26 BULLETIN AMERICAN METEOROLOGICAL SOCIETY Effects of Sunlight and Ammonia on the Action of Silver-Iodide Particles as Sublimation Nuclei * S. E. REYNOLDS, WILLIAM HUME II, and MAX MCWHIRTER New Mexico Institute of Mining and Technology, Socorro ABSTRACT Tests involving exposure of Agl smoke to bright sunlight show a decrease in concentra- tion of effective nuclei (at — 20°C) of approximately two orders of magnitude per hour. The concentration of effective nuclei is increased greatly (as much as two orders of magnitude) by the addition of a little ammonia vapor to the Agl smoke. Smoke samples which have been deactivated completely by exposure to ultraviolet light can be caused to form large numbers of ice crystals by the addition of ammonia vapor. If ammonia is added before exposure to light, the rate of decay is the same or greater, and the effectiveness cannot be restored by fur- ther addition of ammonia. The effect of ammonia is believed to be due to the adsorption of ammonia on the silver iodide surfaces or to the formation of an ammine of silver iodide. INTRODUCTION rected horizontally across the box about 3 in. above the bottom of the liner. N the course of an experiment designed to Two drops of a solution containing 0.1 percent study the effects of certain contaminants on by weight of silver iodide in acetone were placed the electrical behavior of thunderstorms [1] I on a platinum filament. The acetone was per- 450 lbs of ammonia were released, on July 21, mitted to evaporate and the supercooled cloud 1949, from a hilltop near Albuquerque, N. M., in then was seeded by bringing the filament to red such a way that the gas entered a growing cumulus heat in the cloud for a period of ten seconds. The cloud. Silver iodide was being released at the number of ice crystals present was estimated at same time from a nearby point on the ground in frequent intervals according to the method of another experiment to determine the possibilities Schaefer [2] until the count reduced to 104 per for inducing precipitation. A few minutes after cubic meter. The seeding and counts as described the ammonia reached the target cloud torrential were repeated three times to test reproducibility, rains began. These circumstances led some and then a fourth seeding was made which was in months later to an investigation of the effects of all respects similar to the previous seedings ex- ammonia on the action of silver iodide particles cept that a filter paper having two drops of con- as sublimation nuclei and this investigation led in centrated ammonium hydroxide on it was passed turn to a suggestion that silver iodide smoke is through the cloud immediately after the silver deactivated in transit through the atmosphere. iodide seeding. A great increase in the number Since silver iodide is known to be photo-sensitive, of ice crystals present was immediately observable. the first assumption was that sunlight (in particu- The results of these experiments are plotted in lar the short wave-length radiation) caused re- duction of the silver iodide particles. FIGURE 1. The assumption that sunlight causes the de- EXPERIMENTATION activation of silver iodide smoke was first tested by exposing samples of the smoke to the radiation A three cubic foot deep-freeze unit was modi- from a Mineralite ultraviolet source (2540 A). fied so that the temperature could be controlled The samples were obtained by opening an evacu- closely. Water vapor was introduced from a ated 5-gallon carboy in the smoke at a distance beaker of distilled water in the box by means of of about 50 feet from the burner. The carboy was a submerged platinum filament heater. Tem- sealed and returned to the laboratory, where ex- perature of the supercooled cloud, measured with posure to ultraviolet was made through the neck a fine thermocouple, varied from — 13°C to of the bottle. The effectiveness of the smoke — 13.2° C. A collimated beam of light was di- particles as sublimation nuclei was determined by * This work was supported by the Signal Corps of the injecting a little of the air from the carboy into a U. S. Army. A partial report was given at the Van- supercooled cloud before, and at various intervals couver Meeting of the American Physical Society, June 25-28, 1951. after, exposure. Two or three minutes of ir- Unauthenticated | Downloaded 10/06/21 05:41 PM UTC VOL. 33, No. 1, JANUARY, 1952 27 of exposure to natural sunlight.