Optical Properties of Aerosols
When light strikes an aerosol particle, one of two different processes can
occur. First, the light energy received by a particle can be re-radiated without
changing the wavelength. This process is called scattering. Second, the
light energy may be transformed inside the particles into another energy form
(e.g. heat) or re-radiated at a different wavelength. This process is called
absorption.
The sum of the two processes is called extinction or attenuation
(Qe). The proportion (fraction) of light energy received by a particle
that is scattered is called the scattering efficiency (Qs). The fraction
of light energy received by a particle that is absorbed is called the absorption
efficiency (Qa). Qe = Qs + Qa. When
the size of the particle (particle diameter, x) is very small compared to the
wavelength of light (l) striking it , the type of
scattering that occurs is referred to as Rayleigh scattering, or molecular
scattering. Rayleigh scattering is strongly wavelength dependent. When Rayleigh
scattering occurs, Qs is proportional to l-4
, and Qa is proportional to l. Likewise,
for a given wavelength l, Qs is proportional
to x4 and Qa is proportional to x. Particles that are
responsible for Rayleigh scattering, or optically small particles, are
generally , < 0.1 µm, and include air molecules.
If the diameter of the particle is comparable (similarly sized) to the wavelength
of light, the type of scattering that occurs is referred to as Mie scattering.
In Mie scattering, Qe is a more complex function of particle size.
Scattering of visible and UV light caused by aerosols and cloud droplets falls
into the realm of Mie scattering, whereas the scattering of visible and UV light
by a pure atmosphere of gas molecules falls into the realm of Rayleigh scattering.
Obviously, the scattering of light by Earth's atmosphere will consist of some
combination of Rayleigh and Mie scattering.
The direction of scattering is also determined by the optical size of the
particle. Forward scattering redistributes light intensity, whereas backward
scattering is equivalent to reflection. The fraction of the incident radiation
scattered forward after striking an aerosol is called the asymmetry factor.
The asymmetry factor = 1 if 100% of the incident radiation is scattered forward.
The asymmetry factor approaches 0 the more that incident radiation is reflected,
or backscattered. Particles in the Rayleigh range scatter incident light in
a symmetric (isotropic) way, resulting in an asymmetry factor of zero. Particles
in the Mie range tend to scatter more energy forward.
The single scatter albedo (w) is obtained by dividing
the scattering coefficient by the extinction coefficient; w
(r, l) = Qs(r, l)/
Qe(r, l). The higher the single scatter
albedo, the more important scattering processes are in causing light attenuation.
When the extinction coefficient is integrated from the surface of the Earth
to the upper limit of the atmosphere, the optical depth is obtained:
te = pr2Qe
(r, l)
where r = particle radius. The optical depth characterizes the entire attenuation
of solar radiation in a vertical air column due to the presence of aerosol particles.
Knowledge of four quantities as a function of wavelength is necessary to translate
aerosol burdens into first aerosol optical depths, and then a radiative perturbation:
(1) the mass light-scattering efficiency asp,
(2) the functional dependence of light-scattering on relative humidity f(RH),
(3) the single-scattering albedo w , and (4) the
asymmetry factor.