Ammonia is a key ingredient in many of today's air quality challenges. It is an important contributor to fine particle formation and to excess nitrogen deposition. Because it is not a regulated pollutant, its atmospheric concentrations, sources, and sinks are not as well known as for many other air pollutants. This session seeks to highlight ongoing research efforts that add to our knowledge of atmospheric ammonia, its sources, and its environmental impacts. Topics of interest include efforts to characterize ammonia sources, new efforts to measure ammonia concentrations in the ambient atmosphere, studies of ammonia contributions to fine particle and haze formation, and contributions of ammonia to nitrogen deposition. Both observational (in situ and remote sensing) and model-based investigations are welcome.