Transport & Deposition of Air Pollutants

David Gay

CoordinatorNational Atmospheric Deposition ProgramUniversity of Illinois, Champaign, IL217.244.0462, dgay@uiuc.edu

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The Basics

1. The Atmosphere has no boundaries

2. What goes into the Atmosphere, must come out.emissions & deposition

3. It just doesn’t come out in the same placetransport

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If…

1. If you know when, where and how much of a pollutant is emitted

2. If you know where it will move to…

3. If you know how it will change while moving…

4. And if you know how, where, and when it will deposit…

Then you have the answer….

The Cycle we will discuss

Emissions•When•Where•How much &•What compounds

Transport•where•How fast•Will it react chemically on the way? (atmospheric chemistry)

Removal•How, where & when•By what mechanism (deposition, rain, impaction)

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The driver of transport and removal is physics & chemistry of the atmosphere

We are generally only interested in troposphere◦ First 10 kilometers of the atmosphere◦ Where the weather occurs

In the vertical direction◦ Temperature varies with altitude

◦ Typically warm at the surface, cold aloft, which makes the atmosphere buoyant

◦ Density decreases as we move up

◦ Windspeeds typically increase

Emissions

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Pollutant Emissions…..

Emissions are made up of a variety of ◦Solids (particles and aerosols)◦Gases◦Conversion between gases & solids

(and back again)

And a variety of emission sources:◦Natural◦Anthropogenic

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Transport & Chemical Changes

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Transport & Chemical Change

1. Transport is the collection of mechanisms that moves pollution

2. Chemical changes occur during the movement

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Transport of Pollutant has 4 Keys

1. Diffusion◦ Random motion of the molecules

2. Advection◦ movement due to movement of the atmosphere

3. Lifting & Buoyancy & Stability◦ Turning over of the atmosphere

4. Transformation (chemistry)◦ Reaction to form different compounds

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Diffusion an intermingling of molecules, ions, etc., resulting from random thermal agitation, as in the dispersion of a vapor in air.

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Advection the transfer of heat or matter by the flow of a fluid, especially horizontally in the atmosphere or the sea.

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Lifting & Buoyancy & Stability

buoyancy is an upward force exerted by a fluid that opposes the weight of an immersed object.

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Chemical Reactions During Transport

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another.

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It can be very difficult to track movement

◦ Number of sources of same pollutant◦ Several large sources◦ Multiple small sources

◦ Different number of Pollutants◦ Sources that move, such as

Vehicles◦ Meteorological Effects

◦ Wind speeds different at different heights

◦ Updrafts and downdrafts

◦ Topography◦ Mountains and valleys

◦ Diurnal patterns ◦Day to Night◦Heating cooling, changes in wind, solar radiation

◦ Height of release◦In valley◦From stacks

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Different Scales of Transport

◦Local◦Urban◦Regional ◦Continental◦Global

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Some Basics of Transport

Stable atmospheric conditions mean less atmospheric mixing, therefore higher pollutant concentrations at ground-level

◦ No air movement◦ Trapping of air◦ Emissions build◦ e.g., lack of wind; hot, dry days

Unstable atmospheric conditions allow ground-level pollution to readily disperse

◦ Ground-level concentrations reduced ◦ Recharging occurs more frequently◦ Example: cold fronts & thunderstorms

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Some Basics of Transport

Key weather parameters◦ Sunlight

◦ Temperature

◦ Vertical temperature structure

◦ Surface winds

◦ Aloft winds

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Some Basics of Transport (cont.)

Synoptic-Scale Flow (weather systems)

◦ Ridge – high pressure

◦ Trough – lower pressure

◦ Frontal Boundaries

Black lines are isobars= contoured areas of same

atmospheric pressure

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Chemical Reactions: oxidation

• The atmosphere is dominated by oxygenMost primary pollutants become oxidized

–Sulfur–Nitrogen–Ammonia–Carbon compounds

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Chemical Reactions: Photochemistry

Removal

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Depositionairborne pollutants deposited back on land and water, either

◦ Wet Deposition◦ i.e., in precipitation ◦ Washout ◦ Rainout

◦ Dry Deposition◦ Deposit due to gravity◦ Both particles and gases

Often have undergone chemical transformation

Often transported great distances in atmosphere

Wet Deposition: Rainout & Washout

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Removal Processes for Particles: Deposition

When particles collide◦ Get larger through agglomeration (stickiness)Gravity pulls them down◦ Settling out, or sedimentation to the surface◦ “dry deposition”Washed out of the atmosphere◦ “wet deposition”

◦ Snowflakes◦ Rain◦ Hail◦ Mist ◦ Fog

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Removal Processes (cont.)

◦Others leave by impaction and adherence◦ Vegetation◦ Soil ◦ Buildings◦ Metals

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Removal Processes (cont.)

◦Sinks: places where pollutants are deposited◦ Water bodies (ocean!)◦ Soil◦ Buildings◦ Retention properties

Modeling

Atmospheric Model:An atmospheric model is a mathematical model constructed around the full set of primitive dynamical equations which govern atmospheric motions. It can supplement these equations with parameterizations for turbulent diffusion, radiation, moist processes (clouds and precipitation), heat exchange, soil, vegetation, surface water, the kinematic effects of terrain, and convection. 

Several types of models, but the basic idea is:•To calculate the 3-dimensional flow (advection)•the energy flow (sunlight, temperature, etc.), and •the chemical changes (chemical reactions)

• Tend to focus on one particular scale• global, local, urban, etc.

• Many that we would be interested in, and attempt to calculate the chemical reactions

• Typically trying to estimate location and ultimate disposition (wet deposition, etc. of the pollutant)• When• Where• How

Intercontinental Transport of Pollutants

Short Video

Intercontinental flowMovie 1 & 2

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Thank You

Questions?