DOI: 10.48443/665h-rg40
title: Optimization of a gas sampling system for measuring eddy-covariance fluxes of H2O and CO2, 2013

description: Several initiatives are currently emerging to observe the exchange of energy and matter between the earth''s surface and atmosphere standardized over larger space and time domains. For example, the National Ecological Observatory Network (NEON) and the Integrated Carbon Observing System (ICOS) will provide the ability of unbiased ecological inference across eco-climatic zones and decades by deploying highly scalable and robust instruments and data processing. In the construction of these observatories, enclosed infrared gas analysers are widely employed for eddy-covariance applications. While these sensors represent a substantial improvement compared to their open- and closed-path predecessors, remaining high-frequency attenuation varies with site properties, and requires correction. Here, we show that the gas sampling system substantially contributes to high-frequency attenuation, which can be minimized by careful design. From laboratory tests we determine the frequency at which signal attenuation reaches 50 % for individual parts of the gas sampling system. For different models of rain caps and particulate filters, this frequency falls into ranges of 2.5–16.5 Hz for CO2, 2.4–14.3 Hz for H2O, and 8.3–21.8 Hz for CO2, 1.4–19.9 Hz for H2O, respectively. A short and thin stainless steel intake tube was found to not limit frequency response, with 50 % attenuation occurring at frequencies well above 10 Hz for both H2O and CO2. From field tests we found that heating the intake tube and particulate filter continuously with 4 W was effective, and reduced the occurrence of problematic relative humidity levels (RH > 60 %) by 50 % in the infrared gas analyser cell. No further improvement of H2O frequency response was found for heating in excess of 4 W. These laboratory and field tests were reconciled using resistor-capacitor theory, and NEON''s final gas sampling system was developed on this basis. The design consists of the stainless steel intake tube, a pleated mesh particulate filter, and a low-volume rain cap in combination with 4 W of heating and insulation. In comparison to the original design, this reduced the high-frequency attenuation for H2O by ≈ 3/4, and the remaining cospectral correction did not exceed 3 %, even at a very high relative humidity (95 %). This standardized design can be used across a wide range of eco-climates and site layouts, and maximizes practicability due to minimal flow resistance and maintenance needs. Furthermore, due to minimal high-frequency spectral loss, it supports the routine application of adaptive correction procedures, and enables more automated data processing across sites.
description: The test philosophy is that a GSS component has better performance where it results in less IRGA CO2 and/or H2O signal attenuation when used individually and when integrated with other GSS components. Initially nine different types of particulate filters were tested in the laboratory, and it was found that the Swagelok FW 2 μm stainless steel mesh filter (Swagelok, Solon, OH, U.S.A.) provided the most suitable characteristics. This filter was then integrated with three different types of rain caps (Figure 1) and two different intake tubes, and further tested under laboratory conditions.