AIR POLLUTION AND METEOROLOGY Dr.K. Subramaniam, Senior Lecturer (Environmental Health and Safety )

AIR POLLUTION AND

METEOROLOGYDr.K. Subramaniam,

Senior Lecturer (Environmental Health and Safety )

METEOROLOGY OF AIR METEOROLOGY OF AIR POLLUTIONPOLLUTION

• Transport and dispersion

• Removal mechanisms

Important Aspects of Air Pollution Meteorology

• Atmospheric Turbulence

• Scales of Atmospheric/Turbulent Motion

• Plume Behavior

• Planetary Boundary Layer (PBL)

• Effects on Dispersion

• Applications

Meteorological Parameters that Influence Air Pollution

• Turbulence

• Wind Speed and Direction

• Temperature

• Stability

• Mixing Height

Atmospheric Turbulence

• Responsible for dispersion/transport of pollutants

• Refers to the apparently chaotic nature of fluid motions (in this case, atmospheric motions)

• Irregular, almost random fluctuations of such parameters as:i. velocity

ii. temperature

iii. scalar concentrations (pollutants)

Atmospheric Turbulence Sources

• Mechanical Forcing

• Buoyant or Thermal Forcing

Atmospheric Turbulence (Sources)

• Mechanical Forcing:

i. Air flowing over irregular surface

ii. Change in horizontal wind speed with height

• Factors Influencing Mechanical Forcing:

a) Speed of local winds

b) Roughness of terrain over which wind is blowing

Adiabatic Lapse Rate

• It is the temperature profile of what

would happen to a parcel of air that is

raised or lowered vertically, and allowed

to cool or heat from expansion or

contraction with no exchange of energy

or heat.

Atmospheric Turbulence (Sources)

• Buoyant Forcing (Thermal):

– Air rises or sinks based on temperature; heated air becomes less dense & rises on its own; cooled air becomes more dense & sinks

• Factors Affecting Buoyant Forcing

– “Stability” of the atmosphere

– Vertical temperature profile of the atmosphere

– Lapse Rate; specifically the Dry Adiabatic Lapse Rate which is:

1oC/100m = 10oC/km = 5.4oF/1000 ft

Cooler Air Cooler Air

Warmer Air

DRY ADIABATIC PROCESS

Ground

Atmospheric Turbulence (Buoyant Forcing)

Cooler Air Cooler AirWarmer Air

Unstable Conditions - Turbulence is produced

Ground

Displaced warmer air will now rise on its own

(Thermals; Thunderstorm updrafts)



Warmer Air Warmer AirCooler Air

Stable Conditions - Turbulence is suppressed

Ground

Displaced cooler air will sink back to starting point


Neutral Atmospheric Conditions

Environment

Air Parcel

Environment

Ground

Planetary Boundary Layer (PBL)

• Top of the atmospheric boundary layer can be defined as the lowest level in the atmosphere at which the ground surface no longer influences the meteorological parameters through turbulence transfer of mass

• During day this corresponds to Mixing height (up to 3 km in height)

Processes include:

i. Roughness of terrain

ii. Obstructed flow

iii. Heat and energy transfer

The effect of boundary layer stability on plume behavior

In a well-mixed turbulent boundary layer on a hot day (forced by buoyancy), the turbulent eddies may be large and intense enough to advert the whole plume down to the ground. This can result in extremely high plume concentrations in the vicinity of the source.


This is the kind of form assumed for a Gaussian plume, when the boundary layer is well-mixed and turbulent eddies are smaller than the plume scale. The plume forms a cone downstream.


In a stable boundary layer, the plume spreads out horizontally at its level of neutral buoyancy. Vertical motion is weak, so there is little upward spread, but the plume forms a `fan' when viewed from above. The plume is not well-mixed in the vertical, which implies relatively slow dilution, but there are not likely to be high plume concentrations at the ground. Unfortunately, this kind of plume may be the precursor to a `fumigation' event if the inversion is subsequently mixed to ground level.


At early evening, if a surface inversion is developing, vertical motion may be inhibited below the plume while remaining active above: the plume is diluted but does not reach the ground. This is a favorable situation.


There is a strong inversion restricting mixing above, and the plume is mixed throughout the boundary layer. This can occur quite rapidly. For example, after sunrise when the nocturnal inversion is being eroded from below by buoyant eddies, plume-level air of high concentration may be brought down to the surface over a wide area.

