Jacobson M.Z. Atmospheric Pollution

Подождите немного. Документ загружается.

Earth’s gravitational ﬁeld. As a result of these two loss processes (solar wind stripping

and gravitational escape), the ratios of H and He to other elements in the Earth’s

atmosphere today are less than are the corresponding ratios in the sun.

2.3.1. Solid-Earth Formation

The rock-forming elements that reached the Earth reacted to form compounds with

different melting points, densities, and chemical reactivities. Dense compounds and

compounds with high melting points, including many iron- and nickel-containing

compounds, settled to the center of the Earth, called the Earth’s core. Table 2.2 shows

that the total Earth contains more than 34 percent iron and 2 percent nickel by mass,

but the Earth’s crust (its top layer) contains less than 7 percent iron and 0.1 percent

nickel by mass, supporting the contention that iron and nickel settled to the core. Low-

density compounds and compounds with low melting points, including silicates of

aluminum, sodium, and calcium, rose to the surface and are the most common com-

pounds in the Earth’s crust. Table 2.2 supports this contention. Some moderately dense

and moderately high-melting-point silicates, such as those containing magnesium or

iron, settled to the Earth’s mantle, which is a layer of Earth’s interior between its crust

and its core.

During the formation of the Earth’s core, between 4.5 and 4.0 b.y.a., temperatures

in the core were hotter than they are today, and the only mechanism of heat escape to

the surface was conduction, the transfer of energy from molecule to molecule. Because

conduction is a slow process, the Earth’s internal energy could not dissipate easily, and

its temperature increased until the entire body became molten.

At that time, the Earth

’s

surface consisted of magma oceans, a hot mixture of melted rock and suspended crys-

tals. When the Earth was molten, convection, the mass movement of molecules,

became the predominant form of energy transfer between the core and surface.

Convection occurred because temperatures in the core were hot enough for core materi-

al to expand and ﬂoat to the crust, where it cooled and sank down again. This process

enhanced energy dissipation from the Earth’s center to space. After sufﬁcient energy

dissipation (cooling), the magma oceans solidiﬁed, creating the Earth’s crust. The crust

is estimated to have formed 3.8 to 4.0 b.y.a., but possibly as early as 4.2 to 4.3 b.y.a.

(Crowley and North, 1991). The core cooled as well, but its outer part, now called the

outer core, remains molten. Its inner part, now called the inner core, is solid.

Figure 2.7 shows temperature, density, and pressure proﬁles inside the Earth today.

The Earth’s crust extends from the topographical surface to about 10 to 75 km below

continents and to about 8 km below the ocean ﬂoor. The crust itself contains low-

density, low-melting-point silicates. The continental crust contains primarily granite,

whereas the ocean crust contains primarily basalt. Granite is a type of rock composed

mainly of quartz [SiO

(s)] and potassium feldspar [KAlSi

(s)]. Basalt is a type of

rock composed primarily of plagioclase feldspar [[NaAlSi

-CaAl

(s)] and

pyroxene (multiple compositions). The densities of both granite and basalt are about

2,800 kg m

3

Below the Earth’s crust is its mantle, which consists of an upper and lower part,

both made of iron–magnesium–silicate minerals. The upper mantle extends from

the crust down to about 700 km. At that depth, a density gradation occurs due to a

change in crystal packing. This gradation roughly deﬁnes the base of the upper mantle

and the top of the lower mantle. Below 700 km, the density gradually increases to the

mantle–core boundary at 2,900 km.

38 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

The outer core extends from 2,900 km down to about 5,100 km. This region con-

sists of liquid iron and nickel, although the top few hundred kilometers contain liquids

and crystals. The inner core extends from 5,100 km down to the Earth’s center and is

solid, also consisting of iron and nickel, but packed at a higher density. Temperatures,

densities, and pressures at the center of the Earth are estimated to be 4,300C, 13,000

kg m

3

, and 3,850 kbar, respectively (1 kilobar  1,000 bar  10

N m

2

 10

Pa – for comparison, surface air pressures are about 1 bar).

