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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 7.8μm. The precision of the N₂O flux measurements was determined to be 0.2nmolm⁻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 (0.3ppb N₂O/% H₂O). Both effects cause a negative bias of similar magnitude (0.3nmolm⁻² s⁻¹ N₂O flux/mmolm⁻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.
