‫الجامعة المستنصرية‬
‫كلية الهندسة‬
‫قسم هندسة البيئة‬
‫أشراف ‪ :‬أ‪.‬م‪.‬د نغم عبيد كريم‬
‫أعداد ‪ :‬بتول ماجد‬
‫المرحلة الرابعة‬
‫فرع السيطرة على التلوث‬
‫‪2020-3-72‬‬
Introduction
Pollutants enter the atmosphere in a number of different ways.
For example, wind blows dust into the air. Automobiles, trucks
and buses emit pollutants from engine exhausts and during
refueling. Electric power plants, along with home furnaces, give
off pollutants as they try to satisfy mankind's need for energy.
One method of pollution release from stationary point sources
has received more attention than any other: stacks. As the
exhaust gases and pollutants leave a stack, they mix with
ambient air describing a plume. As the plume travels downwind,
the plume diameter grows and it progressively spreads and
disperses
Turbulence is highly irregular motion of the wind.
There are basically two different causes of turbulent eddies:
mechanical turbulence and convective turbulence. While both of
them are usually present in any given atmospheric condition.
Mechanical turbulence is caused by physical obstructions to
normal flow such as mountains, building, trees,... The degree of
mechanical turbulence depends on wind speed and roughness
of the obstructions.
Convective turbulence results from different heating-cooling of
surfaces and air masses. The higher the temperature difference,
the Atmospheric eddies cause a breaking apart of atmospheric
parcels which mixes polluted air with relatively unpolluted air,
causing polluted air at lower and lower concentrations to occupy
successively larger volumes of air. Thus, the level of turbulence
in the atmosphere determines its dispersive ability.
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Atmospheric Stability
Atmospheric stability refers to the tendency for air parcels to
move vertically. when the temperature of the air parcel is
greater than the temperature of the surrounding environment,
then it will rise, and when the temperature of the air parcel is
less than the surrounding environment, then it will sink.
• Environmental lapse rate is less than the dry adiabatic lapse
rate , a rising air parcel becomes cooler and more dense than its
surroundings and tends to fall back to its original position. Such
an atmospheric condition is called stable and the lapse rate is
said to be sub-adiabatic.
• Under stable conditions there is very little vertical mixing and
pollutants can only disperse very slowly. As a result , their levels
can build up very rapidly in the environment.
• Instability and stability may be defined with reference to a
neutrally stable atmosphere where the environmental lapse rate
is equal to the dry adiabatic lapse rate for dry or unsaturated air,
and wet adiabatic lapse rate.
Meteorology and Dispersion Modeling Air Quality and
Meteorology
• Primary Metrological Parameter
– Wind speed, Wind Direction, Atmospheric Stability
• Secondary Metrological Parameter
– sunlight , temperature , precipitation and humidity ,
Topography and Energy from the sun and earth’s rotation drives
atmospheric circulation.
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Lapse rate
The rate of decrease of atmospheric temperature with altitude
is known as lapse rate. In a well mixed air which is dry, for every
300m increase in altitude, the temperature decreases by 1.8˚C.
This vertical temperature gradient is called as lapse rate.
Wherever temperature is high, the concentration of pollutants
will be low.
Negative lapse rate : If the temperature of atmosphere
increases with altitude, the rate of decrease of temperature with
altitude is called as negative lapse rate. When negative lapse rate
occurs, a dense cold stratum of air at ground level gets covered
by warmer air at higher level. during negative lapse rate, the
concentration of pollutants increases near their source of
emission.
Adiabatic lapse rate (ALR): The lapse rate of an air parcel as
it moves upwards in atmosphere, and expands slowly to a lower
environmental pressure without exchange of heat, is known as
adiabatic lapse rate. Normal adiabatic lapse rate is 0.9 to 1˚C per
100m altitude. The smoke or any gaseous pollutants mass, as a
parcel move upwards, by virtue of lower density and higher
temperature, reach atmosphere density equal to air density.
The Environmental Lapse Rate (ELR) : is the rate at which
the measured temperature of the air in the environment outside
the air parcel decreases with height. We send up balloons with
instrument packages called radiosondes to measure the
temperature at different levels above the Earth’s surface.
