factors such as temperature, wind speed, and the presence of surface chemical films (known as surfactants) in the ocean. Over the past 25 years, major advances have been made in understanding air-sea gas exchange in the open ocean, mainly due to improvements in methodology and a number of successful process studies. However, some important questions remain, such as what happens near coastal areas (including inland seas), and how surfactants affect gas exchange. The Baltic Sea Gas Exchange Experiment (Baltic GasEx) is a collaboration between scientists from the US and Germany designed to address these questions. Participants in Baltic GasEx will measure the air-sea gas exchange rates with different techniques in the Baltic Sea before and after the spring bloom, when concentrations and compositions of surfactants will be different. The expeditions will be conducted on a German ship (Alkor), with the ultimate goal of quantifying the relationship between wind speed and gas exchange in an inland sea, and understanding the impact of surfactants on air-sea gas exchange. The project will involve significant international collaboration, public education and outreach through both participating U.S. institutions, and substantial student training in partnership with the international collaborators.NSF funding will support helium-3 and sulfur hexafluoride measurements during Baltic GasEx to determine the gas exchange rate. German colleagues are independently funded to quantify surfactants with AC voltammetry, surface tension, and sum frequency generation; to determine chemical characteristics of the micro layer; and to estimate the gas exchange rate using eddy covariance flux measurements of CO2 and DMS. The proposed experiment is designed to address the question of whether open ocean wind speed/gas exchange parameterizations can be applied to inland seas like the Baltic. Also, the effect of surfactants on gas exchange has been studied extensively in the laboratory, but there is little direct field evidence for the effect of surfactants on gas exchange. The proposed experiment, with expeditions at times with both high and low surfactant concentrations, should shed new light on the effect of natural surfactants on gas exchange. Finally, some wind speed/gas exchange parameterizations proposed for the Baltic Sea appear to be higher than typical open ocean parameterizations. These are typically based on eddy covariance flux measurements of CO2. However, in open ocean experiments where both the helium-3/sulfur hexafluoride approach and eddy covariance of CO2 have been deployed, there seems to be a discrepancy between gas transfer velocities measured with the two techniques. This experiment should show whether a similar difference in gas transfer velocities measured via these two techniques also exists in the Baltic Sea.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.