Dynamics of Aerosol Ice-Forming Efficiency Variability Formed by Different Generator Types under the
Conditions Close to Natural Ones
Shilin A., Drofa A., Ivanov V., Savchenko A., Martines-Castro D., Peres-Sanches K.A., Shilin V.
At present, generators of ice-forming aerosol not aimed at direct introduction of an agent into a supercooled medium (air-borne and ground-based modification means) are widely used. Active particles are present in the atmosphere for a long time (up to tens of hours). Under these conditions currently central are the problems of ice-forming efficiency variability of such aerosols at an impact of environmental factors (temperature, humidity, UV-radiation). Of additional interest are the processes involved in the variability of ice-forming variability of such aerosols with time during their transport into the zone of planned modification.
Such studies were made earlier [1–5], but the activities made in the range of modification means connected with changing the structure and properties of ice-forming active particles need a substantial reappraisal of the data obtained before. When analyzing the recent principal changes in the development of aerosol generators, one should note the following:
The ice-forming efficiency of the agents at a simultaneous decrease of their concentrations in the compounds used was noted. At the same time, the stability of aerosols at their application in real conditions may be somewhat different.
A change of an iodine-containing addition chemical composition. Here, some components of such an addition used now may be stable under environmental effects. And some of them completely decompose in an oxidizing medium or under the effect of UV-radiation.
The latter can result in numerous processes connected with the recovery of ice-forming activity not having taken place earlier. For example, elemental iodine formed at oxidized deconstruction of ammonium iodide can oxidize silver metal – the product of AgI photolysis, forming silver iodide again.
A change of the ratio “active substance/iodizing addition”. In this case, the appearance of qualitative new effects is possible. Such effects are connected with the solubility of silver iodides in the media with high concentrations of an iodide ion, that can result in a possible formation of the solid phase and correspondingly in the activation or deactivation of aerosol under corresponding environmental conditions.
To study the above-mentioned effects, the experiments have been carried out at the RPA “Typhoon” aimed at measurements of aerosol ice-forming efficiency produced by three radically different generators under controlled conditions corresponding to the natural ones. The following systems of aerosol generation were studied:
1) The ground-based liquid-fuelled aerosol generator planned for use in the experiment «Design and implementation of an Artificial Rainfall Enhancement Project, Phase I» (Cuba-Venezuela). Silver iodide will be used as an operating solution with a concentration of about 2% and sodium iodide at a concentration of 1%.
2) The Russian standard pyrotechnic compound AD-1 (AgJ – 8%, KJ – 5%) used in pyrotrechnic flares PV-2b fired from aircraft. The molar ratio of AgJ/KJ is equal to 0.8.
3). The experimental composition with a small content of silver and a high iodide contents developed at the RPA “Typhoon. The formulation of this compound uses a more active ice-forming compound of silver (solid solution of 3AgJ*1 CuJ) in the amount of 2.54% (2% - evaluated in AgJ). Ammonium iodide will be used as an iodizing addition making 30-40% of the pyrotechnic mass of the composition. The molar ratio of 3AgJ*1CuJ/NH4J can reach 0.05.
It should be mentioned that the ratio of silver-copper iodides to the iodizing addition in case (3) is so high that there exists a considerable range of solution concentrations at which no silver-copper iodide solid phase is contained in an aerosol particle (droplet), that is impossible in cases (1 - 2). Thus, in the first two cases the formation of the solid phase of active silver compounds takes place immediately at the moment of condensation of pyrotechnic flare combustion products with a following decrease of the ice-forming activity under the effect of some environmental conditions [6, 7].
In the latter case, due to a considerable excess of alkali metal or ammonium iodides, a radically new process of ice-forming activity variation is observed.
