Monitoring Deposition of Nitrogen-Containing Compounds in a High-Elevation Forest Canopy

Abstract

Measurements of airborne (gaseous and aerosol), cloud water, and precipitation concentrations of nitrogen compounds were made at Mt. Mitchell State Park (Mt. Gibbs, ~2006 m MSL), North Carolina, during May through September of 1988 and 1989, An annular denuder system was used to ascertain gaseous (nitric acid, nitrous acid, and ammonia) and particulate (nitrate and ammonium) nitrogen species, and a chemiluminescence nitrogen oxides analyzer was used to measure nitric oxide and nitrogen dioxide. Measurements of NO3 − and NH4 + ions in cloud and rain water samples were made during the same time period. Mean concentrations of gaseous nitric acid, nitrous acid, and ammonia were 1.14 μg/m3, 0.3 μg/m3, and 0.62 μg/m3 for 1988, and 1.40 μg/m3,0.3 μg/m3, and 1.47 μg/m3 for 1989, respectively. Fine particulate nitrate and ammonium ranged from 0.02 to 0.21 μg/m3 and 0.01 to 4.72 μg/m3 for 1988, and 0.1 to 0.78 μg/m3 and 0.24 to 2.32 μg/m3 for 1989, respectively. The fine aerosol fraction was dominated by ammonium sulfate particles. Mean concentrations of nitrate and ammonium ions in cloud water samples were 238 and 214 μmol/l in 1988, and 135 and 147 μmol/l in 1989, respectively. Similarly, the concentrations of NO3 and NH4 + in precipitation were 26.4 and 14.0 μmol/l in 1988, and 16.6 and 15.2 μmol/l in 1989, respectively. The mean total nitrogen deposition due to wet, dry, and cloud deposition processes was estimated as ~30 and ~40 kg N/ha/year (i.e., ~10 and ~13 kg N/ha/growing season) for 1988 and 1989. Based on an analytical analysis, deposition to the forest canopy due to cloud interception, precipitation, and dry deposition processes was found to contribute ~60, ~20, and ~20 percent, respectively, of the total nitrogen deposition.