2.3.2. Prebiotic Atmosphere

Earth’s second atmosphere evolved as a result of outgassing from the Earth’s mantle. As

temperatures increased during the molten stage, hydrogen and oxygen, bound in crustal

minerals as hydroxyl molecules (OH), became detached, forming the gas-phase hydroxyl

radical. The hydroxyl radical then reacted with reduced gases, such as molecular hydro-

gen [H

(g)], methane [CH

(g)], ammonia [NH

(g)], molecular nitrogen [N

(g)], and

hydrogen sulﬁde [H

S(g)], to form oxidized gases, such as water [H

O(g)], carbon

monoxide [CO(g)], carbon dioxide [CO

(g)], nitrogen dioxide [NO

(g)], and sulfur dioxide

[SO

(g)]. As the molten rock rose to the Earth’s surface during convection, oxidized and

reduced gases were ejected into the air by volcanos, fumaroles, steam wells, and geysers.

After the crust and mantle solidiﬁed, outgassing continued. The resulting second-

ary atmosphere contained no free elemental oxygen. All oxygen was tied up in

oxidized molecules. Indeed, if any free oxygen did exist, it would have been removed

by chemical reaction.

Most outgassed water vapor in the air condensed to form the oceans. The oceans

are a critical part of today’s hydrologic cycle. In this cycle, ocean water evaporates, the

vapor is transported and recondenses to clouds, rain precipitates back to land or ocean

surfaces, and water on land ﬂows gravitationally back to the oceans. Sizable oceans

have been present during almost all of the Earth’s history (Pollack and Yung, 1980). In

1955, geochemist William W. Rubey (1898–1974) calculated that all the water in the

Earth’s oceans and atmosphere could be accounted for by the release of water vapor

from volcanos that erupted throughout the Earth’s history.

THE SUN, THE EARTH, AND THE EVOLUTION OF THE EARTH’S A

TMOSPHERE

Crust

Upper mantle

Lower mantle

Outer core

Inner core

0 4,000 8,000 12,000

1,000

2,000

3,000

4,000

5,000

6,000

Depth below surface (km)

Density (kg m

-3

)

Temperature (°C)

Pressure (kbar)

Inner core

(5,100–6,371 km)

Outer core

(2,900–5,100 km)

Lower mantle

(400–2,900 km)

Upper mantle (50–700 km)

Crust (0–50 km)

(b)(a)

Figure 2.7. (a) Diagram of the Earth’s interior and (b) variation of pressure, temperature, and density in the

Earth’s interior.

Living Organism Term Used

2.3.3. Biotic Atmosphere Before Oxygen

Figure 2.8 shows an approximate timeline of important steps during the e

volution of

the Earth’s atmosphere. Living organisms have been responsible for most of the

changes.

Living organisms can be classiﬁed according to their energy and carbon sources.

Table 2.3 shows that energy sources for organisms include sunlight and both inorganic

and organic compounds. Organisms that obtain their ener

gy from sunlight are

pho-

totrophs. Organisms that obtain their energy from oxidation of inorganic compounds

(e.g., carbon dioxide [CO

(g)], molecular hydrogen [H

(g)], hydrogen sulﬁde [H

S(g)],

the ammonium ion [NH



], the nitrite ion [NO



]) are lithotrophs. Organisms

that obtain their energy from oxidation of organic compounds are conventional

heterotrophs.

The two sources of carbon for organisms include carbon dioxide and organic com-

pounds. Organisms that obtain their carbon from CO

(g) are autotrophs. Organisms

that obtain their carbon from organic compounds are heterotrophs.

Table 2.4 classiﬁes organisms according to their energy and carbon sources.

Photoautotrophs derive their energy from sunlight and their carbon from carbon

dioxide. Photoheterotrophs derive their energy from sunlight and their carbon from

organic material. Lithotrophic autotrophs derive their energy and carbon from

40 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Table 2.3. Classiﬁcation of Organisms in Terms of Their Energy

and Carbon Sources

Energy Source

Sunlight Phototroph

Oxidation of inorganic material Lithotroph

Oxidation of organic material Conventional heterotroph

Carbon Source

Carbon dioxide Autotroph

Organic material Heterotroph

Conventional heterotrophs obtain energy and carbon from organic material.

First shelled invertebrates (0.57)

Primitive fish (0.43–0.5)

Formation of the Earth (4.6)

First land plants (0.395–0.43)

Earliest eukaryotes (1.4)

First prokaryotes (3.5)

Nitrogen fixation (1.5)

Nitrification (1.8)

Billion years ago 4 3 2 1 0

Denitrification (3.2)

Oxygen-producing

photosynthesis

by cyanobacteria (2.3)

Figure 2.8. Timeline of evolution on the Earth.

inorganic material (Fig.

2.9).

