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L. F. Radke and D. Hegg

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L. F. Radke and F. M. Turner

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The original automatic cloud condensation nucleus counter developed by Radke and Hobbs has been improved by reducing the size and weight of the instrument and increasing the sampling rate.

In comparison tests with four other counters, the improved automatic counter was found to give counts within ±50% of the mean of all five counters.

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F. M. Turner and L. F. Radke

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An airborne instrument which provides real-time measurements of the concentrations of ice particles in excess of a certain size has been developed. The instrument utilizes a beam of polarized light and the birefringent nature of ice to detect ice particles but to exclude the counting of water drops no matter how large. Theoretical considerations are discussed which show that the direct use of the birefringence property of ice provides a much larger signal than is possible when only the light scattered from particles is detected. The design, testing and field evaluation of the prototype instrument are discussed.

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L. F. Radke and P. V. Hobbs

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Simultaneous measurements have been made of the concentrations of cloud condensation nuclei, sodium-containing particles, Aitken nuclei, and the magnitude of the light scattering coefficient of the air, for a period of two months in the Olympic Mountains of Washington State.

Large short-term changes in the magnitudes of these four quantities were found to be related to variations in the local meteorological conditions. The most striking changes occurred with the build up and the evaporation of cumulus clouds upwind of the measuring site. The results indicate that growing clouds absorb (and also probably generate) large numbers of particulates, and that these particulates are released when the clouds dissipate. Precipitation also caused significant reductions in the concentrations of particulates in the air.

Longer period variations in particulate concentrations were associated with the diurnal convective cycle and changes in air mass. Continental air contained higher concentrations of cloud condensation nuclei and Aitken nuclei than maritime air, but the Pacific Ocean appeared to be the principal source of sodium-containing particles. However, even in maritime air the measured concentrations of sodium-containing particles were always less than about 1% of the concentrations of cloud condensation nuclei active at 1% supersaturation.

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L. F. Radke and P. V. Hobbs

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A cloud condensation nuclei counter has been developed in which the concentration of water droplets which form on cloud condensation nuclei in a large thermal diffusion chamber is determined electronically by measuring the light scattering coefficient of the cloud. The counter operates completely automatically and may be used to determine the concentration of cloud condensation nuclei in the air (active at a given supersaturation) at any desired interval of time down to a minimum of about 2 min. The counts obtained by this method are in good agreement with direct visual counting of droplets in the thermal diffusion chamber.

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Peter V. Hobbs, L. F. Radke, and S. E. Shumway

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Peter V. Hobbs, Jeffrey L. Stith, and Lawrence F. Radke

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The concentrations of cloud condensation nuclei (CCN) in the plumes from coal-fired electric power plants are generally about 2 to 5 times greater than in the ambient air unaffected by the plumes. However, if the ambient air is very clean, the concentrations of CCN in a coal power plant plume can be up to ∼80 times greater than in the ambient air. The rates of production of CCN due to gas-to-particle (g-to-p) conversion in the plume from one of the plants studied were measured on different occasions to be ∼2 × 1015 and ∼5 × 1013 CCN h−1 per mole of SO2. The maximum current of CCN to be expected in the plume from a coal power plant is ∼1017 CCN s−1. After a travel time of ∼1 h, most of the CCN in power plant plumes have been produced by g-to-p conversion rather than emitted directly from the stack.

The concentrations of ice nuclei in the plumes did not differ significantly from those in the ambient air.

The materials in a plume may be transported rapidly in the vertical if the plume is entrained into a convective cloud. The plume may cause a lowering in the altitude of the cloud base, but any effects that the plume may have on the drop size distribution in a convective cloud are often less than the natural variations. By contrast, in stratiform clouds a plume can produce marked increases in the concentration of small drops (∼10–20 μm diameter) and in the total concentrations of drops

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Peter V. Hobbs, L. F. Radke, and S. E. Shumway

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Measurements of the concentrations of cloud condensation nuclei (CCN) in the air in Washington State have shown that pulp and paper mills, and certain other industries, are prolific sources of CCN. The rate of production of CCN from large paper mills can be as high as 1019 sec−1. Direct observations show that clouds often form downwind of these mills and, in some cases, these clouds produce precipitable particles very efficiently.

