Optimization of an enclosed gas analyzer sampling system for measuring eddy covariance fluxes of H<sub>2</sub>O and CO<sub>2</sub>

Abstract

Abstract. 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) are set to provide the ability of unbiased ecological inference across ecoclimatic zones and decades by deploying highly scalable and robust instruments and data processing. In the construction of these observatories, enclosed infrared gas analyzers are widely employed for eddy covariance applications. While these sensors represent a substantial improvement compared to their openand closed-path predecessors, remaining high-frequency attenuation varies with site properties and gas sampling systems, and requires correction. Here, we show that components of the gas sampling system can substantially contribute to such high-frequency attenuation, but their effects can be significantly reduced by careful system 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 CO 2 , 2.4-14.3 Hz for H 2 O, and 8.3-21.8 Hz for CO 2 , 1.4-19.9 Hz for H 2 O, 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 H 2 O and CO 2 . 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 analyzer cell. No further improvement of H 2 O 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 H 2 O by ≈ 3/4, and the remaining cospectral correction did not exceed 3 %, even at high relative humidity (95 %). The standardized design can be used across a wide range of ecoclimates 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 largely automated data processing across sites.

Quantifying the uncertainty of eddy covariance fluxes due to the use of different software packages and combinations of processing steps in two contrasting ecosystems

“…Generally it is thought that the theoretical approach easily underestimates the high-frequency spectral attenuation for closed-path systems, because of potential variations in the mass flow rates and relative uncertainty in the flow regime (Aubinet et al, 2000). Recent studies, both in the lab and in the field, have also demonstrated the important effects that different dust filters and rain caps (typically mounted at the tube inlet) have on the EC system cut-off frequency (Aubinet et al, 2016;Metzger et al, 2016). Such effects are not included in the theoretical approach.…”

eddy4R 0.2.0: a DevOps model for community-extensible processing and analysis of eddy-covariance data based on R, Git, Docker, and HDF5

“…GitHub provides distributed version control and has become widely used by scientific research groups because it is free, open-source, and provides several features that make it useful for managing artifacts of scientific research (Ram, 2013). Git allows multiple users and developers to simultaneously access and collaborate on a remote repository by means of independent "forks" or replicas of the entire repository (Paarsch and Golyaev, 2016). Figure 3 shows NEON's Git workflow: at any given time (1) the official, stable eddy4R source code resides on NEON's GitHub repository.…”

“…In Lee et al, 2015;Metzger et al, , 2013Metzger et al, , 2016Salmon et al, 2015;Serafimovich et al, 2013;Starkenburg et al, 2016;Vaughan et al, 2016;Xu et al, 2017). eddy4R currently consists of four packages: eddy4R.base, eddy4R.qaqc, eddy4R.turb, and eddy4R.erf.…”

“…Next, eddy4R.qaqc provides the general quality assurance and quality control (QA/QC) tests of Taylor and Loescher (2013), along the Smith et al (2014) model for tracking quality information in large datasets, and functions for de-spiking (Brock, 1986;Fratini and Mauder, 2014;Mauder et al, 2013;Mauder and Foken, 2015;Vickers and Mahrt, 1997). eddy4R.turb provides standard, Reynolds-decomposed turbulent flux calculation (Foken, 2017), accompanied by models for planar fit transformation (Wilczak et al, 2001) and spectral correction (Nordbo and Katul, 2012). Additional functionalities include Fourier transform, the determination of detection limit (Billesbach, 2011), integral length scales, and statistical sampling errors (Lenschow et al, 1994), and flux-specific QA/QC models (Foken and Wichura, 1996;Vickers and Mahrt, 1997).…”

Comparison of the fast Lyman-Alpha and LICOR hygrometers for measuring airborne turbulent fluctuations

“…They have a longer measurement chamber of 12.5 cm compared to the few mm of the Lyman-Alpha. The LICOR sensors in different forms are used at automated field stations for research networks covering large temporal and spatial scales, and they are therefore well characterised concerning the transfer function of different components (Metzger et al, 2016). The closed-path sensors requiring a gas sampling system can be affected by high-frequency attenuation, so inlets and tubes have to 30 be dimensioned reasonably (Aubinet et al, 2016;Metzger et al, 2016).…”