Lithotrophic heter

otrophs

derive their ener

gy from

inorganic material and their carbon from organic material. Conventional het-

erotrophs derive their energy and carbon from organic material. Today, all animals,

fungi, protozoa, and most bacteria are conventional heterotrophs.

About 3.5 b.y.a., the ﬁrst microorganisms appeared on the Earth after a period dur-

ing which the building blocks of life, amino acids, were produced by abiotic

synthesis. Abiotic synthesis is the process by which life is created from chemical reac-

tions and electrical discharges. It was demonstrated in 1953 by American chemist

Stanley Miller (b. 1930), who was working in the laboratory of Harold Urey

(1893–1981). Miller discharged electricity (simulating lightning) through a ﬂask con-

taining H

(g), H

O(g), CH

(g), and NH

(g) and liquid water. He let the bubbling

THE SUN, THE EARTH, AND THE EVOLUTION OF THE EARTH’S A

TMOSPHERE

Figure 2.9. Hot sulfur springs in Lassen National Park, California. Boiling water of geothermal origin is rich

in hydrogen sulﬁde. Lithotrophic autotrophic bacteria thrive in the springs and oxidize the hydrogen sulﬁde

to sulfuric acid, which dissolves the surrounding mineral and converts part of the spring into a “mud pot.”

The steam contains mostly water vapor, but hydrogen sulﬁde, sulfuric acid, and other gases are also pres-

ent. Courtesy of Alfred Spormann, Stanford University.

Table 2.4. Examples of Organisms Classiﬁed According to Their Energ

y and Carbon Sources

Photoautotrophs Green plants, most algae, cyanobacteria, some purple and green bacteria

Photoheterotrophs Some algae, most purple and green bacteria, some cyanobacteria

Lithotrophic autotrophs Hydrogen bacteria, colorless sulfur bacteria, methanogenic bacteria, nitri-

fying bacteria, iron bacteria

Lithotrophic heterotrophs Some colorless sulfur bacteria

Conventional heterotrophs All animals, all fungi, all protozoa, most bacteria

Organism Classiﬁcation Examples

mixture sit for a week and, after analyzing the results, found that he had produced

complex organic molecules, including amino acids. Later experiments showed that the

same results could be obtained with different gases and with UV radiation as opposed

to with an electrical discharge. In all cases, much of the initial gas had to be highly

reduced (Miller and Orgel, 1974).

In the prebiotic atmosphere, the amino acids required for the production of

deoxyribonucleic acid (DNA) were ﬁrst developed. About 3.5 b.y.a., the ﬁrst micro-

scopic cells containing DNA appeared. These cells, termed prokaryotic cells,

contained a single strand of DNA, but had no nucleus. Many early prokaryotes were

conventional heterotrophs because they obtained their energy and carbon from organic

molecules produced during abiotic synthesis. Today, prokaryotic microorganisms

include certain bacteria and blue-green algae.

2.3.3.1. Early Carbon Dioxide

During early biotic evolution, the major energy-producing process, carried out by

prokaryotic conventional heterotrophs, was fermentation, which produced carbon

dioxide gas. One fermentation reaction is

(aq) 2C

OH(aq)  2CO

(g)

Glucose Ethanol Carbon (2.3)

dioxide

which is exothermic (energy releasing). The energy source, glucose in this case, is

only partially oxidized to carbon dioxide; thus, the reaction is inefﬁcient.

2.3.3.2. Early Methane

A source of methane gas in the Earth’s early atmosphere (and today) was metabo-

lism by methanogenic bacteria. Such bacteria obtain their carbon from carbon dioxide

and their energy from oxidation of molecular hydrogen; thus, methanogenic bacteria

are lithotrophic autotrophs. Their methane-producing reaction is

(g)  CO

(g) CH

(g)  2H

O(aq)

Molecular Carbon Methane Liquid (2.4)

hydrogen dioxide water

Methanogenic bacteria use about 90 to 95 percent of carbon dioxide available to them

for this process. The rest is used for synthesis of cell carbon. In Reaction 2.4,

(g)

is reduced to CH

(g). A reaction such as this, in which cells produce energy by break-

ing down compounds in the absence of molecular oxygen, is called an anaerobic

respiration reaction, where anaerobic means in the absence of oxygen. Anaerobic res-

piration produces energy more efﬁciently than does fermentation.

