Thermal Diffusion Chamber Research Articles

A preliminary theoretical analysis of a modified membrane filter technique to detect ice-forming nuclei in a thermal diffusion chamber

A preliminary mathematical analysis of a membrane filter technique for estimating the population of A-sized ice-forming nuclei is presented. The technique consists of drawing an aerosol sample through a membrane filter and then exposing the surface of the filter to controlled conditions of temperature and water vapor supersaturation in a dynamic chamber. The number of ice crystals which form on the filter is related in sometimes subtle ways to the concentration of ice forming nuclei in the aerosol sample. In order to enhance experimental control of supersaturation during filter development, condensation nuclei are also deposited on the filter.

Photonucleation in supersaturated CCl4 vapor investigated in a diffusion chamber

We report an experimental investigation of the nucleation of supersaturated carbon tetrachloride vapor in the presence of an impurity, Cl2, induced by light in the visible part of the spectrum. This photonucleation phenomenon has been studied with a thermal diffusion chamber. The process is shown to be strongly dependent upon the intensity and wavelength of light and upon the supersaturation of the vapor. A tentative interpretation of this photonucleation phenomenon is given, that is, it relies upon the formation of CCl3 free radicals.

都市域における大気中のエアロゾル粒子の凝結核としての不活性度

In order to make clear how much extent urban aerosol particles are inactivated as con-densation nuclei, possibly by the effect of organic materials, an experimental study was made using a vertical thermal diffusion chamber and an impactor. Inactive particles, which grew slowly or not at all under the supersaturation less than about 1%, were collected separated from active particles, and the concentration of inactive particles was compared with that of atmospheric aerosol particles by means of a transmission electron microscope. It was found that inactive particles scarcely existed. This result indicates that most of the atmospheric aerosol particles are not greatly inactivated as condensation nuclei even in the possible pres-ence of organic materials in the urban area.

Condensational enlargement as a supplement to particle control technologies

An experimental study has been undertaken at the Colorado School of Mines to evaluate the feasibility of using condensational enlargement to enhance the collection efficiency of existing dust control technologies for sub-micron particulates. Although the study was begun to consider the problem of respirable dust control in the underground coal mine environment, this enhancement technique shows promise for a more general class of applications. In the experiments to be reported on, particulates are injected into a continuous flow thermal diffusion chamber and subjected to varying environmental conditions; saturation ratio (rel. humidity), temperature, and residence time were the parameters. The results show that condensation on a dust nucleus (≈0.1 micron) can bring it to a size of several microns in a sufficiently short period of time as to indicate engineering applicability of the concept.

Nucleation Experiments with Monodisperse NaCl Aerosols

Nucleation experiments with monodisperse NaCl aerosols showed good agreement with the Köhler theory relating the critical super-saturation Sc to the dry size. Aerosols produced by condensing NaCl showed the same Sc as those produced by evaporating aqueous NaCl solution droplets. This indicates that if there is an energy barrier in going from a dry NaCl particle to a solution droplet, this energy barrier is small. The fact that the evaporation aerosol particles are cubical crystals and the condensation aerosols are amorphous spheres is shown to make no difference in the nucleation threshold. The investigation also gives insights into the performance of the equipment used, especially the commercial electrostatic aerosol classifier and the vertical flow thermal diffusion chamber developed in this laboratory. When operating this chamber in the isothermal mode, a 36% upper limit was found on the uncertainty in Sc due to index of refraction sensitivity in sizing the water drops. Within this range of uncertainty, the isothermal mode data agreed with the Köhler theory.

Ice Nucleation by Aerosol Particles: Experimental Studies Using a Wedge-Shaped Ice Thermal Diffusion Chamber

The principle and feasibility of a new wedge-shaped ice thermal diffusion chamber are demonstrated and the method of estimating the supersaturation in the chamber is shown. Ice nucleation studies were carried out using the chamber and relatively large aerosol particles freely suspended in air. In the studies, the temperature and the supersaturation were independently controlled without substrate. All natural and artificial, organic and inorganic nuclei studied exhibited condensation-freezing as well as deposition modes of ice formation. Conditions for their occurrence and modes of their activation were identified. A forbidden zone for condensation-freezing nucleation above water saturation was found and its temperature dependency, and the extremely steep rise of nucleation rate when the supersaturation increased beyond it, were determined. A unique deposition nucleation behavior near but below water saturation was confirmed with freely suspended aerosol particles. In contrast with deposition and condensation-freezing nucleations, contact nucleation was identified as a slow and therefore long-lasting process except for metaldehyde. The contact nucleation rate of metaldehyde was found to be about 103 times larger than that of AgI. Our results reaffirmed that ice nucleation is a function of temperature as well as supersaturation.

