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

Abstract Eddy covariance (EC) measurements of NH 3 fluxes from a cattle feedlot were made with a high-precision, fast-response (20 Hz) open-path laser-based sensor. The sensor employed a continuous wave, quantum cascade (QC) laser and targeted an isolated absorption feature of NH 3 at 9.06 μm. 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, CO 2 , and CH 4 EC fluxes were measured concurrently on the tower. The open-path NH 3 sensor showed a comparable time response to well-established commercial open-path sensors for CO 2 and H 2 O. 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 NH 3 EC fluxes measured by closed-path sensors. The measured NH 3 EC fluxes were well-correlated with latent heat EC fluxes. During the measurement period, the average daily NH 3 EC flux was 31.7 kg ha −1  d −1 . The flux-variance relationship was used to further validate the performance of the NH 3 EC flux measurement. A 1 σ detection limit of 1.3 ± 0.5 ng m −2  s −1 for NH 3 fluxes measured in 30-min intervals was achieved in this field test. This suite of measurements enabled the evaluation of livestock NH 3 emissions at unprecedented temporal resolution and accuracy in the context of other important agricultural trace gases.