2.3.3.3. Early Molecular Nitrogen

In the Earth’s early atmosphere, the most important source of molecular nitrogen

was ammonia photolysis by the two step process,

(g)  h •N

•

(g)  3H

•

(g)

Ammonia Atomic Atomic (2.5)

nitrogen hydrogen

•N

•

(g)  •N

•

(g)

Atomic nitrogen Molecular (2.6)

nitrogen

42 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Once oxygen levels built up in the air sufﬁciently (about 0.4 b.y.a.), ammonia photolysis

became obsolete because oxygen absorbs the sun’s wavelengths capable of photolyzing

ammonia. About 3.2 b.y.a., some anaerobic conventional heterotrophs developed a new

mechanism of producing molecular nitrogen. In this two step process, called denitriﬁca-

tion, organic compounds react with the nitrate ion [NO



] and the resulting nitrite ion

[NO



] by

Organic compound  NO



(g)  NO



 ..

Nitrate Carbon Nitrite (2.7)

ion dioxide ion

Organic compound  NO



(g)  N

(g)  ..

Nitrite Carbon Molecular (2.8)

ion dioxide nitrogen

Denitriﬁcation is the source of most molecular nitrogen in the air today

2.3.3.4. Anoxygenic Photosynthesis

Most organisms in the Earth’s early atmosphere relied on the conversion of organ-

ic or inorganic material to obtain their energy. During the microbial era,

such

organisms most likely lived underground or in water to avoid exposure to harmful UV

radiation. At some point, certain bacteria, called phototrophs, developed the ability to

obtain their energy from sunlight by a new process, called photosynthesis. An exam-

ple of a photosynthetic reaction by blue, green, and yellow sulfur bacteria, which are

photoautotrophs, is

(g)  2H

S(g)  h CH

O(aq)  H

O(aq)  2S

•

(g)

Carbon Hydrogen Carbohydrate Liquid Atomic (2.9)

dioxide sulﬁde water sulfur

where CH

O(aq) represents a generic carbohydrate dissolved in water. Because

reduced compounds, such as H

S(g), were not abundant on the surface of the Earth,

photoautotrophs ﬂourished only in limited environments. Early sulfur-producing

photosynthesis did not result in the production of oxygen; thus, it is referred to as

anoxygenic photosynthesis.

2.3.4. The Oxygen Age

The earth’s atmosphere lacked molecular oxygen and ozone until the onset of

oxygen producing photosynthesis about 2.3 b.y.a, halfway into the present lifetime

of the earth. For the next 1.9 billion years, oxygen-producing photosynthesis was

carried out primarily by c

yanobacteria (Figure 2.10). During this period, oxygen

buildup was slow. About 1.4 b.y.a., oxygen levels were still only 1 percent of those

today. Land plants evolved from blue-green algae, descendents of cyanobacteria,

about 395–430 million years ago (m.y.a.). Like cyanobacteria, plants photosynthe-

sized to produce oxygen. Subsequent to land-plant evolution, oxygen levels began

to increase rapidly. Today, 21 out of every 100 molecules in the air are molecular

oxygen.

Oxygen-producing photosynthesis in plants is similar to that in bacteria. In both

cases, CO

(g) and sunlight are required, and reactions occur in chlorophylls.

Chlorophylls reside in photosynthetic membranes. In bacteria, the membranes are cell

membranes; in plants and algae, photosynthetic membranes are found in chloroplasts.

THE SUN, THE EARTH, AND THE EVOLUTION OF THE EARTH’S A

TMOSPHERE

Chlorophylls are made of pigments, which are organic molecules that absorb visi-

ble light. Plant and tree leaves generally contain two pigments, chlorophyll a and b,

both of which absorb blue wavelengths (shorter than 500 nm) and red wavelengths

(longer than 600 nm) of visible light. Chlorophyll a absorbs red wavelengths more

efﬁciently than does chlorophyll b, and chlorophyll b absorbs blue wavelengths more

efﬁciently than does chlorophyll a. Because neither chlorophyll absorbs between 500

and 600 nm, the green part of the visible spectrum, green wavelengths are reﬂected by

chlorophyll, giving leaves a green color. Photosynthetic bacteria generally appear pur-

ple, blue, green, or yellow, indicating that their pigments absorb blue, green, and/or

red wavelengths to different degrees.

