Air Meteorology -6

7/30/2019 Air Meteorology -6

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Air Pollution

Meteorology and Control

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Air pollution Meteorology

What is clean air ?

78% Nitrogen + 21% Oxygen + 1% Trace

340 ppm CO2(0.034%)

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Mixing/DispersionMeteorology

VerticalTemperature

Lapse Rate

Horizontal Wind

Speed

Direction

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1. Vertical DispersionVertical mixing of air and dispersion of pollutants depends

on the kind of atmospheric stability prevailing at any giventime

Atmospheric stability depends on the rate of change of airtemperature with altitude, that may prevail at a particulartime and location

But the rate at which the air temperature drops called theenvironmental lapse rate or ambient lapse rate

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Lapse RatesTemperature Change With Height

Lapse rate is the rate of change of temperature withheight

Lapse rate is defined as = -Tz

Atmospheric pressure decrease withincreasing height above the ground

Atmosphere cools with height What rate ? Dry (Adiabatic) 10C/km

Wet (Adiabatic) 6C/km

(Release of heat with condensation)


7/37Temperature (K)

Mesopause

Mesospher

e

Thermosphere

Troposphere

Tropopause

Stratospher

e

Stratopause

TEMPERATURE PROFILE OFATMOSPHERE

Ionosphere


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No exchange of heat of the parcel of airunder consideration with the outside airThe lapse rate is in dependent of theprevailing atmospheric temperature gradientat any given timeA moving parcel of air will always cool down1o C per 100 m it rises in the atmosphere

and will warm up 1o C for every 100 m sinkThe adiabatic lapse rate usually differs fromthe environmental lapse rate becoz of factorssuch as geographic features, wind ,andsunlight


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Contd

Environmental lapse rates are classified as beingeither strong or week (super adiabatic or subadiabatic )

Strong lapse rates are associated with unstableatmosphere, while week lapse rates are associatedwith a stable atmosphere

Increase in actual air temperature with increrasingaltitude is called a temperature inversion stableatmosphere

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Inver

sion

Inver

sion

Subadiabatic

Subadiabatic

Superadiabatic

Superadiabatic

TTT - 1T - 1

100100

mm

ElevationElevation

(m)(m)

Temperature (Temperature (ooC)C)

Adiabatic Lapse RateAdiabatic Lapse Rate


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Lapse RatesSuperadiabatic, Strong, UnstableTemperature Reduction > 1 oC/100m

Subadiabatic, Weak, StableTemperature Reduction < 1 oC/100m

NeutralTemperature Reduction = 1 oC/100m

Inversion (Extreme Subadiabatic)Temperature Increase with Height


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Monroe Power Plant


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Vertical Expansion of Continuous Plumes

Fanning The plume has alarge spread horizontally

and very little vertically.

Typically occurs at night in a very stableboundary layer with strong surfaceinversion and weak variable winds

Fumigation Is when the plume material gets rapidly

brought down to the ground level due to

downward mixing This situation occurs shortly after sunrise

due to surface heating and is slowlyreplaced by an unstable layer that growsup to the top of the plume

This condition is usually short-lived butresults in the highest ground levelconcentrations

Looping occurs in very unstable and convective

conditions during midday and afternoon

Large convection eddies take the plumematerial in successively upward anddownward motions


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Coning This is when the plume looks like a cone in

both the horizontal and vertical scale

This usually occurs under cloudy andwindy conditions

Lofting The plume stays above the surface

inversion

This occurs shortly after transition fromunstable to stable conditions near sunset.

The plume can be thin or become quitethick

Depending on the height of the stack andthe rate of deepening of the inversionlayer, the lofting condition may be verytransitory or it may persist for severalhours

Trapping Plumes released in unstable atmosphere

disperse their material uniformlythroughout the air (the Planetary BoundaryLayer PBL)

Trapping can lead to very high ground levelconcentrations when the inversion layer islow and there are weak winds


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2. Horizontal Dispersion ofpollutants

Horizontal dispersion or spreading of air pollutantsdepends on wind speed and direction

The concentration of air pollutants decreases withincreasing wind speed because, as the pollutants aredischarged from the source, they are more rapidlyseparated and dispersed by the swiftly moving air

Wind velocity data, plotted in a graph called windrose

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Wind RoseWind roses are divided into 16 wind directions

Each wind direction is divided into wind

speeds As the percent of time the wind blows from a

particular directions gets larger, the portion ofthe bar representing the wind speed gets

larger both in length and width

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Wind Rose

20http://www.epa.gov/ttn/naaqs/ozone/areas/wind.htm#dlfi

1 knot = 1.82 km/hr


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Contd

Winds develop because of the combined effects oftemperature gradients and the rotation of Earthpoles while friction and the forces resulting fromEarths rotation deflect the air movement

Important factors that affect circulation patterns includetopography, daily and seasonal variation in surface

Since soil and rock warm up and cool faster thanwater winds shoreline directed toward the water atnight and inland during the day

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Lapse Rates and AtmosphericStability

Strong Lapse Condition (Looping)Superadiabatic lapse rate

-strong instabilities

Associated with clear

daytime conditions ,strong solar heating &light winds

High probability of highconcentrations

sporadically at groundlevel close to stack.

WindWind


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Weak Lapse Condition (Coning)Stable with small-scale

turbulence

Associated with cloudy

moderate to strongwinds

Pollutants travel fairlylong distances beforereaching ground level

in significant amountsOccurs in neutral

atmospheric conditions

WindWind


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Inversion Condition (Fanning)Occurs under large

negative lapse rate

Extremely stableatmosphere

If plume density issimilar to air, travels

downwind atapproximately sameelevation

WindWind


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StabilityInversion Below, Lapse Aloft

(Lofting)Favorable in the sense

that less impacts atground level.

Pollutants go up intoenvironment.

They are created whenatmospheric conditions

are unstable above theplume and stable below

WindWind


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StabilityWeak Lapse Below, Inversion Aloft

(Trapping)Most dangerous plume:

contaminants are all comingdown to ground level.

They are created when

atmospheric conditions arestable above the plume andunstable below.

This happens most often afterthe daylight sun has warmedthe atmosphere, which turns a

night time fanning plume intofumigation for about a half anhour.

WindWind


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ACTUAL

ADIABATIC (1C/100 m)

1000 m (say)

20 C19 C18 C

20 C AirParcel

19 C Air

Parcel at

1100 m

1100 m


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30


1000 m (say)

20 C 21C 22 C

20 C Air

Parcel

21 C Air

Parcel at

1100 m

1100 m

900 m

ACTUAL


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1000 m (say)

20 C19.5 C19C

20 C Air

Parcel

19 C Air

Parcel at

1100 m

1100 m

ACTUAL


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WIND

Adiabatic

Actual


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Scenarios

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Adiabatic

Actual

He

igh

t

Temperature


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Scenarios

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Adiabatic

Actual

He

igh

t

Temperature


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Scenarios

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Adiabatic

Actual

He

igh

t

Temperature


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Stability

36http://www.tpub.com/content/aerographer/14312/css/14312_47.htm

Wet Lapse

Rate

Dry Lapse Rate

Average

Unstable

Stable

Height

Temperature


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ReviewDispersion and Mixing Influenced by :Wind

SpeedDirectionWind Rose

Temperature Lapse RatesDry adiabaticWet AdiabaticStability

Air Meteorology -6

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