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Concentrated agricultural activities and animal feeding operationsin the northeastern plains of Colorado represent an important source ofatmospheric ammonia (NH3). The NH3 from these sources contributesto regional fine particle formation and to nitrogen deposition to sensitiveecosystems in Rocky Mountain National Park (RMNP), located  ∼  80 km to the west. In order to better understand temporal and spatialdifferences in NH3 concentrations in this source region, weeklyconcentrations of NH3 were measured at 14 locations during the summersof 2010 to 2015 using Radiello passive NH3 samplers. Weekly (biweekly in2015) average NH3 concentrations ranged from 2.66 to42.7 µg m−3, with the highest concentrations near largeconcentrated animal feeding operations (CAFOs). The annual summertime meanNH3 concentrations were stable in this region from 2010 to 2015,providing a baseline against which concentration changes associated withfuture changes in regional NH3 emissions can be assessed. Verticalprofiles of NH3 were also measured on the 300 m Boulder AtmosphericObservatory (BAO) tower throughout 2012. The highest NH3 concentrationalong the vertical profile was always observed at the 10 m height (annualaverage concentration of 4.63 µg m−3), decreasing toward thesurface (4.35 µg m−3) and toward higher altitudes (1.93 µg m−3). The NH3 spatial distributions measured using the passivesamplers are compared with NH3 columns retrieved by the InfraredAtmospheric Sounding Interferometer (IASI) satellite and concentrationssimulated by the Comprehensive Air Quality Model with Extensions (CAMx). Thesatellite comparison adds to a growing body of evidence that IASI columnretrievals of NH3 provide very useful insight into regional variabilityin atmospheric NH3, in this case even in a region with strong localsources and sharp spatial gradients. The CAMx comparison indicates that themodel does a reasonable job simulating NH3 concentrations near sourcesbut tends to underpredict concentrations at locations farther downwind.Excess NH3 deposition by the model is hypothesized as a possibleexplanation for this trend

Temporal and spatial variability of ammonia in urban and agricultural regions of northern Colorado, United States