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Experimental Studies of Silver Iodide Pyrotechnic Aerosol Ice Forming Efficiency Dynamics

A.G.Shilin, A.S.Drofa, V.N.Ivanov, A.V.Savchenko, V.A.Shilin
19th International Conference on Nucleation and Atmospheric Aerosols, Colorado State University

   URGENCY OF THE PROBLEM

   The method of modification of supercooled cloud media with glaciogenic agents meant initially direct modification when an anti-hail shell, rocket, cartridge were introduced directly into a cloud. Thus, the process of investigations and development of modification means implied a direct penetration of an agent into the cloud medium and practically immediate action of active ice-forming nuclei. For the realization of such modification methods, as a result of long-term studies, a series of pyrotechnic means formulae with optimal for such conditions use of silver iodide contents (from 8 to 12% of the compound weight) were created. The pyrotechnic compound created demonstrates at present maximum possible ice-forming activity from 2 to 4E+13 of active particles per the compound gram at minus 10oC with rather a high velocity ice nuclei formation (within 1 - 2 minutes). The problems connected with temporal variations of ice-forming activity have not been considered or were not paid attention then.
   But in view of step-by-step dropping of a rocket-artillery method of supercooled cloud seeding from safety considerations, the role increases of means leading to indirect cloud modification (aircraft, unmanned flying vehicles, ground-based generators) during which aerosol is introduced either into the atmosphere in front of the cloud motion path or into the air layers near the ground, so that it could come into the cloud updrafts. In such a case, rather a long time passes before the aerosol comes into the two-phase super cooled zone. It may make from several tens of minutes (when aircraft-borne means are used) to several hours at the use of ground-based generators [1]. Correspondingly, in such conditions ice-forming nuclei undergo the action of numerous external factors (UV- radiation, chemically active atmospheric constituents, the components of chemical pollution) as a result of which, together with the processes of natural ageing of ice-forming substances, aerosol efficiency considerably decreases [2,3]. Nevertheless, the methods of determining the ground-based and aircraft-based generators ice-forming efficiency with time either are not taken into consideration or are considered to proceed with a similar intensity for all the types of ice-forming aerosols and generators.
   Simultaneously, due to a considerable growth of silver cost by about 900% for ten years (from $5 to $45 for a troy ounce), there arouse a question of the development of more effective and cheaper means for weather modification.
   The long-term activities aimed at the search of optimal contents of silver compounds in modification means and also at increasing the stability of ice-forming aerosol with time carried out at the FSBI RPA Typhoon made it possible to conclude that the aerosols of different pyrotechnic compounds differently change their properties with time. A high silver content in the formulae is not always the best solution from the viewpoint of modification means efficiency. It is of an especial urgency for the means not aimed at a direct introduction of aerosol into a super cooled cloud. The present paper concerns the influence of humidity on the processes of decreasing the aerosol ice-forming activity of different pyrotechnic compounds.

   THE EXPERIMENTAL METHOD AND INSTRUMENTATION

   The experiments were made at the aerodynamic stand of FSBI RPA Typhoon for measurements of aerosol particles ice-forming activity. The stand allows one to create an air flow with the velocity up to 100 m/s in the wind tunnel work section of the diameter of 0.8 m, thus modeling a flight of the ice-forming generator in the real atmosphere. The aerosol formed during the generator operation passes through the cascade of turbulators for a more uniform aerosol particles distribution in the flow cross-section. After this a portion of aerosol is taken into the chamber of sample preparation of the volume of 13 m3. The chamber is equipped with the means of air mixing and a simulator of solar radiation. It is possible to sustain a preset temperature and humidity in the chamber. Periodically a preset aerosol volume is taken from the chamber into the climatic chamber Feutron 3001-01 with a fog supercooled at a preset temperature. Ice crystals formed on active particles are trapped onto a special plate and their number is determined with a digital microscope.

   EXPERIMENTAL RESULTS

   For comparative studies taken was the known 12%-AgJ pyrotechnic compound with an ice-forming efficiency of active particles 2E + 13 out of a gram of the compound and the 2%-AgJ compound being developed at the FSBI RPA Typhoon with an increased content of the iodinating component ammonium iodide. The changes of aerosol ice-forming activity at its storage in the aerosol chamber at a fixed temperature and relative humidity of air were studied. The results of ice-forming activity at the humidity of 20, 50 and 80% at exposures of aerosol are given in figure 1.

