Constraining the Fractional Deposition of Ammonia Emissions That Deposit near Confined Animal Feeding Operations: A Modeling Approach

Large livestock production facilities such as Concentrated Animal Feeding Operations (CAFOs) are a major source of ammonia to the atmosphere; once in the atmosphere, ammonia can undergo chemical reactions to form atmospheric aerosols which impact regional air quality and climate, or it can deposit into terrestrial or marine ecosystems where it can cause soil acidification, eutrophication, and loss of biodiversity. There have been numerous studies to measure the emission of ammonia from these large sources. However, a large fraction of ammonia emissions, as measured from the CAFO fenceline, is thought to undergo dry deposition near the feedlot, and the exact fraction of total emissions that undergoes deposition is not well known. Because ammonia deposition and emission fluxes are notoriously difficult to measure, deposition has been investigated with indirect techniques to quantify the flux, and with models. Modeling ammonia deposition is associated with another set of challenges, as ammonia undergoes complex bidirectional fluxes with land surfaces that depend on many hard-to-measure parameters including the following: windspeed, atmospheric and boundary resistances, the stomatal and cuticular resistances inside the plant canopy, and the surface compensation point, which is a function of the nitrogen content and temperature. Many of these parameters vary on small spatial scales, making it difficult to accurately model the interaction of ammonia with the surrounding land surface.

In this study, we use a coupled K-epsilon computational fluid dynamics model, and Lagrangian-stochastic ammonia bidirectional-exchange model to simulate the impact of CAFO emissions on the surrounding landscape. We use this coupled model framework to explore the ammonia deposition parameter space, and to constrain the deposition for various land use types (e.g. crops vs grassland), and for various surface compensation points as estimated from measurement studies of ammonia deposition and literature values of ammonia compensation points. We will present the range of deposition that is expected under our simulated atmospheric conditions, as well as the parameters ammonia deposition is most-sensitive to, and we will discuss these results in the context of land-management practices that can either increase or decrease local recapture of ammonia.