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
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