Vertical Profiles of Ammonia in the Colorado Front Range: Impacts of Source Region and Meteorology

Abstract:

Atmospheric ammonia plays an important role in aerosol particle formation and growth, as well as in nitrogen deposition to sensitive ecosystems. However, significant uncertainties are associated with the distribution and strength of emission sources, and many of the processes that control its atmospheric fate are not fully understood. The high density of agricultural and urban sources located in close proximity to more pristine mountainous areas to the west make the Colorado Front Range a unique area for studying atmospheric ammonia. The meteorology of the region, where heavy monsoon rains can be followed by rapid evaporation, can also impact surface-atmosphere partitioning of ammonia. As part of the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ), vertical profiles of ammonia were measured throughout the boundary layer aboard a moveable platform on the 300 m Boulder Atmospheric Observatory (BAO) tower.

Changes in ammonia concentration and its vertical structure were driven not only by changes in wind direction and estimated source region, but also by fluctuations in surface and atmosphere water content. For example, large increases in atmospheric ammonia mixing ratios were observed following rain events. This may be explained by surface-atmosphere exchange of wet-deposited ammonia associated with rapid evaporation following the event, and likely impacts particle formation. This may also play a role in transport from ammonia-rich agricultural areas towards the mountainous regions to the west during periods of upslope flow. The vertical ammonia concentration gradients observed throughout the structured early morning boundary layer also provide insight into the possible causes of early morning spikes in ammonia – a phenomenon that has been well-documented in many other locations. A box model was used to assess the relative importance of surface emissions due to the evaporation of morning dew versus entrainment of ammonia-rich air from above the nocturnal boundary layer.