Optimizing gas analyzer frequency response and temperature regiments for the NEON eddy covariance system
Rommel C Zulueta, Ph.D.1, Stefan Metzger2,3, Sean P Burns4,5, Hongyan Luo3,6, Theodore Hehn7, Doug Kath7, George G Burba8, Jiahong Li9, Tyler Anderson9, Peter Blanken10 and Jeffrey R Taylor3,11, (1)National Ecological Observatory Network, Instrumented Systems, Boulder, CO, United States, (2)NEON Program, Battelle, Boulder, United States, (3)University of Colorado at Boulder, Institute for Arctic and Alpine Research, Boulder, CO, United States, (4)University of Colorado, Boulder, Department of Geography, Boulder, United States, (5)National Center for Atmospheric Research, Boulder, CO, United States, (6)National Ecological Observatory Network, Battelle, Boulder, United States, (7)National Ecological Observatory Network, CAL/VAL, Boulder, CO, United States, (8)LI-COR, Lincoln, United States, (9)LI-COR Biosciences, Lincoln, NE, United States, (10)University of Colorado Boulder, Department of Geography, Boulder, United States, (11)National Ecological Observatory Network, Fundamental Instrument Unit, Boulder, CO, United States
Abstract:
The eddy covariance technique has typically been used to measure fluxes of mass, momentum, and energy using a 3D sonic anemometer and a trace gas analyzer. Ideally, both of these instruments would have response times >= 10Hz to capture the main contributing scales of turbulent transport. However, depending on the system design and infrastructure limitations, high-frequency spectral corrections can exceed 20% of the initial uncorrected flux. The objective of this study is to devise an optimal design that (i) maximizes system viability, and (ii) minimizes high-frequency spectral loss.
A closed-path LI-COR LI-7200 was selected and multiple combinations of the analyzer and different system components were tested in the laboratory to determine the frequency response of the individual components. This allowed for identifying and improving system bottlenecks in regards to frequency response. Comprehensive field experiments were performed including conditions of condensation which allowed for confirmation of the integral performance of the entire system under field conditions, and aided in determining the optimal setting of the intake tube and particle filter heating. The final system design yields a three-fold improvement of response at 10 Hz and reduces the occurrence of problematic humidity level (RH > 60%) within the analyzer cell by 50%.
