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B. A. Bodhaine and R. F. Pueschel

Abstract

Solar radiation transmission data taken at Mauna Loa exhibit a seasonal variation with the minimum in summer. On the basis of Barrett's model for the depletion of solar radiation by aerosols, it is suggested that these variations are due to the seasonal generation of organic aerosols by the biosphere. It is suggested that the naturally produced atmospheric background aerosol of organic origin causes the typical seasonal turbidity variations. Furthermore, changes in the amplitude or phase of transmission data could be used to indicate whether aerosols from anthropogenic sources would influence the earth's albedo.

Precipitable water calculations suggest that humidity data above Mauna Loa are not accurate enough to make a quantitative estimate of the effect of atmospheric water vapor on Mauna Loa radiation data. However, water vapor apparently cannot account for these variations on the basis of phase angle considerations.

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R. F. Pueschel, B. A. Bodhaine, and B. G. Mendonca

Abstract

Nephelometry, in conjunction with a tube furnace and an Aitken nuclei counter, has been applied to the investigation of the volatile component of the aerosol budget at Cape Kumukahi, Hilo. and Mauna Loa Observatory, Hawaii. It was found that heating of the incoming air sample resulted in a decrease in light scattering above 100C due to the loss of organics and other easily volatilized compounds, and a drastic increase in Aitken nuclei counts at temperatures above 150C in the presence of ammonium sulfate. In the marine aerosol, a decrease in light mattering at about 45C was observed which is probably due to the loss of moisture during the phase transition from droplet to crystal. A second decrease near 120C is probably caused by the volatilization of organics from the droplet aerosol. In heating to 150C, the amount of light-scattering decrease was found to depend on the air mass. On occasions when volcanic effluent was apparently present, an increase in Aitken nuclei was noted in the heated air. Total aerosol mass deduced from light-scattering measurements before heating was in good agreement with aerosol mass measurements determined by standard high-volume filter sampling techniques. The total amount of aerosols in the air mass above the trade inversion is comparable to the quantity found in the marine air man. After penetrating the trade inversion, however, the cation content of the air is significantly reduced and the aerosol volatility is increased.

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R. J. Charlson, T. Silver, A. D. Clarke, and B. A. Bodhaine

Abstract

No abstract available.

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Barry A. Bodhaine, Norman B. Wood, Ellsworth G. Dutton, and James R. Slusser

Abstract

Many different techniques are used for the calculation of Rayleigh optical depth in the atmosphere. In some cases differences among these techniques can be important, especially in the UV region of the spectrum and under clean atmospheric conditions. The authors recommend that the calculation of Rayleigh optical depth be approached by going back to the first principles of Rayleigh scattering theory rather than the variety of curve-fitting techniques currently in use. A survey of the literature was conducted in order to determine the latest values of the physical constants necessary and to review the methods available for the calculation of Rayleigh optical depth. The recommended approach requires the accurate calculation of the refractive index of air based on the latest published measurements. Calculations estimating Rayleigh optical depth should be done as accurately as possible because the inaccuracies that arise can equal or even exceed other quantities being estimated, such as aerosol optical depth, particularly in the UV region of the spectrum. All of the calculations are simple enough to be done easily in a spreadsheet.

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