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.

The absorption properties, expressed as a wavelength-dependent imaginary index of refraction, of the Mount St. Helens ash from the 18 May 1980 eruption were measured between 300 and 700 nanometers by diffuse reflectance techniques. The measurements were made for both surface and stratospheric samples. The stratospheric samples show imaginary index values that decrease from approximately 0.01 to 0.02 at 300 nanometers to about 0.0015 at 700 nanometers. The surface samples show less wavelength variation in imaginary refractive index over this spectral range.

We have measured visible wavelength optical properties of the ash from the 1982 El Chichón eruptions. These measurements were made on ash samples collected at three surface sites at distances between 12 and 80 km from the volcano. The most distant sample is taken as most representative of the silicate ash injected into the stratosphere. The measured optical properties are expressed as a complex refractive index n, with the aerosol absorption expressed as the imaginary component of the refractive index, nIM. Each of these samples showed quite low values of absorption, with nIM at 500 nm ranging from 1.5×10−3 for the 12 km sample to 1.0×10−3 for the 80 km sample. Based on these measurements, we estimate that n for the stratospheric silicate ash is given by n = 1.53 ‐ 0.001i.

The phase I Gametag (Global Atmospheric Measurements Experiment of Tropospheric Aerosols and Gases) aerosol measurements were designed to provide an initial assessment of the levels, types, and optical effects of tropospheric aerosols in remote marine and continental regions and to examine the possible causal relationships between the observed distributions and the dominant factors controlling aerosol population: chemical and physical transformations, source and sink strengths, and transport. We used size‐number data to determine mass concentrations and to estimate extinction, using nominal optical properties. Filter and impactor data have been used to determine aerosol composition, and correlative aircraft measurements have been used to aid in our data interpretation. Our data have been used to generate latitudinal profiles along our Pacific flight tracks. Our continental measurements, in general, show bimodal aerosol size distributions that reflect different source for each mode. The aerosol population consists primarily of crustal aerosols with r ≥ 0.5 μm and sulfate and combustion aerosols with r < 0.5 μm, with only a minor sea salt component. Owing to vertical mixing, there are no qualitative differences between the boundary layer and the free troposphere. Our data indicate that crustal aerosols represent a significant component of a background tropospheric aerosol in western North America and suggest that the possible contribution of the crustal aerosol to extinction should not be ignored. Pacific marine measurements show a qualitative difference between the boundary layer and the free troposphere. The boundary‐layer aerosol population is dominated by a bimodal sea spray aerosol; optical effects and mass concentration are dominated by a mode with a volume mean radius of ∼1 μm. Our measurements show only a small crustal component of the marine boundary‐layer aerosol. Our data indicate a loss of Cl from the sea spray aerosol, with the greatest loss in the small particles. We have inferred a background concentration of 0.2 ppbm for our measured particles that does not appear to be directly related to the sea spray aerosol. We have identified some of these particles as locally produced secondary aerosols; simultaneous measurements of gaseous species support this interpretation. Our Pacific free tropospheric aerosol measurements show a highly variable aerosol component, with local variations in concentration by 1 order of magnitude within a few kilometers. Our measured total aerosol and crustal component concentrations show a general decrease from north to south. Our lowest mean mid tropospheric concentration was seen south of 20°S; we have identified this mean concentration of 0.08 ppbm as a midtropospheric background aerosol.