{"id":33,"date":"2022-07-13T20:40:56","date_gmt":"2022-07-13T19:40:56","guid":{"rendered":"https:\/\/www.oceanblogs.org\/where-the-baltic-sea-touches-the-sky\/?p=33"},"modified":"2022-07-14T11:13:42","modified_gmt":"2022-07-14T10:13:42","slug":"hunting-for-k","status":"publish","type":"post","link":"https:\/\/www.oceanblogs.org\/where-the-baltic-sea-touches-the-sky\/2022\/07\/13\/hunting-for-k\/","title":{"rendered":"Hunting for k"},"content":{"rendered":"\n

By Christa Marandino, Tim Steffens, Tobias Spreitz, David Ho<\/p>\n\n\n\n

***deutsche Version siehe weiter unten***<\/p>\n\n\n\n

How does the ocean influence climate? We know that heat is taken up by the ocean, but what about climate-active trace gases? Does the ocean emit them or take them up? How much is the ocean emitting or absorbing? Is air-sea exchange changing as the climate changes? These types of questions are on the minds of the scientists during the EMB295 research cruise. The research groups of David Ho (University of Hawai\u02bbi at M\u0101noa, USA) and Christa Marandino (GEOMAR, Germany) are trying to determine trace gas transfer between the ocean and atmosphere. They are using two different techniques to quantify the gas transfer velocity, k, used to compute the air-sea fluxes of a range of trace gases that are important for climate, such as carbon dioxide (CO2<\/sub>), nitrous oxide (N2<\/sub>O), methane (CH4<\/sub>), and dimethylsulfide (DMS).<\/p>\n\n\n\n

The technique used by the Ho group involves injecting two inert gas tracers, sulfur hexafluoride (SF6<\/sub>) and the lighter isotope of helium (3<\/sup>He), into the surface ocean and measuring the change in their concentration over time. The technique is generally called the deliberate tracer technique, and it allows us to separate the effects of mixing\/dispersion and gas exchange to determine the gas transfer velocity. In practice, our main task on the ship is to map the tracer every day, and try to be at the center of the tracer patch to sample it with our CTD\/rosette twice a day, at 6 am and 6 pm. It sounds easy, but since we are constantly going in and out of the patch in a lawnmower pattern in order to map its boundaries, we are constantly losing the patch. It becomes a stressful exercise when we approach time for the sampling station. Our unofficial motto is: \u201cit is difficult to schedule finding something you have lost\u201d. The discrete SF6<\/sub> samples are measured onboard the ship using a gas chromatograph equipped with an electron capture detector, whereas the 3<\/sup>He samples will be sent back to the laboratory on land and measured on a helium mass spectrometer. https:\/\/twitter.com\/_david_ho_\/status\/1547122643194847235?s=24&t=VMGrXZONi0xJwHBmv-FIOg<\/a><\/p>\n\n\n\n

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The Marandino group performs eddy covariance measurements to determine k. This technique requires fast and precise measurements of the vertical wind at exactly the same time as fast and precise measurements of gas concentrations, which are mathematically covaried to compute the flux. On board (EMB295), CO2 and DMS are measured using this technique. Wind related measurements are performed on a mast built on to the front of the ship (Figure 2, left). Inlets for CO2<\/sub> and DMS are located on the mast as well. The analytical instrumentation for the gas measurements are located downstream of the mast (e.g., in the lab). This separation of measurements, as well as a moving ship, make eddy covariance data analysis a bit challenging. On top of the normal challenges, the ship\u2019s track during the tracer hunt can make for suboptimal wind conditions for the eddy covariance method. Therefore, it is important that the ship points into the wind during the two CTD stations. For the rest of the crazy cruise track, we take as many opportunities as possible to measure under favorable wind conditions.<\/p>\n\n\n\n

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