Both the imaginary part (n•s•) and the real part (n•) of the complex index of refraction for Saharan aerosols have been determined as a function of wavelength between 300 and 700 nm: n• was determined by means of an immersion oil technique, and n•s• was determined from measurements of the total diffuse reflectance of the aerosol by means of an analysis using the Kubelka-Munk theory. No significant differences in the optical properties of the aerosol were seen among the samples collected at sampling sites on both sides of the Atlantic. Comparison of our results with others based on measurements of the ratio of the direct to the diffuse radiation shows that the two independent methods give remarkably similar results. Concern has been expressed in recent yearsabout the possible climatic effects of aerosols [Bryson, 1968; SCEP, 1970; SMIC, 1971 ]. In order to assess these possible effects a number of radiation models have been developed which describe radiative energy exchange in the earth-atmosphere system and the possible effects of aerosols on this exchange [Atwater, 1970; Ensor et al., 197 l; Shettle and Green, 1974; Chylek and Coakley, 1974]. There have also been a number of experimental studies which have attempted to relate radiation measurements to simultaneous measurements of the aerosol properties, for example, the Complex Atmospheric Energetics Experiment (Caenex) [Kondratyev et al., 1974] and the Global Atmospheric Aerosol and Radiation Study (Gaars) [De Luisi et al., 1976]. According to Kondratyev et al. [ 1976] the study of the Saharan aerosol layer is of great interest from the point of view of the 'climate and aerosol' problem. Accordingly, there was also an extensive program of measurements during Gate (the Garp Atlantic Tropical Experiment) in 1974 on' the radiative properties and effects of the Saharan aerosol layer. The Saharan aerosol layer consists of crust-derived aerosols which are generated by erosion processes in northwest Africa and are then transported by the global circulation westward across the Atlantic. Mass concentrations for this aerosol are high, as are atmospheric turbidities associated with the layer. In addition, measurements over the Atlantic far from the source of the aerosols enable the study of a reasonably well mixed aerosol layer. The measurements during Gate sought to provide the information needed to parameterize the effects of the layer. These measurements included radiation measurements from aircraft above and below the layer [Kondratyev et al., 1976; Ellingson et al., 1975], surface measurements of direct and total solar radiation [Carlson and Caverly, 1977], aerosol phase function measurements [Grams et al., 1976], and aerosol physical properties measurements [e.g., Savoie and Prospero, 1976a]. In addition, Rahn et al. [1976] have reported measurements of the elemental composition of Saharan aerosols during 1973. The radiation measurements have shown that the Saharan aerosol layer can have a significant effect on the radiation balance; so accurate knowledge of the par...

A comparison of several measurements of the size distribution for tropospheric aerosols in which soil‐derived aerosols are significant shows that these aerosol size distributions appear to be characterized by a common mode structure, with the optically important soil particles having radii between 0.5 and 10.0 μm. These measurements suggest that under conditions of low dust loading a reasonable characterization of the size distribution for soil‐derived aerosols is a log normal distribution with a surface mean radius of 1.5 μm and a geometric standard deviation of 2.2.

We have measured crustal aerosol absorption, expressed as a wavelength dependent imaginary index of refraction (nIM) at both visible and infrared wavelengths. Measurements at visible wavelengths show values of nIM that range from ∼0.02 at 300 nm to ∼0.004 to 700 nm. Infrared measurements show an absorption that is dominated by a silicate absorption with a maximum value of nIM ∼ 1 near 10 μm. Comparisons of our measurements with other reported measurements of well‐documented crustal aerosols show a great deal of similarity among the measured values. We have shown that these measured optical properties, and the similarities and differences in the measured values, can be understood in terms of mineralogical characteristics of the aerosol and fractionation processes that occur in the generation and transport of these aerosols.

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We have measured the Kubelka-Munk scattering and absorption coefficients for a barium sulfate white reflectance standard. These measurements have been based on measurements of the absolute reflectance for the particular barium sulfate samples whose scattering and absorption coefficients were measured. This method gives results that are different from earlier measurements; the differences are significant for measurements of the optical properties of atmospheric aerosols.