Tall tower vertical profiles and diurnal trends of ammonia in the Colorado Front Range

Tevlin, A.; Li, Y.; Collett, J.; McDuffie, E.; Fischer, E.; Murphy, J.

Ammonia (NH3) mixing ratios were measured between the surface and 280 m above ground level from a moveable carriage at the Boulder Atmospheric Observatory (BAO) tower in summer 2014 as part of the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ). The campaign median mixing ratio was 3.3 ppb, ranging from below detection limits to 192 ppb. Median vertical profiles show an overall increase in NH3 mixing ratios towards the surface of 6.7 ppb (89 %) during the day, and 3.9 ppb (141%) at night. In contrast to the overall increasing trend, some individual profiles show decreasing NH3 in the lowest 10 m. This suggests that the local surface is capable of acting as either a source or a sink, depending on the relative amounts of NH3 at the surface, and in advected air parcels. We further use this dataset to investigate the variation in diurnal patterns of NH3 as a function of height above the surface. At higher altitudes (100 ± 5 and 280 ± 5 m), NH3 mixing ratios reach a gradual maximum during the day between 9:00 and 16:00 local time, likely driven by changes in source region. At lower altitudes (10 ± 5 m), the daytime maximum begins earlier at about 7:00 local time, followed by a sharper increase at 9:00 local time. At this height we also observe a peak in NH3 mixing ratios during the night, likely driven by the trapping of emitted NH3 within the shallower nocturnal boundary layer.