07-Air Pollutants And

7/28/2019 07-Air Pollutants And

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Air Pollutants andGlobal Climate

Hsin ChuProfessorDept. of Environmental Engineering

National Cheng Kung University


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In this chapter we consider three air pollution

problems in which humans may be making

large-scale changes in our planet.

Air pollution laws in the U.S. and most other

countries are based on the assumption that

air pollution is a local matter.


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Local solutions are not available for global

problems or for problems of pollutants like

acid rain that cross international boundaries.

In addition, some parts of the global climate

overshoot.

They continue to change in the direction they

are changing even after the cause of thechange has been reduced or withdrawn.


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1. Global Warming

Humans are putting gaseous materials into

the atmosphere that may cause the earths

average temperature to rise.

This is called global warming, or the

greenhouse effect.


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Example 1

Estimate the average temperature that the earth would have ifit had no atmosphere.

Solution:The total radiant energy flux from the sun, just outside theearths atmosphere, is 1.353 kW/m2.

The diameter of the earth is 12.75106 m so that, if all theincoming solar energy were absorbed by the earth, the totalheat flow in from the sun would be

2 6 2

2

14

(12.75 10 ) 1.3534 4

1.73 10

kWD flux m

m

kW


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The total heat radiated to outer space would

be this amount plus the amount produced on

earth by nuclear decay and tidal friction with

the moon, which together are less than 0.1%

of the solar energy inflow and can be safely

ignored.


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The outward radiation (assuming a zero temperature

for outer space and blackbody radiation), using the

surface area of the earth rather than the projected

area, is

where = the stefan-Boltzmann constant =

5.67210-11

kW/(m2

K4

). Setting these equal and solving for T, we find 278 K

= 5oC. #

2 4 6 2 11 4

2 4(12.75 10 ) 5.672 10

kWD T m T

m K


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This is approximately 10oC below the

observed average surface temperature of the

earth, which is about 15oC.

Thus, the net effect of having an atmosphere

is to raise the average temperature of the

earth about 10oC above the value it would

have with no atmosphere, if the earthabsorbed all incoming sunlight.


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The earth actually reflects roughly 30% of all

the incoming solar radiation back to outer

space from the tops of clouds, ice surfaces,

oceans, etc. (Technically, the earths albedo

is about 0.3.)

The moon, which has no atmosphere and

hence no clouds, surface water, or ice sheets,reflects about 12% of its incoming solar

radiation.


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If the atmosphere let the same amount of

sunlight in as it actually does but did not

prevent the outward flow of radiant heat, then

we should multiply the incoming solar

radiation in Example 1 by 0.7, finding an

average surface temperature of 254 K =

-19o

C, and a frozen world.


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Example 2

What fraction of the outgoing radiation fromthe earth is blocked by the atmosphere?

Solution:As just discussed, we assume that 30% ofthe incoming solar radiation is reflected away,

and use an average surface temperatureover the whole planet of approximately 15oC= 288.15 K.


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Setting incoming approximately equal to outgoing and solving

for the fraction emitted, we have

We see that for the earths surface temperature to averageabout 15 oC, the atmospheric outward transmission of radiant

energy must be (0.606/0.7), or 86% of the inward transmission

of solar energy. #

2 4

2 4

14

6 2 11 4

0.7(total solar input)fraction emitted

0.7(1.73 10 )

(12.75 10 ) 5.672 10 (288.15 )

0.606

kW

m K

D T

kW

m K


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Clouds block radiation, both inbound and

outbound.

Cloudy days are cool and cloudy nights are

warm relative to clear days and nights at the

same season.

They are more or less equal in their

resistance to incoming and outgoing radiation.


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The same is not true for clear air, which

contains CO2, H2O, CH4, and some other

gases that can absorb radiant energy.

If the wavelengths of the incoming and

outgoing radiant energies were the same,

then these gases would block equal amounts

in both directions. But the wavelengths are quite different.


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Fig. 14.1 (next slide) shows the absorptive

properties of the clear atmosphere (without

clouds, dust, birds, insects) and some

properties of the incoming solar radiation and

the outgoing thermal radiation from the earth.


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The interaction of a photon with a gas

molecule is quite different from that with a

cloud droplet or with a fine particle.

