Download Dispersion of Air Pollutants

Transport of Air
Pollutants
Marti Blad, Ph.D., P.E.
Yavapai Apache Nation
What is the Difference?
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What we will learn
 Pollutants move and spread in air
 Up and down (vertical)
 Away and outward (horizontal)
 Dispersion vs Diffusion
 Slowly “spread” out in all directions (diffusion)
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Diffusion & dispersion
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Transport of Air Pollutants
 Weather conditions are important
 Temperature
 Affect Molecule dance
 Pressure
 Affects density
 Volume Temperature and Pressure related
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3 parts to puzzle
 Source of pollutant
 Stationary vs Mobile
 Control technologies
 Physical, Chemical, Biological
 How transported
 Fate of pollutant
 Toxicology & Chemistry
 Receptor or recipient
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Pollutants moving through air
 2 ways to look at mathematically
 Box: Mass Balance
 Flux = mass / (time x area)
 Follow one particle
 X, y, z, and time
 Mass Transport
 Pollutant has mass so can be tracked
 Models use mass
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Momentum transfer
 Air pollutants can move horizontally
 As fast as wind
 wind speed
 Flow = advection,
 In direction of wind (wind direction)
 Wind Rose
 activity later w James
 Pressure currents in atmosphere
 Diffusion different than Dispersion
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Heat transfer
 Air pollutants can move vertically
 Convection
 Cities as “Heat Island”
 Air temperature changes with altitude
 Ambient air temperature decreases as you go up
 Colder on mountain than in Phoenix
 Pressure changes with altitude
 Pressure decreases as you go up
 Less Molecules on top of you at 7000 ft
 1 atm = sea level
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Changes in meteorology and
climatology
 Molecules have mass and are transported
 Heat transfer
 Temperature difference
 Momentum
 Wind speed
 Wind direction
 Pressure systems
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Atmospheric stability
 When air overhead is cold compared to air near
ground, vertical motion stronger. (Unstable)
 When air overhead is closer to ground
temperature prevents air from moving vertically.
(Stable)
 When air is warmer overhead than near ground,
“inversion” (molecule dance lid)
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How does stability affect
what I can see?
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Smoke plume behavior – a useful indicator
Predict Good burning days
Models use “stability class”
How can pollutants concentrate?
 Under an inversion
 Maximum mixing height
 Mixing height:
 Height plume will rise to given prevailing atmospheric
conditions
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Predict Stability by slope
 temperature change  altitude change
 Can be positive or negative slope
 Ambient Lapse Rate
 Recorded by weather stations
 Dry Adiabatic Lapse Rate (DALR)?
 Theoretical line with constant slope
 Slopes are rates (per time)
 How dry air rises and falls
 Cools 1° C for each 100 meters rise
 Warms 1° C for each 100 meters fall
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DALR
 Dry adiabatic lapse rate
 Air expands as pressure decreases
 Function of elevation
 Rate at which dry air cools at it rises
 Adiabatic = no heat exchange
 Approx. 1° C for every 100 meters
 Speed pollution disperses & diffuses
 Based on “Air stability”
 Relationship between ambient & DALR
 Compare slopes
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Mixing Height: Adiabatic compared to ambient
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Atmospheric stability
 See Pictures comparing Slopes
 Neutral= DALR slope
 Superadiabatic
 Unstable air favors dispersion
 Molecules moving
 Subadiabatic
 Stable air so poor dispersion
 Inversion
 Warm air over cold air
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Slopes of different
conditions
DALR
subadiabatic
Inversion
superadiabatic
Dry adiabatic lapse rate = neutral
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Stability affects plume shape
Series of pictures to help you understand new vocabulary
smoke stacks image from Univ. of Waterloo Environmental Sciences
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Stability affects plume shape
 Superadiabatic
 looping plume
 Adiabatic
 coning plume
 Inversion
 Fanning plume
 Inversion over superadiabatic
 fumigation
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Atmospheric Stability
Classes
 How is stability classified?
 Stability classifications
 A = strongly unstable
 C = slightly unstable
 E = slightly stable
B = moderately unstable
D = neutral
F = moderately stable
 How does stability relate to air pollution?
 UNSTABLE
 Good vertical mixing & dispersion of pollutants
 STABLE or INVERSION
 Poor vertical mixing & dispersion of pollutants
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Vertical Dispersion & Diffusion
 Worst pollution episodes often correspond to
inversion trapping pollution near ground during
calm periods
 Shallow inversions common at night & in winter (can
be especially strong in geographical basins where cold
air pools)
 Deeper inversions can be caused by large-scale
subsidence of air. As air moves toward ground,
compressed and heated. Can lead to a capping
inversion layer 3000 to 6000 feet off ground
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 What do you think is happening here?
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Another Example
http://eol.jsc.nasa.gov/debrief/Iss007/topFiles/ISS007-E-13281.htm
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Upper Air Data
 Soundings
 Radiosondes
 Dropsoundes
 Rocketsondes
 Isotherms
 Isobars
 Isohumes
What We Just Covered
 Pollutants move and spread in air
 Diffusion and Dispersion
 Vertically and horizontally
 Transport phenomena
 Weather conditions dictate transport
 Actual temperature profile=ambient
 DALR = theoretical comparison
 Speed pollution disperses & diffuses
 Based on Air stability class
 Source height & mixing height
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Laboratory: Create an
Inversion
Hands-on exercise in
stability
(see manual)