Using Quantum Cascade Lasers to Quantify Ammonia Emissions from Beef Feedlots.

It is difficult to measure ammonia (NH₃) accurately in real time from beef feedlots, especially in a closed-path measurement system, mostly due to its tendency of adsorption to surrounding materials. A high flow and high vacuum sample stream in the closed-path measurement regime is ideal to minimise the adsorption and responsiveness issue of NH₃. A Quantum Cascade Laser (QCL) trace gas analyser (Aerodyne, Inc.) has no flow restriction on its open-tubular multi-pass absorption cell and can measure NH₃ at a 10 Hz rate. In a few recent studies, we combined QCLs with heated Teflon tubing, a high speed vacuum pump and a virtual impactor to create a sampling system to minimise NH₃ adhesion and retention, with the purpose to improve NH₃ measurement accuracy and responsiveness. In conjunction with this system, we employed integrated horizontal flux (IHF) method to measure NH₃ fluxes from a small scale (20 m x 20 m) feedlot; and eddy covariance method in a large commercial open feedlot. The accurate and real time measurements have dramatically improved the confidence of the flux calculations. We detected NH₃ fluxes from beef feedlots at an average of 10 g NH₃ m^-2 day^-1 with a Gaussian distribution diurnal pattern.