Optimization of an enclosed gas analyzer sampling system for measuring eddy covariance fluxes of H₂O and CO₂

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 analyzer 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 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.