Illuminance turbidity parameters and atmospheric extinction in the visible spectrum

Expressions for the Linke, T Li , and Unsworth-Monteith, T Ui , turbidity parameters in the visible region of the solar spectrum are derived for application in illumination studies. They are based on new determinations of the visible transmittances of various atmospheric constituents, obtained by spectrally integrating the atmospheric transmittances calculated with a spectral irradiance research code. The influence of pressure, of circumsolar radiation, and of different atmospheric constituents (including both stratospheric and tropospheric nitrogen dioxide) on the interpretation of turbidity is discussed. T ui is shown to be virtually free of parasitic variations with air mass or other factors, and to approximate the aerosol optical thickness at 555 nm quite closely. The experimental T Ui has been obtained from measurements of direct illuminance for two days at Albany, New York, and compared to the aerosol optical thickness measured with a multiwavelength radiometer. The observed fair agreement between T Ui and the experimental optical thickness at 555 nm suggests that the latter may be accurately approximated by the former, using equipment that can be easily calibrated in the laboratory-unlike sun photometers. The use of T Ui in subsequent measurement campaigns is recommended rather than that of T Li .

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