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Air Pollutants and

Global Climate

朱信Hsin ChuProfessor

Dept. of Environmental EngineeringNational 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They continue to change in the direction they are changing even after the cause of the change has been reduced or withdrawn.

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1. Global Warming

Humans are putting gaseous materials into the atmosphere that may cause the earth’s 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 if

it had no atmosphere. Solution:

The total radiant energy flux from the sun, just outside the earth’s atmosphere, is 1.353 kW/m2.

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

2 6 22

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(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 σ = 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 42 4

(12.75 10 ) 5.672 10kW

D T m Tm 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 earth absorbed 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 earth’s 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 = -19oC, and a frozen world.

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Example 2 What fraction of the outgoing radiation from

the earth is blocked by the atmosphere? Solution:

As just discussed, we assume that 30% of the incoming solar radiation is reflected away, and use an average surface temperature over 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 earth’s surface temperature to average about 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

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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

kWm 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 the photon.

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For wavelengths shorter than about 0.28µ, the internal transitions involve shifts of electrons in their orbitals around the nuclei of one or more of the atoms that mak up the molecule, but not any change in the relation of one atom to another within the molecule.

For the wavelengths longer than about 1µ, the changes are not within the individual atoms but are those associated with the vibrations of the various atoms in the molecule, relative to each other.

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In the 0.28 to 1µ 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% per micron) because the sun’s spectrum is narrower. (Observe the logarithmic scale for wavelength.)

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Wein’s law for blackbody radiatin is

which shows that for the temperature of the sun’s surface, about 6000 K, the peak intensity is at 0.50µ, corresponding to visible light.

For the earth’s surface temperature of about 288 K the peak intensity is 10.3µ, 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- µ 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 for incoming 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-µ window.

The same is also true for chlorofluorocarbons, or CFCs.

They are collectively called greenhouse gases.

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Human activities are increasing the concentrations of greenhouse gases in the atmosphere.

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

However, humans do not directly influence its concentration in the atmosphere, and it is not normally a part of the discussion of the greenhouse 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 average cloudiness.

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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- µ window for 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 the world.

Temperature increases much smaller than those needed to melt the ice caps would cause the deserts and the temperature zones to extent farther from the equator.

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Agricultural area that are currently highly productive 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 future emissions of all greenhouse gases, the global mean temperature will increase by 0.2 to 0.5oC (best estimate 0.3oC) per decade for this 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 earth’s atmosphere as long as the earth has had an atmosphere.

The current carbon dioxide concentration in the world atmosphere is approximately 380 ppm (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 of years.

About 1750 humans began to burn increasing amounts of fossil fuels, and the CO2 content of 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 CO

2 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 50 years), and that growth rate shows little sign of slowing.

2

2

Global fuelCO emissions perglobal per capita

combustion (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 use perhaps 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.

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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 many anaerobic biological processes.

It is the principal component of “swamp gas”, which is produced by bacterial decay of woody matter, and is a major component of the waste gases produced by 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 0.01 ppm/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 significant role.

The remaining important greenhouse gas is nitrous oxide, N2O, which formerly was often used 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- µ 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 earth’s average temperature can also be altered by an increase in the content of fine particles of the atmosphere.

For atmospheric particles to have effects lasting more 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 stratosphere have 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 two after 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 µ) and hence effective in scattering light and reducing the amount of incoming sunlight.

However, these particles are much smaller than the wavelength of outgoing infrared radiation, and hence less effect in scattering it.

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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 earth’s 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 µ (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 ground level, but a beneficial ultraviolet shield in the stratosphere.

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Destruction of the ozone layer is mostly caused by elemental chlorine atoms; the mechanism apparently 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 ignore other 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 nonconsumed catalyst 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 the lower atmosphere up to the ozone layer.

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The only naturally occurring chemical that can transport much chlorine high enough into the atmosphere to damage the ozone layer is methyl chloride, CH3Cl, which is produced in large quantities by biological processes in the shallow oceans.

Most of it is destroyed in the troposphere, but an estimated 3% of worldwide methyl chloride emissions reaches the stratospheric ozone layer.

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Chemically active ultraviolet light in the 0.2- to 0.28-µ 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 the ozone 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 have hydrogen atoms can be attacked in the atmosphere by the OH radical; for this reason most of them do not survive to reach the stratosphere.

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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 for use 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 and a blowing agent in the production of plastic foams.

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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 CFCs in 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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As far as we know, the only process for removing CFCs from the atmosphere is their slow transport to the top of the ozone layer, where they are attacked by shortwave ultraviolet light and thus destroyed, releasing their Cl to participate in the reactions with ozone.

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Trichloroethane, CH3CCl3, is not a CFC because it has hydrogens and thus can be attacked by atmospheric OH.

But that attack is relatively slow, so that an estimated 9% of this material that is emitted to the atmosphere makes its way to the stratosphere and participates in ozone destruction.

Table 14.3 (next slide) shows the concentrations, lifetimes, and expected contribution to delivery of elemental chlorine to the stratosphere for these chemicals.

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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 contribute to ozone depletion by the reaction:

NO + O3 → NO2 + O2

which is swift and practically irreversible. But it is not a catalytic reaction like the chlorine

reaction; 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 CFCs are 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 have accompanied 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 carbon dioxide in the air.

2 2 2 3 3 CO 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 is reached.

The transport distances between emission and precipitation are generally hundreds of miles, 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 to ground as acid precipitation in a 1000 km by 1000 km area in the northeastern U.S. and southeastern Canada, and that the average precipitation over that area is 1 m/yr, by how much would this sulfur dioxide (if all converted to H2SO4) change the pH of the 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 10

precipitationm 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 #

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+ 515

2 12.5% 2.9 10 increase 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 amounts of 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 those metals in the water.

These dissolved metals may be the true agents of destruction for fish or plants.