air pollution - improve disp

8/8/2019 Air Pollution - Improve Disp

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AIR POLLUTIONAIR POLLUTION

CONTROLCONTROL


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There are 3 alternatives in air pollutionThere are 3 alternatives in air pollutioncontrol :control :

Improve dispersionImprove dispersion Use a downstream pollution controlUse a downstream pollution controldevicesdevices

Reduce emissions by process change (Reduce emissions by process change (

Pollution prevention )Pollution prevention )


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IMPROVE DISPERSIONIMPROVE DISPERSION

Factors affecting dispersion are :Factors affecting dispersion are : Lapse RateLapse Rate

Pressure systemPressure system WindsWinds MoistureMoisture


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Lapse RateLapse Rate AA lapselapse raterate isis thethe changechange ofof temperaturetemperature

withwith unitunit riserise ofof elevationelevation.. The average adiabatic lapse rate in theThe average adiabatic lapse rate in the

troposphere is 6.5 C per kilometertroposphere is 6.5 C per kilometer Lapse rates are assumed to beLapse rates are assumed to be negativenegative

(cooling with height)(cooling with height) Environmental lapse rateEnvironmental lapse rate the specificthe specific

change in temperature with altitude atchange in temperature with altitude atany particular time and locationany particular time and location

E

nvironmental lapse rates can changeE

nvironmental lapse rates can changehourhour--toto--hour and dayhour and day--toto--dayday Environmental lapse rates are measuredEnvironmental lapse rates are measured

by weather balloonby weather balloon


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Atmosphere stabilityAtmosphere stability

We determine the stability of the air byWe determine the stability of the air bycomparing the temperature of a rising parcelcomparing the temperature of a rising parcel(moist or dry adiabatic lapse rate) to that of its(moist or dry adiabatic lapse rate) to that of itssurroundings (environmental lapse rate).surroundings (environmental lapse rate).

Air is stable when, after being lifted or lowered, itAir is stable when, after being lifted or lowered, it

returns to its original position.returns to its original position. Air is unstable when, after being lifted orAir is unstable when, after being lifted or

lowered, it will move further away from itslowered, it will move further away from itsoriginal position.original position.


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Absolutely Stable ( Subadiabatic )Absolutely Stable ( Subadiabatic )

The atmosphere is always AbsolutelyThe atmosphere is always AbsolutelyStable when the environmental lapse rateStable when the environmental lapse rateis less than the adiabatic lapse rate.is less than the adiabatic lapse rate.

Air lifted from any level will sink back to itsAir lifted from any level will sink back to itsstarting position when released.starting position when released.

Since absolutely stable air strongly resistsSince absolutely stable air strongly resistsupward vertical motion, it will, if forced toupward vertical motion, it will, if forced to

rise, tend to spread out horizontally.rise, tend to spread out horizontally.


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Absolutely StableAbsolutely Stable


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Absolutely Unstable ( Superadiabatic )Absolutely Unstable ( Superadiabatic )

Absolute Instability results when theAbsolute Instability results when theenvironmental lapse rate is greater thanenvironmental lapse rate is greater thanthe dry adiabatic lapse rate.the dry adiabatic lapse rate.

Air parcels lifted from any level willAir parcels lifted from any level will

continue to rise when they are releasedcontinue to rise when they are releasedbecause they are warmer and lessbecause they are warmer and lessdense than the air around them.dense than the air around them.

Deep layers in the atmosphere areDeep layers in the atmosphere are

seldom, if ever, absolutely unstable.seldom, if ever, absolutely unstable.Absolute instability is usually limited toAbsolute instability is usually limited toa very shallow layer near the ground ona very shallow layer near the ground onhot, sunny days.hot, sunny days.


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Absolutely UnstableAbsolutely Unstable


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Inversions ( Very stable )Inversions ( Very stable )

occurs when air temperature increases withoccurs when air temperature increases withaltitudealtitude occurs frequently but is generally confined to aoccurs frequently but is generally confined to a

relatively shallow layerrelatively shallow layer


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Effect of atmospheric stability onEffect of atmospheric stability on

air pollutant dispersionair pollutant dispersion

LoopingLooping : The plume is: The plume isformed in unstableformed in unstablecondition. The flow patterncondition. The flow pattern

follows closely that of a flowfollows closely that of a flowpast bluff body forming apast bluff body forming aturbulent wake behind it.turbulent wake behind it.

ConingConing : A typical buoyant: A typical buoyantplume which grows byplume which grows bymixing. It is formed inmixing. It is formed inneutral atmosphericneutral atmosphericcondition. The plume iscondition. The plume isusually bifurcated and risesusually bifurcated and rises

by its own buoyancy.by its own buoyancy.

natural adiabatic lapse

actual temperature lapse


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FanningFanning : The plume is seen to: The plume is seen toreach its equilibrium levelreach its equilibrium level

quickly. The plume starts offquickly. The plume starts offbifurcated by meets strongbifurcated by meets stronginversion and travels stably. It isinversion and travels stably. It isformed in stable atmosphericformed in stable atmosphericcondition.condition.

FumigatingFumigating : The plume is: The plume isformed by being trapped by aformed by being trapped by astrong stable inversion abovestrong stable inversion above

and a well mixed unstable airand a well mixed unstable airbelow it. It occurs usually in thebelow it. It occurs usually in themorning when sunshine causesmorning when sunshine causesthe layer beneath the inversionthe layer beneath the inversionto be mixed by convection.to be mixed by convection.


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LoftingLofting : If the inversion: If the inversionis weak, the plume mayis weak, the plume may

penetrate it and thenpenetrate it and thencontinue to spreadcontinue to spreadupwards but notupwards but notdownwards due to thedownwards due to the

inversion.inversion. ThermallingThermalling : It is: It is

formed when theformed when thethermals are wellthermals are well

developed, and thedeveloped, and theplume is carried awayplume is carried awayby convection and isby convection and isbroken into lumps.broken into lumps.


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FlaggingFlagging : When the efflux: When the effluxvelocity of the plume is notvelocity of the plume is notlarge enough to carry thelarge enough to carry the

effluent, some pollutants areeffluent, some pollutants aretrapped near the stack hole.trapped near the stack hole.This is then brought down byThis is then brought down bygravity.gravity.

BifurcationBifurcation : The buoyancy of a: The buoyancy of ahot plume generates a doublehot plume generates a doublevortex circulation in the plumevortex circulation in the plumewith an updraught. This is duewith an updraught. This is dueto internal convective motion ofto internal convective motion of

the plumethe plume


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