Abstract
The chemical composition of both liquid droplet and leaf surface determines the behaviour of a cloud droplet on interception by vegetation. In some circumstances, particularly in wind-driven cloud, droplets may not adhere to foliage. Once on the leaf surface, the contact angle between the droplet and surface will determine the amount of water held by the canopy. Factors influencing this contact angle are discussed in the light of field measurements from Scots pine. Chemical modification of canopy water in terms of dissolution and ion exchange is briefly considered, including oxidation of SO2. Evaporation of water from the canopy leads to changes in solute concentrations, which influence fluxes of volatile solutes and transfer of involatile solutes between a leaf and surface water. In all these processes the physical and chemical nature of the leaf surface is of the greatest importance, and is expected to depend on both the environment in which the tree grows and the tree species.
The chemical interaction of a cloud or rain droplet with a leaf surface begins as soon as it is intercepted by the leaf. Transfer of small droplets typical of cloud (5–50 pm diameter) is effected by the turbulence of the atmosphere, and gravitational settling is not significant when compared with potential deposition rates to forests at windspeeds typical of high elevations. At high windspeeds, mean droplet velocity (and hence kinetic energy) is large, and this must be dissipated on collision with a leaf. If the leaf is already covered by a layer of water, this energy may be expended as heat by elastic deformation of the water layer. However, not all leaf surfaces are wetted efficiently, and the droplet may have sufficient energy to be reflected, or “bounce” off the leaf, while losing some of its momentum. The factors affecting droplet reflection have been studied in relation to pesticide formulation and application, where droplets (usually much larger, 100-250 pm) fall under gravity at terminal velocities in air. However, the formalism developed (Hartley & Brunskill, 1958) may be used also for smaller droplets in wind-driven cloud.
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© 1988 Kluwer Academic publishers
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Cape, J.N. (1988). Chemical Interactions between Cloud Droplets and Trees. In: Unsworth, M.H., Fowler, D. (eds) Acid Deposition at High Elevation Sites. NATO ASI Series, vol 252. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-3079-7_39
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DOI: https://doi.org/10.1007/978-94-009-3079-7_39
Publisher Name: Springer, Dordrecht
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