Effects of Sunlight and Ammonia on the Action of Silver-Iodide Particles As Sublimation Nuclei *

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 concentration 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 further 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-

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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 prevent 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 before 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 concentration 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 supercooled 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.

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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 overnight. 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. Tests

* The aerosol tracer technique was developed by W. D. Crozier and Ben Seely of New Mexico Institute of Min- ing and Technology under Office of Naval Research FIG. 3. Decay rate of Agl particles in 10' X 20' tank sponsorship. with cellophane cover (August 1, 1950).

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processes are not immediately clear. A tentative explanation can, however, be offered on the basis of the experiments described. Vonnegut [4] has shown that as the size of the particles produced by a silver iodide generator is decreased beyond a certain point (the total number increased), the number of effective nuclei produced per gram of silver iodide consumed is greatly decreased according to cold chamber tests. Vonnegut has reported that this results from the fact that the reaction rate of the small particles is so low that these particles precipitate on the walls of the chamber before nucleating ice crystals. The data in FIGURE 1 suggest that the ammonia does not increase the probability that a given silver iodide particle will act as a sublimation nucleus (i.e., reduce the half life of the aero- sols in the cloud)* but rather that it increases the total number of effective particles present. This suggests that the ammonia causes a large number of small particles having a zero probability of reaction to become effective, rather than merely changing the reaction rate of these particles. However, measurements indicate that the rate of precipitation on the cold box walls is so much greater than the rate of ice crystal formation at — 13°C that the slope of the curves of FIGURE 1 is determined principally by precipitation to the walls; therefore, the conclusion from the equivalence of the slopes of the two curves that the total number of effective particles is increased is not justified. Nonetheless, it is rea- sonable to assume that the ammonia exerts a major influence on the reaction of the smaller particles of silver iodide smoke and causes the formation of a much larger number of ice crystals

FIG. 4. Effect of NH3 on Agl particles exposed to during the observation period (15 to 20 minutes). sunlight in 10' X 20' tank with cellophane cover (July Ammonia might increase the size of the silver 28, 1951). iodide particles either by being adsorbed on the surfaces of the particles or by forming one of the FIGURE 4 shows the result of the addition of ammines of silver iodide at the surfaces of the about 20 cc of concentrated ammonium hydroxide particles. In view of the affinity of water for am- to the tank after exposure to sunlight for about monia, it would not be surprising if a nucleus thus 3 hours under conditions of partial cloud cover. composed of silver iodide and ammonia were effective in initiating glaciation. Bond affinities be- DISCUSSION tween water and ammonia are suggested by the The impairment of the action of silver iodide persistence of ammonia as a contaminant in water nuclei by exposure to sunlight is not surprising in and by freezing experiments of Workman and view of the photolytic property of the substance, Reynolds [1]. and the importance of this property of the par- The observation that a sample of air containing ticles in their use as artificial sublimation nuclei silver iodide particles can be irradiated with ultra- is readily apparent. The cause for the increased activity of the nuclei in the presence of ammonia * It should be understood here that the depth of the unit is such that once an ice nucleus is formed in the vapor and the importance of this phenomenon in supercooled cloud it will fall to the bottom of the unit attempts to control the initiation of sublimation in about one minute.

