Dry deposition of ammonia, nitric acid, ammonium, and nitrate to alpine tundra at Niwot Ridge, Colorado

Micrometeorological measurements and ambient air samples, analyzed for concentrations of NH3, HNO3, NH4+, and NO3−, were collected at an alpine tundra site on Niwot Ridge, Colorado. The measured concentrations were extremely low and ranged between 5 and 70 ng N m−3. Dry deposition fluxes of these atmospheric species were calculated using the micrometeorological gradient method. The calculated mean flux for NH3 indicates a net deposition to the surface and indicates that NH3 contributed significantly to the total N deposition to the tundra during the August–September measurement period. Our pre-measurement estimate of the compensation point for NH3 in air above the tundra was 100–200 ng N m−3; thus, a net emission of NH3 was expected given the low ambient concentrations of NH3 observed. Based on our results, however, the NH3 compensation point at this alpine tundra site appears to have been at or below about 20 ng N m−3. Large deposition velocities (>2 cm s−1) were determined for nitrate and ammonium and may result from reactions with surface-derived aerosols.