Evaluation of a lower-powered analyzer and sampling system for eddy-covariance measurements of nitrous oxide fluxes
Nitrous oxide (N<sub>2</sub>O) fluxes measured using the eddy-covariance method capture the spatial and temporal heterogeneity of N<sub>2</sub>O emissions. Most closed-path trace-gas analyzers for eddy-covariance measurements have large-volume, multi-pass absorption cells...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-03-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://www.atmos-meas-tech.net/11/1583/2018/amt-11-1583-2018.pdf |
| Summary: | Nitrous oxide (N<sub>2</sub>O) fluxes measured using the
eddy-covariance method capture the spatial and temporal heterogeneity of
N<sub>2</sub>O 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 N<sub>2</sub>O 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 N<sub>2</sub>O and carbon dioxide (CO<sub>2</sub>). 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 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
N<sub>2</sub>O and CO<sub>2</sub> 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: −26.1 to 31.6 °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 (3.5 Hz compared to 2.30 Hz) and similar to that of a
closed-path CO<sub>2</sub> eddy-covariance system (4.05 Hz), using shorter tubing
and no dryer, that was also collocated at the site. Values of the N<sub>2</sub>O
fluxes were similar between the two spectrometer systems (slope = 1.01,
<i>r</i><sup>2</sup> = 0.96); CO<sub>2</sub> fluxes as measured by the short-tubed
eddy-covariance system and the two spectrometer systems correlated well
(slope = 1.03, <i>r</i><sup>2</sup> = 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. |
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| ISSN: | 1867-1381 1867-8548 |