Aspect Ratio of Thermal Diffusion Chambers

Abstract The supersaturation distribution inside thermal diffusion chambers of cylindrical symmetry with a linear vertical temperature gradient on the wall was computed with various aspect ratios using the vapor pressure field. The results were compared with previous works. It was shown that the effect of the wall on the supersaturation inside the chamber is much reduced when a linear temperature gradient is maintained on the wall instead of an isothermal condition.

A segmented thermal diffusion chamber for continuous measurements of CN

A new segmented thermal diffusion chamber for measurement of CN has been constructed and found to operate satisfactorily. In the segmented chamber aerosol laden air is passed through a vertical cylindrical tube consisting of hot and cold segments with water saturated walls. Air in the center of the tube reaches a temperature and water vapor pressure midway between the temperature and vapor pressure of the hot and cold segments. This produces a supersaturation in a manner similar to that which generates supersaturations in a static thermal diffusion chamber operated at the same temperature difference. An analysis of this chamber shows that introducing the sample along the axis and surrounding it with filtered sheath air will prevent the sample from encountering fluctuating supersaturations which exist along the walls and give the dilution necessary for use with optical counters.

Rocket Effluent: Its Ice Nucleation Activity and Related Properties

To investigate the possibility of inadvertent weather modification from rocket effluent, aerosol samples were collected from an instrumented aircraft subsequent to the Voyager 1 and 2 launches. The aerosol's morphology, concentration, and size distribution were examined with an electron microscope. The elemental compositions of individual particles were analyzed with an X-ray energy spectrometer. Ice nucleus concentration was measured with a thermal diffusion chamber. The particles' physical and chemical properties were related to their ice nucleation activity. A laboratory experiment on rocket propellant exhaust was conducted under controlled conditions. Both laboratory and field experimental results indicated that rocket propellant exhaust can produce active ice nuclei and modify local weather in suitable meteorological conditions.

Performance of a Dual-Range Cloud Nucleus Counter

An instrument for measuring the concentrations of cloud condensation nuclei as a function of nucleus critical supersaturation (Se) is described. In the range 0.1% ≤ Se ≤ 1%, the instrument is operated as a vertical flow thermal diffusion chamber. In the range 0.016% ≤ Se ≤ 0.1%, the chamber is operated at 100% humidity, and the equilibrium radii of the nuclei are measured with an optical counter. This paper presents a large body of data taken in the laboratory to evaluate the performance of the instrument. The data indicates that the instrument is internally consistent over a wide range of operating conditions.

雲核測定用熱拡散型チャンバーの改良および製紙工場地域での測定

雲核測定用熱拡散型チャンバーの改良および製紙工場地域での測定

An Automatic Light Scattering CCN Counter

Abstract An automatic instrument for the measurement of cloud condensation nucleus (CCN) concentrations utilizing a thermal gradient diffusion chamber and light scattering has been developed. The concentration of droplets (activated CCN) in an illuminated volume is determined by the measurement of the peak light intensity scattered at 45°. The CCN concentration is linearly related to the scattered light signal at a fixed supersaturation S, but the sensitivity exhibits an S0.55 dependence over a range of supersaturations. Calibration of the system against the photographic method verifies the linear dependence of scattered light on number concentration. The main features of the system are the automatic sampling, measurement and recording of CCN droplet concentration by means of scattered light with the capability of direct calibration by means of the photographic method. Results from four days of continuous hourly measurements of the CCN spectra show the influence of meteorological events on CCN. Included i...

Atmospheric ice nuclei: No detectable effects from a coal‐fired powerplant plume

Atmospheric ice nuclei were measured upwind and within the effluent plume of a coal‐fired powerplant during February 1976. Aerosol particles were captured on two types of membrane filters (Nuclepore and Millipore) and processed in two different thermal diffusion chambers, one calibrated to produce a 100% saturation relative to water and the other to produce a slight supersaturation relative to water. Consequently, the ice nuclei measured were active in the modes that are dominant in diffusion chambers, viz., deposition nucleation and condensation‐followed‐by‐freezing nucleation. Results indicate that plume particles do not act as ice nuclei between the temperatures of −10 and −20°C, nor do combustion gases in the plume deactivate natural ice nuclei.

