Separation and dispersion of conditionally sampled eddies through an intake tube
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With conditional sampling, atmospheric fluxes are estimated from the difference in mean concentration of a gas between upward and downward moving eddies and a measure of the standard deviation of the vertical Wind velocity. Two basic components of a conditional sampling system are a sonic anemometer to measure the direction and standard deviation of the vertical wind velocity, and a valve to separate air sampled from updrafts and downdrafts. Use of the Campbell Scientific CSAT3 three-axis sonic anemometer to measure the vertical direction of the eddies and to trigger a splitter valve requires building a physical delay into the air sampling line to counter the electronic delay that is built into the anemometer. This physical delay in the form of an intake tube creates a source of dispersion that may reduce concentration differences and affect estimates of fluxes. Our objectives were to determine the proper match of electronic and physical delays to separate eddies, and then to measure the amount of dispersion from the combined mixing in the tube and the splitter valve. A conditional sampling system to measure CO2 fluxes was constructed using a CSAT3 anemometer and a three-way valve. A physical delay was created by adding tubing on the upstream side of the valve. A simple timing circuit was used to control a highspeed valve that pulsed CO2 in front of the intake tube to mark the locations of eddy transitions. Differences in concentrations between updraft and downdraft lines were measured as flow rates and electronic delays were varied. Tube length based on electronic delay, flow rate, and intake tube geometry was found to be adequate for separation of updrafts and downdrafts. Dispersion in the intake tube was significant and predictable. The effect of the splitter valve on dispersion was minimal. A correction factor to adjust concentration differences in the lines for dispersion may be warranted, hut only for eddy reversal frequencies > 2 Hz.
