Sunday, July 3, 2011

Sampling CO2 in the air and sea

- Matt Czikowsky [SUNY Albany]

During this cruise, concentrations of carbon dioxide in the surface seawater and atmosphere are being monitored with the goal of continuously measuring the difference in these concentrations, known as ΔpCO2 (or delta pCO2).
The difference in CO2 concentrations between atmosphere and sea is important in terms of the CO2 flux between the surface ocean and atmosphere: CO2 levels in the surface seawater greater than those in the air (positive delta pCO2) results in a release of CO2 into the atmosphere, while CO2 levels in the seawater that are less than atmospheric (negative delta pCO2) result in the ocean taking up CO2.
The air-sea CO2 flux can be estimated by multiplying the delta pCO2 level by an empirical exchange coefficient called the “piston velocity.” Since the measured CO2 fluxes vary as a function of wind speed and atmospheric stability, the piston velocity is a function of these atmospheric variables as well. Since our group from the State University of New York at Albany is making direct CO2 flux measurements using eddy covariance on the mast during this cruise, we can make estimates of the piston velocity using these delta pCO2 measurements.

Our group is deploying two systems to measure delta pCO2. The primary system consists of an electronics box (Figure 1, built at UAlbany by Jim Albrecht) which contains a CO2 gas analyzer, valves to control system flows, and a logger to collect the data as well as a showerhead equilibrator used for sampling the surface seawater CO2 concentration. The system performs a measurement by flowing seawater from the ship’s intake into the showerhead equilibrator where it is sprayed through nozzles into a volume partially filled with water. Mixing occurs between the seawater and air in the equilibrator chamber, resulting in the CO2 level in the headspace of the equilibrator to be in equilibrium with that of the seawater.
The equilibrated air stream containing the CO2 level of the seawater is then pumped to the electronics box, where some of the air is sent to gas analyzer, while another portion of the sample is returned to the equilibrator chamber to form a closed loop. A water manometer is located at the top of the equilibrator, which helps to regulate the pressure and water level in the equilibrator chamber. Once the seawater CO2 measurement is complete, a series of valves switch in the electronics box to allow atmospheric air from the foremast of the ship to be sampled.

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