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) in 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 mu gm(-3); highest values of 12 h daily averages reached 4.15 mu gm(-3). Similarly, the concentrations of ammonia (NH3) were high: seasonal 12 h daytime averages of 1.11-1.56 mu gm(-3); highest values of 12 h daily averages reached 3.75 mu gm(-3) Also the concentrations of nitrate (NO3-) and ammonium (NH4+) in fine (< 2.2 mu m 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 mu gm(-2)h(-1), respectively. The calculated deposition velocities for NO3- and NH4+ to ponderosa pine ranged from 0.054 to 0.327 cms(-1) and from 0.107 to 0.322 cms(-1), respectively. During the 1990 summer season, NO3- and NH4+ washed from branch surfaces provided 24.2 and 19.7 g Nha(-1) per month, respectively; estimated foliar internal uptake of nitrogen dioxide (NO2) and HNO3 vapor 24.2 g Nha(-1) per month; and the estimated internal uptake of NH3 47.2 g Nha(-1) per month. The elevated levels of air pollutants and N deposition could adversely affect the natural ecosystems of the western Sierra Nevada.