Eddy covariance measurement of ammonia fluxes: Comparison of high frequency correction methodologies.

Several methods are available for evaluating and correcting the underestimation of trace gas fluxes measuredby eddy covariance (EC) method and due to the design and the setup of the employed equipment.Different frequency correction factors (CF) to apply to the measured EC ammonia (NH3) fluxes have beenapplied using the following approaches: (i) the spectral theoretical transfer function (CFTheor) with andwithout phase shift; (ii) the in situ ogive method (CFO); and (iii) the inductance correction method (CFL).The NH3 fluxes were measured in an experimental field located in south Italy, above a sorghum cropsubmitted to Mediterranean semi-arid climate and fertilized with urea. A fast analyser based on TunableInfrared Laser Differential Absorption Spectroscopy (TILDAS) coupled with a quantum cascade (QC)laser was used to measure NH3 concentration at high frequency. The results showed that correction ofthe values of NH3 EC fluxes over natural surfaces are necessary for taking into account the dumpingof high frequency contribution due to the coupling of ammonia fast analyser by QC-TILDAS and sonicanemometer. In particular, the calculated flux losses ranged between-23% (inductance method, withthe CF threshold selected to 2.5) and-43% (experimental-ogive method); while the two theoretical transferfunction approaches gave comparable loss estimates, i.e.-30 and-31% for the methods with timelag and phase shift, respectively.