Optimization of an enclosed gas analyzer sampling system for measuring eddy covariance fluxes of H<sub>2</sub>O and CO<sub>2</sub>
Journal Article
Overview
abstract
<p><strong>Abstract.</strong> 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 open- and 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<span class="thinspace"></span>% 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<span class="thinspace"></span>Hz for CO<sub>2</sub>, 2.4–14.3<span class="thinspace"></span>Hz for H<sub>2</sub>O, and 8.3–21.8<span class="thinspace"></span>Hz for CO<sub>2</sub>, 1.4–19.9<span class="thinspace"></span>Hz for H<sub>2</sub>O, respectively. A short and thin stainless steel intake tube was found to not limit frequency response, with 50<span class="thinspace"></span>% attenuation occurring at frequencies well above 10<span class="thinspace"></span>Hz for both H<sub>2</sub>O and CO<sub>2</sub>. From field tests we found that heating the intake tube and particulate filter continuously with 4<span class="thinspace"></span>W was effective, and reduced the occurrence of problematic relative humidity levels (RH<span class="thinspace"></span> &gt; 60<span class="thinspace"></span>%) by 50<span class="thinspace"></span>% in the infrared gas analyzer cell. No further improvement of H<sub>2</sub>O frequency response was found for heating in excess of 4<span class="thinspace"></span>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<span class="thinspace"></span>W of heating and insulation. In comparison to the original design, this reduced the high-frequency attenuation for H<sub>2</sub>O by ≈ 3∕4, and the remaining cospectral correction did not exceed 3<span class="thinspace"></span>%, even at high relative humidity (95<span class="thinspace"></span>%). 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.</p>;
