Unusual Conditions of Charging of Aerosol Particles of Industrial Origin·)

V. G. Morachevsky and N. A. Dubrovich

Abstract

The processes of the charge separation in the clouds in a real atmosphere in the absence of strong electric fields are considered. Attention is called to the basic phenomenon - the existence of double electric layers at the inter­faces. Existing data on the dependence of the electric properties of these layers on the concentration of various chemical substances are reviewed. One of the main sources of the charge separation - the so-called crystallisa­tion potential - is discussed in detail. The authors conclude that the variation of the electric characteristics of the double electric layers may have a determining influence on the electric properties of clouds.

Preface

The processes of the charge separation in clouds in a real contaminated atmosphere are determined by the existence of double electric layers at the air-water, ice-air and water-ice interfaces. In the absence of strong electric fields the slightest changing of the electric properties of these double layers may have a determining influence on the macro-electric properties of the clouds. At present, the electric properties of the surfaces of water and ice cannot be considered to be well studied. As can be seen from the review presented below the dependence of these properties on the chemical content of the cloud­water can be evaluated only qualitatively. In order to show in which way the changing of the properties ofthe interface may influence the charge separation processes, the occurrence of the potential difference between water and ice during crystallisation is discussed in some detail. This process is one of the main sources of charge separation in clouds. We think that the time has come for the "physicists of the clouds" to use physico-chemical methods and to study the basic electric properties of cloud droplets and ice particles.

Air-Water Interface

We have performed an extensive research of the electric properties of the air-water and air-solution interfaces. The results are in good qualitative agreement with the results obtained by other authors (4,6,8). The research presented sometimes insuperable difficulties because the slightest contamination of the samples changed the results drastically. One must not forget that even for pure distilled water surface, the potential is not definitely determined: the values from 0.2 to 1 Volt are often mentioned. Nevertheless, the qualitative picture of the influence of the various soluble chemical substances on the value of the surface potential of the solution is now clear. Inorganic salts such as halides and sulfates of the alkaline metals in concentrations not higher than 10- 3 -10 - 5 N change the sign of the surface potential. Surface-active organic compounds in the same concentrations reduce the surface potential, and the influence of the non-electrolytes is stronger than that of electrolytes. When an ion approaches the surface work is to be done against the hydration forces, that is why the role of the surfactants is determined simply by their position on the surface ofthe solution. However, sometimes the surface-active substances increase the surface potential, but in all such cases one can always detect a hydrophylic group which is negatively charged and by structural reasons approaches the surface. We find it premature to present here the tables of our results, for the reasons aforementioned.

Ice-Air Interface Since Faraday, the problem ofthe structure of the air-ice interface was discussed many times (3,5,9).

Nowadays, the existence of a quasiliquid layer on the surface of the ice is ascertained beyond doubt. On the surface of that layer with the depth of some tens of monomolecular layers, a part of the water molecules contacting with air is oriented just like on the surface of water. Ice may be considered posi-

*) Paper was accepted by the Executive Panel but could not be presented. It was, therefore, not considered in any discussion.

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H. Dolezalek et al. (eds.), Electrical Processes in Atmospheres© Dr. Dietrich Steinkopff Verlag GmbH & Co. KG., Darmstadt 1976

tively charged with respect to air. This quasiliquid layer may be described as adsorption of water on ice. That layer has all the qualities of the adsorbed phase, e.g., the dielectric constant is lower, the specific mass higher, the specific heat is also larger than that of pure water. The layer undoubtedly exists but the surface conductivity, the surface potential and other physico-chemical parameters of the layer are not yet measured. And just as in the case of water the role of the contamination of the surface is not fully determined.