Unauthenticated | Downloaded 10/06/21 05:41 PM UTC 30 BULLETIN AMERICAN METEOROLOGICAL SOCIETY violet light until it is capable of producing very ever, that the number of effective nuclei intro- few or no ice particles in a supercooled cloud, but duced into a growing cumulus cloud from a then can be caused to produce many (103 per cc ground-operated generator might be enhanced of air sample introduced) ice particles by the addi- greatly (especially under conditions of bright sun- tion of a little ammonia vapor to the cloud, fur- light) by the release of ammonia from a point on nishes further evidence as to the method of action the ground a few miles from the generator. The of the ammonia. It is well known that the "speed" convergence which accompanies the growth of of photographic plates is governed largely by the such a cloud would cause the co-mingling of the grain size, the smaller grain sizes providing the generator product and the ammonia near the base more insensitive plates. It seems quite possible of, or within, the cloud. This co-mingling would, then that the very small and very numerous parti- for a limited time at least, result in the presence cles, the reaction of which the ammonia can affect of a greatly-increased number of sublimation nu- greatly, would remain undissociated and capable clei active at relatively warm (— 4°C to — 20°C) of response to ammonia after most or all of the temperatures.f larger, initially effective particles had been dis- sociated by irradiation. There is some experi- CONCLUSIONS mental evidence to show that continued exposure Silver iodide smoke from a hot platinum fila- gradually reduces the response to ammonia. ment can be caused to produce approximately 100- Further evidence that the ammonia acts on par- fold more ice crystals in a supercooled cloud in a ticles normally too small to be affected rapidly by laboratory cold chamber at — 13 °C by adding a light absorption is provided by the observation little ammonia vapor to the cloud. Silver iodide that if the ammonia vapor is added to the sam- particles as produced by butane flame generators ple before irradiation the airborne silver iodide for nucleation of supercooled clouds are photo- particles in the sample are inactivated and show sensitive ; the number of effective nuclei in an air no response when ammonia is added to the super- sample containing initially 1011 per cubic meter cooled cloud. The reasoning here is that the decreases at the rate of about two orders of mag- size of the very small particles is increased (or nitude per hour under exposure to bright sunlight. effectively increased) to such an extent that they An air sample (bearing silver iodide particles more readily absorb light radiation. This reason- from a butane generator) which has been de- ing and the data of FIGURE 4 (which show that activated by exposure to sunlight produces large the rate of decay increases somewhat after am- numbers of ice particles in a supercooled cloud monia is added to the sample) are consistent with the well-known practice of using ammonia for fOn June 1, 1950, in the vicinity of Socorro, New the sensitization of photographic plates. Mexico, 750 lbs of ammonia were released into a growing The results of these tests are not interpreted cumulus cloud at a distance of about one mile from an operating silver iodide generator. A few minutes after as meaning that ground-operated silver iodide the estimated (on the basis of a ceiling balloon filled with generators are incapable of producing cloud modi- helium and weighted to give slight lift) time at which fication even at great distances from the point of the ammonia reached the cloud, precipitation began and increased to cloudburst proportions in the next thirty operation under some circumstances. Prelimi- minutes. A total of 1.14 in. of rain fell at Socorro, New nary tests have shown that thin cloud cover * Mexico from this storm. While it cannot be concluded greatly reduces the rate of decay of effectiveness that the ammonia contributed to the heavy rainfalls of June 1, 1950, or July 21, 1949 (both days were char- of silver iodide particles. acterized by much shower activity over New Mexico), The experiments with ammonia vapor and this possibility should not be ignored. silver iodide particles seem to show clearly that It is recognized that this possibility is dependent upon despite the fact that ammonia might greatly en- the quantity of ammonia vapor required to produce the effects described. While the amount of ammonia used hance the effect of particles as dispensed from in the laboratory experiments seemed small, it was not silver iodide generators, it will not decrease the measured, and the amount required to produce detectable rate of decay of effectiveness of these particles effects in the atmosphere may be very great. Quanti- tative determinations are necessary not only to evaluate under exposure to sunlight but may rather in- properly this possibility but to determine the importance crease this rate. The possibility does exist, how- of naturally occurring ammonia (1 to 20 lbs of ammonium ions per cubic mile of air with an average of * According to Coblentz and Stair (Jn. Res. Nat. about 2 lbs per cubic mile according to a series of meas- Bur. Stds., V. 15, Aug. 1935) the integrated ultraviolet urements by New Mexico Institute of Mining and intensity through light fog was measured to be less than Technology at Socorro) in the silver iodide ice nucleat- 10% as great as observed with clear sky. ing process.

