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W. A. Hoppel

Abstract

Data on the submicron aerosol size distribution, the supersaturation spectrum, and the relationship between dry size and critical supersaturation taken during a cruise of the USNS Hayes from Norfolk, Virginia, to Piraeus, Greece, are presented. Measured values of the critical supersaturation required to nucleate natural aerosols of known size are given and used to convert the size distribution to the supersaturation spectrum. The supersaturation spectrum derived in this manner shows good agreement with the supersaturation spectrum measured directly. The observed decrease in particles smaller than 0.06 µm in an air mass which moved 100 mi out to sea could be explained by coagulation and mixing with marine air. The absence of particles below about 0.02 µm on most occasions in marine air was consistent with the finding that the number of CCN active at 0.8% was nearly equal to the total CN count.

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W. A. Hoppel
and
T. A. Wojciechowski

Abstract

Limitations on the accuracy of CCN counts obtained with thermal gradient diffusion cloud chambers (TGDCC) are discussed. Evidence presented here regarding 1) the absence of a well-defined plateau when the number of CCN in the TGDCC are plotted as a function of time, 2) the discrepancy between CCN counts obtained by instantaneous photography and the time-exposure method, and 3) the difficulty in reaching a sufficiently small minimum detectable size throughout the range of supersaturations, points to the possibility of considerable error in instantaneous CCN counts. The time-exposure method is demonstrated as a means of eliminating the problems associated with the instantaneous method and of extending the measurements to lower supersaturations.

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W. A. Hoppel
and
J. H. Kraakevik

Abstract

Measurements of both polar conductivities, the densities and mobility distributions of small ions, electric field intensity, and other related atmospheric variables were made from an airplane from 30 m to 5.6 km. A Blackwood type mobility chamber and a Gerdien condenser were used simultaneously to determine the mobility distribution and the conductivity. The mobility instrumentation and the technique for obtaining the ion spectrum from the measurements are discussed. The results of five flights 100 miles cast of the Virginia coast and one over Lake Ontario are presented.

The mobilities of ions in the free atmosphere lie between 0.3 and 5 cm sec−1 per v cm−1 at STP with a peak near the low mobility end and a broadening toward the high end. The average mobility reduced to STP is about 1.2 cm sect −1 and the negative-to-positive ratio is nearly 1.1, both with no significant dependence upon altitude from 3.0 to 5.2 km. The mobility is smaller and the ratio closer to unity than values given for artificially produced ions in filtered air. The positive and negative ion densities are nearly equal, with the conductivity ratio about equal to the mobility ratio. Conductivities and ion densities computed from the mobility spectrum are compared with direct measurements.

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W. A. Hoppel
and
B. B. Phillips

Abstract

The system of differential equations which governs the equilibrium charge distribution around a charged cloud located in an atmosphere with constant ionization is solved numerically for clouds with differing water content. The equations include the effects of ionic recombination, and the ionic conduction currents to cloud droplets. The results show that the electric field external to the cloud is greatly reduced by a shielding charge distribution which first develops at the clear air-cloud boundary and then slowly moves inward toward the charge center. If ions are generated locally within the cloud, they produce an inner region of charged droplets of opposite polarity to the outer screening layer. An increased electric field maximum will exist between the regions of charge. The effects of turbulence and electroconvection within boundary layers and the time-dependence of the shielding layer charging with and without turbulent transport are discussed. The role of the shielding charge in determining the electric field recovery time following lightning discharges is specified and the ionic concentration and electrical conductivity within thunderstorms is considered.

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W. A. Hoppel
and
B. B. Phillips

Abstract

No abstract available.

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W. A. Hoppel
and
J. E. Dinger

Abstract

No abstract available.

