Using modelling tools to advance the understanding of ammonia dry-deposition and bidirectional flux processes next to large animal feeding operations
Lassman, William, author
Pierce, Jeffrey R., advisor
Collett, Jeffrey L., Jr., advisor
Fischer, Emily V., committee member
Ham, Jay M., committee member
Ammonia in the atmosphere is a trace gas that can play a big role in the Earth's climate, as well as human and ecological health. Due to its stickiness and solubility, ammonia can enter the biosphere via wet and dry deposition, where excess ammonia input often results in soil acidification, disruption of natural ecological equilibria, and loss of biodiversity. Additionally, ammonia is the most abundant alkaline species in the atmosphere and can react with atmospheric acids to form aerosols, which can affect the earth's radiative balance as well as human health. Ammonia emissions tend to be associated with agricultural sources, such as fertilized fields or animal waste at concentrated Animal Feeding Operations (CAFOs). Consequently, ammonia emissions tend to be dynamic and highly heterogeneous, and ammonia surface-fluxes are difficult to measure. However, in regions with many large CAFOs, ammonia can be an important regional pollutant, especially if there are sensitive ecosystems or other regional sources of atmospheric acids present. In this dissertation, I study ammonia dry-deposition fluxes immediately downwind of CAFOs using a variety of modelling tools. First, I discuss original research where I use a coupled a K-epsilon model with a Lagrangian-Stochastic ammonia bidirectional exchange surface model to simulate the dispersion and deposition of ammonia downwind of an idealized CAFO. Based on these simulations, the amount of ammonia that undergoes dry deposition depends greatly on the land surface downwind of the CAFO; replacing bare soil or unmanaged grassland with leafier surfaces such as cropland or forests can increase the fraction of total ammonia emissions that deposits from 2 - 10% to 30 - 50%, though this is sensitive to the ammonia emission potential in the model plant canopy. Next, I describe a separate study where I use a 3-D Large-Eddy Simulation model to simulate the dispersion of ammonia and methane from a CAFO with a time-resolved modelling tool. I use this modelling system to produce synthetic observations, which are used to develop an inversion approach to quantify the ammonia dry deposition near a CAFO with colocated mobile measurements of ammonia and methane. While I demonstrate that such an inversion technique is feasible with surface-based measurements, considerable value is added, in terms of minimizing method bias and increasing method precision, by mounting measurements on a small Unmanned Aerial System (sUAS). Finally, I present measurements of PM2.5 concentration and composition that were made in Palapye, Botswana. Botswana is a developing country with a hot and arid climate. Beef and livestock production are important economic activities in Botswana; however, the agricultural practices differ considerably from the CAFOs discussed in the rest of the dissertation. Furthermore, these livestock activities occur against a backdrop of emissions and air pollutants that differ considerably from the United States and Europe. The measurements show that PM2.5 concentrations were on average 9 μg m-3 during the 5-week measurement period. While below levels that are typically considered hazardous, there was considerable variability in the measured concentrations, and the measurement period is too short to conclusively determine that air pollution is not a public health concern in this region. The aerosol composition is dominated by carbonaceous species, probably from biomass burning, though inorganic sulfate also is abundant in the aerosol phase. As Botswana continues to undergo economic development, the types of emissions and pollution present will continue to change.
Includes bibliographical references.
Includes bibliographical references.