Ice-Water Interface

In all the models of the crystallisation processes, the existence of the double electric layer at the interface is always assumed (1, 2,10). The structure of the interface is in most cases described as follows: the ions of the solute are adsorbed on the surface of the ice, above them lies a layer of oriented water molecules and the charge is neutralised by a diffuse cloud of ions in the liquid phase. The ions which are able to penetrate into the crystal structure of the ice are assumed to play the main role at the interface. But, as we have seen in the previous paragraph, the surface of ice does not differ greatly from a water surface. We think it is possible to assume that in the first degree of approximation, all the three discussed interfaces may be considered identical. In particular, we can check this assumption since in the case of the water-ice interface a most sensitive function of the structure of the interface is known: the so­called crystallisation potential. That is the potential difference which exists between water and ice during crystallisation. Ice becomes positively charged, water acquires a negative charge. It was found (2), that this potential changes the sign when simple inorganic salts (see above) in the same concentra­tions (10 - 3 -10 - 5 N) are dissolved in water. The potential is lowered or even reduced to zero in the presence of surface-active organic substances. That is why we can suppose that water-air and ice-water interfaces are identical. Just as in the case of the discriminative capture of the atmospheric ions on the surface of the cloud drops, the moving crystallisation front discriminatively captures only positive ions. Let us consider the processes related to such mechanism.

Charge Separation during Crystallisation

Ice may be called a protonic semiconductor. The mobility of protons in ice is of the same order as in one of the most typical semiconductors - silica (silica 2.5, ice appro 0.5 V cm sec). The concentration of the ion-accepting centers is also approximately of the same order (silica 1017, ice 1015 per cm3). The main role in our model is ascribed to the native water ions - H + and 0 H -. For the latter the potential barrier which presents the moving crystallisation front is practically insurmountable. One can expect a redundancy of H30+ or H+ ions in the rear of the moving crystallisation front and the redundancy of OH- ions before the front with subsequent neutralisation of the double layer. It does not happen only because the protons are subjected to thermodiffusion. They begin to move to the colder end of the sample. The temperature gradient becomes the determining factor in the process. Indirect evidence of such a mechanism was obtained by (7). It was noticed that during the routine experiments in which we measured the crystallisation potentials, at the moment of the switching off the refrigerator the potential difference began instantly to decrease, though the front was still moving. We may say that the temperature gradient is the driving force and the double layer is the determining factor in the process.

Of course, the explanation presented here is oversimplified but this is done on purpose. We meant to emphasize the role of the double electric layer.

It can be easily shown that all the other processes of the initial charge separation in the clouds in the absence of electric fields are also determined by the structure of the double electric layers. The knowledge of the physico-chemical characteristics of the double electric layers will permit us to predict the course of the charge separation processes in the clouds.

However, even now two important conclusions can be formulated: 1. The contaminations present in the atmosphere - particularly in industrial regions - can

strongly change the electrical properties of the clouds. 2. The substances which change the properties ofthe interfaces may become active agents in lightning

suppression.

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References

1. Drost-Hansen, W., J. Colloid Interface Sci. 25, 131 (1967). - 2. Eyerer, P., Advan. Colloid Interface Sci. 3, 223 (1972). - 3. Faraday, M., Proc. Roy. Soc. London 10,440 (1860). - 4. Hermans, I. I., Rech. Trav. Chim. 60, 747 (1941). - 5. Jelinek, H., J. Colloid Interface Sci. 25, 192 (1967). - 6. Kamiensky, B., Electrochim. Acta 1, 272 (1959). - 7. Psalomstchikov, V. F., (personal communication) Leningrad Hydrometeorological Institute. -8. Siwek, B., Zest. nauk. Pro chem. 264, 16 (1971). - 9. Weyl, w., J. Colloid. Sci. 6, 389 (1951). - 10. Workman, E. J. and S. E. Reynolds, Phys. Rev. 78, 254 (1950).

Authors' addresses:

v. G. Morachevsky Leningradskij Gidromet. Inst. Malo-Okhtinskij Prospect 98 Leningrad 194018 USSR

N. A. Dubrovich Leningradskij Gidromet. Giteski Inst. Malo-Okhtinskij Prospect 98 Leningrad 195196 USSR

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