Nitrogenous air pollutants in a mixed conifer stand of the western sierra nevada, California

Nitrogenous (N) air pollutants were monitored during three summer seasons (1988–1990) a mixed coniferous stand at Whitaker Forest in the western Sierra Nevada, California. In comparison with other forest locations, the recorded levels of nitric acid (HNO3) vapor were elevated: seasonal 12 h daytime averages of 1.11–1.97 μgm−3; highest values of 12h daily averages reached 4.15 μgm−3. Similarly, the concentrations of ammonia (NH3) were high: seasonal 12 h daytime averages of 1.11–1.56 μgm−3; highest values of 12 h daily averages reached 3.75 μ−3. Also the concentrations of nitrate (NO3−) and ammonium (NH4+) in fine ( < 2.2 μm in diameter) particulate fractions were higher than in the other remote mountain locations in North America. When expressed on a molar basis, NH3 was the most abundant N air pollutant and represented almost 50% of the total N. NO3+and NH4+ deposition fluxes to ponderosa pine (Pinus ponderosa Dougl. ex Laws.) branches during the three summer seasons ranged from 5.93 to 30.75 and from 1.46 to 5.95 μgm−2 h−1, respectively. The calculated deposition velocities for NO3− and NH4+ to ponderosa pine ranged from 0.054 to 0.327 cm s−1 and from 0.107 to 0.322 cm s−1, respectively. During the 1.990 summer season, NO3− and NH4 washed from branch surfaces provided 24.2 and 19.7 g N ha−1 per month, respectively; estimated foliar internal uptake of nitrogen dioxide (NO2) and HNO3 vapor 24.2 g N ha−1 per month; and the estimated internal uptake of NH3 47.2 g N ha−1 per month. The elevated levels of air pollutants and N deposition could adversely affect the natural ecosystems of the western Sierra Nevada.