Measurements of N2O fluxes from fertilized grassland using a fast response tunable diode laser spectrometer

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Abstract
A fast response tunable diode laser spectrometer was used to make N2O flux measurements by both eddy correlation and concentration gradient techniques during a methods intercomparison field program in April 1992 at a site in Stirling, Scotland. A description of the site and the results of the intercomparison are presented in companion papers. Sufficient instrumental precision and time resolution for N2O flux determination using both techniques were obtained by application of the recently developed two‐tone frequency modulation coupled with fast scanning of the laser. The use of a dedicated digital signal processor allowed zero‐overhead on‐line data handling at a rate of 10 Hz such that the time response of the system was only limited by the gas exchange time in the multipass sample cell (200 ms). Vertical concentration gradients that lead to a difference of ≤1 part per billion by volume in the N2O mixing ratio at 0.06‐ and 1.05‐m elevation were statistically resolved within 1 min. Eddy correlation measurements with intake heights of 2.25 m and 2.75 m were made in conjunction with two different sonic anemometers. The software developed for reduction and analysis of the 10‐Hz eddy correlation data was based on time efficient FFT methods and performed time‐base matching of the data set, drift correction, coordinate rotation, and evaluation of the covariances and the frequency power distributions. N2O fluxes determined with this technique were in the range of 38–113 ng N m−2 s−1. An unusual shape of the N2O concentration covariance function can be ascribed to fetch inhomogeneities, and we conclude that the eddy correlation data set contains information that can be used to characterize the spatial variability in N2O emission in addition to its spatial integral.