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

• Open-path NH3 EC flux measurements were demonstrated at a cattle feedlot.
• A new open-path, quantum cascade laser-based sensor was used in an EC setup.
• A detection limit of 1.3 ± 0.5 ng m−2 s−1 NH3 flux was achieved.
• NH3 flux had a strong diurnal cycle and correlated well with sensible/latent heat.

Eddy covariance (EC) measurements of NH3 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 NH3 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, 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 31.7 kg ha−1 d−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 ± 0.5 ng m−2 s−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.