Ammonia Emissions from Livestock in the United States: From Farm-Level Models to a New National Inventory

The role of nitrogen as an environmental pollutant has long been established, but the significance and amount of ammonia emitted from livestock production has not been well-characterized in the United States. In order to better characterize the impacts of ammonia as an air pollutant and its role in the formation of fine particulate matter, we have used a semi-empirical process-based model to estimate ammonia emissions from beef cattle, swine, layer chickens, and broiler chickens in the United States. The semiempirical model is used so that the ammonia emissions can be simulated using a mass balance on nitrogen through the farm, tracking nitrogen from housing, storage, and application of manure, in addition to being constrained by a number of tuned parameters which ensure that the resulting emission factors are realistic. In addition, model inputs, including manure pH, manure volume, manure nitrogen content and manure dry matter content are taken from the existing literature. These previous studies were the first means by which the FEMs were evaluated, with the FEMs able to capture 20-70% of the variability in reported emissions. Next, observations from the National Air Emissions Monitoring Study were used to further test and evaluate the model, including the evaluation in its performance to characterize seasonal as well as day-to-day variability in emissions of ammonia; here again, seasonal variability was well-characterized with slightly more mixed results for daily variability in ammonia emissions. Then, emissions from these single farms were summed over the entire populations of cattle, swine and poultry in the US, using data from the National Climate Data Center and USDA National Animal Health Monitoring Survey for appropriate meteorology and manure management practices in different locations. Emissions total 1.7 Tg from our model: 723 Gg from swine, 435 Gg from beef, 202 Gg from dairy, and 357 Gg from poultry. These inventories were compared to the National Emissions Inventory which estimates 1.9 Tg of ammonia emissions from livestock in 2011. This work has demonstrated the model’s ability to characterize the variability in ammonia emissions from livestock and can be used to understand the air quality impacts of this variability.