Download - ENERGYPHYX 1020USU 1360 CHAPTER 9 - AIR POLLUTION 2002 1 CHAPTER 9 - AIR POLLUTION

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ENERGYPHYX 1020 USU 1360C

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CHAPTER 9 - AIR POLLUTION

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Our Atmosphere (1)• Many charts showing atmospheric properties give the impression that the

atmosphere is a thick mantle around the earth.– In fact this is an erroneous impression because altitude scales are not plotted in the

context of the earth’s radius.– The atmosphere is a thin layer of gases clinging to the earth by gravitational force– 50% of the atmosphere is below an altitude of 18,000 feet (~5.5 km)– 99% of the atmosphere is below an altitude of 20 miles (~33 km)– The radius of the earth is 3,958 miles (6,370 km)

• Up to about 100 km altitude the atmosphere is composed mainly of nitrogen (N2) (78%) and oxygen (O2) (21%)– The remaining 1% is composed of trace elements , carbon dioxide, neon, argon,

helium, ozone … which although in small quantities play an important role in the habitability of the earth for animals and plants.

– The following table shows the relative abundance of the atmospheric constituents

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Our Atmosphere (2)

G

G

TR

AC

E G

AS

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G Greenhouse gas

G

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Our Atmosphere (3)

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Atmospheric Regions

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Local Pollution

• Most of the pollution in our atmosphere comes from the conversion of fossil fuel chemical energy to heat energy by the process of burning.

• In this section we will focus on local pollution in the lower troposphere down to ground level.

• Global pollution effects associated with the same energy conversion processes will be discussed separately

• Carbon dioxide does not contribute to local pollution, so even though it is produced in the largest quantities by burning fossil fuels the discussion of this pollution will be deferred

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Thermal Inversions• Local pollution can be minimized if the polluting material can be

dispersed rapidly from its point of origin.– The dispersal may be horizontally by winds.– It may be vertically upwards by convection currents.– It may be vertically downwards by rain.

• Thus in calm, dry weather (anti-cyclonic) we must rely on vertical dispersion.– The stability of the lower atmosphere with respect to vertical mixing is

determined by the vertical temperature variation from the ground upwards.– The range of vertical temperature profiles which occur are shown in the

following diagram.

• The average temperature-altitude relation is called The Adiabatic Lapse Rate (ALR)– Adiabatic - no heat in or out of a rising packet of air– Lapse - temperature decreases with altitude– Average ALR is -0.65°C/100m (3°F/1000 feet)

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Atmospheric Vertical Stability

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Inversion Temperature Profiles

• Positive temperature gradients lead to stable air and build-up of pollutants.

Radiative InversionHigh PressureSubsidence

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Major Sources of Pollutants

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SMOG• Smog is the term given to fog caused by pollutants rather than for

resulting from high levels of moisture in the atmosphere.• Smog results from a wide range of pollutants:

– Carbon Monoxide CO– Nitrogen Oxides NO, NO2

– Sulfur Dioxide SO2

– Ozone O3

– Particulates Small solid particles– Aerosols Small liquid droplets

• If these pollutants are emitted into a stable, windless atmosphere characterized by an increasing temperature with altitude, they will accumulate and build up in concentration to form a smog.– It may be more of a haze with adequate visibility for driving as formed by

vehicles in large cities– It may be dense with low visibility for driving as in the “London Fogs”

• If the pollutants build up for several days of stable atmosphere the results can be fatal to some of the population.

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Smog Kills

• Smoke represents particulates from open coal fires.

• Also the SO2 levels are high because of the high sulfur content of coal

London Smog, 1952

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Photochemical Smog• The smog which occurs in many large cities (including Salt Lake City) is

called photochemical smog.– It is characterized by a hazy appearance which often has a yellowish/brownish tinge due

to the color of NO2

– The oxides of nitrogen are a result of burning fossil fuel (gasoline) at high temperature in the engine cylinders when the nitrogen in the air combines with the oxygen.

– Smog can be very irritating to the eyes and the lungs and the CO in it is a poisonous gas reducing the ability of the blood to carry oxygen.

– The name comes from the fact that sunlight interacts with the pollutants, particularly the aerosols, to form complex molecules of hydrogen, carbon and nitrogen call PAN (peroxyacyl nitrates).

• The formation of city smog can be made worse by geographical features such as mountain ranges surrounding the city which reduce horizontal dispersion of the smog.