The oxygen-producing photosynthesis process in green plants is

6CO

(g)  6H

O(aq)  h C

(aq)  6O

(g)

Carbon Liquid Glucose Molecular (2.10)

dioxide water oxygen

where the result, glucose, is dissolved in water in the photosynthetic membrane of the

plant. The source of molecular oxygen during photosynthesis in green plants is not

carbon dioxide, but water. This can be seen by ﬁrst dividing Reaction 2.10 by 6, then

adding water to each side of the equation. The result is

(g)  2H

O(aq)  h CH

O(aq)  H

O(aq)  O

(g)

Carbon Liquid Carbohydrate Liquid Molecular (2.11)

dioxide water water oxygen

44 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Figure 2.10. Hot spring in Yellowstone National Park, Wyoming. The hot, mineral-rich water provides ideal

conditions for colored photosynthetic cyanobacteria to grow at the perimeter of the spring, where the

temperatures drop to about 70 C. The colors identify different photosynthetic bacteria with different tem-

perature optima. Courtesy of Alfred Spormann, Stanford University.

A comparison of Reaction 2.11 with Reaction 2.9 indicates that because the source of

atomic sulfur in Reaction 2.9 is hydrogen sulﬁde, the analogous source of oxygen in

Reaction 2.11 should be water. This was ﬁrst hypothesized in 1931 by Cornelius B.

Van Niel, a Dutch microbiologist working at Stanford University, and later proved to

be correct experimentally with the use of isotopically labeled water.

With photosynthesis in cyanobacteria and green plants came the production of molec-

ular oxygen and ozone, which helped to shield the Earth’s surface from UV radiation.

Molecular oxygen absorbs far-UV radiation, and ozone absorbs far-UV, UV-C and a large

portion of UV-B radiation. With the slow production of these gases, the surface of the

Earth became protected from such radiation. The increase in ozone allowed organisms to

migrate to the top of the oceans and land plants to develop 430–395 m.y.a.

2.3.5. Aerobic Respiration and the Oxygen Cycle

The introduction of molecular oxygen and ozone 2.3 b.y.a. also resulted in biological

changes in organisms that shaped our present atmosphere. Most important was the

development of aerobic respiration, which is the process by which molecular oxygen

reacts with organic cell material to produce energy during cellular respiration.

Cellular respiration is the oxidation of organic molecules in living cells.

Whereas aerobic respiration may have developed ﬁrst in prokaryotes (bacteria and

blue-green algae), its spread coincided with the development of another type of organism,

the eukaryote, about 1.4 b.y.a. A eukaryotic cell contains DNA surrounded by a true

membrane-enclosed nucleus. This differs from a prokaryotic cell, which contains a single

strand of DNA but not a nucleus. Unlike prokaryotes, many eukaryotes became multicel-

lular. Today, the cells of all higher animals, plants, fungi, protozoa, and most algae are

eukaryotic. Prokaryotic cells never evolved past the microbial stage.

Almost all eukaryotic cells respire aerobically. In fact, such cells usually switch

from fermentation to aerobic respiration when oxygen concentrations reach about

1 percent of the present oxygen level (Pollack and Yung, 1980). Thus, eukaryotic cells

probably developed only 1.4 b.y.a., after the oxygen level increased to above 1 percent

of its present level.

The products of aerobic respiration are carbon dioxide and water. Aerobic respira-

tion of glucose, a typical cell component, occurs by

(aq)  6O

(g) 6CO

(g)  6H

O(aq)

Glucose Molecular Carbon Liquid (2.12)

oxygen dioxide water

This process produces energy more efﬁciently than does fermentation or anaerobic

respiration. Thus, Reaction 2.12 was an evolutionary improvement.

Table 2.5 summarizes the current sources and sinks of O

(g). The primary source is

photosynthesis. The major sinks are photolysis in and above the stratosphere and aero-

bic respiration.

2.3.6. The Nitrogen Cycle

With the development of aerobic respiration came the evolution of organisms that

affect the nitrogen cycle. This cycle centers around molecular nitrogen [N

(g)], the

most abundant gas in the air today (making up about 78 percent of it by volume).

THE SUN, THE EARTH, AND THE EVOLUTION OF THE EARTH’S A

TMOSPHERE

Figure 2.11 summarizes the major steps in the nitrogen cycle. Four of the ﬁve steps are

carried out by bacteria in soils. The ﬁfth involves nonbiological chemical reactions

occurring in the air.

The direct source of molecular nitrogen in the air is denitriﬁcation, the two step

process carried out by anaerobic bacteria in soils that was described by Reactions 2.7

and 2.8. The second step of denitriﬁcation produces nitric oxide [NO(g)], nitrous oxide

O(g)], or N

(g), with N

(g) being the dominant product. N

(g) is also produced

chemically from N

O(g), which can form from NO(g).

