Interannual variability in acidic deposition on the Mt. Mitchell area forest

Recently the forest decline at high elevation mountains in the eastern U.S. has been suggested to be associated with the deposition of acidic substances on the forest canopy through dry, wet and cloud deposition pathways. To determine the relative importance of these deposition mechanisms, a field study was initiated in May 1986, at Mt. Mitchell, NC. Since Mt. Mitchell is frequently immersed in clouds (immersion time being in the range of 28–41%), our investigations were primarily focused on the collection of cloud water and the monitoring of meteorology and ambient air quality. The precipitation data and related chemistry were obtained from a nearby NADP (National Atmospheric Deposition Program) site (Clingman's Peak). To estimate the dry and cloud deposition, the deposition velocities for gases and the rates of cloud deposition for cloud droplets are calculated with the ATDD (Atmospheric Turbulence and Diffusion Division, National Oceanic and Atmospheric Administration) model and a cloud deposition model (CDM), respectively. The wet deposition is obtained from NADP annual reports. Computations show the deposition velocities for SO₂, NO₂ and O₃ to be in the range of tenths of cm s⁻¹. The mean rate of cloud deposition is about 0.13–0.21 mm h⁻¹. The rainfall ranged from 40 to 60 cm during the growing seasons (from mid-May to the end of September) of 1986–1988. Using these deposition parameters and the 3 year database, the deposition fluxes of sulfur (S) compounds are found primarily contributed through cloud capture mechanism (60%) followed by incident precipitation (25%) and dry deposition (15%). As to the deposition fluxes of nitrogen (N) compounds, cloud, wet and dry deposition contributedf about 50%, 25% and 25%, respectively. A comparison of deposition estimates at Mt. Mitchell with those at other sites shows that the sulfate deposition at sites exceeding 1200 m MSL in elevation in Bavaria, F.R.G., and the eastern U.S. is a almost identical within error limits. Reasons for large uncertainties in deposition estimates are also discussed as the mechanisms for redistribution of the deposited material on the forest canopy.