A comparison of precipitation and streamflow records in Washington for the period 1929–46 with those for 1947–66 shows that a number of areas have had a mean annual precipitation during the second period more than 30% greater than that in the first period. In nearly all cases these areas are in the vicinity of large industrial sources of CCN and it is suggested that the higher precipitation in recent years is a consequence of the increased numbers of very efficient CCN emitted into the atmosphere by these sources.

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F. G. Meyer, J. A. Curry, C. A. Brock, and L. F. Radke

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Since the Ptarmigan flights in the 1950s, the springtime visibility reduction in the Arctic has been identified with pollution aerosol. However, observed values of the dry aerosol extinction coefficient are too small to explain the observed visibility reductions. Water uptake by the aerosol appears to be an important component of the Arctic turbidity. Furthermore, the presence of lower-tropospheric ice crystals may have a dominant effect on the visibility in the Arctic. Data obtained from a series of meteorological, chemical, and cloud microphysical measurements made by the University of Washington research aircraft during April 1983 and 1986 are used as input for a visibility model. The model calculates the water uptake by the dry aerosols as a function of relative humidity and determines the single-scattering properties of the aerosols and the ice crystals. Path radiances are calculated using an extension to the delta-Eddington approximation introduced by Hering. Modeled visual ranges indicate that aerosols alone cannot account for the low visibilities. In certain cases, the inclusion of ice crystals along with the aerosols can produce visual ranges which are as low as those observed. A comparison between visual ranges obtained using the model and estimated using Koschmieder's equation showed that Koschmieder's equation will generally underestimate the visual range.

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J. E. Penner, R. J. Charlson, J. M. Hales, N. S. Laulainen, R. Leifer, T. Novakov, J. Ogren, L. F. Radke, S. E. Schwartz, and L. Travis

Anthropogenic aerosols are composed of a variety of aerosol types and components including water-soluble inorganic species (e.g., sulfate, nitrate, ammonium), condensed organic species, elemental or black carbon, and mineral dust. Previous estimates of the clear sky forcing by anthropogenic sulfate aerosols and by organic biomass-burning aerosols indicate that this forcing is of sufficient magnitude to mask the effects of anthropogenic greenhouse gases over large regions. Here, the uncertainty in the forcing by these aerosol types is estimated. The clear sky forcing by other anthropogenic aerosol components cannot be estimated with confidence, although the forcing by these aerosol types appears to be smaller than that by sulfate and biomass-burning aerosols.

The cloudy sky forcing by anthropogenic aerosols, wherein aerosol cloud condensation nuclei concentrations are increased, thereby increasing cloud droplet concentrations and cloud albedo and possibly influencing cloud persistence, may also be significant. In contrast to the situation with the clear sky forcing, estimates of the cloudy sky forcing by anthropogenic aerosols are little more than guesses, and it is not possible to quantify the uncertainty of the estimates.

In view of present concerns over greenhouse gas-induced climate change, this situation dictates the need to quantify the forcing by anthropogenic aerosols and to define and minimize uncertainties in the calculated forcings. In this article, a research strategy for improving the estimates of the clear sky forcing is defined. The strategy encompasses five major, and necessarily coordinated, activities: surface-based observations of aerosol chemical and physical properties and their influence on the radiation field; aircraft-based observations of the same properties; process studies to refine model treatments; satellite observations of aerosol abundance and size distribution; and modeling studies to demonstrate consistency between the observations, to provide guidance for determination of the most important parameters, and to allow extension of the limited set of observations to the global scale. Such a strategy, if aggressively implemented, should allow these effects to be incorporated into climate models in the next several years. A similar strategy for defining the magnitude of the cloudy sky forcing should also be possible, but the less firm understanding of this forcing suggests that research of a more exploratory nature be carried out before undertaking a research strategy of the magnitude recommended for the clear sky forcing.

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