A gas molecule will absorb a light photon if

the gas molecule can make an internal

rearrangement that requires the same

amount of energy as that carried by thephoton.


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For wavelengths shorter than about 0.28, the

internal transitions involve shifts of electrons in their

orbitals around the nuclei of one or more of the

atoms that make up the molecule, but not anychange in the relation of one atom to another within

the molecule.

For the wavelengths longer than about 1, the

changes are not within the individual atoms but arethose associated with the vibrations of the various

atoms in the molecule, relative to each other.


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In the 0.28 to 1 window, the photons have

too little energy to cause shifts of electron

orbitals, and too much energy to be in tune

with intermolecular vibrations.

The H2O absorption peaks shown on Fig.

14.1 are caused by the various intermolecular

vibration modes of the water molecule.


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The lower part of Fig. 14.1 shows the

distribution of energy in sunlight and in the

infrared radiation from the earth.

These are idealized values for blackbody

radiators at 6000 and 288 K, which

correspond roughly to the average surface

temperatures of sun and earth.


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The quantity plotted is the fraction of the total

emitted energy per micron of wavelength,

which has a higher maximum (14% per

micron) for the sun than for the earth (7% permicron) because the suns spectrum is

narrower. (Observe the logarithmic scale for

wavelength.)


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Weins law for blackbody radiatin is

which shows that for the temperature of the suns

surface, about 6000 K, the peak intensity is at 0.50,

corresponding to visible light.

For the earths surface temperature of about 288 Kthe peak intensity is 10.3, which is in the infrared

region.

3

max

wavelength of 2.987 10

maximum emission

m K

T


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Comparing the lower and upper parts of Fig. 14.1,

we see that sunlight comes to the surface practically

unimpeded except for cloudy areas, whereas the

peak radiation from the earth is close to the 8- to 12- window, which is not as wide nor as completely

open as the window for solar energy.

This is the main reason that the atmosphere is less

transparent for outgoing infrared energy than it is forincoming solar energy.


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Fig. 14.1 shows that CO2, CH4, N2O, and

H2O all have some absorption in the 8- to 12-

window.

The same is also true for chlorofluorocarbons,

or CFCs.

They are collectively called greenhouse

gases.


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Human activities are increasing theconcentrations of greenhouse gases in theatmosphere.

Of these gases, the strongest contributor toreducing the transparency of the 8- to 12- window is water vapor.

However, humans do not directly influence itsconcentration in the atmosphere, and it is notnormally a part of the discussion of thegreenhouse effect.


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Fig. 14.2 (next slide) shows a very simplified

view of the interactions and feedback loops

involved in the global temperature.


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Increasing the global temperature by adding

greenhouse gases will have positive and

negative effects on the albedo by increasing

cloudiness and reducing snow and ice over,and will further close the IR window by

increasing the average water vapor content

of the atmosphere and the averagecloudiness.


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Fig. 14.3 (next slide) shows the calculated

relative contributions of the various green-

house gases to the reduction of transparency

of the atmosphere in the 8- to 12- windowfor the period 1980 to 1990.


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If rising temperatures were to melt the ice cap

in Antarctica, the world sea level would rise

several hundred feet, flooding most of the

coastal cities and agricultural areas of theworld.

Temperature increases much smaller than

those needed to melt the ice caps wouldcause the deserts and the temperature zones

to extent farther from the equator.


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Agricultural area that are currently highlyproductive would become dryer and hotter,while sub-Arctic regions would become

warmer and wetter. The best current estimates are that, for a

business as usual projection of futureemissions of all greenhouse gases, the global

mean temperature will increase by 0.2 to0.5oC (best estimate 0.3oC) per decade forthis century.


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The corresponding projection of world sea

level is for a rise of 3 to 10 cm/decade (best

estimate 6 cm/decade) over the same period.


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1.1 Carbon Dioxide

CO2 is a colorless, tasteless gas that

provides the carbonation in soft drinks and

sparkling wines.

It has been part of the earths atmosphere as

long as the earth has had an atmosphere.

The current carbon dioxide concentration in

the world atmosphere is approximately 380ppm (year of 2006).


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Geologic records show that the CO2 content

of the world atmosphere before about A.D.

1750 was 280 10 ppm and did not move out

of that range for hundreds or thousands ofyears.