* A cylindrical steel tank 10 feet in diameter by 20 feet high was set up, and the top was cut open to a diameter of 9 feet. A removable plug was provided near the bottom for sampling with a syringe. Water sprays were arranged outside the tank at the top in such a way that the entire exterior tank surface was covered with running water to pre- vent heating of the air in the tank to temperatures above that of the free atmosphere. A %-horse- power electric fan was suspended near the center of the tank with the blast directed vertically down- ward to provide air mixing. Two interchange- able covers were made for the top of the tank to prevent the loss of air and airborne particles. One of these was made of linoleum with the paper side coated with ceresin wax and was used with the paper side toward the inside of the tank. The second cover was fabricated from clear cellophane .001 in. thick. The cellophane was tested for light transmission and found to be satisfactory for transmission of most of the ultraviolet light which reaches the lower atmosphere. It should be pointed out that this material is thinly coated with a waxy substance for water proofing and sealing. The interior of the tank was washed down be- fore each run. After carefully sealing one of the covers in place with Scotch tape and starting the electric fan, the silver iodide generator flame was inserted through a pipe opening at the bottom of the tank and run for 2 seconds to give a high con- centration of silver iodide particles in the tank. The air in the tank was sampled with a large (6" X 15") syringe at 5 minutes after charging and at about 15-minute intervals thereafter. At each sampling a measured, small amount of the FIG. 1. Effect of ammonia on Agl seeding at — 13 °C. air from the syringe was discharged into super- cooled clouds in two cold boxes, one at about radiation reduced by 3 to 4 orders of magnitude - 13° C and the other at about - 20°C, and the the number of ice particles formed by a given number of ice particles produced was estimated. volume of smoke. Typical results of the experiment as conducted In the course of these preliminary tests it was with the opaque cover are shown by FIGURE 2. found that an apparently completely inactivated Results of the experiment as conducted using the sample of silver iodide smoke produced large transparent cover are shown by FIGURE 3. A low numbers of ice particles if a little ammonia gas rate of decay of activity at — 20°C in the early was added to the supercooled cloud. It was found stages of the exposure was recorded; this low rate also that if a little ammonia gas was added to may be only apparent since the estimates of con- the sample before irradiation, deterioration under centration are least accurate at very high concen- exposure progressed at the usual rate, or perhaps trations. The individual estimates of concentra- somewhat more rapidly, and the activity of a tion as given probably are reliable only to the sample so treated could not be enhanced by add- nearest order of magnitude. ing ammonia to the supercooled cloud. *A preliminary report of the results of these experi- Following these preliminary tests, equipment ments was published in the Bulletin of the American was provided to study the effects on silver iodide Meteorological Society, Vol. 32, No. 2, Feb., 1951. Unauthenticated | Downloaded 10/06/21 05:41 PM UTC 28 BULLETIN AMERICAN METEOROLOGICAL SOCIETY made in the usual manner at 0800 the next morn- ing showed that the concentration of effective nuclei had changed from 2 X 103 to 4 X 103, or an insignificant amount. The copper sulphate aerosol concentration was about 5 X 105/m3 at this time. Another similar test (without copper sulphate tracer) showed a slight decrease in the number of effective silver iodide particles after the exposed smoke had remained in the tank over- night. These experiments indicate no appreciable re- versal of the decay process, while experiments by Edward C. Y. Inn [3] gave some inconsistent evidence that the ice-nucleating properties of silver iodide, once destroyed by exposure to light, might be restored if the particles were permitted to "rest" in a darkened environment for a period of several hours. FIG. 2. Decay rate of Agl particles in 10' X 20' tank with opaque cover (July 31, 1950). A copper sulphate aerosol * was dispersed in the tank and the concentration of this substance also was determined at frequent intervals to estab- lish the effectiveness of the seal around the tank cover. The rate of change of concentration of this aerosol also is shown on both FIGURES 2 and 3 for ready comparison with the rate of change of concentration of effective sublimation nuclei. Data for the silver iodide and copper sulphate curves were taken concurrently. After the last readings shown by FIGURE 3 were made, the opaque cover was placed over the cellophane cover and the material was permitted to remain overnight in the sealed tank.