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Built on the same platform as the LI-7810 CH4/CO2/H2O and LI-7815 CO2/H2O analyzers, the LI-COR Environmental LI-7820 N2O/H2O analyzer is designed primarily for field measurements of soil flux, but with short-term precision and drift specifications making the instrument suitable also for long-term background measurements of atmospheric N2O. Based on Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS), the instrument employs a near-infrared tuneable diode laser source coupled with a low-volume (6.41 cm3), high-finesse cavity to greatly increase the potential optical path-length. Here we describe the principle of OF-CEAS as implemented in the LI-7820, including a description of the optical bench and derivation of mole fraction values from recorded absorption spectra. We present results from internal testing used to define instrument specifications, including the determination of short-term precision, long-term drift, Allan deviation, dynamic range, temperature stability, and handling of water correction. In addition, results from independent tests are also presented and discussed. These tests include static, laboratory-based measurements, as well as measurements made in mobile settings; road vehicles and light aircraft. In addition we briefly present a case study detailing the definition of minimum detectable soil flux of N2O where high precision, fast measurement of N2O mole fraction is required to measure fluxes that are typically very low for the majority of the growing season. We show that a minimum detectable flux of 0.05 nmol/m2/s can be measured effectively in less than two minutes, with a chamber diameter of 20 cm. We also demonstrate that with the application of a suitable calibration protocol, the LI-7820 OF-CEAS based analyzer delivers data suitable long-term background measurements of atmospheric N2O.
Topic : Theme 2: Novel GHG concentration and flux methods and sensors.
Reference : T2-C8
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