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Smog Reduction Measures• “London Fog” type of air pollution can be virtually eliminated by

drastically reducing open burning of fossil fuel or biomass in fireplaces.– This was very effective in London which has not had severe pollution fog

since 1962.

• Reduction of vehicle pollutant emissions– Computer control of fuel-air mixture ensured more complete combustion

of the gasoline.– Sealed fuel systems– Elimination of the carburettor

– Catalytic converter to decompose NO to N2 and O2 before it leaves the tailpipe .

• Forced gasoline refiners to reduce lead in gasoline to prevent poisoning of the catalyst

– Annual emission testing mandatory in large urban districts.

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Sulfur Dioxide• We saw that many forms of coal have a high sulfur content• When this coal is burned in power plants the sulfur combine with oxygen in

the air to form sulfur dioxide (SO2)• SO2 escapes into the atmosphere where it reacts with water droplets in clouds

to form sulfuric acid (H2SO4)• This eventually falls as rain composed of dilute sulfuric acid rather than pure

rainwater. So called acid rain.• The pollution is localized near power plants, and has had devastating effects

on plant life and lake fish, particularly in the industrial NE .• The acid rain is also very bad for buildings ornamented with limestone or

marble or held together with mortar, all of which dissolve in sulfuric acid.• Newer power plants contain and upgrades to older plants are being made to

reduce SO2 in the flue gases– Produces more solid waste– Increases cost of generating electricity

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Particulates• The widespread use of coal-fired power plants is responsible for

pollution by particulates.– The solid particles in the ash in the flues of coal furnaces are 20 times

greater than the aerosols emitted by gasoline or oil burning

• The location of coal fired plants near centers of population put more people at risk from the health consequences of particulates than from natural events such as volcanoes or forest fires.

• Also certain industrial processes such as steel manufacture and cement manufacture produce large amounts of pollutants.

• There are techniques for “scrubbing” the flue gases to reduce the emission of particulates, again adding to the cost of the product being made.

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Controls Work• Through the efforts of the Environment Protection Agency

atmospheric pollution is showing a downward trend after ~70 years of approximately matching the trend of the increase in population growth.

Note that after the early 1970’s there has been a continuous downward trend in the common local pollutants

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Learning Objectives (1)• Be aware that the atmosphere is a thin layer of gasses over the

earth’s surface• Know that 50% of atmosphere is below 5.5 km. 99% below 33 km

in the context of the earth’s radius of 6370 km• Know that the principal constituents of the earth’s atmosphere are

nitrogen (78%) and Oxygen (21%)• Be aware that there are many trace gases which, even in small

quantities, can greatly affect the quality of the air we breathe.• Know that the atmosphere is divided into vertical regions

determined by maxima and minima in the atmospheric temperature.

• Know that the lowest region of the atmosphere is called the troposphere and the region above is the stratosphere.

• Be aware that local pollution occurs primarily in the troposphere.• Be aware that the conversion of fossil fuel chemical energy to

other forms of energy produces atmospheric pollution.

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Learning Objectives (2)• Know three methods by which pollution may be dispersed.• Be aware that the vertical stability of the atmosphere is determined

by the vertical temperature variation.• Be familiar with the term Adiabatic Lapse Rate (ALR) to describe

the average rate of decline of atmospheric temperature with altitude.• Know that the ALR is about 3°F/1000 feet.• Understand that lower lapse rates of increasing temperature with

altitude is increasingly stable and inhibits vertical mixing.• Know what is meant by a temperature inversion and why it occurs at

ground level and at higher levels.• Realize that highway vehicles are the major producers of local

pollutants.• Be aware that the major local pollution in urban areas is caused by

carbon monoxide and oxides of nitrogen.

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Learning Objectives (3)• Know that other forms of deadly smog can be produced from particulates

and sulfur dioxide produced by burning coal.• Understand that in sunny urban areas the smog situation is worsened by

the formation of photochemical smog.• Know that the sunlight causes smog gases to react to for compounds with

the generic acronym PAN.• Be aware that sulfur dioxide in the atmosphere is dissolved by water

droplets to eventually fall as acid rain.• Realize that the sulfuric acid in acid rain is very harmful to fish, plants

and buildings.• Understand that controls have been applied to reduce local pollution

gases • Open coal fires are restricted.• Anti pollution measures are applied to cars.• Particulate removal from coal fired power plants• Know that there has been a decrease in total pollutants since 1970 after

70 years of growth

Download - ENERGYPHYX 1020USU 1360 CHAPTER 9 - AIR POLLUTION 2002 1 CHAPTER 9 - AIR POLLUTION

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