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Briant L. Davis

Abstract

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.

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Briant L. Davis

Abstract

A cloud chamber mounted on a commercial x-ray diffractometer has been used to obtain ice crystal shapes, numbers, and volume information from clouds seeded with AgI-NaI aerosols. Data obtainable from this system include automatic printout of integrated diffraction intensities, photocell traces for fog densities, four-point temperature readout for the chamber, and optional Formvar replication.

Aerosol efficiency data for the AgI-NaI particles are comparable to those generally published with the exception of two anomalies: 1) a pronounced dip in the efficiency curve at −12C interpreted to mark the transition from condensation-freezing to sublimation nucleation, and 2) a maximum in the curve at ∼−20C associated with a maximum in proportion of plates in the plate-column crystal mixture. At −20C and an aerosol dilution density of 5 × 106 cm−3 we observe about 5 × 1015 active nuclei per gram of AgI. At dilution densities in the 108 cm−3 range the efficiencies are reduced an order of magnitude.

X-ray and Formvar replica data indicate that plates are most common from −17 to −22C, with thick plates and short columns occurring above this temperature and long hollow columns and frozen water drops at progressively higher temperatures. We also observe the mean of the volume ratios of plates to columns for all runs to be 0.5, in fair agreement with other observations.

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Briant L. Davis

Abstract

Examination of the theory of heterogeneous nucleation of ice has led to a number of observations which indicate several inconsistencies between this theory and experimental observations. Using activity curves for AgI-bearing smokes proposed by Fletcher, we have calculated the hypothetical efficiency curves from log-normal distributions of such smokes. We have also generated ΔT vs radius (R a) data from fitted curves to experimental activity data to test the values of contact and other parameters in the theory. As a result of the above analysis, we observe that:

  1. Using a KI-AgI activity curve and log-normal smoke distributions, the resulting efficiency curve shows a sharp-kneed trend completely unlike the smooth sweeping trends of actual curves observed by experiments.

  2. Some of the data involving means of distributions of smoke sizes are considerably different from those estimated from electron micrographs.

  3. When inserted into the theoretical expressions the generated ΔT and R a data result in values of σ12, m and δ (pit area factor) and/or trends of f(m,x i) and x i which are incompatible with the theoretical treatment and reasonable values for these physical constants.

Recognizing that a semi-empirical theory of this type has its place in the development of a scientific discipline, it is nevertheless apparent that analyses of this type based upon scanty experimental values for the physical parameters contained therein will continue to present an inaccurate picture of nucleation processes unless a greater effort is given to the determination of such parameters as contact angle, interfacial energies, and actual surface structure of nucleant materials.

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Briant L. Davis

Abstract

An x-ray diffraction study of AgI-NaI aerosols produced from a standard solution generator has revealed the formation of several hydrated phases. The aerosol samples were collected on glass filter paper and analyzed continuously in a controlled atmosphere chamber at 23C, with exploratory runs at 0C, at vapor pressures ranging from 2.5 mb to saturation values.

At 4.5 (+1, −1.5) mb and 23C the aerosol transforms from a 3.9:1 weight ratio of dry AgI and NaI to 3.1:1 weight ratio of AgI and NaI·2H2O. At 8.1 (±1) mb there is a 60% loss in free AgI with formation of 3AgI·2NaI·nH2O (4<n<10). At 11.7 (±1) mb the double salt phase breaks down to form pure AgI immersed in an aqueous solution of NaI and AgI. Exploratory runs made at low temperatures reveal other compounds, one of which may be NaI·5H2O.

Kinetic observations at the lower temperatures indicate that the hydrous phases may persist in aerosols released from airborne generators but would be eliminated before cloud base is reached when the aerosol is produced from ground generators.

