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METEOROLOGY AND
NATURAL PURIFICATIONPROCESSES
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Scales of motion An interaction of four atmosphere properties
elements
Relates mass move movement of air
Can be designated as macroscale, mesoscale or
microscale
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Macroscale/global scale
Motion involves planetary patterns of circulation,
grand sweep of air currents over hemisphere
Occur on scales of thousand of kilometers
Exemplified by semipermanent high and lowpressure areas over oceans and continents
The air movement on macroscale influenced by:
earths rotation - which affect the windvelocity and direction (Coriolis force)
thermal convection
the distribution of land and water masses
High and lowpressure area, cold
& warm fronts,
hurricanes, winter
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Mesoscale
Circulation pattern developed under influence
of regional or local topography
Occur on scales of hundreds kilometers
Air movement is affected by configuration of
earths surface
Phenomenaland and sea breeze, mountainand valley winds
Present as vital concern in air pollution control
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Microscale Occur over areas of less than 10km
Exemplified by dispersion of smoke plumes
Occur within friction layer - layer of air that is
influenced by friction caused by the surface
Air movement
Affected by mechanical turbulence from the
frictional stress
Affected by thermal turbulence from radiant
heat
Vital concern in air pollution control
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Heat
Major catalyst of climatic conditions
Comes from sun as short-wave radiation in the
form of visible light
Suns ray
Some may be reflected back to space
Scattered by intervening air moleculesgives
clear sky its deep blue color, red sunrises andsunsets
Absorbed by ozone, water vapor, CO2, earth
surface
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Trophospheric heating
Heat transfer introposphere
Greenhouseeffect
Evaporation-condensation
cycle
Conduction Convection
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Greenhouse effect
Absorbed byearth
Solarenergy
Heatenergy
Retained by water vapor and CO2
Earths reradiation retained,temperature increase
Emitted to
space as
long-waveradiation
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Evaporation-condensation cycle
Evaporationrequires energy which is
absorbed from atm and stored in water vapor
Condensationrelease heat energy
E-C - tends to move heat from lower regions
to higher regions
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Conduction
Heat transfer by direct physical contact of air
and earth
Convection
Process initiated by the rising of warm air andthe sinking of cold air
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Temperature measurement
Degree-daystemperature designation of
particular interest
Measure of heating and fuel requirements and
hence air pollution potential from fossil fuels
burning
Calculation =
Preselected comfortable tempaverage daily temp
for a year
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Lapse rate
defined as the rate at which air temperature
changes with height
varies widely depending on location and time of
day approximately 6 to 7C per km
called as ambient / environment lapse rate
can be determined for a particular place at aparticular timesending up a balloon equipped
with thermometer
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temperature changes within parcel caused by
increases or decreases of molecular activity occur adiabatically due to only the change in
atmospheric pressure as a parcel moves vertically (P compression - heating ; P expansion - cooling)
A dry air parcel rising in the atm cools at the dryadiabatic rate of 9.8C/1000m = dry adiabatic lapse
rate
is a fixed rate, entirely independent of ambient air
temperature
Air is considered dry, as long as any water in it
remains in a gaseous state
Dry Adiabatic Lapse rate
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the slope of the line
remains constant
regardless of its initial
temperature
Dry Adiabatic Lapse rate
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A rising parcel of dry air containingwater vapor will continue to cool at the
dry adiabatic lapse rate until it reaches
its condensation temperature
some of the water vapor begins to
condense
Condensation releases latent heat in
the parcel, thus the cooling rate of the
parcel slows, so called the wet
adiabatic lapse rate is not constant but depends on
temperature and pressure
assumed to be approximately 6 to
7C/1000 m.
Wet Adiabatic Lapse rate
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Atmospheric Stability determined by the temperature difference between an air
parcel and the air surrounding
The difference can cause the parcel to move vertically (i.e., it
may rise or fall)
characterized by four basic conditions unstable, stable,
neutral andinversion
these conditions are directly related to pollutant
concentrations in the ambient air
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Unstable Conditions
surrounding atmosphere has alapse rate greater than the
adiabatic lapse rate (cooling at
more than 9.8C/1000 m)
This is a superadiabatic lapse
rate
so that the rising parcel will
continue to be warmer than the
surrounding air.
In unstable cond., the air parcel
tends to move upward or
downward and to continue that
movement
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Neutral Conditions
When the environmental lapse rateis the same as the dry adiabatic
lapse rate
Vertical air movement is neither
encouraged nor hindered neutral condition is important as
the dividing line between stable
and unstable conditions
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Stable Conditions
When the environmental lapse
rate is less than the adiabatic
lapse rate (cools at less than
9.8C/1000 m)
This is a subadiabatic lapse rate
the air is stable and resists
vertical motion
Stable conditions occur at night
when there is little or no wind
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Inversions
occurs when air temperature increases with altitude
Plumes emitted into inversion layer do not disperse
very much as they are transported with the wind.
