Anatomy of Summertime Upslope Events in Northeastern Colorado: Ammonia (NH 3 ) Transport to the Rocky Mountains.

The Transport and Transformation of Ammonia (TRANS 2 Am) airborne field campaign occurred over northeastern Colorado during the summers of 2021 and 2022. A subset of the TRANS 2 Am flights investigated easterly wind conditions capable of moving agricultural emissions of ammonia (NH 3 ) through urban areas and into the Rocky Mountains. TRANS 2 Am captured 6 of these events, unveiling important commonalities. (1) NH 3 enhancements are present over the mountains on summer afternoons when easterly winds are present in the foothills region. (2) The abundance of gas-phase NH 3 is 1 and 2 orders of magnitude higher than particle-phase NH 4 + over the mountains and major agricultural sources, respectively. (3) During thermally driven circulation periods, emissions from animal husbandry sources closer to the mountains likely contribute more to the NH 3 observed over the mountains than sources located further east. (4) Transport of plumes from major animal husbandry sources in northeastern Colorado westward across the foothills requires ∼5 h. (5) Winds drive variability in the transport of NH 3 into nearby mountain ecosystems, producing both direct plume transport and recirculation. A similar campaign in other seasons, including spring and autumn, when synoptic scale events can produce sustained upslope transport, would place these results in context.