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 targetsIntervening atmosphereCharacteristics 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.

Figure 10.1

Visibility

Influence of gases and particles on visibility

Light scattering by gasesous molecules and particles

Light absorption by gases and particlesDETERMINES THE VISIBILITYFor 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.

10.3-

Lv= 3.92/bext (Lv: is the distance at which a black object is visible)

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.

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.