Atmospheric deposition to a high-elevation forest at Whiteface Mountain, New York, USA

Abstract

Atmospheric deposition rates have proven difficult to quantify in mountainous settings but must be estimated in order to assess the potential impact of air pollution on high-elevation ecosystem processes. We measured major ion concentrations in precipitation, cloud water and aerosols as well as the concentrations of the pollutant gases SO₂ and HNO₃ vapor in a subalpine spruce-fir forest at Whiteface Mountain, New York, USA, for a 4-yr period beginning in June of 1986. Deposition of S, N and major elements to this forest at 1050-m elevation was estimated using direct flux measurements and deposition velocities that were calculated with inferential models. The validity of model flux estimates was evaluated using mass-balance calculations for chemically conservative elements. Combined estimates of cloud water plus dry deposited ion fluxes based on micrometeorological modeling agreed with mass-balance derived estimates of these fluxes to within ~ 25%. Annual N deposition averaged 16.7 kg N ha^-1 yr^-1 with 32% contributed by cloud water deposition and 25% by HNO₃ vapor deposition. Annual S deposition averaged 16.3 kg S ha^-1 yr^-1 with 37% deposited by clouds and 11% due to SO₂ deposition. These values are substantially lower than previous estimates of S and N deposition to high-elevation forests of the northeastern USA and characterize pollutant deposition rates in the elevational range of average cloud base. Scavenging of aerosols and gases by cloud droplets with subsequent deposition to the forest canopy was found to be 12 to 30 times more efficient than dry deposition processes for transferring atmospheric N and S, respectively, to the forest. Direct effects of the atmospheric flux of N and S to the forest have been detected, including canopy assimilation of N and high SO₄ fluxes in soil solutions.