Frequency Response Evaluation of a Low-power Closed-path Eddy Covariance System for Measuring Nitrous Oxide Fluxes

The eddy covariance (EC) method has been widely used to capture the temporal and spatial patterns of nitrous oxide (N₂O) emissions from a wide variety of agricultural ecosystems. Technological advancements in the recent years have brought new tunable infrared laser-based closed-path gas analyzers suitable for EC measurements. To achieve high sensitivity and low measurement noise, these analyzers use multi-pass optical cells with long sensing path. A drawback of these cells is the relatively large internal volume requiring high-flow rate, high-power pumps to attain fast response to changes in gas concentration.  Additionally, these cells are prone to contamination and require in-line filters. In this study we evaluate the frequency response of a novel, low-power, field deployable N₂O closed-path EC system consisting of: (1) a gas analyzer with a small volume single-pass optical cell, (2) a 3 m sulfonated tetrafluoroethylene ionomer intake tube acting as water vapor permeable membrane to dry the air sample, (3) a cyclone type, non-barrier inertial particle separator (IPS) to mitigate the effects of particulates contamination of the optical sample cell, and (4) a small, low-power pump module with an automatic pressure and flow control. The performance of the new N₂O EC system is evaluated in-situ 3 m above a fertilized agricultural wheat field and compared to a co-located fast-response H₂O and CO₂ open-path gas analyzer and sonic anemometer (IRGASON). Tube delays, determined by cross-covariance of N₂O with vertical wind, were consistent over time and varied between 0.2 and 0.5 s. Spectral and co-spectral analysis of vertical wind, temperature, H₂O, CO₂ and N₂O showed good agreement. Ogive functions demonstrated that the new system has adequate frequency response to capture >90% of the N₂O fluxes for a wide range of wind speeds and atmospheric stabilities and is suitable for deployment in remote areas.