About 1750 humans began to burn increasing

amounts of fossil fuels, and the CO2 contentof the global atmosphere has risen.


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Fig. 14.4 (next slide) shows CO2

concentrations from 1960s to 1990s.

During that period, the annual increase in

CO2 concentration was 1.5 ppm/yr.


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Fig. 14.5 (next slide) shows the estimated

reservoirs and flows for carbon on earth (To

convert from carbon to CO2 multiply by

44/12).


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The global annual fuel combustion CO2 emissions

are:

The first term, the global population, is growing at

about 1.4% per year (population doubles every 50years), and that growth rate shows little sign of

slowing.

2

2

Global fuel

CO emissions perglobal per capitacombustion (1)population fuel use unit of fuel use

CO emissions


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The second term is highly variable from

country to country.

To compare energy uses, we need a proper

standard energy unit.

The most intuitive unit is the minimal energy

intake, as food, that a normal human needs,

about 2750 kcal/day (4 million BTU/yr). Using it, we can make Table 14.1 (next slide).


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In the U.S., they use a total of about 79 times

as much fuel as the minimum needed to feed

themselves.

If we had made a similar table for the

average person in the U.S. is 1850, or the

average person in the Third World today, we

would have seen that they used or useperhaps three to five times the energy they

needed as food.


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Americans use probably 15 to 30 times as

much fuel per person as they do and live a

much more physically comfortable life.

If the people in the Third world are to live as

Americans do, then world fuel consumption

will grow dramatically.


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The third term in Eq. (1) depends on the

hydrogen/carbon ratio of the fuel burned.

For equal amounts of energy released, the

relative CO2 release rates are approximately

coal, 1.0; oil, 0.8; and natural gas, 0.6.


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It is possible to capture CO2 from combustion

exhaust gas and prevent its release, but only by

using chemicals like CaO, whose production leads

to the release of more CO2. the only methods we now know to slow or stop the

buildup of CO2 in the atmosphere are to reduce the

use of fossil fuels and to stop the deforestation of

the tropical rain forests. Solar, wind, hydroelectric, geothermal, and nuclear

energy release much less CO2.

1 2 Oth G h G


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1.2 Other Greenhouse Gases,

Aerosols

CFCs are apparently next in greenhouse effect after

CO2 and will be discussed in next section.

Next in importance is methane, the principal

component of natural gas, which is formed in manyanaerobic biological processes.

It is the principal component ofswamp gas, which

is produced by bacterial decay of woody matter, and

is a major component of the waste gases producedby landfills and sewage treatment plants.


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It is also emitted by almost all animals; our

domestic dairy and meat cattle and pigs are a

significant worldwide source.

In preindustrial times the world atmosphere

contained ~ 0.7 ppm of methane.

Over the past century that has increased to

~1.7 ppm, and it is increasing by about 0.01ppm/yr.


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A methane molecule is roughly 20 times as

strong an infrared absorber as a CO2

molecule, so that even at this low

concentration methane can play a significantrole.

The remaining important greenhouse gas is

nitrous oxide, N2O, which formerly was oftenused as a dental anesthetic (laughing gas).


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N2O is not believed to have any harmful

effects as an air pollutant except in its role as

a greenhouse gas.

One N2O molecule is roughly 200 times as

effective as one CO2 molecule in reducing

the transmission in the 8- to 12- window.


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The sources and sinks for N2O are not as

well known as those for the other greenhouse

gases.

There is some concern that the NOX control

technologies that reduce NO with NH3 and its

near chemical relatives may produce

significant amounts of N2O. Table 14.2 (next slide) summarizes current

information on greenhouse gases.


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The earths average temperature can also be

altered by an increase in the content of fine particles

of the atmosphere.

For atmospheric particles to have effects lastingmore than a few days, they must be injected into the

stratosphere (above about 36,000 ft) because there

is little mixing between the stratosphere and the

troposphere, so that particles in the stratospherehave life times measured in years.


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Few human activities place many particles in

the stratosphere.

However, such particles can be injected into

the stratosphere in large quantities by major

volcanic eruptions.

There they cause a lowering of the global

temperature, generally for only a year or twoafter the eruption.


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They lower global temperature because they

are generally close in size to the wavelength

of light (0.4 to 0.7 ) and hence effective in

scattering light and reducing the amount ofincoming sunlight.

