Condensation raindrop formation

Any isotope fractionation occurring in such a way that the products are isolated from the reactants immediately after formation will show a characteristic trend in isotopic composition. As condensation or distiUation proceeds, the residual vapour or liquid will become progressively depleted or enriched with respect to the heavy isotope. A natural example is the fractionation between oxygen isotopes in the water vapour of a cloud and the raindrops released from the cloud. The resulting decrease of the iso/i o ratio in the residual vapour and the instantaneous isotopic composition of the raindrops released from the cloud are shown in Fig. 1.4 as a function of the fraction of vapour remaining in the cloud. 

When a cloud forms, water vapor condenses on aerosol particles present in the atmosphere. Regardless of the size of a cloud droplet, at least one aerosol particle is required for its formation. A major portion of the radionuclides present in the initial raindrop is caused by this aerosol particle. Although increases in the drop size owing to coalescence would tend to keep the radionuclide concentration constant, increases in size owing to the condensation of moisture on drops already present would tend to decrease the radionuclide concentrations. Therefore, one would expect a maximum in radionuclide concentrations in small drops. Raindrops tend to be larger in heavier rains. Therefore, the decrease in the 38C1 and 39C1 activities with an increase in rainfall rate could be caused by an increase in the drop size. 

Seidl created a model based on the state of the surface film (e.g. expanded or condensed), the equilibrium spreading pressure, and the area per film molecule to describe organic film formation from fatty acids, then applied it to rainwater and aerosol particles [245]. He concluded that, in most cases, only dilute films (with concentrations below that necessary to form a complete monolayer) would form on aerosols and raindrops, and such films would not affect their physical or chemical properties. However, dense films were predicted to form on aerosols in the western U.S., mainly attributable to biomass burning. Mazurek and coworkers developed a model to describe structural parameters (elastic properties, etc.) of fatty acid films on rainwater without requiring knowledge of the surfactant concentration or composition by using surface pressure-area and surface pressure-temperature isochors and the rain rate and drop diameter distribution [33]. This model can be used to identify the origin of specific compounds and an approximate chemical composition based on the force-area characteristics of collected rainwater films. 

Figure 2.200. Heavier water molecules will consequently be concentrated in the liquid the vapour will become depleted. The reversed process is observed during condensation, for example, during the formation of raindrops from humid air. As the extent of this process is temperature dependent, isotopic thermometers are formed and ultimately isotopic signatures of materials and processes are created.

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