No abstract

Cloud collection principles are briefly reviewed; the performance of two collectors in common use is compared. Two cloud event studies (one during winter, the other during summer) are presented to contrast typical time‐dependent histories of chemical concentrations for the two seasons. During winter, the oxidation rate of SO2 is low, resulting in measured sulfate concentrations independent of SO2 concentrations. During summer, SO2 is rapidly oxidized by H2O2 with a residue of H2O2 persisting in cloud water after SO2 has been converted. Changes in the gas phase concentrations of SO2 and H2O2 are shown during a cloud event. Both gases drop to very low concentrations in cloud with H2O2 tending to recover after available SO2 has been oxidized. The meteorology for each event is presented to illustrate the role of atmospheric structure in the transporting of emitted material and in determining cloud water compositions. Finally, a cloud chemistry climatology for winter and summer is presented, based upon 6 years of observations at Whiteface Mountain.

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.

No abstract

Cloud collection principles are briefly reviewed; the performance of two collectors in common use is compared. Two cloud event studies (one during winter, the other during summer) are presented to contrast typical time-dependent histories of chemical concentrations for the two seasons. During winter, the oxidation rate of S0 2 is low, resulting in measured sulfate concentrations independent of S0 2 concentrations. During summer, S0 2 is rapidly oxidized by H 2 0 2 with a residue of H 2 0 2 persisting in cloud water after S0 2 has been converted. Changes in the gas phase concentrations of S0 2 and H 2 0 2 are shown during a cloud event. Both gases drop to very low concentrations in cloud with H 2 0 2 tending to recover after available S0 2 has been oxidized. The meteorology for each event is presented to illustrate the role of atmospheric structure in the transporting of emitted material and in determining cloud water compositions. Finally, a cloud chemistry climatology for winter and summer is presented, based upon 6 years of observations at Whiteface Mountain.