Introduction Aerosol scattering is responsible for many
atmospheric events - sunsets - halos around the sun or moon -
rainbows - white (extensive scattering from the surface) and black
(complete scattering where light cannot penetrate) clouds -
visibility degradation from pollution Aerosol light scattering is
also a powerful method used by instruments that measure aerosol
size and concentration - these instruments are sensitive and do not
manipulate particles 3
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Light Scattering Regimes D p < 0.05 m described by molecular
scatteringaka Rayleigh Scattering D p > 100 m described by
geometric optics (diffracted, reflected, refracted rays) 0.05 m
< D p < 100 m D p on the order of , described by Mie Theory
NOTE: All scattering can be derived via Mie Theory, developed by
Gustav Mie in 1908 using Maxwells theory of Electromagnetic
Radiation. Limiting cases such as D p > allow for
simplifications to be made. 4
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Definitions c = speed of light = 3x10 10 cm/s = f* For visible
light, = 400-700 nm m = refractive index relates the change in
velocity that light experiences upon going from one medium to
another (a material related property) m = c/v p = speed of light in
a vaccum/speed of light in a material, p 5
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Index of Refraction 6
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7 scattering absorption Scattering portion measured with Snells
Law:
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Index of Refraction 8 Absorption often measured via
spectrophotemtry Bulk absorption For electrically conductive
material For most aerosol particles
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Relative Index of Refraction (m r ) 9 Used to describe a two
phase system, i.e. a particle in air For air For vacuum For aerosol
particles in air
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Intensity of Light 10 detector Incident light scattered light
Light arriving at a surface:
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Intensity of Light 11 Light from a point source: A
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Size Parameter () 12 - Added to simplify light scattering
equations - Makes = ratio of particle size to wavelength of
radiation For d p on the order of m
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Electromagnetic Theory 13 Light possesses wave/particle duality
we will treat it as the electric wave component of EM radiation
Light can be: 1) unpolarized (sunlight) 2) parallel polarized 3)
perpendicular polarized
Extinction 15 Definition: the attenuation of light along an
axis resulting from scattering and/or absorption Particles
Extinction is dependent upon the chemical composition of particles
as well as particle size, shape, orientation and number. Light
Extinction is also dependent upon the polarization and frequency of
the incident beam.
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Extinction 16 Mathematically, how do we quantify the results of
extinction? I0I0 I Lambert-Beer Law
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Extinction 17 Lambert-Beer Law For a monodisperse aerosol:
Extinction coefficient (L -1 ) Particle area Extinction efficiency
# concentration Represents fractional loss in intensity per unit
length
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Extinction 18 Extinction Efficiency Represents the relative
ability of a particle to remove light from a beam compared with
blocking or interception by the projected area of the particle Does
not have to approach 1 in fact: For polydisperse aerosols:
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Example Problem 19 If: What is: a) Number concentration in #/m
3 b) Mass concentration in g/m 3 ? Lambert-Beer Law
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Example Problem 20
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Extinction 21 Recall: Therefore, there is no single equation to
calculate for all d p
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Extinction 22 For d p > 4 m Extinction Paradox
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Extinction Paradox 23 Based on the condition that extinction
must be observed at long relative distances For coffee cup 100 km
(rarely observed condition) d obs >>
Scattering 26 Responsible for optical effects caused by
aerosols Basis for many aerosol measuring instruments Important for
visibility and radiation balance Think of an aerosol particle as a
light source with its own angular distribution of light
intensity
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Scattering 27 Physical basis The scattering of EM radiation by
any system is related to the heterogeneity of that system (the
physics remains the same)
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Scattering 28
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Scattering 29 Two cases In this case, the whole particle sees
the same E-field and scatters in phase d p
