Simultaneously Measuring Changes in Methane and Ammonia Emissions Resulting from Altering Beef Cattle Diet

Beef cattle are notoriously responsible for significant emissions of methane (CH₄), a potent greenhouse gas resulting from enteric fermentation in cattle stomachs. Less well-known is the substantial emissions of ammonia (NH₃) from cattle urine interacting with soil, which can affect local air quality. Altering beef cattle diets may be used to mitigate cattle CH₄ emissions, but the effects of these new diets on NH₃ emissions remain largely under characterized, since ammonia can be difficult to measure. The challenge is to simultaneously measure any reduction in CH₄ emissions in an operational feedlot while precisely monitoring NH₃ emissions. Here, we apply open-path dual comb spectroscopy (DCS) to perform a fence-line measurement of both CH₄ and NH₃ enhancements from two groups of cattle on different feed and to calculate fluxes over multiple months of measurement.

We deployed our DCS instrument at a small feedlot located at the Kansas State University Southeast Research-Extension Center in Mound Valley, Kansas, USA. Open-path DCS is particularly well suited for this application because it enables fast and precise simultaneous detection of multiple species without suffering from inlet effects that could bias measurements of lower-volatility compounds. Additionally, two open-path laser beamlines from the DCS system allow for both an upwind (background) and a downwind (enhancement) measurement given the prevailing wind at the site. We calculate CH₄ and NH₃ fluxes by inputting our high time-resolution concentration and wind velocity data into an inverse dispersion model implemented with WindTrax. The two groups of cattle are fed a rolled grain finishing diet with one group having corn grain and the other sorghum grain. Sorghum is considered a more sustainable grain than corn because it requires less fertilizer and water and is resilient to heat. We compare CH₄ and NH₃ fluxes between the two cattle groups to assess the net impact of substituting sorghum for corn on total greenhouse gas emissions and local air quality.

Our open-path DCS technique is uniquely advantaged for calculating the most accurate fluxes, since it samples continuously (rather than just when cattle are eating), but it requires mindful correction for potential interferences. Because of strong H₂O absorption lines in the near infrared (6000 cm^-1 to 6615 cm^-1), any errors in the H₂O molecular line list have the potential to bias our NH₃ measurements. To reduce these interferences, we carefully fit the H₂O absorption using the TCCON 2020 data release, which produced better fit residuals than HITRAN 2020. In this work, we implement an improved H₂O line list based on the TCCON 2020 data and our observed NH₃/H₂O data. We apply these precise measurements to quantify net emissions of both CH₄ and NH₃ for both cattle feeds.