Air Pollution and Air Pollution Control

AirTroposphere• “ambient air” nearest to Earth's surface• weather occurs here• Properties:

– approximately 80% of the atmosphere's mass – 99% of its water vapor and aerosols– average depth of approximately 17 km (11 mi)

in the middle latitudes– deeper in the tropics, up to 20 km (12 mi)

Air

• Troposphere– Ozone here is considered a pollutant

• Stratosphere – situated between about 10 km (6 mi) and 50

km (30 mi) altitude– location of protective ozone layer: absorbs

high energy UV B and UV C energy waves from the Sun

• Three more layers above

The Earth’s Atmosphere

• Earth's Atmosphere Has Changed Dramatically since the Time it was Formed– some changes the result of chemical and

physical characteristics of the earth and the gases

The Earth’s Atmosphere: Current Theory of Evolution of Atmosphere

Early Earth • no oxygen at that time• Instead high in CO2, H2O

vapor, and probably NH3, H2 and CH4

Now• considerable reduction in

CO2; significant increase in O2

If true,• Where did the “C” of CO2

go?

Current Composition of Atmosphere

78.08% N2

20.95% O2

0.9% Ar

0.04% CO2

• trace amounts of gases, vapors, particulates– naturally occurring: Ne, He, Kr, Xe, H2, CH4, O3, N2O

these have remained relatively constant for at least several hundred million years

Current Composition of Atmosphere

whatever humans emit is considered ‘trace amounts’

– example CFC = 0.000000002%

But still can have a significant impact

Atmosphere Bombarded by Solar Radiation: Greenhouse Effect Keeps Earth Warm

Some is reflected back to space by stratospheric ozone or clouds while others penetrate

Air Pollutants

Air Pollution

Accumulation of substances in the atmosphere that can cause harmful health effects to living things or can negatively affect the public welfare

Fate - Matter cannot be created nor destroyed, it only changes form:

• Pollutants just don’t disappear, they may– Dilute - Dispersion– Change form - Degradation

• ‘Sink’ – Deposition, Transfer to other compartments

Effects of Air Pollutants

• Wildlife health including forests

• Impact on agriculture

• Buildings and other property

• Visibility and navigation

• Human health effects:

• Irritation of the respiratory passages including nose, throat,

trachea and bronchii

• Eye irritation

• Increased susceptibility to the common cold

• Neural disorders (from toxics like lead)

• Other toxic effects

Air Pollution Sources

• Anthropogenic• Geogenic• Biogenic

Anthropogenic Sources• Stationary Sources

– Stack emissions– Fugitive emissions– Major stationary sources– Small sources

• Mobile Sources• Accidental spills

Air Pollutant Types and Sources

Energy-related?

(ozone related)

Other Sources

• Geogenic– volcanoes– dust storms

Other Sources

• Biogenic– decomposition gases– organic molecules released from plants and

animals• Pollen• Insect parts

– cow gas?

Progress Being Made?

Other Pollutants

Hazardous Air Pollutants (HAPS)• asbestos, benzene, mercury, vinyl

chloride, radionuclides, numerous others– Each has its own sources and effects– Toxicity/carcinogenicity, persistence, and

bioaccumulation the key issues

General Air Pollution Control Strategies

• Source Shutdown• Source Location• Fuel Substitution and Process Changes

– Very successful with autos

• Correct Operation and Maintenance, Efficiency• Emissions Trading• ‘End of Pipe’ Pollution Control Technologies

‘End of Pipe’ controls

• Consider only after other options are exhausted?

• Selection heavily regulated by environmental statutes

Pollution Control Technologies

• Form of substance dictates control technology used– Gases/vapors– Particulates

• Physical and chemical properties of the pollutant also dictate which particular methods will be effective

Control Technologies

• Various levels of control technologies exist• Examples:

– Best Available Control Technology (BACT)– Lowest Achievable Emissions Rate (LAER)

• Ultimately, choice depends on:– Form of contaminant– Regulatory requirements for a particular industry– Quantity generated– Installation costs– Maintenance and operational costs

Characteristics of Pollutants

• Physical state• Particulate Size (µm)• Size distribution of particulates• Density• Reactivity

Characteristics of Carrier Gases

• Flow rate (cfm)• Moisture content• Temperature• Flammability

Particulate Emissions Control

Common control devices include• Settling chambers• Cyclones• Wet scrubbers• Electrostatic precipitators• Fabric filters (baghouse filters)

Settling Chambers

Cyclones

Wet Scrubbers

Electrostatic Precipitator

Baghouse Filters

Gaseous Emissions Control

Four Major Techniques• Adsorption• Absorption• Condensation• Combustion

Adsorption

• Attachment of gas/vapor to a solid

Absorption• Dissolving gaseous

pollutants in a liquid

Condensation

• Removal of gas or vapor by condensing it into liquid form

• Uses changes in temperature and/or pressure

• The pollutant(s) must be condensable at achievable temperature

Combustion