AgI Particles Research Articles

Numerical Simulation of Ice-Phase Convective Cloud Seeding

Abstract A two-dimensional time-dependent cloud model which covers a region 19.2 km × 19.2 km in the x and z directions with 200 m grid intervals, has been used to simulate silver iodide (AgI) seeding effects on strong convective clouds. The model is a set of conservation equations for momentum, energy and mass (air and water contents). One extra conservation equation is applied to trace the seeding agent which advects and diffuses along the flow field and interacts with the supercooled cloud fields. Contact and deposition nucleation are simulated. Only inertial impact and Brownian collection are considered as possible mechanisms for contact nucleation. Most of the AgI particles work as deposition or sorption nuclei in this study. Three different soundings are tested. Most of the effort is used in testing sounding H1, from Miles City, Montana, 29 July 1975. Seeding at a different place (see H1/P1), at a different time (case H1/T1), and with different amounts of AgI (cases H1/M1 and H1/M2) are simulated. T...

Investigation of the electric moments of AgI particles in aqueous solutions by electric birefringence

Aqueous suspensions of AgI were studied by the electric birefringence method. The rotatory diffusion constantθ and the relation between two electric moments of the particles (defined by the ratio β) were determined from the dependence of the electric birefringence on the frequency of the applied field (θ=229s−1; β=2,37). These parameters were also calculated from the electric birefringence data in rapidly reversed electric pulses. The obtained values areθ=237s−1 and β=1,09. The correspondence of theθ-values is satisfactory. The discrepancy of the β-values may be due to the polydispersity of the investigated suspension. The dependence of electric birefringence on the frequency of the applied field, the difference between the initial slopes of the curvesΔn/E=2 andΔn/E~2 and the type of the birefringence at field reversal show that the orientation of the AgI particles is due to the interaction of two electric moments with the applied field, which are directed along the larger dimension of the particles. One of both electric moments is the induced dipole moment connected with the displacement of the ions in the double electric layer. The other moment shows a slow relaxation between 10 Hz and 1 kHz and may be related to either the permanent or a “slow” induced dipole moment.

Relationship of Size and Activity for AgI Smoke Particles

The ability of AgI particles to nucleate ice by the freezing mechanism was measured with the Goetz aerosol centrifuge as a function of particle size, temperature and time. When the cooled centrifuge deposit was exposed to a puff of supersaturated nitrogen, each particle was immersed in a supercooled droplet which, if it contained an active AgI particle, froze and formed a visible ice crystal. The fraction of active particles was found by comparing the number of ice crystals with the size distribution of all the particles on the deposit. The mean nucleation rate per particle depended on particle size for a given temperature, which suggested that the surface of the particles was smooth with respect to the size of the critical ice embryos. The measured rates were about a factor of 1014 ± 16 smaller than predicted by Fletcher's theory. Activity spectra, calculated from the measured rates by numerically integrating the activities of all the particles in log-normal size distributions, showed the importance of the nucleation time lag at temperatures warmer than about −12°C or when the AgI generator produces very small particles.

An Investigation of the Relative Importance of Vapor Deposition and Contact Nucleation in Cloud Seeding with AgI

In order to determine the relative frequency of contact and deposition nucleation in AgI seeded supercooled clouds, laboratory experiments were carried out in which the supercooled cloud drops were tagged with insoluble fluorescent particles. Ice crystals resulting from seeding this cloud with AgI were examined microscopically to locate fluorescent particles. Presence of such a particle in the center of an ice crystal was interpreted as evidence that freezing of a drop through contact nucleation initiated crystal formation. Absence of a tracer particle from an ice crystal center was considered to be indicative of crystals formed by the vapor deposition mechanism. Evaluation of the experimental data (limited, so far, to the temperature range of −15 to −17C) indicate that only 2–4% of the ice-forming AgI particles in the 100–500 Å diameter range act as contact nuclei. Several auxiliary experiments were devised to show the validity of the experimental methods employed. The results are discussed in view of drop-nuclei collision efficiencies and other investigators' findings.