(g) is slowly removed from the air by nitrogen ﬁxation. During this process,

nitrogen-ﬁxing bacteria, such as Rhizobium, Azotobacter, and Beijerinckia, convert

(g) to ammonium [NH



], some of which evaporates back to the air as ammonia gas

[NH

(g)]. Another source of ammonium in soils is ammoniﬁcation, a process by

which bacteria decompose organic compounds to ammonium. Today, an anthro-

pogenic source of ammonium is fertilizer.

Ammonium is converted to nitrate in soils during a two step process called nitri-

ﬁcation. This process occurs only in aerobic environments. In the ﬁrst step,

nitrosofying (nitrite-forming) bacteria produce nitrite from ammonium. In the second

step, nitrifying (nitrate-forming) bacteria produce nitrate from nitrite. Once nitrate is

formed, the nitrogen cycle continues through the denitriﬁcation process.

(g) has few chemical sinks. Because its chemical loss is slow and because

its removal by nitrogen ﬁxation is slower than is its production by denitriﬁcation,

(g)’s concentration has built up over time. Table 2.6 summarizes the sources and

sinks of N

(g).

46 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Table 2.5. Sources and Sinks of Atmospheric Molecular Oxygen

Photosynthesis by green plants Photolysis and kinetic reaction

Chemical production in the Aerobic respiration

stratosphere and above Dissolution into ocean water

Rusting

Chemical reaction on soil surfaces

Fuel combustion and biomass burning

Sources Sinks

(g)

Molecular nitrogen

O(g)

Nitrous oxide

NO(g)

Nitric oxide

Nitrite ion

Nitrate ion

(g), NH

Ammonia, ammonium ion

Nitrogen fixation

(aerobic)

Denitrification

(anaerobic)

Ammonification

Nitrite ion

Nitrification

(aerobic)

Organic compounds

containing

Atmospheric reaction

Figure 2.11. Diagram showing bacterial processes affecting the nitrogen cycle.

Figure 2.12. Estimated change in composition during the history of the Earth’s second atmos-

phere.

Modiﬁed from Cattermole and Moore (1985).

100

01234

Percentage of total

air by volume

Billions of years ago

(g)

2.3.7. Summary of Atmospheric Evolution

Figure 2.12 summarizes the estimated variations in N

(g), O

(g), CO

(g), and H

(g)

during the evolution of the Earth’s second atmosphere. The ﬁgure shows that the

atmosphere of the early Earth may have been dominated by carbon dioxide. Nitrogen

gradually increased due to denitriﬁcation. The oxygen concentration increased follow-

ing the onset of oxygen-producing photosynthesis 2.3 b.y.a. It reached 1 percent of its

present level 1.4 b.y.a., but did not approach its present level until after the evolution of

green plants around 400 m.y.a.

2.4. SUMMARY

The sun formed from the condensation of the solar nebula about 4.6 b.y.a. Solar radia-

tion incident on the Earth originates from the sun’s photosphere. The photosphere emits

radiation with an effective temperature near 6,000 K. The solar spectrum consists of

UV, visible, and near-IR wavelength regimes. The Earth formed from the same nebula

as the sun. Most of the Earth’s growth was due to asteroid and meteorite bombardment.

The composition of the Earth, percentage-wise, is similar to that of stony meteorites.

Dense compounds and compounds with high melting points settled to the center of the

Earth. Light compounds and those with low melting points became concentrated in the

crust. The ﬁrst atmosphere of the Earth, which consisted of hydrogen and helium, may

have been swept away by an enhanced solar wind during early nuclear explosions in the

sun. The second atmosphere, which resulted from outgassing, initially consisted of car-

bon dioxide, water vapor, and assorted gases. When microbes ﬁrst evolved, they

converted carbon dioxide, ammonia, hydrogen sulﬁde, and organic material to methane,

molecular nitrogen, sulfur dioxide, and carbon dioxide, respectively. Oxygen-producing

photosynthesis led to the production of oxygen and ozone. The presence of oxygen

THE SUN, THE EARTH, AND THE EVOLUTION OF THE EARTH’S A

TMOSPHERE

Table 2.6. Sources and Sinks of Atmospheric Molecular Nitrogen

Bacterial denitriﬁcation Bacterial nitrogen ﬁxation

Atmospheric reaction and photolysis of N

O(g) Atmospheric reaction

High-temperature combustion

Sources Sinks