Atmospheric Dispersion,
Transport and Deposition
EOH 468
Spring 2008
Dr. Peter Bellin, CIH, Ph.D.
Dispersion
• Atmospheric process affect dilution.
• Wind speed and lapse rate impact on
emissions.
• Planetary Boundary Layer (PBL)
– Up to 2000 meters from surface
– Relatively stable layer in troposphere above
PBL.
• Mixing in PBL is variable in short term.
Wind Speed
• Varies with height.
• Profile is affected by topography.
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Wind Speed
• The wind profile affects dilution (image
from Boubel, 1994)
Wind Direction
• Affected by meteorology, and topography.
• Depending on conditions, small changes
can have big impact on pollution
concentration.
• Quite variable
• Long range transport.
Turbulence
• Mechanical
– Smaller in scale, regular pattern
– Work around edges of a plume
• Thermal
– Larger in scale, more variable
– Can transport larger bundles of air.
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Mechanical turbulence
• Caused by air moving over and around
structures/vegetation
• Increases with wind speed
• Affected by surface roughness
Turbulence
• Turbulence will
usually dilute
pollutants
• Downwash is an
exception.
Thermal turbulence
• Caused by heating/cooling of the earth’s
surface
• Flows are typically vertical
• Convection cells of upwards of 1000- 1500
meters
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Thermal turbulence
• Effects
– Enhances mixing/pollutant dispersion
– Downwash may result from mechanical
turbulence
• Increases ground level concentrations
Atmospheric stability
• Back to temperature profiles
• Atmospheric stability affects plume behavior and
dispersion.
• Dry adiabatic lapse rate is constant, air cools as
it rises.
– -9.8 degrees C per 1000 meters.
• Air contains water; it will cool at dry lapse rate
until water starts condensing, then cool at
(variable) wet adiabatic lapse rate.
– -6.5 degrees C per 1000 meters.
Atmospheric Stability
• Environmental Lapse
rate refers to the
actual temperature
profile, as impacted
by meteorological
variables.
• This is what can lead
to temperature
inversions, and stable
or unstable air.
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Atmospheric Stability
• The relationship
between
environmental and
dry lapse rates
determines stability of
air.
• Here, we see a
mixing height.
Atmospheric stability
• As parcels of air rise,
they are cool less
than the environment
and buoyancy is
enhanced. Unstable
air.
• Sunny days with low
wind speeds.
Atmospheric Stability
• Neutral conditions.
Environmental Lapse
rate is equal to dry
lapse rate.
• Dividing line between
stable and unstable
conditions.
• Windy days or cloud
cover
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Atmospheric Stability
• Stable conditions.
• Air rises, but then is
cooler than
surrounding air, so
falls back down.
• Night time with little or
no wind.
Atmospheric Stability
• Radiational inversion.
• Ground cools at night,
air is stable near the
ground.
Inversions
• Types
– Radiational
– Subsidence
– Frontal
– Advective
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Inversions
• Frontal -warm air overrides cooler air
• Advective - warm air flows over a cold
surface or cold air
Radiational inversions
• Result from radiational cooling of the
ground
• Occur at night -nocturnal
• Typically surface based
Radiational inversions
• Occur on cloudless nights
• Are intensified in river valleys
• Cause pollutants to be “trapped”
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Radiational inversion
Radiational inversions
• Breakup after sunrise
• Breakup results in elevated ground level
concentrations
• Breakup described as a fumigation
Radiational inversions
• Elevated inversions are formed over urban
areas
– Due to heat island effect
– Due to dust dome
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Dust dome and inversion
Subsidence inversion
• Associated with high-pressure systems
• Inversion layer is formed aloft
• Covers hundreds of thousands of square
kms
• Persists for days
Subsidence inversion
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Subsidence inversion
• Migrating high-pressure systems
• Semi-permanent marine high-pressure
systems
– Results in large number of sunny calm days
– Inversion layer closest to the ground on continental
side
– Responsible for air stagnation over Southern
California
Mixing Height
• Height of air that is relatively vigorously
mixed and where dispersion occurs
• Varies from one region to another
Mixing Heights
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Atmospheric Stability
• Associated with high
pressure systems.
• Subsiding air warms,
becomes warmer
than air below:
inversion forms
• Stable for longer
periods than
radiational inversions.
Stability and plume behavior
Stability and plume behavior
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Stability and plume behavior
Stability and plume behavior
Stability and plume behavior
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Plume Height
• Plume height
depends on
temperature of plume,
and air at top of stack.
Dispersion Modeling
• Models can be used
to predict dispersion
and transport of stack
emissions.
• The figure represents
a Gaussian plume
Air Pollution Modeling
• Now essential for effective planning and
control.
• See EPA web site:
http://www.epa.gov/scram001/dispersionin
dex.htm
• Modeling photochemical smog:
http://www.epa.gov/scram001/photochemi
calindex.htm
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Urban Plume
• Consider all the sources in an urban
region.
• Large geographic area
• Regional transport with weather systems.
• Photochemistry will occur in the plume.
Long Range Transport
• Historical assumption: dispersion results in
minimal effects distant from the source.
• 1970’s: observed long range transport of
ozone, with elevated levels at night.
• Acid rain issue, for example in the NE US.
• Arctic haze resulted form pollution emitted
in Europe and Asia.
• Natural and anthropogenic pollution.
Long Range Transport
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Removal/deposition
• Atmospheric lifetimes
– Average of the life histories of all molecules of
a substance
– May also be described as residence time
– Can be characterized by half life
• Time to decrease to 50% of initial value
Removal Mechanisms
• Chemical reactions do this, as substances
are converted, for example, from gas to
particle.
• Sedimentation
– < 20 µ treat as gases
– 20 to 100 µ move with the plume, but drop
due to gravity
– > 100 µ particles particles fall rapidly
Removal Mechanisms
• Dry Deposition refers to particles
impacting on surfaces of vegetation. This
is described by a deposition velocity, and
can include chemical reactions.
• Wet Deposition refers to scavenging in
clouds (cloud droplets), or below clouds
(rain droplets).
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Deposition processes
• Dry deposition
– Transfer of gas/particulate phase substances
to ground, water, vegetation by:
•
•
•
•
Impaction
Diffusion
Settling
Physiological uptake
Deposition rates
Depositional processes
• Wet deposition
– Processes by which gases/particles are
brought to the earth’s surface in aqueous form
• Absorption in cloud/rain droplets
• Particles serve as condensation nuclei in in-cloud
processes
– Described as “rainout”
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Emission inventories
• Emission inventories are essential for
input in air pollution modeling.
• Tutorial is here:
http://www.epa.gov/air/oaqps/eog/course4
19a/index.html
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