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

摘要

Nitrous oxide (NO) fluxes measured using the eddy-covariance method capture the spatial and temporal heterogeneityof NO emissions. Most closed-path trace-gas analyzers for eddy-covariance measurements have large-volume, multi-passabsorption cells that necessitate high flow rates for ample frequency response, thus requiring high-power sample pumps. Othersampling system components, including rain caps, filters, dryers, and tubing can also degrade system frequency response. Thisfield trial tested the performance of a closed-path eddy-covariance system for NO flux measurements with improvements to useless power while maintaining the frequency response. The new system consists of a thermoelectrically cooled tunable diode laserabsorption spectrometer configured to measure both NO and carbon dioxide (CO). The system features a relatively small, single-pass sample cell (200 ml) that provides good frequency response with a lower-powered pump (~ 250 W). A new filterless intakeremoves particulates from the sample air stream with no additional mixing volume that could degrade frequency response. Asingle-tube dryer removes water vapor from the sample to avoid the need for density or spectroscopic corrections, whilemaintaining frequency response. This eddy-covariance system was collocated with a previous tunable diode laser absorptionspectrometer model to compare NO and CO flux measurements for two full growing seasons (May 2015 to October 2016) in afertilized cornfield in Southern Ontario, Canada. Both spectrometers were placed outdoors at the base of the sampling towerdemonstrating ruggedness for a range of environmental conditions (minimum to maximum daily temperature range: −26.1 to31.6 °C). The new system rarely required maintenance. An in situ frequency response test demonstrated that the cutoff frequencyof the new system was better than the old system (3.5 Hz compared to 2.30 Hz), and similar to that of a closed-path CO eddy-covariance system (4.05 Hz) using shorter tubing and no dryer that was also collocated at the site. Values of the N2O fluxes weresimilar between the two spectrometer systems (slope = 1.01, r = 0.96); CO fluxes as measured by the short-tubed eddy-covariance system and the two spectrometer systems correlated well (slope = 1.03, r = 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.