| Summary: | Agriculture generates ~83% of total US ammonia (NH<sub>3</sub>) emissions, potentially adversely impacting sensitive ecosystems through wet and dry deposition. Regions with intense livestock production, such as the dairy region of south-central Idaho, generate hotspots of NH<sub>3</sub> emissions. Our objective was to measure the spatial and temporal variability of NH<sub>3</sub> across this region and estimate its dry deposition. Ambient NH<sub>3</sub> was measured using diffusive passive samplers at 8 sites in two transects across the region from 2018–2020. NH<sub>3</sub> fluxes were estimated using the Surface Tiled Aerosol and Gaseous Exchange (STAGE) model. Peak NH<sub>3</sub> concentrations were 4–5 times greater at a high-density dairy site compared to mixed agriculture/dairy or agricultural sites, and 26 times greater than non-agricultural sites with prominent seasonal trends driven by temperature. Annual estimated dry deposition rates in areas of intensive dairy production can approach 45 kg N ha<sup>−1</sup> y<sup>−1</sup>, compared to <1 kg N ha<sup>−1</sup> y<sup>−1</sup> in natural landscapes. Our results suggest that the natural sagebrush steppe landscapes interspersed within and surrounding agricultural areas in southern Idaho receive NH<sub>3</sub> dry deposition rates within and above the range of nitrogen critical loads for North American deserts. Finally, our results highlight a need for improved understanding of the role of soil processes in NH<sub>3</sub> dry deposition to arid and sparsely vegetated natural ecosystems across the western US.
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