air polln & met

8/2/2019 Air Polln & Met

1/15C ECRD Publications

AIR POLLUTIONWhy is Meteorology so

important

Air Pollution knows no barriers.Air Pollutants are free to travel / disperse in

any direction once they are emitted. Several Parameters of Ground Level

Conc. (GLC) are determined byMeteorology

Value & Location of Peak GLC

Duration and timing of Peak GLC

Extent of Area affected

Adverse Met. Conditions can lead to abuild up of Air Pollutants leading to

large-scale damage & death London SMOG episode

Grand Canyon disaster



Met Parameters affect different

aspects of Dispersion Modeling

Wind Direction - needed to define thecoordinate system. Usually hourly averages are used.

Some models use seasonal windroses.

Wind Speed used for: Plume rise calculations

Determination of Stability Class

Main GLC calculation

Ambient Temperatureneeded for: Calculation of Plume rise

Determination of Stability Class in some models.

Atmospheric Stability affects: Plume rise calculations

Determination of Sigma Y and Sigma Z

Extrapolation of wind speed to elevated height.

Mixing Height affects:

Plume Rise

Sigma Z

Dispersion behavior



Dispersion of Air PollutionBasic Phenomena & Terms

Lapse rate - change of temperaturewith height

Normally we expect temperature todecrease with height

Adiabatic Lapse rate -without exchange of energy

Dry Adiabatic Lapse rate -0.98

oC/100m.

Inversion -Temperature increaseswith height

IsothermalInversion

Adiabatic

Temperature



Dispersion of Air Pollution

Stability Class - the mostimportant factor

Temperature

The air parcel is hotterthan surrounding air &thus Buoyant

Super Adiabatic conditionsare said to be UNSTABLE

Temperature

The air parcel is cooler thansurrounding air & tends to sinkdown

Inverted Lapse rate signifies

STABLE conditions

Height



Stability Class affects thelocation & value of GLC

Stability Class is used as an

index of the Capacity of theatmosphere to disperse

pollutants. Unstable conditions A or B

indicate a high capacity todisperse.

Effects are usually localized. Can lead to high GLC near the

source of the emissions.

Stable conditions E & F imply

that dispersion will be poor. Pollutants can travel large distances

Under adverse conditions can leadto a build up of pollutants.



Criteria for Determinationof Stability Class



Atmospheric Stability has two

components

SigmaZ

SigmaY

Down-wind direction X

Down-wind direction X



Dispersion of Air PollutionSigma Theta - a direct measure of

Horizontal Dispersion - Sigma Y

Stable conditions and a steady winddirection implies little dispersion

Fluctuating Wind Direction causes the

Plume to meander increasing horizontaldispersion of Pollutants.

Plume dispersesgradually & symmetrically

Large Sigma Theta implies lower GLCspread over a wider area



MIXING HEIGHTThe atmosphere is not a

uniform mediumFor a Surface based layer Mixing Height

is taken upto the top of the layer.

For an Elevated layer Mixing Height istaken upto the base of the layer.



Mixing Height Varies from

place to place and with changeof Season

Mixing Height is controlled by:

Incoming Solar Radiation Albedo - Rate of Radiative cooling

Function of

Land use

Vegetative cover

Nature of Rocks / Soil etc of the site

Climatic factors such as Winds,

Hot and Cold Fronts etc.

Mixing Height is thus Site Specific.



Air Pollutants Disperse by a

Variety of MechanismsHorizontal Dispersion is primarily

dependent on

Wind Speed & Direction

Atmospheric Stability

Dispersion in the Vertical Plane ismainly governed by:

Vertical Turbulence - Stability

Mixing Height

Stable Layers or Inversions restrictVertical spread of Pollutants



Measurement of MixingHeight

Holzworth Model

Temperature

Height

Temperature Soundings at 00GMT (05:30 IST)and 12GMT (17:30 IST) and the surfacetemperature are used to estimate the mixingheight as given below:

Adiabatic Lapse Rate

Measured Lapse

Mixing Height

Max. Surface Temperature



Measurement of MixingHeight

Temperature Profile Measurement

Temperature

Height

The Lapse rate is measured using a temperaturesensor attached to a tethered balloon or a slowrising balloon.

Measured Lapse

Mixing Height

Mixing Height is taken upto

The top of the surface based layer

The base of the lowest elevatedinversion layer.



Modeling of Air Pollution

Applications & Requirements Modeling of Dispersion of Air

Pollutants can be used for: Apportioning share in air quality

deterioration among contributing sources Proper siting of Industry to minimize

adverse impact on air quality

Taking timely action to avoid exposure

Selecting safe periods for unavoidable

emissions such as during process start-upor shut-down.

Meaningful Modeling Requires: Reliable data on relevant Met Parameters

Realistic Inventory of Sources

Validation of the Model throughphysical measurement


15/15

Air Pollution Modeling mustsupplement Physical

Monitoring

Physical measurement of air quality is

constrained by limitations of resourcessuch as:

Equipment

Manpower

Consumables

Monitoring Stations are located at

critical points and values obtained are: Indicative of trends or

Confirm compliance with standards

To protect community health it is

important to be able to predictconcentration levels

Prompt timely corrective action

Avoid exposure

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