N 2O exchange over managed grassland: Application of a quantum cascade laser spectrometer for micrometeorological flux measurements

Abstract

A novel quantum cascade laser absorption spectrometer (QCL-AS) was tested to monitor N 2O exchange fluxes over an intensively managed grassland using the eddy covariance approach. The instrument employs a continuous wave quantum cascade laser to scan over the absorption features of N 2O, CH 4 and water vapor at 7.8 μm. The precision of the N 2O flux measurements was determined to be 0.2 nmol m −2 s −1 but the accuracy can easily be affected by water vapor interferences twice as large. These water vapor interferences are not only due to the respective gas dilution effect but also due to an additional cross-sensitivity of the N 2O analyzer to water vapor (0.3 ppb N 2O/% H 2O). Both effects cause a negative bias of similar magnitude (0.3 nmol m −2 s −1 N 2O flux/mmol m −2 s −1 H 2O flux) in the flux measurements. While the dilution (or density) correction is a well known and routinely applied procedure, the magnitude of the analyzer cross-talk may depend on the specific instrumental setup and should be empirically determined. The comparison with static chamber measurements shows the necessity of the cross-talk correction; otherwise the QCL-AS based eddy covariance system would yield unrealistically large uptake of N 2O.