N2O exchange over managed grassland: Application of a quantum cascade laser spectrometer for micrometeorological flux measurements

A novel quantum cascade laser absorption spectrometer (QCL-AS) was tested to monitor N2O 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 N2O, CH4 and water vapor at 7.8 μm. The precision of the N2O 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 N2O analyzer to water vapor (0.3 ppb N2O/% H2O). Both effects cause a negative bias of similar magnitude (0.3 nmol m−2 s−1 N2O flux/mmol m−2 s−1 H2O 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 N2O.