Numerical Simulation of a Widely Used Cloud Nucleus Counter

Abstract The performance of the conventional horizontal plate thermal diffusion chamber as a cloud nucleus counter was studied. Numerical calculations were performed in order to follow the simultaneous drop growth and sedimentation in the spatially and temporally nonuniform supersaturation field. The effect of nuclei distribution and smallest detectable drop size were investigated. The results indicate order of magnitude uncertainty in the count at 0.1% supersaturation, and a factor of 2 uncertainty at 1.0% supersaturation.

Measurement of Growth Rate to Determine Condensation Coefficients for Water Drops Grown on Natural Cloud Nuclei

Abstract Growth rate measurements were made for water drops grown on nuclei in atmospheric air samples taken in Rolla, Missouri. Rolla, having a population of 15,000 and very little industry, is relatively free of urban pollutants. The measurements were made in a vertical flow thermal diffusion chamber at supersaturations of 0.5 and 1%. The time to grow from near dry radius to the final radius (6 to 7.5 µm) was measured. If one assumes the thermal accommodation coefficient is unity, the measurements indicate an average value of 0.026 for the condensation coefficient. The temperature ranged from 22.5 to 25.7°C.

A Determination of the Condensation Coefficient of Water from the Growth Rate of Small Cloud Droplets

Abstract An experiment is described, in which the rate of growth of cloud droplets, in a thermal diffusion chamber, is measured accurately to yield the value of the condensation coefficient. The data obtained from measurements of 40 individual droplets grown at four different supersaturations (0.40,0.49,0.62 and 0.72%) suggests that the values of the condensation coefficient are 0.037, 0.033, 0.031 and 0.033, respectively; the average value is 0.033 with a standard deviation of 0.005. Probable sources of error are discussed and rejected.

Supersaturation Development in a Vertical-Flow Thermal Diffusion Chamber

Abstract Analytical expressions are derived for temperature and vapor pressure profiles in a vertical-plate, steady-flow, thermal diffusion chamber designed to count cloud condensation nuclei. The influence of longitudinal diffusion of heat and water vapor on the supersaturation rise time is shown to be quite significant at low sample velocities. The effect of different velocity profile shapes is also investigated.

Experimental Investigation of Transient Supersaturations in a Thermal Diffusion Chamber

Abstract The effect of transient supersaturations on the concentration of cloud condensation nuclei (CCN) in a conventional parallel-plate type thermal diffusion chamber is experimentally investigated by controlling the relative humidity of the sample and the temperature of the top and bottom plates individually. It is found that the error in CCN counts is quite significant around 90% relative humidity and might be dominant at cloud bases where the relative humidity approaches 100%. The error is a combined result of thermal and hydrodynamical transients. It is suggested that a chamber having a continuous and laminar inflow of the sample which has the steady-state temperature profile already existing in it is but suited for reliable CCN measurements. The calculations based on the theory of Saxena, Burford and Kassner help explain the obtained results which also support the earlier theoretical predictions of Fitzgerald. The results seem to disagree with recent experimental observations reported by Radke and...

A Cloud Nucleus Counter with Long Available Growth Time

A vertical plate, steady-flow, thermal diffusion chamber designed for use as a cloud condensation nucleus counter is described. In this instrument, phoretic forces are shown to limit the maximum available growth time to a value eight times larger than the growth time available in conventional horizontal plate chambers. This additional growth time is shown to be necessary when operating at supersaturations below 0.2%. Experiments and calculations concerning convection currents in the vertical chamber are also presented.

An Investigation of the Size-Supersaturation Relationship of Soluble Condensation Nuclei

The activation supersaturation of several artifically prepared NaCl aerosols was measured in the laboratory using a thermal gradient diffusion chamber. Size distributions of these aerosols and total particulate concentrations were determined with a Whitby Aerosol Analyzer and a Gardner Small Particle Detector. From a combination of these data, the relationship was derived between supersaturation and necessary minimum size of the particles for nucleation. This experimentally obtained correlation indicates that, at a given supersaturation, a NaCl particle, has to be two to three times larger than theory predicts in order to be active as a cloud condensation nucleus. Possible explanations for the discrepancy are discussed.