However, these particles are much smaller

than the wavelength of outgoing infraredradiation, and hence less effect in scattering

it.

2 St t h i O D l ti


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2. Stratospheric Ozone Depletion

and Chlorofluorocarbons

The second global problem concerns the

possible destruction of the stratospheric

ozone layer.

At ground level, O3 is a strong eye and

respiratory irritant and a major component of

photochemical smog.

It may also act as a greenhouse gas.


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In the stratosphere, 10 to 20 km above the

earths surface, is a layer of low-density air

containing 300 to 500 ppb of ozone.

Ozone is the only component of the

atmosphere that absorbs significantly at the

wavelength below 0.28 (far ultraviolet).


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If that ozone layer were removed, we would

expect large amounts of ultraviolet light to

reach the surface of the earth.

The high-energy photons are expected to

cause increased rates of skin cancer in

animals and harmful effects on plants.

Thus ozone is a harmful pollutant at groundlevel, but a beneficial ultraviolet shield in the

stratosphere.


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Destruction of the ozone layer is mostly caused byelemental chlorine atoms; the mechanismapparently involves two reactions:

Cl + O3 ClO + O2 (2)

ClO + O3 Cl + 2 O2 (3) Other reactions are going on in the stratosphere that

modify and complete with these two, but if we ignoreother reactions, add these two reactions, and cancel

like terms, we see that the overall reaction is:2 O3 3 O2

with no net consumption of Cl atoms.


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Thus one Cl atom can convert many ozone

molecules to ordinary oxygen molecules.

One sees estimates from 104 to 106 O3

molecules destroyed by one Cl atom.

This mechanism is often referred to as

catalytic destruction of ozone, because the

chlorine atom acts as a nonconsumedcatalyst for the reaction.


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Most of the chlorine in the world is in the form

of chemically stable NaCl either dissolved in

the oceans or in underground salt deposits

formed by the evaporation of ancient oceans.

Elemental chlorine, a very reactive chemical,

has a short lifetime in the lower atmosphere

and has few natural ways to get from thelower atmosphere up to the ozone layer.


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The only naturally occurring chemical thatcan transport much chlorine high enough intothe atmosphere to damage the ozone layer is

methyl chloride, CH3Cl, which is produced inlarge quantities by biological processes in theshallow oceans.

Most of it is destroyed in the troposphere, but

an estimated 3% of worldwide methylchloride emissions reaches the stratosphericozone layer.


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Chemically active ultraviolet light in the 0.2- to 0.28-

range, which enters the ozone layer but does not

penetrate below It, is strong enough to split up

methyl chloride and the other chlorine compounds,releasing Cl atoms, which initiate Eq. (2).

Before we had synthetic halogen compounds,

methyl chloride was probably the principal natural

destroyer of the ozone layer; its destruction of theozone was in balance with natural production

mechanisms, leading to a steady-state ozone layer.


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Starting about 1900, humans began releasing

into the atmosphere synthetic chlorine-

containing compounds in significant amounts.

Those like methyl chloride that havehydrogen atoms can be attacked in the

atmosphere by the OH radical; for this reason

most of them do not survive to reach thestratosphere.


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Carbon tetrachloride, CCl4, has no hydrogen;

most of it is believed to reach the

stratosphere and to participate in the

destruction of the ozone layer. CFCs (compounds containing chlorine,

fluorine, and carbon, commonly called Freons)

were first developed by General Motors foruse in household refrigerators.


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One of their virtues is their chemical inertness;they are nontoxic, nonflammable, invisible,tasteless, odorless, non-almost everything

else. They replaced toxic sulfur dioxide and

ammonia in household refrigerators.

Later, their inertness led to the widespread

use of CFCs as propellants in spray cans anda blowing agent in the production of plasticfoams.


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Then in the 1950s we began to use air conditioners

in autos.

The CFCs used as refrigerants in these are much

more likely to leak to the atmosphere than the CFCsin refrigerators and home air conditioners because

the shaft-sealing problem on belt-driven auto air

conditioners is more difficult than that on electric-

driven refrigerators and home air conditioners. Auto air conditioners became a major source of CFC

emissions.


