Rapid Ammonia Deposition Measured Near Concentrated Animal Feeding Operations

Abstract:

Concentrated animal feeding operations (CAFOs) emit massive amounts of ammonia (NH₃) to the atmosphere. Current measurements of NH₃ are generally conducted far away from the sources (satellites, airplanes, etc.). There is insufficient knowledge about the dry deposition rate of NH₃ near the sources, which might contribute to the large discrepancies between measured concentrations at CAFOs and those from models. During the 2014 NASA DISCOVER-AQ campaign, we designed a series of tests to measure the deposition rate of NH₃ by utilizing a suite of sensors, including a LICOR LI-7700 methane sensor and Princeton University’s custom open path NH₃ sensor, which was mounted on top of a small SUV. Our mobile sampling technique enables us to follow feedlot emission plumes to see how ambient NH₃ concentration decays as gases moves away from the CAFO. The mobile platform is used to perform upwind and downwind sampling to characterize the NH₃ emission source. We tracked the change of the enhancement of NH₃ concentration relative to the enhancement of CH₄ concentration (ΔNH₃:ΔCH₄), while transecting the plume of individual cattle feedlots. Measured data shows that the high concentration of NH₃ seen at the source decreases quickly as one moves further downwind from it. A time constant of approximately ten minutes has been calculated from the decay of the ΔNH₃:ΔCH₄ ratios while moving away from the sources. We also will compare our measurements with those of NASA’s P-3B aerosol measurements to show that the majority must be lost to dry deposition. This rapid deposition suggests that large amounts of NH₃ are being deposited in very close proximity to these CAFOs, which is consistent with previous findings of locally high soil pH near NH₃ sources. Our results will be used to better characterize nitrogen deposition from cattle feedlots and estimate NH₃ lifetime.