B33A-0621
Sampling Line Heating Improves Frequency Response of Enclosed Eddy Covariance Gas Analyzers

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Gerardo Fratini1, Stefan Metzger2, James C Kathilankal3, Derek Trutna1, Hongyan Luo4, Sean P Burns5, Peter Blanken6 and George G Burba1, (1)LI-COR Biosciences, Lincoln, NE, United States, (2)NEON, Fundamental Instrument Unit, Boulder, CO, United States, (3)LI-COR Inc, Lincoln, NE, United States, (4)National Ecological Observatory Network, Fundamental Instrument Unit, Boulder, CO, United States, (5)National Center for Atmospheric Research, Boulder, CO, United States, (6)University of Colorado, Boulder, Boulder, CO, United States
Abstract:
One of the challenges when measuring eddy-covariance fluxes with closed gas analyzers is high frequency attenuation due to the passage of the sampled air through a gas sampling system (GSS). The problem is particularly relevant for gases that undergo strong sorption processes, such as H2O. Recent “enclosed” analyzer designs (e.g. LI-7200, LI-COR Biosciences Inc.) mitigate the problem by allowing a reduced length of the intake tube (<1 m). Further improvements can come from carefully designed filtering and heating systems that reduce hygroscopic particulates and H2O adsorption on GSS surfaces.

Because the sorption processes of H2O increase exponentially with air relative humidity (RH), low-pass filtering effects can be reduced by reducing RH inside the GSS, for example by increasing air temperature via heating. In this work, we evaluate the effects of several heating strategies with the aim of optimizing the LI-7200 performance while limiting the implied increase in power consumption.

From field tests we found that 4 W of heating applied uniformly to a rain cap-integrated 2 µm particulate filter (FW-series, Swagelok) and a 700 mm stainless steel tube with 4.8 mm inner diameter reduces the occurrence of problematic RH levels (>60%) in the LI-7200 by ≈50%. As a result, the system half-power frequency increased by ≈1 Hz, and the remaining cospectral correction did not exceed 3%, even at very high ambient RH (95%).

While little further improvement was found for increased heating powers, it is possible to optimize the sequence of GSS components and their heating: we found that positioning the particulate filter ≈20 cm downstream of the rain cap and concentrating 2/3 of the heat in this first 20 cm, and 1/3 in the remainder of the tube, provides optimal performances. Using model cospectra and a range of realistic measurement and environmental conditions, we estimated H2O spectral corrections to reduce by ≈50-70%, getting very close to those of CO2 in most conditions of interest.

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