Advancements in atmospheric nitrous oxide eddy covariance flux measurements

Nitrous oxide (N₂O) is stratospheric ozone depleting, long-lived green-house trace gas with a 100-year global warming potential 298 times greater than carbon dioxide, (IPCC, 2013). Microbial production processes in agricultural soils contribute significant portion of the total N₂O emissions. Nitrogen fertilization and manure management are the main drivers for the N₂O fluxes at the soil-atmosphere interface, which are highly variable in space and time. The eddy covariance (EC) technique provides a continuous landscape-scale flux estimates with high temporal resolution. The availability of mid-infrared tunable diode lasers (TDL) operating at room temperature allowed the development of high precision fast response gas analyzers suitable for the EC method. In this study we describe the design and evaluate the performance of a novel, field-deployable low-power N₂O EC system. The system consists of compact closed-path TDL absorption spectrometer with a 1 m single-pass, small volume optical cell which allows the use of low power DC pump. It features a cyclone-based inertial particle separator acting as a non-barrier filter to prevent contamination of the optical components with minimal attenuation of N₂O fluctuations. The effects of absorption line broadening and dilution due to water vapor are minimized using sulfonated tetrafluoroethylene ionomer intake tube acting as water vapor permeable membrane to dry the air sample. The new N₂O EC system was deployed in manure fertilized agricultural cornfield 3 m above the canopy and was collocated with an open-path CO₂ and H₂O infrared gas analyzer and ultrasonic anemometer (IRGASON). Spectral analysis and Ogive functions demonstrated that the new EC system achieves adequate performance and excellent frequency response to measure N₂O fluxes under a wide range of meteorological conditions. Further refinements for stable and prolonged unattended operation are described.