Polymer adsorption and its effect on the stability of hydrophobic colloids. II. The flocculation process as studied with the silver iodide-polyvinyl alcohol system

In a series of investigations on the interaction between hydrophobic colloids and polymers, a study has been made on the flocculation of silver iodide sol by polyvinyl alcohol. The most salient finding is that the extent of flocculation depends critically on the way of mixing of sol and polymer. The flocculation is most effective if equal amounts of nearly completely covered AgI particles and uncovered particles are mixed. An explanation is found in terms of bridging of particles by the polymer. The apparent irreversibility of the adsorption of polymer molecules is essential for the dependence of the flocculation on the mixing order. As a certain critical amount of salt is needed to bring about flocculation, the flocculation process has to be referred to as sensitization. The critical salt concentration decreases strongly with increasing valency of the counterion, approximately in the ratio 100:10:1 for KNO 3, Ca(NO 3) 2 and La(NO 3) 3. The salts reduce the Stern potential and compress the diffuse double layer to such an extent that the attachment of adsorbed loops on other particles is no longer electrostatically inhibited. The flocculation efficiency increases somewhat with increasing molecular weight and acetate content of the polymer. The better the polymeric flocculant, the less critical the way of mixing. From preliminary kinetic measurements it followed that the aggregation between polymer covered and uncovered particles is a bimolecular process. Silver iodide sediments formed upon flocculation with polyvinyl alcohol are loose and open, their volume decreases only little upon standing. On the other hand, silver iodide sediments obtained by coagulation with salts have a higher volume during the first hours after their formation, but this volume decreases gradually. The influence of the mixing order on flocculation has probably a wide array of practical applications.

Evaluation of NCAR Ice Nucleus Counter. Part I: Basic Operation

Abstract A detailed examination was made of the variables affecting the operation of the National Center for Atmospheric Research (NCAR) ice nucleus counter. The design criteria for proper flow control to achieve consistent cloud formation were established. The loss of ice crystals by settling to the bottom of the cloud chamber instead of exiting to the ice crystal counter was found to be constant. The most important variable is the concentration of the cloud condensation nuclei added to the sample stream to control cloud supersaturation. The rate of activation for AgI can be increased considerably by increasing the super-saturation by the new method of combining the AgI particles with NaCl particles serving as low supersaturation condensation centers. This also gives a much faster nucleation rate of potential importance in cloud seeding. The response of kaolin and phloroglucinol under varying supersaturation was found to be in agreement with previous studies, the former being responsive to high and the l...

Solubility of Small AgI Particles in NaI and KI Solutions: Significance to Cloud Seeding

Utilizing standard solubility data in the literature it is seen that for 2:1 mole ratio AgI:NaI (or KI) aerosol compositions only AgI particles <0.01 μ radius are in danger of complete dissolution into hygroscopically derived solution envelopes containing NaI, or KI. This condition develops only momentarily when sufficient water vapor has accreted to form a saturated solution about the AgI particle. Taking a typical AgI-NaI aerosol such as that studied by Mossop and Tuck-Lee as representative of generated aerosols, only a small part of the AgI can be dissolved under the most favorable atmospheric conditions; particles representing the median size for this aerosol, of 0.042 μ radius, will lose only 35 weight percent of the AgI present. Very similar relations hold for KI-bearing aerosols.

Size and Nucleating Ability of AgI Particles

Size and Nucleating Ability of AgI Particles

Freezing of Freely Suspended, Supercooled Water Drops by Contact Nucleation

The freezing of freely suspended, supercooled water drops by contact nucleation has been studied. Water drops were balanced in an updraft of a large vertical wind tunnel and allowed to supercool to the ambient temperature. Ice crystals introduced into the updraft were, as expected, the most effective nucleants in freezing the drops at ambient temperatures colder than and up to 0C. The results of this experiment using silver iodide and clay as contact nucleants closely agree with earlier work performed with a constant rate of cooling apparatus. The effective temperature for 100% nucleation efficiency in the case of AgI particles was −4 to −5C and for silicate particles −7 to −10C.

