Illumination Independent Aerosol
Optical Properties

Extinction
• Scattering
• Absorption
Volume scattering function (phase)
 Transmittance

Extinction =
The extinction coefficient is made up of
particle and gas scattering and absorption:
bext = bsg + bag + bsp + bap
where s, a, g, and p refer to scattering,
absorption, gases, and particles, respectively.
Components of Scattering and
Extinction
Animations of scattering and
absorption
One Approximation for Estimating bext
bext = (esf )f s (RH)[Ammon iated SO4 ] + (enf )f n (RH)[NH 4 NO3 ]
+ (eocmf ) f ocm (RH)[OMC] + (esoilf )[SOIL]
(esc )f s (RH)[Ammon iated SO4 ] + (enc )f n (RH)[NH 4 NO3 ]
+ (eocmc ) f ocm (RH)[OMC] + (esoilc )[SOIL]  10[lacf  lacc ].
Extinction as Function of Size
B ext, i =


D

4
2
Q e ( m i ,  ) n i (D)d(D)
o

B ext, i =
 E ( m ,x, ) f (x)dx
e
i
i
-
where Ee is mass extinction efficiency, f i(x) is
the aerosol mass distribution dm/dx of the ith
species, x=ln[D/Do], and λ is the wavelength.
Externally Mixed Model
B ext,i =  i m i
where

 i =  E e ( m i , x,  ) f i (x)dx
o
and for multiple species
B ext =   i m
i
i
Scattering Efficiency (Q)
Single Particle Efficiency
Particle Size Distribution
Scattering Efficiency as Function of
Size
Carbon Extinction Efficiency
Phase Function for Soil and Sulfate
Phase Function for Carbon
Forward and Backward Scattering
Pollutant Species
Primary vs. Secondary Particles and
Gases (Pollutants)


Primary particles and gases are those
emitted into the atmosphere directly from
some source.
Secondary particles and gases are
formed in the atmosphere by chemical
reactions, by condensation growth, and/or
by coagulation.
Particle Size Distribution
Sources of Primary Particles
Anthropogenic

Wind Blown Dust
Naturally Occurring

Roads, Over Grazing,
Farming practices, Mining


Deserts
Biomass Burning

Land Clearing Practices

Emissions from
fossil fuel
combustion

Fly Ash
Condensation of Hot Vapors
Wind Blown Dust
Volcanoes
Fires
Plant Particles
(pollen)

Sea Salt Spray
(NaCl)
Sources of Primary Gases Important to
Secondary Particle Formation
Compound
Anthropogenic
Naturally Occurring
SO2
Fossil-Fuel
Smelters
Oil Refining
Volcanoes
NOx
Fossil-Fuel Combustion
Mobil Sources
Soil Release (Fertilizer)
Soil Release
NH3
Farm Animals
Volatile
Organics
Mobile Sources
Wild Animals
Vegetation
Ocean
Vegetation
Lightning
Hygroscopic Aerosols




Water uptake by particles in the atmosphere
Aerosol particles grow and scatter more light
Deliquescence - the RH value at which the
crystal begins to absorb water and becomes
a solution droplet
Hysteresis - water is retained on the particle
at RH values lower than predicted by
equilibrium
Growth of Sulfate
Hygroscopic Growth of Particles
Ammonium Sulfate D/Do Curves
Two Measured f(RH) Curves and
Theoretical Estimated
F(rh) for Grand Canyon
Estimated f(rh) for Sulfate and
Organics
bscat,water ( RH )  ao  a1[ SULFATE]  a2 [OMC ]  ......  an [Other Species ]
Internally Mixed Aerosol
FM   m
i
i
however,
Bext    i m
i
i
Mass Removal Issues
Partial Scattering Efficiency
From previous equations, it is apparent that
changes in visibility that correspond to
changes in aerosol species concentrations
can be expressed by forming the derivative
τr/Ci, where Ci refers to the concentration of
particulate species i, and that this derivative
will have terms containing the derivative
bext/Ci. Therefore, define partial scattering
efficiency as:
e p = (  bext / Ci )c j , etc
Extinction for External and Internal
Mixture
D/Do Curves for Partial Scattering
Calculation
Partial Scattering Efficiency for
External and Internally Mixed Aerosols