Optimizing gas analyzer frequency response and temperature regiments for the NEON eddy covariance system

Friday, 19 December 2014
Rommel C Zulueta1, Stefan Metzger1,2, Sean P Burns3,4, Hongyan Luo1,2, Theodore Hehn5, Doug Kath5, George G Burba6, Jiahong Li6, Tyler Anderson6, Peter Blanken4 and Jeff R Taylor1,2, (1)National Ecological Observatory Network, Fundamental Instrument Unit, Boulder, CO, United States, (2)University of Colorado at Boulder, Institute for Arctic and Alpine Research, Boulder, CO, United States, (3)National Center for Atmospheric Research, Boulder, CO, United States, (4)University of Colorado, Boulder, Boulder, CO, United States, (5)National Ecological Observatory Network, CAL/VAL, Boulder, CO, United States, (6)LI-COR Biosciences, Lincoln, NE, United States
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%.