Separation and dispersion of conditionally sampled eddies through an intake tube

With conditional sampling, atmospheric fluxes are estimated from the differ ence in mean concentration of a gas between upward and downward moving eddi es and a measure of the standard deviation of the vertical wind velocity. T wo basic components of a conditional sampling system are a sonic anemometer to measure the direction and standard deviation of the vertical wind veloc ity, 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 electro nic delay that is built into the anemometer, This physical delay in the for m of an intake tube creates a source of dispersion that may reduce concentr ation differences and affect estimates of fluxes. Our objectives were to de termine the proper match of electronic and physical delays to separate eddi es, and then to measure the amount of dispersion from the combined mixing i n the tube and the splitter valve. A conditional sampling system to measure CO2 noses was constructed using a CSAT3 anemometer and a three-may valve. A physical delay was created by adding tubing on the upstream side of the v alve. A, simple timing circuit was used to control a highspeed valve that p ulsed CO2 in front of the intake tube to mark the locations of eddy transit ions. Differences in concentrations between updraft and downdraft lines wer e measured as now rates and electronic delays were varied, Tube length base d on electronic delay, now rate, and intake tube geometry was found to be a dequate 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 differe nces in the lines for dispersion may be warranted, but only for eddy revers al frequencies > 2 Hz.