Depending on individual chemical properties of the soluble iodide in the humidity range of 40 – 70%, absorption of water vapors by an aerosol particle occurs
with the formation of the solution and the transition of silver iodides into the dissolved state. Further on, at the solution dilution the concentration of soluble
iodides decreases, and at some time moment the formation of the solid phase of silver active compounds begins. Up to this moment practically all the processes
of ice-forming activity decrease, typical of the silver iodide solid phase, do not occur. The aerosol form in itself (particle or droplet) does not manifest
any ice-forming properties, because there are no active ice-forming substances in the solid phase. The stages of an aerosol particles lifetime and break-down
factors (surface ageing, adsorption of admixtures, UV-radiation) affecting the particles activity can be presented in this case in the following way:
Stage of the process
State of an aerosol particle
Effect of break-down factors. Particle ice-forming activity
1. Formation of aerosol at the moment of high-temperature gas jet condensation
A solid particle. Depending on the composition of an iodizing addition it can be either a solid iodide solution or heavy metal iodides and iodizing addition can condensate separately.
Break-down factors are present. The particle is active in ice formation.
2. The beginning of water vapor transport to the particle surface at the moment of silver compounds solid phase formation.
A particle or drop covered with water. No solid phase of ice-forming-active substances. The beginning of solution formation and its break-down are controlled by chemical properties of an iodizing addition, ratio of the components and environmental conditions.
The particle is incapable of ice formation. No break-down factors.
3. Silver compounds solid phase formation at iodide solutions dilution
A particle covered with water with a solid newly-formed phase of efficient silver compounds. Ice-forming activity is maximal, an ideal situation for heterogeneous ice nucleation.
Break-down factors begin to decrease particle ice-forming efficiency. Dynamics of ice-forming efficiency is similar to the aerosol dynamics of generators 1 and 2.
4. Repetition frequency of ice-forming variation cycle at particle falling into the zone of low humidity
In case of particle falling into the zone of low humidity a repetition of the cycle “dissolution-formation of solid phase” with the recovering of ice-forming properties is possible.
Break-down factors cease to act.
So, in contrast to aerosol formed in traditional systems, the following principal differences are typical for aerosol with a considerable prevalence of an iodizing addition:
The existence of non-active state of the particle after the moment of aerosol formation, the processes of ice-forming substance break-down are absent;
Maximum activity in some range of humidity and temperature;
A possibility of ice-forming properties recovering when a particle falls into the zone of low humidity;
The properties mentioned can be to some extent controlled by the variation of the iodizing addition composition and the ratio “iodizing addition/active silver compounds’.
The specialists of RPA “Typhoon” obtained a great number of experimental data on studying ice-forming efficiency of aerosols generated by different systems. The experiments have been carried out with a specially designed stand ensuring studies of generators operation in the conditions maximally close to the real ones. The analysis of aerosol ice-forming efficiency was made after a representative aerosol sample was taken from the wind tunnel and carried into the climate chamber. During the experiments studied were the effect of humidity, temperature and UV-radiation on aerosol ice-forming efficiency. For this, an aerodynamic stand with the following parameters was used:
The generators under study are full-sized models or industrial specimens;
The velocity of air flow is from 1 to 100 m/s;
Measurements of temperature and humidity along the whole path of ice-forming aerosol;
Aerosol sampling into the chamber of sample preparation of the volume of 13 m3 with the exposure time up to 12 hr at different temperatures, humidity and UV-radiation.
Assessment of aerosol ice-forming efficiency – manually or programmed processing of digital images exposed in the thermostat climate chamber.
As an example, Figure 1 gives the measurement results of ice-forming active nuclei amounts at minus 10oC out of one gram of the pyrotechnic compound depending on time of their exposure in the climate chamber at a relative humidity of 70%.
From Fig. 1 it is seen that under certain conditions the efficiency of ice-forming particles generated by system (2) may decrease by 3 orders of the value during 40 minutes.
The ice-forming efficiency of particles generated by system (3 drops to a significantly lesser extent. On the whole, the results of experiments performed by us, demonstrate a number of advantages of using systems (3) for the generation of ice-forming aerosol especially in the situations when aerosol should not be introduced directly into a supercooled medium. Aside from a high stability of aerosol produced by such systems, one should expect a possibility of partial recovery of its ice-forming activity at coming into the zone of low humidity. Such a recovery can be caused both by recrystallization of active ice-forming substances and chemical reactions of the main break-down products, first of all, metallic silver with elementary iodine formed at photolysis of the iodizing addition compounds.
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