Open-path eddy covariance measurements of ammonia fluxes from a beef cattle feedlot

Eddy covariance (EC) measurements of NH₃ fluxes from a cattle feedlot were made with a high-precision, fast-response (20Hz) open-path laser-based sensor. The sensor employed a continuous wave, quantum cascade (QC) laser and targeted an isolated absorption feature of NH₃ at 9.06μm. It was deployed on a 5-m tall flux tower beside a 22,000-animal cattle feedlot in Colorado, USA for two weeks. Sensible heat, latent heat, CO₂, and CH₄ EC fluxes were measured concurrently on the tower. The open-path NH₃ sensor showed a comparable time response to well-established commercial open-path sensors for CO₂ and H₂O. The average high-frequency flux loss over the measurement period was 6.6%, mainly resulting from sample path averaging. The sensor showed significant improvement over NH₃ EC fluxes measured by closed-path sensors. The measured NH₃ EC fluxes were well-correlated with latent heat EC fluxes. During the measurement period, the average daily NH₃ EC flux was 31.7kgha^-1d^-1. The flux-variance relationship was used to further validate the performance of the NH₃ EC flux measurement. A 1σ detection limit of 1.3±0.5ngm^-2s^-1 for NH₃ fluxes measured in 30-min intervals was achieved in this field test. This suite of measurements enabled the evaluation of livestock NH₃ emissions at unprecedented temporal resolution and accuracy in the context of other important agricultural trace gases.