Plumes emitted above or below an inverted layer donot penetrate that layer, rather these plumes are
trapped either above or below that inverted layer
High concentrations of air pollutants are oftenassociated with inversions since they inhibit plume
dispersion
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2.5 Lapse rate and dispersion
By comparing ambient and adiabatic lapse
ratethe dispersion of gases emitted from a
stack (plume) can be predicted
Plume types - important because they help us
understand under what conditions there will
be higher concentrations of contaminants at
ground level.
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2.5.1 Looping plume
Ambient lapse rate is superadiabaticstronginstabilities
Atm serve as effective vehicle of dispersion
Stream of pollutant undergoes rapid mixing Any wind causes large eddies may carry the
plume down to the ground
Higher stacks may be needed for areas oflooping plume is likely
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2.5.2.Neutral plume
Occur when ambient lapse rate = dry adiabatic
lapse rate
Tend to rise directly into atm until it reaches
air of density similar to the plume
Tend to cone when
wind velocity greater than 20mi/h
Cloud cover blocks solar radiation by day and
terrestrial radiation by night
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2.5.3.Coning plume
Ambient lapse rate is subadiabatic
Stable with small-scale turbulence
Associated with overcast moderate to strongwinds
Limited vertical mixing, air pollution increase
Pollutants travel fairly long distances beforereaching ground level in significant amounts
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2.5.4.Fanning plume
Under extreme inversion condition (due to
negative lapse rate),
In the presence of inversion, dispersion is
minimal due to little turbulence
If plume density is similar to air, travels
downwind at approximately same elevation
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2.5.5.Lofting plume
Superadiabatic lapse rate above the emission
source and inversions conditions exist below
the source
Has minimal downward mixing
Pollutants dispersed downwind
Favorable in the sense that fewer impacts at
ground level.
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2.5.6.Fumigating plume
Most dangerous plume: contaminants are allcoming down to ground level.
They are created when atmospheric
conditions (inversion layer) are stable abovethe plume and unstable below
(superadiabatic).
This happens most often after the daylight sun
has warmed the atmosphere, which turns a
night time fanning plume into fumigation for
about a half an hour.
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2.5.7.Trapping plume
Similar to conditions provoke by fumigating
plume
Inversion layer prevails both above and below
the emission source
Results in coning plume below the source and
above the inversion layer
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3.Pressure system and dispersion High and low pressure system
are caused by location of continents, difference in surfaceroughness and radiation, wind energy etc
Responsible for many weather changes
High-pressure system
Related to clear skies, light winds, and stable atm
Reflect the relative uniformity of air masses
Pollutants likely to build up when stagnant over an area for
several days
Low-pressure system
Unstable atmospheric conditions - associated with cloudy
skies, gusty winds, bring wind and rain
Dispersion of pollutant is likely and air pollution problems
are minimal - Less contaminant build up
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4. Winds and dispersion
Wind velocity
determine the travel time of particulate and
dispersion rate air contaminant
Affected by topographic conditions
Conc of air contaminant in plume inversely
proportional to wind velocity
Differing conductive capacity of landmass andwater masscontribute to air pollution
problems
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5. Moisture and dispersion
Water vapor
Affect the amount of solar radiation received and
reflected by earth
Serves to scatter or absorb radiation energy
Washout process
removing particulates and soluble gases by
precipitation
Detrimental effects RainfallSO2 react with water to form sulfurous acid
(acid rain) which increase rate of corrosion
Low pH of acid rain influence algae and plant life
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6. Model
Maximum mixing depth- Help establish whetheran area is a proper site for contaminant-causing
human activities
measured at night or early in the morning.
An air parcel at a temperature (maximum surface
temperature for the month) 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
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- Gaussian dispersion - describes the transport and
diffusion of a gas (or particle) from a source to a
receptor according to stability class and other
parameterized characteristics of the atmosphere.
The conc (C) of gas at ground level for distance downwindcan be calculated by using equation 8-3 in page 501
Equation 8-4 can be used if y=0 (concentration along
plume centerline only are needed)
Equation 8-5 can be used if H=0 (ground level burning)
Go through Example 8.1 and 8.2
Dispersion model
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Stack design
Meteorological data are necessary for expressing
dispersion equations
For optimum stack designlocal variables must
be considered
Local variables Mechanical turbulence from nearby buildings
Irregular terrain
Using different criteria for short-term releases,
explosions, for instantaneous release of nuclear
fission products
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Hollands equation and Davidson &
Bryant
Where h = rise of plume above the stack, m= stack gas velocity, m/s
d = inside stack diameter, m
u = wind speed, m/s
p = atmospheric pressure, millibars
T = stack gas temperature minus airtemperature, K
Ts = stack gas temperature, K
Unstable cond h must be increased by 1.1 to 1.2
Stable cond h must be decreased by 0.8 to 0.9
Go through Example 8-3
H = h + h
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Self Reading on
Effects of Air Pollution on
Meteorological Conditions
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Tutorial
8.22, 8.23, 8.25, 8.29, 8.30, 8.31