Complex index of refraction between 300 and 700 nm for Saharan aerosols

Abstract

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...

Columnar aerosol single‐scattering albedo and phase function retrieved from sky radiance over the ocean: Measurements of Saharan dust

“…This spectral behavior is consistent with ironbearing minerals such as Saharan dust [Sokolik and Toon, 1999]. However, the absorption is significantly less than models commonly computed from laboratory measurements of the complex refractive index of Saharan dust [Patterson, 1977].…”

“…These with range of models constructed from laboratory measurements of imaginary refractive index of Saharan aerosol samples. Upper limit of dust models represents a long-range mineral dust model constructed using refractive index of Patterson [1977], geometric mean radius (r g ) of 0.5, natural logarithm of geometric standard deviation (ln s g ) of 0.79 [Schütz, 1980], lower limit represents interpolated values of Carlson and Caverly [1977] and Carlson and Benjamin [1980]. Results from the work of d 'Almeida [1987] fall between these values.…”

Airborne Measurements of Optical Properties of Tropospheric Aerosols over an Urban Area

“…Many investigators have attempted to estimate this quantity, and found that the real and imaginary in dices of refraction were typically within the range of 1.45*1.55 and 0.005*0.02, respectively, for the tropospheric aerosols (e.g. Grams et al, 1974;Patterson et al, 1977;Reagan et al, 1980;Tanaka et al, 1983). The value of refractive index, m = 1.50-0.0li was assumed in this study.…”

Airborne observations of dust aerosol over the North Atlantic Ocean during ACE 2: Indications for heterogeneous ozone destruction

“…This instrument classifies particles in 31 size bins according to their forward scattering properties. Calculations from the measured scattering cross section to the particle size have been performed using a refractive index of 1.55-0.004i for Saharan dust particles [Patterson et al, 1977]. A detailed description of the FSSP-300 is given by Baumgardner et al [1992].…”