Nucleation of water drops by Brownian contact with AgI and other aerosols

AbstractAitken‐sized (0.01μm) particles of AgI, CuI, and ‘contaminated’ Ag2O were able to nucleate various sized water droplets in free fall (20–80μm in radius) by a direct, diffusive (Brownian) contact mechanism. The AgI aerosol was found to be the most efficient nucleator, showing a probability of 100 per cent at −16°C, 50 per cent at about −14°C, and 1 per cent at −11°C. These nucleation characteristics for AgI can be modelled from heterogeneous nucleation theory if the value of the contact parameter is assumed to be 0.70. Ag2O, passing through carrier tubing previously contaminated with iodine, was found to be only a slightly less efficient nucleating agent than AgI. CuI was found to have a nucleating efficiency corresponding to that which would theoretically be expected from an aerosol with a contact parameter of 0.55. Aerosols of the ‘pure’ oxides of silver, platinum, and chromium were unable to cause nucleation (to within detectable limits) at temperatures as low as −18°C. Auxiliary experiments indicated that the AgI aerosol had a small, but finite (10−5), nucleating efficiency at −5°C. Aitken‐sized particles of AgI did not lose their nucleating ability when captured at temperatures higher than 0°C.It is postulated that contact nucleation by the Brownian capture of Aitken particles may be an important enough process in the atmosphere to contribute significantly to the large amounts of ice which have been observed at relatively high (−10°C) temperatures in some clouds.

A Contact Nucleation Model for Seeded Clouds

Abstract A simple model is developed for clouds seeded with an AgI-acetone generator with the assumption that the AgI particles will behave only as contact nuclei rather than sublimation nuclei. The model was tested against limited data for the case of cumulus clouds typical of summer clouds in Flagstaff, Ariz.; the results compared well with the observations.

Microcinematographic Studies of Contact Nucleation

Abstract Water drops of 2 mm diameter were supercooled on a glass slide, sprayed with AgI particles, and photographed at low magnification with a 16-mm movie camera at a speed of 64 frames per second to determine the nature of freezing by contact nucleation. In each case; nucleation was initiated at the point of particle contact and continued over the entire surface of the drop. The interior of the drop then froze toward the center at a much slower rate. The surface structures and crystallization rates by contact nucleation compare favorably with those found by other workers where water was frozen in containers.

Some Size Distribution Measurements of AgI Nuclei with an Aerosol Spectrometer

Abstract The Goetz Aerosol Spectrometer, generally considered to possess only a fair ability in resolving size distributions of polydispersed aerosols, operates properly following a modification to the geometry of the entrance to the instrument's deposition channels. Its accuracy is demonstrated with an electron microscopic evaluation of a collecting surface deposit of a thermally produced polydispersed AgI aerosol with particle sizes ranging from 60 to 1000A In diameter. Thus calibrated, the instrument was utilized to investigate the activity of the same aerosol as freezing nuclei. The AgI particles on the hydrophobic chrome-plated collecting foil were nucleated by sorption at water saturation for temperatures of −15 and −20C. The results appear to reflect the influence of the Kelvin effect since the activity decreased at a faster rate than predicted by the “surface area rare” and since it showed a sharp cutoff corresponding to Fletcher's theoretical size temperature predictions for ideal sublimation nuc...

Ice-nucleating properties of dust collected above 80 kilometers

A specially constructed apparatus was used to compare the ice-nucleating properties of dust particles collected above 80 km with those of soil and Agi particles. The dust particles were found to be relatively inactive in ice nucleation when compared with Agi and soil particles of terrestrial origin.

Small Particle Detection and Identification by Crystal Growth: Preliminary Work on AgI Particle Detection

Small Particle Detection and Identification by Crystal Growth: Preliminary Work on AgI Particle Detection

Electrification Experiments with Agl in the System: Water Vapor, Liquid Water and Ice

The charge on an AgI particle, freely suspended in an air current of terminal velocity, has been measured at various temperatures, humidities and cooling rates. The wall of the flow pipe serves as a source or sinkfor water vapor, simulating neighbor droplets and ice particles in a cloud. The data show that the AgI particle sorbs water below the dew point and thereby acquires negative charge, that the sorbed water freezes at 0C, and that the electrification is reverisble. There are sudden changes in charge at the onset of sorption at the dew point and at freezing and melting, but the largest changes take place in prolonged and extensive sorption or desorption. The electrification follows an exponential rate law, indicating an autocatalytic process. Ice formation at 0C was also obtained with a dense cloud of AgI and correspondingly small condensate droplets in the expansion chamber. It is concluded that nucleation occurs at 0C, and that super-saturation pertains to the growth of the ice to detacable size. The data are compared with data in the literature. It is concluded that contradictions among published data are the results of differences in experimentalconditions, especially substrate effects in experiments with supported drops. The mechanisms of nucleation,growth and electrification are discussed.

