Da Pan, Assistant Professor, School of Civil and Environmental Engineering, Georgia TechHuman activities have significantly increased atmospheric levels of Nr, contributing to secondary inorganic aerosol formation and disrupting biogeochemical processes. It is estimated that 40% PM2.5 can be attributed to Nr emissions globally. Meta-analyses indicate that elevated Nr deposition lowers soil carbon decomposition rates, enhancing terrestrial CO2.uptake. However, excessive Nr deposition also leads to adverse effects such as biodiversity loss, soil acidification, and eutrophication. Additionally, increased nitrogen deposition is linked to higher nitrous oxide (N2O) emissions. Balancing the benefits and harmful effects of Nr emissions and deposition requires comprehensive knowledge of its spatiotemporal patterns, composition, and ecosystem impacts. Yet, large-scale and long-term analyses of secondary inorganic aerosol formation and Nr deposition based on model simulations have substantial uncertainties. In this presentation, I will show improved constraints on secondary inorganic aerosol formation using decade-long in situ observations from multiple monitoring networks across the United States. I will discuss a shift in the secondary inorganic aerosol formation regime between 2011 and 2020, making rural areas less sensitive to changes in ammonia concentrations and shortening the effective atmospheric lifetime of reduced forms of Nr. This leads to potential increases in reactive nitrogen deposition near ammonia emission hotspots, which provides unique opportunities to evaluate ecosystem responses and the interaction between carbon and nitrogen cycles. Finally, I will identify locations that have experienced significant Nr deposition changes and possess comprehensive atmospheric and ecological observations for potential natural experiments.