PARTICLE COMPOSITION AND REFRACTIVE INDICES

Aerosol particles

Samples of natural small particles are often composed of a variety of different minerals. Although the refractive indices at visible wavelengths of these constituent minerals may be known, the refractive index for the mixture may not be easy to derive from these values. To determine the quantitative mineral composition or the complex refractive index usually a bulk sample is needed, and this is seldom available. For cases where the refractive index is not accurately known, we provide in the database a qualitative estimate of the mineral composition, and an estimate of the real part of the refractive index n based on values found in the literature for the constituent minerals. Less information is usually available for the imaginary part of the refractive index k, because the natural variability within a mineral can be quite large. However, for silicates values at visible wavelengths mostly are in the range 10-2 - 10-5. An indication of whether the value of k is relatively high or low is given by the color of the powdered sample, which is shown on photographs included in Facts and Figures parts of the database. 
 

Hydrosol particles

All values for the refractive index of hydrosol particles are given relative to water, and are valid for a wavelength of 632.8 nm in air. 

The real and imaginary part of the refractive index for phytoplankton particles may, for example, be estimated by using cell size and cell number data and absorption measurements, following the method of Morel and Bricaud (1986). This method is not rigorous. However, in the absence of more accurate data, such values provide a reasonable starting point. Also, literature data for estimates of the real part of the refractive index for the phytoplankton samples are available, but in only a limited amount of cases. However, because the chemical composition of the phytoplankton is more or less known, the bulk refractive index can be inferred. This has been done, for example, by Aas (1981), who assumed that phytoplankton consists of a mixture of 70 to > 80% water and < 20-30% other constituents, i.e., mainly organics. In general, assuming no internal structures, the bulk real part of the refractive index varies from n = 1.015 to 1.08. For phytoplankton species for which no experimental estimate is available, n = 1.04 seems to be a reasonable value (Morel and Bricaud, 1986). 

Because bulk values of the imaginary part of the refractive index differ considerably for distinct species, choosing one central value is in this case not a good option. However, the probable range of k can be estimated. Morel and Bricaud (1986) derived a typical value of k of 0.0025 for absorption due to Chl a by assuming a typical value of 1.5% for the ratio of the mass of Chl a to the dry mass of an algal cell. The presence of other pigments may increase this value. However, in general, the bulk value of k is small (< 0.01 or 0.02). In some cases, it may become negligible, and the cell can be regarded as transparent. If all else fails, an estimate of the imaginary part of the refractive index of phytoplankton samples may be obtained from fitting results of Mie calculations to measured elements of the scattering matrix.



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