Evaluation of a lower-powered analyzer and sampling system for eddy-covariance measurements of nitrous oxide fluxes

Abstract

Nitrous oxide (N2O) fluxes measured using the eddy-covariance method capture the spatial and temporal heterogeneity of N2O emissions. Most closed-path trace-gas analyzers for eddy-covariance measurements have large-volume, multi-pass absorption cells that necessitate high flow rates for ample frequency response, thus requiring high-power sample pumps. Other sampling system components, including rain caps, filters, dryers, and tubing, can also degrade system frequency response. This field trial tested the performance of a closed-path eddy-covariance system for N2O flux measurements with improvements to use less power while maintaining the frequency response. The new system consists of a thermoelectrically cooled tunable diode laser absorption spectrometer configured to measure both N2O and carbon dioxide (CO2). The system features a relatively small, single-pass sample cell (200ĝ€mL) that provides good frequency response with a lower-powered pump ( ĝ1/4 ĝ€250ĝ€W). A new filterless intake removes particulates from the sample air stream with no additional mixing volume that could degrade frequency response. A single-Tube dryer removes water vapour from the sample to avoid the need for density or spectroscopic corrections, while maintaining frequency response. This eddy-covariance system was collocated with a previous tunable diode laser absorption spectrometer model to compare N2O and CO2 flux measurements for two full growing seasons (May 2015 to October 2016) in a fertilized cornfield in Southern Ontario, Canada. Both spectrometers were placed outdoors at the base of the sampling tower, demonstrating ruggedness for a range of environmental conditions (minimum to maximum daily temperature range: ĝ'26.1 to 31.6ĝ€°C). The new system rarely required maintenance. An in situ frequency-response test demonstrated that the cutoff frequency of the new system was better than the old system (3.5ĝ€Hz compared to 2.30ĝ€Hz) and similar to that of a closed-path CO2 eddy-covariance system (4.05ĝ€Hz), using shorter tubing and no dryer, that was also collocated at the site. Values of the N2O fluxes were similar between the two spectrometer systems (slopeĝ€ Combining double low line ĝ€1.01, r 2 Combining double low line ĝ€0.96); CO2 fluxes as measured by the short-Tubed eddy-covariance system and the two spectrometer systems correlated well (slopeĝ€ Combining double low line ĝ€1.03, r 2 Combining double low line ĝ€0.998). The new lower-powered tunable diode laser absorption spectrometer configuration with the filterless intake and single-Tube dryer showed promise for deployment in remote areas.