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)