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

摘要

pstrongAbstract./strong Concentrated agricultural activities and animal feeding operations in the northeastern plains of Colorado represent an important source of atmospheric ammonia (NHsub3/sub). The NHsub3/sub from these sources contributes to regional fine particle formation and to nitrogen deposition to sensitive ecosystems in Rocky Mountain National Park (RMNP), located a??a??a??span class="thinspace"/span80span class="thinspace"/spankm to the west. In order to better understand temporal and spatial differences in NHsub3/sub concentrations in this source region, weekly concentrations of NHsub3/sub were measured at 14 locations during the summers of 2010 to 2015 using Radiello passive NHsub3/sub samplers. Weekly (biweekly in 2015) average NHsub3/sub concentrations ranged from 2.66 to 42.7span class="thinspace"/span?μgspan class="thinspace"/spanmsupa??3/sup, with the highest concentrations near large concentrated animal feeding operations (CAFOs). The annual summertime mean NHsub3/sub concentrations were stable in this region from 2010 to 2015, providing a baseline against which concentration changes associated with future changes in regional NHsub3/sub emissions can be assessed. Vertical profiles of NHsub3/sub were also measured on the 300span class="thinspace"/spanm Boulder Atmospheric Observatory (BAO) tower throughout 2012. The highest NHsub3/sub concentration along the vertical profile was always observed at the 10span class="thinspace"/spanm height (annual average concentration of 4.63span class="thinspace"/span?μgspan class="thinspace"/spanmsupa??3/sup), decreasing toward the surface (4.35span class="thinspace"/span?μgspan class="thinspace"/spanmsupa??3/sup) and toward higher altitudes (1.93span class="thinspace"/span?μgspan class="thinspace"/spanmsupa??3/sup). The NHsub3/sub spatial distributions measured using the passive samplers are compared with NHsub3/sub 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 NHsub3/sub provide very useful insight into regional variability in atmospheric NHsub3/sub, 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 NHsub3/sub concentrations near sources but tends to underpredict concentrations at locations farther downwind. Excess NHsub3/sub deposition by the model is hypothesized as a possible explanation for this trend./p.