Transport of Air Pollutants Marti Blad, Ph.D., P.E. Yavapai Apache Nation.
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Transport of Air Pollutants
Marti Blad, Ph.D., P.E.Yavapai Apache Nation
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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)
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 transferTemperature difference
MomentumWind 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?
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
Inversion
superadiabatic
subadiabatic
Dry adiabatic lapse rate = neutral
DALR
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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 B =
moderately unstable C = slightly unstable D = neutral E = slightly stable 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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http://eol.jsc.nasa.gov/debrief/Iss007/topFiles/ISS007-E-13281.htm
Another Example
Upper Air Data
Soundings
Radiosondes Dropsoundes
Rocketsondes
Isotherms
Isobars
Isohumes
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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
Laboratory: Create an Inversion
Hands-on exercise in stability
(see manual)
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