Urban Emissions of Ammonia in North American Megacities

Ammonia (NH₃) is an unregulated air pollutant in the US. It contributes to fine particle formation and nitrogen deposition. Anthropogenic sources, particularly agriculture, combustion, and industrial activities, dominate emissions of NH₃. However, observations of the atmospheric sources, sinks, and phase partitioning of NH₃ are limited compared to other major anthropogenic pollutants. In contrast to declining emissions of nitrogen oxides (NO_x) from combustion sources, the emissions of NH₃ from combustion and agricultural activities have grown, and the deposition of reduced nitrogen has increased. Here, we explore the relationships between NH₃ and other urban pollutants. We leverage airborne trace gas measurements including NH₃, methane (CH₄), ethane (C₂H₆), carbon monoxide (CO), carbon dioxide (CO₂), and nitrogen oxides (NO_x) collected from the NASA DC-8 aircraft during the Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) field campaign conducted in summer 2023. We will use these measurements to quantify urban enhancement ratios, such as ΔNH₃/ΔCO and ΔNH₃/ΔCO₂, from the major North American metropolitan areas sampled during AEROMMA. We will then compare these ratios with reported values from prior ground-based and airborne studies.