Comparison of Urban Ammonia Observations in Major U.S. Cities

Ammonia (NH₃) is an unregulated air pollutant in the United States. 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, NH_x (NH₃ + NH₄), and other urban pollutants. We use airborne measurements of NH₃, ammonium (NH₄), methane (CH₄), ethyne (C₂H₂), acetonitrile (CH₃CN), and carbon monoxide (CO) 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 report on NH₃ and NHx abundance, NH_x phase during both low/no smoke and smoke-impacted flights, NH_x:CO and NH₃:C₂H₂ area ratios, and comparisons of NH_x:CO ratios from previous studies in the major U.S. metropolitan areas sampled during AEROMMA. NH_x abundance and phase varied based on smoke presence and city. LA under low/no smoke conditions had the highest average NH₃ concentration (4.3 ± 0.5 ppb) while NYC under low/no smoke conditions had the lowest average NH₃ concentration (0.8 ± 0.1 ppb). NH_x:CO ratios were mostly consistent (within error) between cities and prior studies, ranging from 0.035 ± 0.011 ppb/ppb in Chicago during smoke-impacted conditions to 0.052 ± 0.017 ppb/ppb in NYC during low/no smoke conditions. However, the AEROMMA NH_x:CO ratios disagreed with ratios derived from the EPA’s National Emissions Inventory (NEI). All low/no smoke NH_x:CO relationships during AEROMMA had r² values > 0.91. NH_x:C₂H₂ ratios also show a strong correlation during low/no smoke conditions. The strong correlations with CO and C₂H₂ demonstrate that NHx is likely associated with vehicle emissions in these areas.