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Plume Behaviour
• Plume: The dispersion of emitted gases from the source of
their production is known as plume and the source is known as
stack.
• Mixing or dispersion of the waste gases and products into the
atmosphere = plume behaviour.
• Effects of plumes are considered local within 500 metres of the
stack, and regional beyond this.
Types of plume
the following are different types of plumes characterised by
different relative positions of environmental lapse rate and
adiabatic lapse rate:
a. Looping plume
b. Coning plume
c. Fanning plume
d. Lofting plume
e. Fumigating plume
f. Trapping plume
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a- Looping Plume:
• Looping plume is of wavy character and occurs in super
adiabatic environment (ELR>ALR), which produces highly
unstable atmosphere because of rapid mixing. In an unstable
atmosphere, rapid air movements take place vertically, both
upward and downward and the plume becomes plume.
• As a result of this, high concentrations of pollutants may occur
near the ground. To disperse these pollutants, it is advisable to
design high stack where atmosphere is generally super adiabatic.
b-Coning Plume:
• It is calculated that when horizontal wind velocity exceeds
32km/hour and under the condition of cloud blocking solar
radiations at day time and terrestrial radiation at night, neutral
plume tends to form cone like structure known as plume.
• Under sub-adiabatic conditions (ELR < ALR), when there is
limited vertical mixing and environment is slightly stable, the
plume also attains cone like structure and is coning plume.
c- Fanning Plume:
• Usually occurs at night, or 1200m-1800m above ground.Under
extreme inversion condition (due to negative lapse rate), fanning
plume is obtained. Under condition of inversion, stable
environmental condition exists just above the stack and plume
does not move upwardly but horizontally
• In areas where such conditions are caused by radiation
inversions, highrise stacks, rising higher than the usual inversion
layer may be adopted .
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d- Lofting Plume:
• Under conditions of strong super adiabatic lapse rate just
above the stack and negative lapse rate (inversion) just below
the opening of stack, lofting plume is obtained. downward
mixing is less because the downward movement is prevented by
inversion. They are created when atmospheric conditions are
unstable above the plume
• The dispersion of pollutants therefore becomes rapid and
pollutants cannot come down to the ground. Such kind of a
plume is ideal for dispersion of air pollutants and protection of
living beings to a great extent.
e- Fumigating Plume:
• Fumigating plume is just opposite to lofting plume. Under
conditions of negative lapse rate (inversion) just above the stack
and strong super adiabatic lapse rate below the stack, the type
of plume obtained is known as fumigating plume.
• Under these set of conditions, the pollutants cannot escape
above the stack, rather they come down near the ground due to
turbulence and mixing. Fumigating plume is therefore extremely
bad for dispersion of pollutants.
f- Trapping Plume:
• When the inversion layer exists above the stack and as well as
below the stack, the plume neither goes up nor goes down,
rather, it gets confined or trapped between these two inversion
layers .This plume is not ideal for dispersion of pollutants as it
cannot go above a certain height.
• Such type of plume is therefore, termed as trapping plume .
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Different types of plume behavior for various atmospheric .
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Maximum Mixing Depth (MMD)
 The dispersion of pollutants in the lower atmosphere is
greatly aided by the convective and turbulent mixing that
takes place.
 The vertical extent to which this mixing takes place
depends on the environmental lapse rate which varies
diurnally, from season to season and is also affected by
topographical features.
 The depth of the convective mixing layer in which vertical
movement of pollutants is possible, is called the maximum
mixing depth (MMD) .
 The maximum mixing depth (sometimes called the mixing
height) is obtained by projecting the dry adiabatic lapse
rate line to the point ofintersection with the atmospheric
temperature profile
 These profiles are usually measured at night or early in the
morning.
 An air parcel at a temperature warmer than the existing
ground level temperature rises and cools according to
adiabatic lapse rate.
 The level where its temperature becomes equal to the
surrounding air gives the MMD value.
 Urban air pollution episodes are known to occur when
MMD is 1500 m or less.
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