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  • Author or Editor: T. A. Wojciechowski x
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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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S. TWOMEY
and
T. A. WOJCIECHOWSKI

Abstract

Airborne measurements at flight level of cloud nuclei have been carried out during approximately 100,000mi of flight over wide areas of the world. A thermal diffusion chamber was used to obtain cloud-nucleispectra which present the concentration of active nuclei as a function of supersaturation over the rangeof supersaturation of interest in cloud formation. The results confirm previous conclusions that continentalair masses are systematically richer in cloud nuclei and indicate that the median of cloud nuclei over theopen ocean is reasonably predictable and varied little from region to region. Over continents the medianspectrum does not seem to vary greatly even when North America was compared to Africa and Australia;however, the variability about the median is much greater over land when compared to ocean areas. Theresults indicate that the lifetime of cloud nuclei, at least over the oceans, is about 3 days. The results requirea widespread and relatively uniform source of cloud nuclei both over the oceans and over the land, and itis indicated that the sources of nuclei are not largely dependent on industrial or other man-made pollution.

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H. E. Gerber
,
W. A. Hoppel
, and
T. A. Wojciechowski

Abstract

Polydisperse NACl and (NH4)2SO4 salt particles were passed through a Goetz centrifuge, which transmitted only those particles smaller than the cutoff size of the centrifuge. In one set of experiments the sample from the centrifuge was passed directly into the thermal gradient cloud chamber and the threshold supersaturation for activating nuclei was recorded. In the second set of experiments the output sample from the centrifuge was passed through a mobility analyzer which transmitted a nearly monodisperse aerosol of a known size to the thermal-gradient cloud chamber. In the first experiment the threshold supersaturation agreed well with the critical supersaturation predicted by theory for particles of radii equal to the cutoff size of the Goetz centrifuge. Likewise, the critical supersaturation found for the nearly monodisperse aerosols transmitted through the mobility analyzer agreed well with that predicted by theory.

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James G. Hudson
,
W. A. Hoppel
, and
T. A. Wojciechowski

Abstract

A number of careful laboratory comparisons were made between two very different CCN counters. The two instruments gave consistent results which were within about 20% on the average.

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S. Twomey
,
H. B. Howell
, and
T. A. Wojciechowski

Abstract

No abstract available.

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J. E. Dinger
,
H. B. Howell
, and
T. A. Wojciechowski

Abstract

Measurements of the concentration of cloud nuclei, which are activated at a supersaturation of 0.75%, were made aboard an aircraft at various altitudes in a subsident air mass over the North Atlantic Ocean and on the cast coast of Barbados, West Indies. The measurements were made on air samples at normal temperatures as well as on air samples heated to various temperatures up to 600C. In this way the volatility of the cloud nuclei was compared to the volatility as measured in a similar manner in the laboratory on nuclei artificially generated and of a known composition.

The measurements at Barbados showed that ∼50% of the cloud nuclei were nonvolatile at the temperatures used and thus were similar to artificially generated nuclei composed of sea salt; the remaining nuclei were destroyed by temperatures ≳320C. The aircraft measurements showed the fraction of volatile cloud nuclei to increase with altitude with all nuclei being volatile above the inversion layer.

These measurements indicate that in a subsident marine atmosphere only a fraction of the cloud nuclei at the sea surface are composed of sea salt, this fraction decreasing with altitude such that the sea salt nuclei are confined to the lower few kilometers. Based on the work of other investigators it is suggested that the volatile cloud nuclei are sulfates or sulfuric acid particles which result from the oxidation in the atmosphere of S02 or H2S.

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