Transcript CHAPTER 10

CHAPTER 4
EFFECTS ON THE
ATMOSPHERE,SOIL AND WATER
BODIES
The Physics of visibility
visibility of the atmosphere affected from
several factors
 The optical characteristics of the
ilimunation source
 Viewed targets
 Intervening atmosphere
 Characteristics of the observer eyesight.

Contrast
If the contrast betwen an object and
surroundings decrease, it becomes
difficult to observe the object
 The lowest limit of contrast for human
observers is called threshold contrast
and determines the maximum distance
that we can see objects.
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Figure 10.1
Visibility
Influence of gases and particles on
visibility

Light scattering by gasesous molecules and
particles
 Light absorption by gases and particles
DETERMINES THE VISIBILITY
For instance nitrogen dioxide in the lower
atmosphere absorbs shorter wavelengths of
the light,that is why we see redish yellow color
in polluted atmosphere.
Carbocaneous nature of particulate matter is
important regarding absorptive properties of
PM.
Scattering of light by gaseous molecules
and particles depends wavelength of the
light.
 This process is most responsible for
visibility degradation.

The scattering from molecules and very tiny particles (< 1 /10 wavelength) is
predominantly Rayleigh scattering. For particle sizes larger than a wavelength,
Mie scattering predominates.
The blue color of the sky is caused by the scattering of sunlight off the
molecules of the atmosphere. This scattering, called Rayleigh scattering, is
more effective at short wavelengths (the blue end of the visible spectrum).
Therefore the light scattered down to the earth at a large angle with respect
to the direction of the sun's light is predominantly in the blue end of the
spectrum.
Source: http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html

Mie scattering is not strongly wavelength
dependent and produces the almost
white glare around the sun when a lot of
particulate material is present in the air.
It also gives us the the white light from
mist and fog.
Light extinction in the atmosphere
-dI=bext I dx
Where –dI: decrease in intesity
bext extinction coefficient (is a
function of the degree of scattering and
absorption)
dx: pathlength of light.
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10.3-
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Lv= 3.92/bext
(Lv: is the distance at which a black object is visible)

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bext: brg+bag+bscat+bap
brg: scattering by gaseous molecules(Rayleigh
scattering)
 bag: absorption by NO2 gas
 bscat:scattering by particles
 bap:absorption by particles
10.5
Scattering and absorption by
particles

10.7
Importance of particles
Particles in the range of 0.1-1
mikrometre scatter light very effectively
 Contribution of absorption to extinction
coefficient is generally less than
scattering.
 Characteristics of particulate matter
determines the scattering properties.
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Fig10.8
FORMATION OF ATMOSPHERIC HAZE

Mist and fog formed condensation of water
vapor on particulates
 Haze: reduced visibility due to fine particles or
NO2 in the atmosphere.
 Particles must be in the range of 0.1-1
micrometer.
 The major component of the atmospheric haze
is the sulfate particulate matter along with
Nitrate particles. Graphitic material , fine fly
ash and organic aerosols also makes
contribution.