Satellite-detected ammonia changes in the United States: Natural or anthropogenic impacts

Ammonia (NH₃) is the most abundant alkaline component and can react with atmospheric acidic species to form aerosols that can lead to numerous environmental and health issues. Increasing atmospheric NH₃ over agricultural regions in the US has been documented. However, spatiotemporal changes of NH₃ concentrations over the entire US are still not thoroughly understood, and the factors that drive these changes remain unknown. Herein, we applied the Atmospheric Infrared Sounder (AIRS) monthly NH₃ dataset to explore spatiotemporal changes in atmospheric NH₃ and the empirical relationships with synthetic N fertilizer application, livestock manure production, and climate factors across the entire US at both regional and pixel levels from 2002 to 2016. We found that, in addition to the US Midwest, the Mid-South and Western regions also experienced striking increases in NH₃ concentrations. NH₃ released from livestock manure during warmer winters contributed to increased annual NH₃ concentrations in the Western US. The influence of temperature on temporal evolution of NH₃ concentrations was associated with synthetic N fertilizer use in the Northern Great Plains. With a strong positive impact of temperature on NH₃ concentrations in the US Midwest, this region could possibly become an atmospheric NH₃ hotspot in the context of future warming. Our study provides an essential scientific basis for US policy makers in developing mitigation strategies for agricultural NH₃ emissions under future climate change scenarios.