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Briant L. Davis and Michael W. Ekern

Abstract

By means of the Lambert projection, wind vector data may be plotted onto a frequency map. The resulting diagram portrays the “wind fabric” for the data sample of a single station or for groups of stations. The area-true distribution of wind vectors is thus given in great detail and allows several standard tests for homogeneity and anomaly significance. By using a “swinging plate” device the wind energy density and wind power can be calculated for any velocity-compass heading sector of the diagram desired and allows for a universal scaling of the velocity variable of the diagram.

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Briant L. Davis and Richard A. Schleusener

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Donald N. Blair and Briant L. Davis

Abstract

The effect of storage of AgI-NaI aerosol particles in moist and dry air upon the efficiency of nucleation of such particles has been investigated. Samples were obtained from a standard solution generator and tested in an optical cold stage at −15C under a controlled atmosphere. The “wet” and “dry” samples were paired and studied together in the optical cold chamber to eliminate several environmental variables. In 16 out of 21 runs the aerosol aged under dry conditions showed a higher nucleation efficiency, there also being a possible weak positive correlation of the ratio of efficiencies of dry to wet samples with time. Differences between these results and those of other workers can best be explained by differences in aging treatment.

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Briant L. Davis, L. Ronald Johnson, Bryan J. Johnson, and Robert J. Hammer

Abstract

A field instrument for sampling sulfate and nitrate particulate matter in a controlled chemical environment has been constructed and field tested. The instrument contains HNO3 and NH3 denuders and an ambient air path, all connected by manifold to a PM-10 size-selective inlet. Ambient aerosols are collected on acid-washed quartz filters mounted in the front compartment of dual stainless steel cassettes. The rear cassette chamber contains backup filters which allow denuder breakthrough detection and specific gas measurements. The aerosol collection system has been designed specifically for use with x-ray diffraction analysis methods. An investigation of denuder coating reactions reveals that multistage reactions take place with ammonia absorbed by oxalic acid, and that nitromagnesite is formed by reaction of nitric acid with magnesium oxide coatings. Reference intensity constants needed for quantitative x-ray diffraction analysis of denuder reactions, filter pack reactions, and ambient atmospheric particulate products have been measured. They have been used in several applications, including the estimation of ammonia gas concentrations in the system gas stream and quantitative analysis of compounds observed in field “shakedown” tests. These field tests demonstrated that the system is mechanically reliable, although improvements to this system might include an increased pumping capacity and use of teflon filters to increase the signal-to-noise ratio of x-ray diffraction analysis.

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Briant L. Davis, L. Ronald Johnson, Dana T. Griffen, William Revell Phillips, Robert K. Stevens, and David Maughan

Abstract

A quantitative study by x-ray diffraction, optical polarizing microscopy, and x-ray fluorescence spectrometry of fallout and ambient ash from three Mt. St. Helens eruptions has revealed a consistent picture of the mineralogical and elemental composition. The major components observed are amorphous glass, plagioclase of about An50 composition, minor amounts of quartz and other SiO2 polymorphs, as well as ferromagnesian constituents such as hypersthene, magnetite, etc. Free SiO2 exists in all samples, but in no case (even considering large uncertainties in the cristobalite analyses) does the free SiO2 exceed 8% of the total aerosol mass. The variation in the glass/plagioclase ratio with particle size observed in four samples suggests that much of the crystalline material exists in the finer particle size fraction of the ash.

Reduction of the mineralogical constituents to component oxides has made it possible to compare the diffraction results with direct elemental determination by x-ray fluorescence; the two data sets are generally in good agreement. Exceptions to this are in the Fe-Mg content and Al2O3/SiO2 ratio. In the former case, the differences are explained by the lack of ferromagnesian accessory minerals detectable by the x-ray diffraction method. All data support an andesite classification for the ash. Record precipitation observed in the northern Rocky Mountains during May coincides roughly with the volcanic plume trajectory, which raises the question of a possible seeding influence on storms over the area. The mineralogical makeup of the plume, however, is not particularly suited to precipitation enhancement through nucleation of the ice phase.

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