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 measured by 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 been applied using the following approaches: (i) the spectral theoretical transfer function (CFTheor) with and without 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 crop submitted to Mediterranean semi-arid climate and fertilized with urea. A fast analyser based on Tunable Infrared 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 of the values of NH3 EC fluxes over natural surfaces are necessary for taking into account the dumping of high frequency contribution due to the coupling of ammonia fast analyser by QC-TILDAS and sonic anemometer. In particular, the calculated flux losses ranged between -23% (inductance method, with the CF threshold selected to 2.5) and -43% (experimental-ogive method); while the two theoretical transfer function approaches gave comparable loss estimates, i.e. -30 and -31% for the methods with time lag and phase shift, respectively.