Concentrated agricultural activities and animal feeding operations in the northeastern plains of Colorado represent an important source of atmospheric ammonia (NH 3 ). The NH 3 from these sources contributes to regional fine particle formation and to nitrogen deposition to sensitive ecosystems in Rocky Mountain National Park (RMNP), located ∼ 80 km to the west. In order to better understand temporal and spatial differences in NH 3 concentrations in this source region, weekly concentrations of NH 3 were measured at 14 locations during the summers of 2010 to 2015 using Radiello passive NH 3 samplers. Weekly (biweekly in 2015) average NH 3 concentrations ranged from 2.66 to 42.7 µg m −3 , with the highest concentrations near large concentrated animal feeding operations (CAFOs). The annual summertime mean NH 3 concentrations were stable in this region from 2010 to 2015, providing a baseline against which concentration changes associated with future changes in regional NH 3 emissions can be assessed. Vertical profiles of NH 3 were also measured on the 300 m Boulder Atmospheric Observatory (BAO) tower throughout 2012. The highest NH 3 concentration along the vertical profile was always observed at the 10 m height (annual average concentration of 4.63 µg m −3 ), decreasing toward the surface (4.35 µg m −3 ) and toward higher altitudes (1.93 µg m −3 ). The NH 3 spatial distributions measured using the passive samplers are compared with NH 3 columns retrieved by the Infrared Atmospheric Sounding Interferometer (IASI) satellite and concentrations simulated by the Comprehensive Air Quality Model with Extensions (CAMx). The satellite comparison adds to a growing body of evidence that IASI column retrievals of NH 3 provide very useful insight into regional variability in atmospheric NH 3 , in this case even in a region with strong local sources and sharp spatial gradients. The CAMx comparison indicates that the model does a reasonable job simulating NH 3 concentrations near sources but tends to underpredict concentrations at locations farther downwind. Excess NH 3 deposition by the model is hypothesized as a possible explanation for this trend.