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There are many different CFCs; the two most

widely used are CFC12, CF2Cl2, and CFC11,

CFCl3 (the first digit in the number of carbon

atoms, the second is the number of F atoms). CFCs have no H, so they cannot be attacked

by atmospheric OH.


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Trichloroethane, CH3CCl3, is not a CFC because ithas hydrogens and thus can be attacked byatmospheric OH.

But that attack is relatively slow, so that an

estimated 9% of this material that is emitted to theatmosphere makes its way to the stratosphere andparticipates in ozone destruction.

Table 14.3 (next slide) shows the concentrations,

lifetimes, and expected contribution to delivery ofelemental chlorine to the stratosphere for thesechemicals.


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Some other gases can attack the ozone layer, e.g.,

NO from stratospheric air-planes and relatively inert

N2O, if we release much of it at ground level.

NO, released by high-flying aircraft, can contributeto ozone depletion by the reaction:

NO + O3 NO2 + O2

which is swift and practically irreversible.

But it is not a catalytic reaction like the chlorinereaction; one NO molecule only destroys one O3

molecule.


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The only method we know to protect the

ozone layer is to limit the emission of those

materials that can harm it.

Many of the proposed substitutes for CFCsare chlorohydrofluorocarbons, HCFCs, which

contain at least one H atom, and hence are

susceptible to OH attack in the atmosphere.


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3. Acid Rain

The average acidity of rainfall in Scandinavia,

the northeastern United States and Canada,

and parts of Europe has increased over the

past 40 years. There seems no question that this change is

primarily due to the increased emissions of

sulfur oxides and nitrogen oxides that haveaccompanied the greatly increased economic

activity in or upwind of these regions.


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The common name for acid precipitation is

acid rain, but the complete description

includes acidic rain, acidic snow or hail, acids

adsorbed on falling dust particles, etc. The normal technical measure of acidity is pH.

Table 14.4 (next slide) shows pH of various

substances.


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Rain falling through a perfectly unpolluted

atmosphere will arrive at the earth with a pH of about

5.6 because of the carbon dioxide in the atmosphere,

which reacts with rainwater by these reactions:

H2CO3 is a weak acid, with the acid concentration in

the rain depending on the concentration of carbondioxide in the air.

2 2 2 3 3CO H O H CO H HCO


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Generally, any rain with a pH less than the

5.6 is considered acidic, but damage to

plants and animals does not begin to become

apparent until a pH of about 4.5 or less isreached.

The transport distances between emission

and precipitation are generally hundreds ofmiles, so that local control seems imossible.


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Example 3 Table 1.1 shows that the total annual U.S.

emissions of SO2 in 1997 were 20.4 million tons.

If we assume that 25% of that was in the Midwest-Ohio Valley area, and that 50% of that came toground as acid precipitation in a 1000 km by 1000km area in the northeastern U.S. and southeasternCanada, and that the average precipitation over that

area is 1 m/yr, by how much would this sulfurdioxide (if all converted to H2SO4) change the pH ofthe rainwater?


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Solution:

An estimated 25% 50% = 12.5% falls on the

affected area.

6 12 11

2

Annual U.S.20.4 10 18.5 10 2.9 10

SO emissions 64

1 ton = 2000 lbs

molton g mol

g

6 6 12 3 15Annual area depth 10 10 1 10 10precipitation

m m m m L


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Each mol of SO2 produces two mols of H+, so the

increase of H+ above the naturally occuring value is

The original rainfall is assumed to have a pH of 5.6,

or a H+ concentration of:

Adding these two values, we find an H+ = 7.48 10-

5 mol/L, or pH = -log (H+) = -log (7.48 10-5) = 4.13

#

11

+ 5

15

2 12.5% 2.9 10increase in H = 7.23 10 /

10 L

molmol L

5.6 610 10 2.51 10 /pHH mol L


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How harmful acid precipitation is to a given

area is strongly dependent on the buffering

capacity of the soil.

If the local soil contains significant amountsof limestone, CaCO3, the acid will react by

CaCO3 + H+ Ca2+ + HCO3

-

thus removing the H+

ion.


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It is believed that in some areas the

increased rainfall acidity has speeded the

dissolution of metals from the soil, e.g.,

aluminum, thus raising the content of thosemetals in the water.

These dissolved metals may be the true

agents of destruction for fish or plants.

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