Scattering of electromagnetic waves by composite spherical particles: experiment and effective medium approximations

Abstract

We have measured the differential scattering cross sections (phase functions I(22)) and the normalized extinction and scattering cross sections (efficiences) of composite spherical particles. The size parameter x = 2pir/lambda was around 2pi. Composite spheres consisted of nonabsorbing matrix containing a small amount (1.6 and 2.7% by volume) of highly absorbing inclusions. Such composite particles may represent a realistic model of fog or cloud droplets containing small amounts of carbon or a composite atmospheric aerosol particle. We have compared measured data with those calculated using seven different effective medium approximations. We have found that the approximations of Bruggeman and Maxwell Garnett, the generalization of dynamic effective medium approximation derived by Chylek and Srivastava, and the experimental waveguide method of determination of the effective refractive index lead to an acceptable agreement between calculated and measured values. The reduced x(2) values for these approximations ranged between 0.6 and 2.0. The remaining three approximations (volume averages of refractive indices or dielectric constants and the Maxwell Garnett relation with matrix and inclusion materials interchanged) lead to reduced x(2) values between 4.0 and 12.0 demonstrating characteristics.

Characterisation of episodic aerosol types over the Australian continent

“…For the case of internal mixing, the effective refractive index of the aggregate was calculated from the three components based on the Bruggeman mixing rule (Chýlek et al, 1988). The refractive index of hematite was taken from Sokolik and Toon (1999), and that of quartz was from Krekov (1992); both were listed in Table 1 of Koven and Fung (2006).…”

Modelling the optical properties of fresh biomass burning aerosol produced in a smoke chamber: results from the EFEU campaign

“…This allows us to derive the real part of the OC refractive index as well as the imaginary part of the BC fraction using the EC a mass fraction (thermographic method) as proxy for the BC volume fraction. Thereby, the effective refractive index of the internal OC-BC mixture was determined with the mixing rule of Maxwell-Garnett (MG) (Maxwell Garnett, 1904), which is preferable to simple volume mixing for particles composed of non or slightly absorbing matter containing a small amount of spherical, stronglyabsorbing inclusions (Chýlek et al, 1988). Alternatively, stratified spheres with particles composed of a BC core and a shell of organic carbon assuming a constant ratio of core diameter to particle diameter were used.…”

In situ and remote-sensing measurements of the mean microphysical and optical properties of industrial pollution aerosol during ADRIEX

“…The volumemixing and the Maxwell-Garnett mixing methods (e.g. Chylek et al, 1988) were both used to calculate the effective 'dry' and 'wet' (at RH = 60% for the Adriatic aerosol) refractive indices by the internal mixing of scattering and absorbing components; the methods were shown to produce very similar results. The effective complex refractive index for the Po Valley sub-micron pollution aerosol at 0.55 µm was m wet = 1.445-0.0084i, m dry = 1.492-0.0088i.…”