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Donald E. Hagen

Abstract

A numerical cloud model is presented which can describe the evolution of a cloud starting from moist aerosol-laden air through the diffusional growth regime. The model is designed for the direct support of cloud chamber laboratory experimentation, i.e., experiment preparation, real-time control and data analysis. In the model the thermodynamics is uncoupled from the droplet growth processes. Analytic solutions for the cloud droplet growth equations are developed which can be applied in most laboratory situations. The model is applied to a variety of representative experiments.

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Donald E. Hagen, James L. Kassner Jr., and Ronald C. Miller

Abstract

A piston-typo expansion cloud chamber, capable of producing short pulses of nucleation, was used to study the homogeneous nucleation of water over a wide range of temperature and supersaturation. A large effort was made to remove impurities capable of acting as heterogeneous nuclei from the system. The chamber was found to be capable of performing experiments that were relatively free of interfering impurities and the results resolved several anomalous nucleation phenomena appearing in the literature: the knee in drop concentration vs supersaturation data, the temperature dependence of the critical supersaturation, and the decay of the nucleation rate with time. The anomalies were found to be due to impurities active at different temperature and supersaturation regimes.

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Darryl J. Alofs, Donald E. Hagen, and Max B. Trueblood

Abstract

The relation between dry diameter (X 0) and critical supersaturation (Sc) for atmospheric submicron aerosol particles is investigated using a long term air sampling program at Rolla, Missouri. The particles are passed through an electrostatic aerosol size classifier, and then through an isothermal haze chamber. Results are reported in terms of an apparent volume fraction of soluble material, εv defined such that for particles composed only of ammonium sulfate and water insoluble compounds, εv is the actual volume fraction of soluble material. The probability distribution of εv is found to be approximately Gaussian in the εv range 0.2 to 1.3. The mean εv is 0.5, for electrostatic aerosol classifier settings of 0.2, 0.3, and 0.4 μm diameter.

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Donald E. Hagen, Josef Podzimek, and Max B. Trueblood

Abstract

During the FIRE IFO II project, aircraft were available for airborne sampling in and around cirrus cloud. Aerosols can play a role in the cloud formation process through the heterogeneous nucleation mechanism, and in turn, once formed, cirrus clouds can impact the ambient aerosol through scavenging and other collection mechanisms. University of Missouri aerosol sampling facilities were employed on these aircraft for in situ collection and characterization of the particulates near cirrus cloud level. Tandem differential mobility analyzer and impactor techniques were used to measure aerosol size distribution, hydration capability, and particle composition information. Evidence of aerosol layering was observed near the tropopause, and there was a tendency toward depletion of the ambient aerosol at both ends of the condensation nuclei (CN) size distribution. A large variability in the fine particle CN concentration was found, ranging from several tens to several thousands per cubic centimeter. The size distribution of particles larger than 0.5 micro;m roughly followed a lognormal relationship and large particle concentrations varied between 0.127 and 1.70 cm−3. The particulates were found to be of mixed character, primarily inert with a small percentage of soluble material. A large variability in particulate concentrations was found.

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Donald E. Hagen, Rodney J. Anderson, and James L. Kassner Jr.

Abstract

Experimental data on ice nucleation, presented in an earlier paper, are analyzed to yield information about the homogeneous nucleation rate of ice from supercooled liquid and the heights of energy barriers to that nucleation. The experiment consisted of using an expansion cloud chamber to nucleate from the vapor a cloud of supercooled pure water drops and the observation of the fraction of drops which subsequently froze. The analysis employed standard classical homogeneous nucleation theory. The data are used to extract the first experimental measurement (albeit indirect) of the activation energy for the transfer of a water molecule across the liquid-ice interface at temperatures near −40°C. The results provide further evidence that the local liquid structure becomes more icelike as the temperature is lowered.

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Rodney J. Anderson, Ronald C. Miller, James L. Kassner Jr., and Donald E. Hagen

Abstract

Observations of the homogeneous nucleation of water vapor in an expansion cloud chamber have been carried out for the temperature range −50 to +17°C in the carrier gases argon and helium. We have found that the onset of the ice phase in freshly nucleated drops always occurs in the form of a two-stage process, condensation followed by homogeneous freezing at temperatures near −40°C. Ice particles appear as brilliant spherical particles in the cloud of liquid drops which scatter much less light. The critical gas temperature associated with the observation of ice nucleation depends on the type of carrier gas, the duration of the minimum final temperature, and whether there are ions or re-evaporation nuclei present. These effects and the analysis of the total homogeneous nucleation rate (liquid drops plus ice particles) strongly support the conclusion that the ice particles result from the freezing of liquid water drops which have been nucleated homogeneously from the vapor phase. A somewhat higher critical freezing temperature is observed in the absence of an electric clearing field. This probably is an indication that ice particles preferentially form on ions or simply that droplets which nucleate slightly earlier on ions have a chance to grow to a larger size, thus increasing the droplets’ probability of freezing. An ice memory effect has also been observed in nucleation which occurs on re-evaporation nuclei remaining from previous expansions. lens and re-evaporation nuclei raise the threshold temperature of ice nucleation about 1 and 2°C, respectively, above the critical spontaneous freezing temperature (−41°C). Consequently, they would be expected to have little impact on atmospheric processes.

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Kenneth Sassen, David O'C. Starr, Gerald G. Mace, Michael R. Poellot, S.H. Melfi, Wynn L. Eberhard, James D. Spinhirne, E.W. Eloranta, Donald E. Hagen, and John Hallett

Abstract

In presenting an overview of the cirrus clouds comprehensively studied by ground-based and airborne sensors from Coffeyville, Kansas, during the 5–6 December 1992 Project FIRE IFO II case study period, evidence is provided that volcanic aerosols from the June 1991 Pinatubo eruptions may have significantly influenced the formation and maintenance of the cirrus. Following the local appearance of a spur of stratospheric volcanic debris from the subtropics, a series of jet streaks subsequently conditioned the troposphere through tropopause foldings with sulfur-based particles that became effective cloud-forming nuclei in cirrus clouds. Aerosol and ozone measurements suggest a complicated history of stratospheric-tropospheric exchanges embedded within the upper-level flow, and cirrus cloud formation was noted to occur locally at the boundaries of stratospheric aerosol-enriched layers that became humidified through diffusion, precipitation, or advective processes. Apparent cirrus cloud alterations include abnormally high ice crystal concentrations (up to ∼600 L−1), complex radial ice crystal types, and relatively large haze particles in cirrus uncinus cell heads at temperatures between −40° and −50°C. Implications for volcanic-cirrus cloud climate effects and usual (nonvolcanic aerosol) jet stream cirrus cloud formation are discussed.

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