Here we compare the optical properties of porous particles of two kinds:
-
aggregates -- clouds of (cibic) subparticles represented
by 1 (or 8, 27, etc) dipole
(the subparticles are randomly distributed within a sphere);
-
homogeneous spheres with an effective refractive index.
The parameters are
the refractive index of the subparticles m,
the number of dipoles forming a subparticle N (at the moment N = 1),
the porosity (the volume fraction of the subparticles) P,
the difraction parameter x = 2 pi r / lambda ,
where lambda is the wavelength of incident radiation
and r the radius of the quasispherical aggregate
as well as of the corresponding homogeneous sphere
(its effective index is calculated from P and m
by using the effective medium theory, EMT).
To compute the properties of the aggregates and
of the homogeneous spheres the DDA method and the Mie theory with
the Bruggeman rule of the EMT were utilized, respectively
(see for more details the paper of Voshchinnikov et al., 2007).
Below we show the behaviour of the efficiency factors
Qext (x), Qsca (x), Qabs (x),
the asymmetry parameter g (x) and albedo (x) as well as
for x = 3 and 100 the phase function i (theta) and
polarization of scattered radiation p (theta)
for different values of the parameters m and P.
The refractive index m is chosen to be
1.200+0.000i (typical of biological particles in the visual part of the spectrum),
1.330+0.010i (dirty ice),
1.578+1.038i (center of the 10 micron silicate feature),
1.680+0.030i (silicate),
1.750+0.058i (soot),
1.980+0.230i (amorphous carbon).
The effective refractive indices of the homogeneous spheres
are indicated in Table.
The porosity is taken to be P = 0.33 and 0.9.
Effective refractive indices calculated using EMT (the Bruggeman rule)
P = 0 | P = 0.33 | P = 0.9 |
1.2000+0.0000i | 1.1328+0.0000i | 1.0193+0.0000i |
1.3300+0.0100i | 1.2183+0.0066i | 1.0310+0.0009i |
1.5780+1.0380i | 1.3764+0.6710i | 1.0762+0.0754i |
1.6800+0.0300i | 1.4471+0.0196i | 1.0588+0.0022i |
1.7500+0.5800i | 1.4916+0.3781i | 1.0707+0.03932 |
1.9800+0.2300i | 1.6431+0.1507i | 1.0795+0.0137i |
For given porosity, the particles of the same size parameter
xporous = xcompact/(1-P)1/3
(see for more details Voshchinnikov et al., 2007)
and hence of the same mass are compared in the following figures
organized in the tables.
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