Structural properties of the silver iodide-aqueous solution interface

A survey of existing literature on the water structure around AgI particles shows that a complete description of this topic can not yet be given. In order to contribute to the knowledge of the AgI-aqueous electrolyte solution interface, the shift of the point of zero charge due to the replacement of water molecules by alcohol molecules and the influence of temperature on double layer parameters are analyzed and discussed. From the thermodynamic analysis of the temperature dependency, yielding the interfacial excess entropy η σ , and from the alcohol adsorption experiments, it is inferred that at 25°C water dipoles are not preferentially oriented at a surface charge of about −1 μC.cm −2, coinciding with the adsorption maximum of alcohols. From this point on, dipole orientation increases upon increasing positive and negative charge. However, at positive charges the effect is obscured by the structure-breaking effect of specifically adsorbed nitrate ions. At 85°C, the relation between η σ and surface charge is more pronouced than at 25°C, whereas the cation specificity has completely disappeared.

INFLUENCE OF ATMOSPHERIC STABILITY LAYERS ON THE EFFECT OF GROUND-BASED CLOUD SEEDING, I. EMPIRICAL RESULTS

creased. Neither A nor B is yet identified. The identification of these conditions A and B is, then, an important outstanding problem. A promising approach to this problem consists in a scrutiny of weather modification experiments already completed, with the primary purpose of verifying empirically the various a priori hypotheses underlying the current efforts at weather modification. The basic premise of rain stimulation with AgI seeding is the discovery by Vonnegut of the ice crystal nucleating property of the AgI smoke at temperatures below the threshold of -4?C. If the seeding is done from the ground, hopes of success depend upon the presence of strong updrafts capable of carrying the AgI particles into the supercooled parts of the clouds before the Agl is deactivated by photolysis. Ordinarily, temperature inversions and isothermal layers (stability layers, for short) are obstacles to convective updrafts and are expected to prevent cloud seeding from the ground from being effective. The purpose of the present paper is to provide evidence that, at least in mountainous regions, the mechanism of the effects of cloud seeding involves some unexpected

A comparison of models for the electric double layer

The relation between surface potential ψ 0 and surface charge density Γ of stabilizing ions is obtained for a model which represents the negative silver iodide-aqueous 1-1 electrolyte plane interface, with or without specific adsorption of cations. In the case of adsorption, the Gouy-Chapman-Stern-Grahame theory of the electric double layer is applied, with corrections due to discreteness-of-charge effect and counter ion-size. Also, the integral capacity of the inner region is assumed to decrease linearly with increase in ∥ Γ ∥. The general behavior of the experimental ( Γ, ψ 0) plots can be reproduced by the theory. Mirnik's claim that the condition of thermodynamic equilibrium between AgI and aqueous phases with respect to stabilizing (I −) ions implies log Γ proportional to pI (or ψ 0) is shown to be incorrect. An alternative treatment by Mirnik of counter-ion adsorption on negative AgI particles, which is based on his ion-exchange theory, is examined. Certain objections are raised to this theory—a particular difficulty being Mirnik's conclusion that stabilizing I − ions are not in equilibrium between the AgI and aqueous phases. Mirnik's ion-exchange theory of coagulation is also discussed and arguments are given to show that this theory is incorrect.

Determination of the ion adsorption by the radioactive tracer technique

Using radioactive152Eu the adsorption of the Eu3+ ions on the surface of the silver iodide precipitates in the presence of rhodamine 6 G, strychnine and triton-X-305 (T-X-305) was studied. The effect of the desorption on rhodamine 6 G and strychnine is determined. The obtained results show that a complete desorption of the Eu ions is obtained if the concentration of these compounds is ten times lower than that needed for inorganic ions of the same valency. The effect of T-X-305 is opposite to that of the above mentioned compounds. It is also determined that by increasing the concentration of T-X-305 the adsorption of the Eu ions is enhanced, this being connected with the change of the dispersity of the system. The electron micrographs show that the change of the dispersity of the AgI particles is caused by various concentrations of T-X-305. It has been found that the adsorbed amount of the Eu ions depends upon the way of preparation of the AgI systems.