Anatomy of Ammonia (NH3) Transport during Summertime Upslope Events in Northeastern Colorado

The Transportation and Transformation of Ammonia (TRANS²Am) airborne field campaign occurred in northeastern Colorado during the summers of 2021 and 2022. A subset of the TRANS²Am flights were designed to investigate the timescales and factors that influence the loss of ammonia (NH₃) in plumes originating from agricultural facilities during transport and mixing with other urban pollutants (i.e., nitric acid (HNO₃)). Of particular concern were upslope events that move agricultural emissions through urban areas and into Rocky Mountain National Park (RMNP). Agricultural emissions are an important source of nitrogen, which can then deposit and have negative effects on the nitrogen cycle of sensitive ecosystems such as the RMNP. TRANS²Am captured 6 of these upslope events; the 3 that occurred during 2021 were impacted by wildfire smoke, and the 3 that occurred during 2022 were not. We observed some important commonalities between events: 1) we were often able to observe NH₃ enhancement over the mountains, 2) the observed gas-phase NH₃ was ~2 orders of magnitude higher than the observed particle-phase NH₄⁺, 3) emissions sources closer to the mountains are the major contributor of the NH₃ observed over the mountains during TRANS²Am, which can be explained by the nature of summertime upslope events (i.e., topography driven and weaker than in spring), and 4) transport from emission sources took ~6 hours on average. We also found that each upslope event was characterized by unique wind patterns that influenced transport. For instance, some were characterized by strong winds that effectively transported plumes from specific agricultural facilities up into the mountains and further west. Others were characterized by weaker winds that resulted in the recirculation of NH₃-rich plumes back to the east aloft. We will present a summary of our findings as well as a closer look at different case studies.