Development of an Air Sampling System for the True Eddy Accumulation Technique

摘要

The major advantages of the true eddy accumulation technique are that it is a direct method for flux measurements and applicable to any trace gases which need either a slow response sensor or the installation of an adsorbent for condensation. However,it is difficult to accurately take the sample air with high time resolution according to vertical wind velocity and direction. Therefore, we have developed an air sampling system for the true eddy accumulation technique. The air sampling system is designed to take updraft and downdraft air samples in proportion to vertical wind velocity with high time resolution using syringes connected to a 40 Hz stepping-motor. The system is also designed to accumulate air samples in separate reservoirs for updraft and downdraft, and to convey air samples to the gas analyzer alternately to detect their gas concentration automatically. To test how precisely air samples are taken with high time resolution, the sample volumes by the system under conditions of differentsuction durations and input voltages were measured. The results of this experiment showed the system with the back flow route was able successfully to take air samples in proportion to vertical wind velocity on a base of 7 Hz considering the problem ofsuction errors. The preliminary test in a grass field showed that the accumulated sample volume during one measurement sequence was proportional to the amount of accumulated vertical wind velocity. The linear relations were recognized between CO_2 flux measured using the system and that measured using the eddy correlation technique. These results lead to the conclusion the developed air sampling system for the true eddy accumulation technique is available to measure CO_2 flux.