Interaction of Forest Canopies with Atmospheric Constituents: So2 , Alkali and Earth Alkali Cations and Chloride

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From the view point of the receiving ecosystem, a distinction is made between precipitation deposition and interception deposition. Precipitation deposition occurs by gravity and is the sum of deposition with rain and snow and as dust particles; the receiving surface has no influence on the deposition rate. Interception deposition is the sum of impaction of aerosols and droplets (fog, cloud droplets), and of gas absorption; the receiving surface influences the deposition rate greatly by its size, kind and chemistry. An approach is presented to calculate the rate of interception deposition on forest canopies and to account for the various processes. The approach rests on the measurement of precipitation (wet) deposition and throughfall. Flux balance equations are used to account for sink and source terms of the canopy. The approach is demonstrated with data from a beech and a spruce forest covering the period from 1969 to 1981. The data indicate that the S02 absorption in the canopy (dry deposition) is limited by the ability of the trees to buffer the protons produced during SO2 absorption in water films at inner and outer plant surfaces. Under conditions of high soil acidity and low tree vitality, the buffer ability of the trees can cease. This limits dry deposition of SO2 to very low rates.