Near‐Source Ammonia (NH ₃ ) Fluxes and Evolution Observed From Airborne Measurements in Plumes Up To 25 km Downwind From Large Beef Cattle Facilities

Abstract The Transport and Transformation of Ammonia (TRANS 2 Am) campaign took place in northeastern Colorado in the summers of 2021 and 2022. One of the goals of TRANS 2 Am was to study the evolution of ammonia (NH 3 ) in plumes from large animal feeding operations. This work investigates the nearfield deposition of NH 3 in four plumes from large animal husbandry facilities observed during TRANS 2 Am using a mass balance approach with methane (CH 4 ) as a conservative tracer in the timescales of plume transport. Since the plumes in TRANS 2 Am were not sampled in a pseudo‐Lagrangian manner, an empirical model is developed to correct for variations in summertime NH 3 emissions as a function of time of day. The mass balance approach results show that the average summertime NH 3 decay time to 80% and 60% against deposition in plumes from large animal sources is 43 (+11/−7) and 85 (+19/−14) min, respectively. Additionally, we present estimates of net fluxes every 5 km downwind of the plume. Under the hot, dry summertime midday conditions sampled during TRANS 2 Am, we found that within 10–15 km of the large emission sources studied, NH 3 undergoes net deposition. Beyond this distance, the complex environmental exchange of NH 3 between the atmosphere and the surface suggests that fresh NH 3 emissions from smaller nearby sources could switch the direction of the net flux to emission.