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April 22, 2010 – Dissolved gases (Shari Yvon-Lewis)
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 The focus of this cruise is to look at the how saturated or under saturated the ocean is with respect to methyl bromide and other gases in the atmosphere. We study this deviation from equilibrium, or saturation anomaly, by measuring the concentration in the surface water and in the air repeatedly every day. We use something called an equilibrator (figure 1) to measure the concentration of the gases in the water. In the equilibrator, the seawater pumped in through a pipe in the hull of the ship flows in through the top and rains down through a plate with many small holes. It looks like it is raining inside. The water flows out a drain in the bottom after it reaches a certain level. The air in the top of the equilibrator is pumped out and back in again recirculating through the equilibrator. Any gases dissolved in the water will equilibrate with that air. An  instrument inside the ship’s lab called a gas chromatograph with mass spectrometer (GCMS – figure 2) has a computer that periodically allows air from the equilibrator to be sampled automatically and analyzed by the GCMS. In between samples from the equilibrator, the GCMS analyzes samples of air pumped straight from the bow, where we have a sample line near the CO2 samplers discussed in an earlier blog. We can then compare the air and equilibrator samples to see if there is more methyl bromide or other gases in the water or in the air. If there is more in water, then it is supersaturated, and the gas fluxes from the water to the air. If there is more in the air, then it goes the other way. The gases we are measuring are produced by algae in the ocean, and some are actually chewed up by bacteria in the ocean. At the end of this cruise, we will have a long dataset that we can compare with a cruise from 16 years ago that measured some of these gases in the same place. This way we can see if the ocean sources and sinks are changing over time. |
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