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G. M. Frick
and
W. A. Hoppel

Abstract

High-resolution aerosol, trace gas, and cloud microphysical measurements were made from an airship during transects across ships exhaust plumes advecting downwind of ships in the marine boundary layer (MBL). This study was part of the Office of Naval Research Monterey Area Ship Track experiment designed to understand the mechanisms by which ships produce cloud tracks visible in satellite imagery. Measurements made below clouds and close to the ships are used to define the concentrations and source strength of effluents, and the size distribution of ship-generated aerosols. Measurements made during crossings inside the cloud indicate that ship-generated aerosol increases the number and decreases the radii of cloud droplets. Case studies of four ships are presented, two of which produced cloud tracks and two that did not. Of the two that did not produce cloud tracks, one did not produce a cloud track because of unfavorable background aerosol loading; the other, because the ship-produced particles were too small relative to the background aerosol. A simple cloud microphysical model that assumes the MBL dynamics remains the same inside and outside the plume is used to study differences in background and plume cloud formation and reveals the intricate relationship among the size and number of background aerosols; MBL dynamics (as it effects cloud supersaturation); and the concentration, size, and composition of ship-generated particles.

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G. M. Frick
and
W. A. Hoppel

The use of an airship as a platform to conduct atmospheric chemistry, aerosol, and cloud microphysical research is described, and results from demonstration flights made off the Oregon coast are presented. The slow speed of the airship makes it an ideal platform to do high-spatial resolution profiling both vertically and horizontally, and to measure large aerosol and cloud droplet distributions without the difficulties caused by high-speed aircraft sampling. A unique set of data obtained during the demonstration flights show the effect that processing marine boundary layer aerosol through stratus clouds has on the aerosol size distribution. Evidence of new particle formation (nucleation of particles) was also observed on about half the days on which flights were made.

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W. A. Hoppel
and
S. G. Gathman

Abstract

The method of determining the eddy diffusivity over the open ocean either from profiles of the average electric filed or from average values of space charge and electric field measured near the sea surface is discussed. Recent measurements of atmospheric electric variables and meteorological parameters off the east coast of Barbados make it possible to compare values of diffusivity calculated by the atmospheric electric method with values obtained from the bulk aerodynamic method. The results of this comparison show that the atmospheric electric method holds considerable promise under conditions set forth in this paper. The relationship between fluctuations in the electrical variables and fluctuations in the air velocity within the electrical boundary layer is discussed.

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James W. Fitzgerald
,
William A. Hoppel
, and
Michael A. Vietti

Abstract

The relative humidity dependence of the size and scattering coefficient of atmospheric aerosol particles was measured at Washington, DC during the period 26–31 July 1979. Particle growth curves (i.e., curves of the ratio r/r 0, of particle radius to particle dry radius, versus relative humidity) were calculated using measured values of the particle composition parameter B ≡ ν¯ϕεM wρ0ws, where ρ0 is the density of the particle in dry state, ρ w and Mw the density and molecular weight of water, ε the mass fraction of soluble material in the particle. s the molecular weight of the soluble material, ν¯ the mean number of moles of ions per mole of solute, and ϕ¯ the mean value of the practical osmotic coefficient. To determine B, a mobility analyzer was used to transmit dry particles of nearly uniform size to a thermal gradient diffusion cloud chamber where the supersaturation (Sc ,) necessary to activate the particles was measured. Then, a relationship between Sc , r 0 and B was employed to determine B. The measurements were made for particles in the size range between 0.03 and 0.08 μm radius. The value of B ranged from 0.10 to 0.40 and averaged 0.23 for the period. For B = 0.23 and r 0 = 0.05 μm, the computed values of r/r 0 are 1.22, 1.37, 1.57 and 1.90 at relative humidities of 80, 90, 95 and 98%, respectively.

The light-scattering coefficient as a function of (decreasing) relative humidity was measured directly by controlling the relative humidity of the air flowing to an integrating nephelometer. A smooth variation of scattering coefficient with humidity was observed. On the average, the scattering coefficient changed by a factor of 3.5 between 30 and 95% relative humidity. In general, the measured dependence of the scattering coefficient on relative humidity was consistent with that computed from the particle growth curves.

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