Unauthenticated | Downloaded 10/06/21 05:41 PM UTC VOL. 33, No. 1, JANUARY, 1952 31 when ammonia vapor is added to the cloud, but if REFERENCES ammonia is added to the air sample before ex- [1] E. J. Workman and S. E. Reynolds: "Electrical posure, the rate of decay is somewhat increased Phenomena Occurring During the Freezing of Dilute Aqueous Solutions and Their Possible Re- and nucleating effects are not enhanced by addi- lationship to Thunderstorm Electricity," The tion of ammonia to the supercooled cloud. - Physical Review, Vol. 78, No. 3, 1 May 1950. [2] V. J. Schaefer: "The Occurrence of Ice Crystal Nuclei in the Free Atmosphere," G. E. Laboratory, ACKNOWLEDGMENT Project Cirrus Occasional Report No. 20, 15 Jan- uary 1950. The authors are indebted to Dr. E. J. Work- [3] Edward C. Y. Inn: "Photolytic Inactivation of Ice- Forming Silver Iodide Nuclei," Bulletin of the man for the design of the experiment to study Amer. Met. Soc., Vol. 32, No. 4, April 1951. the effects of sunlight on silver iodide smokes and [4] Bernard Vonnegut: "Nucleation of Supercooled Wa- ter Clouds by Silver Iodide Smokes," Chemical for guidance in carrying out this experiment. Reviews, Vol. 44, No. 2, April 1949.

NYU Research Fellowships and Assistantships NEWS AND NOTES in Meteorology and Oceanography for 1952-53

The following three fellowships are supported by the Spring Meeting of USA National Committee research fund of the Department of Meteorology and of URSI-IRE Professional Group on Oceanography, New York University. They will be Antennas and Propagation awarded to properly qualified candidates who will have the Bachelor's degree by September 1952, with courses in A meeting of the USA National Committee of the mathematics through differential equations and at least International Scientific Radio Union (URSI) and the twelve semester hours of physics. The candidates need Institute of Radio Engineers Professional Group on not have taken any courses in meteorology or oceanog- Antennas and Propagation is being held at the National raphy. Bureau of Standards, Washington, D. C., on April 21, Recipients of the fellowships will be expected to pursue 22, 23 and 24, 1952. a full-time program of study leading toward an advanced Sessions to be held will include ones on Tropospheric degree in meteorology or oceanography. The recipients Radio Propagation, Ionospheric Radio Propagation, Ter- may, however, elect to take one course less than a full- restrial Radio Noise, and Radio Astronomy. time program in order to supplement their income by work on sponsored research projects in the Department A preliminary program and advance registration forms of Meteorology, up to twelve hours per week at an will be available after March 10, 1952. These and further hourly rate of $1.85. information concerning the meetings may be obtained Benefits of each fellowship are as follows: from A. H. Waynick, The Pennsylvania State College, State College, Penna. (1) Full tuition and fees for nine months and an additional cash award of $1000.00, payable in ten monthly installments. Personal Note (2) Full tuition and fees for nine months and an Prof. C. IV. Thornthwaite, Johns Hopkins University, additional cash award of $500.00, payable in has been elected President of the Commission on Clima- five monthly installments. tology of the World Meteorological Organization. The (3) Full tuition and fees for nine months. election took place at the first congress of the WMO, For application forms, write to Assistant Dean in successor to the International Meteorological Organiza- charge of the Graduate Division, College of Engineering, tion, in Paris. Dr. Thornthwaite's Laboratory of Clima- New York University, New York 53, N. Y. Completed tology, at Seabrook, N. J., recently has had two distin- forms and supporting material should be in the hands of guished visitors giving a series of lectures and consulting the Assistant Dean by April 1, 1952. with the staff. They are B. J. Gamier, formerly Head A number of assistantships on sponsored research proj- of the Department of Geography at the University of ects will be available in 1952-53. An assistant may work Otago, New Zealand, and now of Ibadan, Nigeria, who from ten to twenty hours per week, depending upon how was there June 20 to July 19, and Ernst Frankenberger, many courses he is taking. The rate of pay varies from meteorologist of the Meteorological Service of Northwest $1.85 per hour (Graduate Student Assistant) up to a Germany, there from Sept. 21 to 26. maximum of about $2.30 (Research Assistant).

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