Air pollution and meteorology
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AIR POLLUTION AND METEOROLOGY. Dr.K . Subramaniam , Senior Lecturer (Environmental Health and Safety ). METEOROLOGY OF AIR POLLUTION. Transport and dispersion Removal mechanisms. Important Aspects of Air Pollution Meteorology. Atmospheric Turbulence

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AIR POLLUTION AND METEOROLOGY

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Air pollution and meteorology

AIR POLLUTION AND METEOROLOGY

Dr.K. Subramaniam,

Senior Lecturer (Environmental Health and Safety )


Meteorology of air pollution

METEOROLOGY OF AIR POLLUTION

  • Transport and dispersion

  • Removal mechanisms


Important aspects of air pollution meteorology

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

Meteorological Parameters that Influence Air Pollution

  • Turbulence

  • Wind Speed and Direction

  • Temperature

  • Stability

  • Mixing Height


Atmospheric turbulence

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:

    • velocity

    • temperature

    • scalar concentrations (pollutants)


Atmospheric turbulence sources

Atmospheric Turbulence Sources

  • Mechanical Forcing

  • Buoyant or Thermal Forcing


Atmospheric turbulence sources1

Atmospheric Turbulence (Sources)

  • Mechanical Forcing:

    • Air flowing over irregular surface

    • Change in horizontal wind speed with height

  • Factors Influencing Mechanical Forcing:

    • Speed of local winds

    • Roughness of terrain over which wind is blowing


Adiabatic lapse rate

Adiabatic Lapse Rate

  • It is the temperatureprofile 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 sources2

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


Atmospheric turbulence buoyant forcing

DRY ADIABATIC PROCESS

Cooler Air

Cooler Air

Warmer Air

Ground

Atmospheric Turbulence (Buoyant Forcing)


Atmospheric turbulence buoyant forcing1

Unstable Conditions - Turbulence is produced

Cooler Air

Warmer Air

Cooler Air

Displaced warmer air

will now rise on its own

(Thermals; Thunderstorm updrafts)

Ground

Atmospheric Turbulence (Buoyant Forcing)


Atmospheric turbulence buoyant forcing2

Stable Conditions - Turbulence is suppressed

Warmer Air

Warmer Air

Cooler Air

Displaced cooler air

will sink back to starting point

Ground

Atmospheric Turbulence (Buoyant Forcing)


Atmospheric turbulence buoyant forcing3

Neutral Atmospheric Conditions

Environment

Environment

Air Parcel

Ground

Atmospheric Turbulence (Buoyant Forcing)


Planetary boundary layer pbl

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:

    • Roughness of terrain

    • Obstructed flow

    • Heat and energy transfer


The effect of boundary layer stability on plume behavior

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.


The effect of boundary layer stability on plume behavior1

The effect of boundary layer stability on plume behavior

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.


The effect of boundary layer stability on plume behavior2

The effect of boundary layer stability on plume behavior

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.


The effect of boundary layer stability on plume behavior3

The effect of boundary layer stability on plume behavior

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.


The effect of boundary layer stability on plume behavior4

The effect of boundary layer stability on plume behavior

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.


Effects of pbl height on stack pollutant dispersion

Effects of PBL Heighton Stack Pollutant Dispersion

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

Horizontal Winds

PBL Top

PBL


Effects of pbl height on stack pollutant dispersion1

Effects of PBL Height on Stack Pollutant Dispersion

PBL Top

Buoyant

Turbulence

PBL

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


Effects of pbl height on stack pollutant dispersion2

Effects of PBL Height on Stack Pollutant Dispersion

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

PBL Top

Buoyant

Turbulence

PBL


Temperature profile in atmosphere

Temperature Profile in Atmosphere

1.INVERSIONS

2.ATMOSPHERIC STABILITY


Effects of stability on stack pollutant dispersion

Effects of Stability on Stack Pollutant Dispersion

Unstable Conditions: leads to greater dispersion of pollutants

PBL Top

PBL


Effects of stability on stack pollutant dispersion1

Effects of Stability on Stack Pollutant Dispersion

Stable conditions: lead to less dispersion of pollutants

PBL Top

PBL


Effects of stability ground source pollutant dispersion

Effects of Stability(Ground Source Pollutant Dispersion)

Buoyant

Turbulence

XXX

Unstable Conditions: Lead to lower concentration of

pollutants at surface


Effects of stability ground source pollutant dispersion1

Effects of Stability(Ground Source Pollutant Dispersion)

Stable Conditions: Leads to greater concentration of

pollutants at surface

XXX


Wind speed and direction

WIND SPEED AND DIRECTION

  • Mesoscale circulation

  • Large scale circulation


Mesoscale circulations affecting dispersion

Mesoscale Circulations Affecting Dispersion

Land-Sea Breeze: Daytime (Sea Breeze)

Upper Level Return Flow

Air Warmed over Land Expands

(Becomes Less Dense)

Air Cooled over Water Contracts

(Becomes More Dense)

Sea Breeze (arises due to density differences)

Cooler Water

Warmer Land

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


Mesoscale circulations affecting dispersion1

Mesoscale Circulations Affecting Dispersion

1. Mountain/Valley Winds

Day:

Night:

Warm

Mtn

Cool

Mtn

2. Urban/Heat Island (Night)

PBL Top

CITY


Large scale circulation

Large Scale Circulation

  • Transboundary air pollution

  • Acid deposition

  • Ozone transport


Applications of air pollution meteorology

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

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 direction1

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


Air pollution and meteorology

A Wind Rose


Air pollution and meteorology

A Wind Rose


Wind speed with height

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

Impact of Fixed Geographic Features

  • TERRAIN EFFECTS

  • Sea breeze

  • Valley wind

  • Drainage wind

  • Flow patterns due to topographical features


Temperature gradient

TemperatureGradient

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

  • Reasons:

    • heating by the ground

    • heating by the sun

    • cloud cover

    • evaporative cooling over the oceans

    • expansion of gases due to air movement


Stability and lapse rate

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

Stability Classes

A = very unstable

B = moderately unstable

C = slightly unstable

D = neutral

E = slightly stable

F = stable


Why is stability important

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.


Inversions

Inversions

  • 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:

  • Are very common

  • Are easy to recognize

  • Affect the dispersal of very small spray droplets suspended in the air

  • Do not increase the amount of off-site movement

  • Can increase the potential for offsite affects & the distance at which affects can be observed


Atmospheric stability

Atmospheric Stability

  • Indicator of atmospheric turbulence

  • Depends on static stability, thermal and mechanical turbulence

  • Unstable : Lapse rate > dry adiabatic lapse rate

  • Neutral : Lapse rate = dry adiabatic lapse rate

  • Stable : Lapse rate < dry adiabatic lapse rate

  • Turner method: solar angle, cloud cover and wind speed


Importance of meteorology

IMPORTANCE OF METEOROLOGY

  • Dispersion

  • Transport

  • Wind speed and direction

  • Temperature

  • Stability

  • Mixing height


Any questions

Any questions?


Thank you

Thank you…


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