Measurement Report: Urban Ammonia and Amines in Houston, Texas

Abstract. Ammonia and amines play critical roles in secondary aerosol formation, especially in urban environments. However, fast measurements of ammonia and amines in the atmosphere are very scarce. We measured ammonia and amines with a chemical ionization mass spectrometer (CIMS) at the urban center in Houston, Texas, the fourth most populated urban site in the United States, during October 2022. Ammonia concentrations were on average 4 parts per billion in volume (ppbv), while the concentration of an individual amine ranged from several parts per trillion in volume (pptv) to hundreds of pptv. These reduced nitrogen compounds were more abundant during the weekdays than on weekends and correlated with measured CO concentrations, implying they were mostly emitted from pollutant sources. Both ammonia and amines showed a distinct diurnal cycle, with higher concentrations in the warmer afternoon, indicating dominant gas-to-particle conversion processes taking place with the changing ambient temperatures. Studies have shown that dimethylamine is critical for urban new particle formation (NPF), but currently, there are no amine emission inventories in global climate models (as opposed to ammonia). Our observations show that amines in general positively correlated with ammonia, indicating that it is reasonable for global models to use scaled-down ammonia concentrations (e.g., 0.1 %) as a proxy of urban dimethylamine concentrations to simulate urban NPF processes.