Coastal megacities like Los Angeles and New York City experience some of the worst air quality in the United States The summer 2021 AEROMMA field campaign will address precursor emissions, pollutant formation, and transport between megacities and marine environments. Gas-phase ammonia (NH3) is an essential observation during the AEROMMA study. NH3 is an unregulated air pollutant that contributes to fine particle formation and nitrogen deposition. However, our 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 NOx from combustion sources, the emissions of NH3 from combustion and agricultural activities have grown, and the deposition of reduced nitrogen has increased. NH3 emissions, particularly from vehicular sources in urban areas, are highly under constrained. This project will deploy a flight-ready quantum-cascade tunable infrared laser direct absorption spectrometer aboard the NOAA WP-3 aircraft to provide observations of gas-phase NH3 for the AEROMMA field campaign. Through the NH3 measurements and analysis objectives, we will be able to provide information about the abundances and emissions of NH3 relative to other regulated pollutants as well as the chemical processes leading to fine particle formation (specifically ammonium aerosol formation) in these megacity and marine environments. This information will aid in guiding emission control strategies and policies aimed at optimizing air quality standards for protecting human health.