Trends in U.S. Ammonia Emissions and Lifetime (Invited Presentation)

Gaseous ammonia (NH₃) is the primary alkaline gas in the atmosphere. It contributes to ambient fine particulate matter formation, and its adverse health effects, by combining with acidic gases such as sulfuric and nitric acid. NH₃ is directly emitted, is not formed by photochemical reactions, and has become the primary contributor to reactive nitrogen deposition over most of the USA. Via deposition, NH₃ also degrades ecosystems. Unlike the decreasing trend seen in other PM_2.5 precursor emissions over the last 20 years, NH₃ emissions in the United States have increased by approximately 20%. The corresponding change in PM_2.5 sensitivity to precursor gases has also affected the residence time of NH₃ and therefore the extent of its influence on ecosystems via deposition. Here, we illustrate the relative contributions of NH₃ sources to total NH₃ emissions, as reported in the U.S. Environmental Protection Agency’s (EPA) National Emissions Inventory (NEI). This includes agricultural sources (livestock waste and fertilizer application) and increasingly acknowledged contributions from onroad mobile sources and fires. We briefly outline the bottom-up estimation methods for NH₃ emissions from some of these sources in the NEI, show their relative emission magnitudes in the 2020 NEI, illustrate sector-based emission trends over time, and point to future work to be done to improve current methods and add missing sources of NH₃ to the NEI. We will also use the EPA’s Air Quality Time Series (EQUATES), which provides 18 years of consistently configured regional chemical transport model (CMAQ) simulations, to explore spatiotemporal trends in NH₃ loss pathways.