PBL below stack top: little or no concentration of pollutants at the surface

PBL Top

Horizontal Winds

PBL

Effects of PBL Height on Stack Pollutant Dispersion

PBL well above stack top: decreased concentrations of pollutants at the surface

PBL Top

BuoyantTurbulence

PBL


PBL just above stack top: increased concentrations of pollutants at the surface

PBL Top

BuoyantTurbulence

PBL


Temperature Profile in Atmosphere

1. INVERSIONS

2. ATMOSPHERIC STABILITY

Unstable Conditions: leads to greater dispersion of pollutants

PBL Top

PBL

Effects of Stability on Stack Pollutant Dispersion

Stable conditions: lead to less dispersion of pollutants

PBL Top

PBL

Effects of Stability on Stack Pollutant Dispersion

Unstable Conditions: Lead to lower concentration of pollutants at surface

XXX

BuoyantTurbulence

Effects of Stability (Ground Source Pollutant Dispersion)

Stable Conditions: Leads to greater concentration of pollutants at surface

XXX

Effects of Stability (Ground Source Pollutant Dispersion)

WIND SPEED AND DIRECTION

• Mesoscale circulation

• Large scale circulation

Air Cooled over Water Contracts(Becomes More Dense)

Air Warmed over Land Expands(Becomes Less Dense)

Land-Sea Breeze: Daytime (Sea Breeze)

Cooler WaterWarmer Land

Reverses at Night as Water Remains Warmer than Land to Make Land Breeze

Sea Breeze (arises due to density differences)

Upper Level Return Flow

Mesoscale Circulations Affecting Dispersion

1. Mountain/Valley Winds

Day: Night:

WarmMtn

CoolMtn

2. Urban/Heat Island (Night)

CITY

PBL Top

Mesoscale Circulations Affecting Dispersion

Large Scale Circulation

• Transboundary air pollution

• Acid deposition

• Ozone transport

Applications of Air Pollution Meteorology

• Atmospheric Dispersion Modeling

• Study of Accidental Release of Hazardous Substances

Including Radioactive Nuclides

• Applications of air quality meteorology can be used for

dispersion modeling, i.e., predicting the path of the

pollutant concentration and for calculations of ground

sources, such as hazardous waste spills.

• Let’s first look at dispersion modeling.

Air Pollution Meteorology

• Meteorology very important factor in developing strategies for air pollution control

• State of the lower troposphere (PBL) plays large role in dispersion of pollutants and plumes:– Mechanical Turbulence– Buoyant Turbulence– Circulation

Wind Speed and Direction

• The average ground level wind

speed is about 4.5 m/s.

– “Calm” wind is less than 0.5m/s

• Wind speed almost always

increases with height.

–ground friction slows lower level

winds

A Wind Rose

A Wind Rose

Wind Speed With Height

• Deacon’s power law:

u2 / u1 = (z2 / z1)p

where:

u1 is the wind speed at elevation z1

u2 is the wind speed at elevation z2

and p is an exponent that depends on stability

and ground characteristics

Note: Wind speed measured by the NWS is

usually obtained at z = 10 meters (z1)

Impact of Fixed Geographic Features

• TERRAIN EFFECTS• Sea breeze

• Valley wind

• Drainage wind

• Flow patterns due to topographical features

Temperature Gradient

• Air temperature is not uniform with altitude at a given location.

• Reasons: a) heating by the groundb) heating by the sunc) cloud coverd) evaporative cooling over the oceanse) expansion of gases due to air

movement

Stability and Lapse Rate

• The lapse rate determines how readily parcels of air move upward or downward.

• In stable atmospheres = vertical movement is opposed by the temperature gradient

• In unstable atmospheres = vertical movement is enhanced

• In neutral atmospheres = neither

Stability Classes

A = very unstable

B = moderately unstable

C = slightly unstable

D = neutral

E = slightly stable

F = stable

Why is stability important?

• Stability affects plume rise.

• Plume rise can be calculated using information about the stack gases and meteorology.

• Stability can effect the dispersion and appearance of plumes being emitted from stacks.

InversionsInversions

• An inversion is a situation of increasing temperature with height.

• Pre-dawn mornings have an inversion that reached up to about 1000 ft (100m).

• Atmospheres within an inversion are extremely stable, with damped vertical mixing.

Surface Temperature Inversions:Surface Temperature Inversions:

a)Are very common

b)Are easy to recognize

c) Affect the dispersal of very small spray

droplets suspended in the air

d)Do not increase the amount of off-site

movement

e)Can increase the potential for offsite affects &

the distance at which affects can be observed

Atmospheric Stability

i. Indicator of atmospheric turbulence

ii. Depends on static stability, thermal and mechanical turbulence

iii. Unstable : Lapse rate > dry adiabatic lapse rate

iv. Neutral : Lapse rate = dry adiabatic lapse rate

v. Stable : Lapse rate < dry adiabatic lapse rate

vi. Turner method: solar angle, cloud cover and wind speed

IMPORTANCE OF METEOROLOGY

• Dispersion

• Transport

• Wind speed and direction

• Temperature

• Stability

• Mixing height

Any questions?

Thank you…

Documents

AIR POLLUTION AND METEOROLOGY Dr.K. Subramaniam, Senior Lecturer (Environmental Health and Safety )