Rain-out of trace gases

After cloud formation the sorption of soluble trace gases begins immediately. Rain-out is particularly important in those cases in which an absorbed gas reacts irreversibly in the liquid phase with another constituent. Examples of this type of transformation are provided by S02, NH3, N02 etc. In the following we will deal mostly with S02 removal which is discussed in a considerable body of papers. However, the main points of the rain-out of NH3 and N02 will also be mentioned. 

The rain-out of reactive gases is determined by the transport of gas molecules to the droplet surface, their molecular diffusion into the liquid water, and by their rate of chemical transformation. It is shown (Beilke and Gravenhorst, 1978 Hales, 1978) that, in case of sulfbr dioxide removal, the rate of this latter process is much smaller than that of diffusion, since the equilibrium between S02 in the gas phase and sulfur(IV) in liquid water is reached within less than 1 s (in this form of notation, a Roman numeral in parentheses means the valence of the elements). For this reason the oxidation of sulfur(IV) in cloud drops is the rate-determining process in S02 removal. 

To understand this removal mechanism let us consider a heterogeneous system in which S02 in the gas phase is in equilibrium with sulfur(IV) in liquid water. This equilibrium is represented by the following three equations  

In these equations sulfur (IV) in the aqueous phase is the sum of SOz H20, HSO3 and SO,-, while [5.11] describes a simple physical dissolution. Considering the foregoing discussion, the problem is to determine the nature and rate of the sulfur (IV)- sulfur (VI) transformation of bisulfite (HSOj) or sulfite (SOf ) ions (unfortunately there is no agreement among different authors as to whether the bisulfite ion, the sulfite ion, or both is the reactive species). It is well documented that the oxidation of sulfur (IV) to form sulfate ions can be proceeded by the following processes  

The overall reaction of sulfite ions with absorbed oxygen is as follows  

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For the rain-out of trace gases, equation [5.10] can be applied. According to model experiments and calculations of Beiike and Georgii (1968) Eg (g gas) ranges from 0.01 to 0.1 in the case of sulfur dioxide. On the other hand, Hales (1978) suggests that about 1 % of the airborne S02 will dissolve in the aqueous phase at a liquid water content of 1 g m" 3 this figure is equivalent to the lower limit proposed by the previous two authors. 

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