DRY DEPOSITION AND CUTICULAR UPTAKE OF POLLUTANT GASES

Abstract Transport characteristics of several gases (O2, O3, CO2, H2S, SO2, CH3SH, NO2 and H2O) for cuticles isolated from leaves and fruits of a variety of plant species were investigated. It was shown that: (1) Cuticular permeances determined at high partial pressure depend strongly on the condensability of the respective gas. They rise exponentially with increasing boiling point of the gases; (2) The partition coefficient for SO2 between the dry cuticle and the surrounding gas phase rises when the partial pressure is decreased. This behavior is explained tentatively as a combination of gas adsorption at internal surfaces of the cuticle and dissolution of the gas in the cuticular polymer network. This conception implies significant interactions between different gases present in the air (e.g. SO2 and water vapor). The use of a model allows the mathematical extrapolation to gas concentrations which occur in the environment, and for which the partition coefficients or permeances cannot be determined directly. The mathematical treatment corresponds to the ‘dual-mode sorption’ model for polymers which are in the glassy state. A great number of deposition experiments with typical pollutant gas concentrations have shown clearly that the deposition velocities to plant surfaces after stomatal closure had been accomplished were much higher than the corresponding cuticular permeances observed so far. Although the dual-mode sorption model predicts higher cuticular permeances at ambient pollutant concentrations, other hypothetical reasons for the discrepancy between observed surface flux velocities and cuticular permeances have to be taken into account. Incomplete stomatal closure and steady-state reaction or adsorption processes in contact with the cuticle are discussed.

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