meteorology and natural purification processes1

8/11/2019 Meteorology and Natural Purification Processes1

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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

meteorology and natural purification processes1

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