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

摘要

Concentrated agricultural activities and animal feeding operationsin the northeastern plains of Colorado represent an important source ofatmospheric ammonia (NH). The NH 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 NH concentrations in this source region, weeklyconcentrations of NH were measured at 14 locations during the summersof 2010 to 2015 using Radiello passive NH samplers. Weekly (biweekly in2015) average NH concentrations ranged from 2.66 to42.7 µg m, with the highest concentrations near largeconcentrated animal feeding operations (CAFOs). The annual summertime meanNH concentrations were stable in this region from 2010 to 2015,providing a baseline against which concentration changes associated withfuture changes in regional NH emissions can be assessed. Verticalprofiles of NH were also measured on the 300 m Boulder AtmosphericObservatory (BAO) tower throughout 2012. The highest NH concentrationalong the vertical profile was always observed at the 10 m height (annualaverage concentration of 4.63 µg m), decreasing toward thesurface (4.35 µg m) and toward higher altitudes (1.93 µg m). The NH spatial distributions measured using the passivesamplers are compared with NH 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 NH provide very useful insight into regional variabilityin atmospheric NH, 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 NH concentrations near sourcesbut tends to underpredict concentrations at locations farther downwind.Excess NH deposition by the model is hypothesized as a possibleexplanation for this trend.