Dependence of ice-forming activity on aerosol exposition in aerosol chamber at different humidity - 20% humidity Dependence of ice-forming activity on aerosol exposition in aerosol chamber at different humidity - 50% humidity
Dependence of ice-forming activity on aerosol exposition in aerosol chamber at different humidity - 80% humidity

FIGURE 1. Dependence of ice-forming
activity on aerosol exposition in
aerosol chamber at different humidity
a - 20% humidity, b -50% humidity,
c - 80% humidity,
 12%-AgJ pyrotechnic compound;
 2%-AgJ compound

   As it follows from the above information, the activity of aerosols produced by pyrotechnic generators considerably varies with time. The higher the relative humidity is, the greater is the aerosol activity. Moreover, every value of relative humidity can be correlated with the time when the 2% AgJ aerosol activity becomes equal to that of the compound with 12% of AgJ (Fig. 2).

FIGURE  2. Effect of humidity on the time of attaining the equilibrium state of the activity of aerosols of 2%-AgJ and 12%-AgJ compounds. Dependence of time after which the aerosol of 2%-AgJ compound activity  becomes equal to that of the 12%-AgJ compound aerosol
FIGURE 2. Effect of humidity on the time of attaining the equilibrium state of the activity of aerosols of 2%-AgJ and 12%-AgJ compounds. Dependence of time after which the aerosol of 2%-AgJ compound activity becomes equal to that of the 12%-AgJ compound aerosol.

   So from Fig. 2 it follows that after the moment when the ice-forming aerosol is formed, depending on the medium relative humidity, the processes accompanied by the decrease of its activity start. The processes of decreasing the activity are more pronounced for more active ice-forming aerosol (formed by the 12%-AgJ compound). It is especially significant at a humidity close to 100%, i.e. in the zone where a cloud begins to form. The extent of ice-forming activity decrease may make more than one order of the value during some minutes, the mostly clear being this process for the compounds with high silver contents. The compounds with low silver contents and a high content of iodine compounds decrease their activity with rather a lower velocity. Therefore, in some cases the use of aerosol with 2%-AgJ compound may be more efficient than the 12% of AgJ compounds.

POSSIBLE MECHANISMS OF THE EFFECT OBSERVED

   When analyzing the results obtained, one can note that the behavior of the aerosols of the pyrotechnic compounds with low AgJ contents and a significant excess of NH4J differs in principle from the aerosols with 12% and more of AgJ. As far as the content of technological additives in the pyrotechnic compound is limited and cannot be higher than 20 25%, for the 12% of AgJ formula amount of the iodine component is relatively low and does not exceed 10 12%, i.e. the ratio of the iodine component to AgJ is about 1:1. At the same time, the compounds with low contents of AgJ may have the ratio of the components of NH4J : AgJ at the level of 10 : 1 and more. At such a ratio of soluble iodides to silver iodide soluble complexes of [AgJn]1-n are formed. There exists rather a wide range of solution concentrations when a aerosol particle does not contain the solid phase of silver iodide. The aerosol particle under consideration looses its ability to form ice, but silver iodide present in the solution does not undergo the processes of ageing. The lifetime of such a solution is determined by the air relative humidity and depends on a certain iodide contained in the iodine components.
   Ammonium iodide rather a hygroscopic component begins to absorb water from air even at a relative humidity from 40 to 50%. Correspondingly, under this situation the combustion products formed at combustion of the 2%-AgJ compound contain ice-forming components in the form of a solution. At a further transport of water to the surface of an aerosol particle silver iodide forms a solid phase and from this moment is subjected to decreasing activity.
   For the compounds with 12% of AgJ the process described above is impossible in principle at any humidity. Correspondingly, there exists a region for aerosol of 2%-AgJ compounds where destructive processes do not act, ageing of 12%-AgJ compound aerosol begins just from the moment of its formation.

RFERENCES

   1. Association Nationale dEtude et de Lutte contre les Fléaux Atmosphériques (Association to Suppress Atmospheric Plagues). http://www.anelfa.asso.fr
   2. Shevchuk E.A., Kim N.S., Shilin A.G., Shkodkin A.V. Ice nuclei inhibition by organic molecules adsorption. // Int.Aerosol Symp., Atmospheric Aerosols, Moscow, March 21-25, 1994, v.1, p.1-3.
   3. Shilin A.G., Kim N.S., Shkodkin A.V. Ice forming capability of various aerosols under haloid influence. // Proc. XIII Int.Conf. on Nucleation and Atmospheric Aerosols, Salt Lake City, USA, 1992.