Ammonia Observations Over Large North American Cities

Ammonia (NH 3 ) contributes to fine particle formation and nitrogen deposition. Observations of NH 3 are limited compared to other major anthropogenic pollutants. Here, we explore the relationships between NH 3 and other urban pollutants using measurements collected from the NASA DC‐8 aircraft during the Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) field campaign in summer 2023. We report on NH 3 abundance, NH x (NH 3 + NH 4 + ) phase partitioning, enhancement ratios of NH 3 to carbon monoxide (CO), ethyne (C 2 H 2 ), and nitrogen oxides (NO x/y ), and comparisons of enhancement ratios to previous studies. NH 3 abundance and NH x phase partitioning varied based on city. Between Los Angeles (LA), Chicago, and New York City (NYC), LA had the highest average NH 3 concentration (4.3 ± 4.0 ppb) whereas NYC had the lowest average NH 3 concentration (0.8 ± 0.4 ppb). ΔNH 3 :ΔCO ratios ranged from 0.014 ± 0.002 ppb ppb −1 in NYC to 0.037 ± 0.004 in LA. The strong correlations with CO, C 2 H 2 , and NO x/y suggest that vehicles are the dominant source of NH 3 during the summer months in these locations. The ΔNH 3 :ΔCO ratio over NYC observed during the AEROMMA campaign is consistent with National Emissions Inventory (NEI) emissions for on‐road vehicles, whereas the ΔNH 3 :ΔCO ratios observed over LA and Chicago during AEROMMA are significantly higher than expected from the NEI on‐road emissions.