Spatial distribution of ammonia concentrations and modelled dry deposition in an intensive dairy production region

Agriculture generates ~83% of total U.S. ammonia (NH3) 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 NH3 emissions. Our objective was to measure the spatial and temporal variability of NH3 across this region and estimate its dry deposition. Ambient NH3 was measured using diffusive passive samplers at 8 sites in two transects across the region from 2018-2020. NH3 fluxes were estimated using the Surface Tiled Aerosol and Gaseous Exchange (STAGE) model. Peak NH3 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 50 kg Nitrogen ha/yr, compared to < 1 kg Nitrogen ha/yr in natural landscapes. Modeling work highlighted a need for better understanding of soil emission potential in environments with high soil pH and low leaf area. Research toward better understanding soil processes is needed to improve understanding of ammonia dry deposition to arid and sparsely vegetated natural ecosystems across the western U.S