Determination of field scale ammonia emissions by eddy covariance using chemical ionization mass spectrometry

A system for fast ammonia (NH₃) measurements with high-temperature chemical ionisation mass spectrometry (HT-CIMS) based on a commercial Proton Transfer Reaction-Mass Spectrometer is presented. It uses electron transfer reaction as ionisation pathway and features a drift tube of PEEK and silica-coated steel. Heating the instrumental inlet and the drift tube to 180°C enabled an effective time resolution of about 1 s and made it possible to apply the instrument for eddy covariance (EC) measurements. EC fluxes of NH₃ were measured over agricultural fields in Switzerland, following fertilizations with cattle slurry. Sample air was aspirated through a 23 m long PFA tube heated to 150°C. This setup minimised damping of fast NH₃ concentration changes between the sampling point and the actual measurement. For a measurement height of 1.2 m, high-frequency attenuation loss of the NH₃ fluxes of 20 to 40% was quantified and corrected for using an empirical ogive method. The NH₃ emissions after broad spreading of the slurry showed an initial maximum as high as 150 μg/m2/s with a fast decline in the following hours. The flux detection limit of the EC system was about 5 ng/m2/s while the accuracy of individual flux measurements was estimated 16% for the high-flux regime. Slurry was usually spread on the fields in sequential tracks over a period of one to two hours. In order to calculate field emissions, measured EC/HT-CIMS fluxes were combined with flux footprint analysis of individual slurry spreading tracks to parameterise the NH₃ volatilisation with a bi-exponential time dependence. The EC/HT-CIMS technique was compared to an independent method using backward Lagrangian Stochastic dispersion modelling (bLS) and concentration measurements by three optical open path Fourier Transform Infrared (FTIR) systems. The cumulated emissions obtained by the EC/HT-CIMS and the bLS/FTIR methods agreed within 20%, a difference typical for NH₃ flux quantification. Both methods were further tested for consistency with independent impinger concentration measurements. These analyses showed good agreement without systematic deviations for the EC/HT-CIMS results but some systematic over- or underestimation for the bLS/FTIR results.