Limitations of an Eddy-Covariance System in Measuring Low Ammonia Fluxes

• Published in: Boundary-layer meteorology Vol. 180; no. 1; pp. 173 - 186
• Main Authors: Ferrara, Rossana Monica, Di Tommasi, Paul, Famulari, Daniela, Rana, Gianfranco
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01-07-2021; Springer; Springer Nature B.V
• Subjects: Ammonia; Ammonia emissions; Atmospheric Protection/Air Quality Control/Air Pollution; Atmospheric Sciences; Confidence limits; Covariance; Detection; Earth and Environmental Science; Earth Sciences; Eddy covariance; Emission measurements; Evaluation; Fertility; Fluctuations; Fluxes; Infrared lasers; Infrared spectrometers; Lasers; Legumes; Manures; Measurement; Measuring instruments; Meteorology; Notes and Comments; Random errors; Response time; Signal to noise ratio; Soil fertility; Soil improvement; Solubility; Surface boundary layer; Surface layers; Sustainability; Time lag; Tunable lasers; Vortices; Limit of detection; White noise; Ammonia volatilization; Fast-pulsed quantum-cascade laser; Time lag
• ISSN: 0006-8314; 1573-1472
• DOI: 10.1007/s10546-021-00612-6

Green manuring of legume crops can improve soil fertility and sustainability. To evaluate its agronomic and environmental effectiveness, gaseous losses of ammonia (NH
3
) in the surface layer need to be quantified by direct measurements in the field. However, the application of the eddy-covariance technique to atmospheric NH
3
is challenging: its high reactivity, water solubility, and low background concentrations all hinder the response time of closed-path sensors for fast measurements of NH
3
concentration. Ammonia emissions following green manuring were measured for 21 days using a flux system equipped with a fast-pulsed quantum-cascade tunable-infrared-laser spectrometer. The noisy cross-covariance function for this configuration indicates flux measurements are close to the limit of detection; the low signal-to-noise ratio further increases the uncertainties, introducing a mirroring effect on the fluxes, which results in the rapid alternation between emission and deposition, within the limit of detection (around 13 and 20 ng m
−2
 s
−1
, at the 95 and 99% confidence limits, respectively). An evaluation of the measurement errors is presented, focussing on three technical aspects of the eddy-covariance system: (1) time lag, (2) random error, and (3) limit of detection. The NH
3
fluxes measured by the spectrometer are close to its limit of detection, with a random error of the same order as the flux.