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- - - × × , force metric tons)! 15 10 × kg (5 kg (5 metric the atmosphere tons), is 18 metric and oceans tons) (1.4 DISTRIBUTION 18 21 10 × 10 OR OR 10 × × Newton’s second law of motion tmosphere SALE SALE A

FOR FOR kg; 6 or kg; 1.4 or Because the atmosphere is composed the of The The massof the air is in constantmotion, giving atmosphere The is an extremely complex entity Origin of the Earth and ■ 21 24 NOT NOT 10 According to the best scientific information, the universe is believed to have begun approximately billion15 years ago. At that time all matter in the universe was confined to a single space. An and liquid particles suspended above the surface that are too tiny gravity for to pull downward. Solid aerosols include ice crystals, volcanic soot parti cles, salt crystals from the ocean, and soil particles; liquid aerosols include clouds and fog droplets. lightest elements gravitationally attracted to the Earth, many assume that has it little mass. no or Compared withthe massthe of solidEarth (6 10 ■ considerable impact to the surface environment. example, For a tornadocan causecatastrophic devastation to a location. In the case a tornado, of the mass air of has substantial acceleration. Ac cording to is the product mass of and acceleration. The two factors combine, in this situation, to produce a force capable of devastation. that must be viewed simultaneously many on levels, bothtemporally and inthree spatial dimensions (west–east, north–south, and vertical). Atmospheric processes can be difficult tounderstand. appre To ciate the nature the of atmosphere we properly, must first understand origins the of atmosphere the and its changes since the origin the of planet. indeed light. But the atmosphere has a substantial total mass 5 of

- - - LLC. LLC. . This —solid —solid Learning, Learning, omposition C

second law ofsecond law aerosols Bartlett Bartlett & & Atmospheric Structure Structure Atmospheric and Jones Jones © © —states that energy there (and, hydrostatic equilibrium is a collection gases of near held

atmosphere 2 Chapter Technically, the atmosphere is a subset the of air Carbon Cycle Constant and Variable Gases air contains only not gases also but the planet. When in you look the sky the atmo sphere appears to continue infinitely, but if Earth were the size an of the apple, atmosphere would have the thickness skin. the of apple’s because is it composed solely gases. of By contrast, where a balance isgenerally achieved between the downward-directed gravitational forceand the upward-directed force buoyancy. of This bal ance is termed extremely thin and delicate zone known as the atmosphere makes life as we know possible it on properties the of universe—the thermodynamics fore, mass, because energy and mass are related by Einstein’s theory relativity) of moves from areas of higher concentration this (in case, Earth’s lower to areasatmosphere) lower of concentration (outer atmosphere The space). thus representsplace a The the Earth by gravity. is It also pulled away from the Earthbecause vacuuma exists in the harsh con- ditions space. of One the of most fundamental Atmospheric Structure Summary Key Terms Review Questions Questions for Thought Chapter at a Glance at Chapter and Atmosphere Origin of the Earth Atmospheric Composition Faint Young Sun Paradox 9781284028775_CH02_0011.indd 11 © Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION

12 Chapter 2 — Atmospheric Structure and Composition

­explosion of unimaginable proportion sent this couraged by frequent impacts with large planetes- matter—mostly hydrogen—outward in all direc- imals, which were essentially very small planets of tions. Over time, gravity caused matter to collect condensed debris moving over wildly eccentric or- in various areas of space to form galaxies. Within bits about the Sun. These planetesimals contrib- the galaxies smaller amounts of matter condensed uted heavier elements and mass to the growing gravitationally into stars. Star formation began to Earth while shattering and melting its hot surface. occur when hydrogen was compressed under its A collision between Earth and a planetesimal is own gravitational weight. If the mass involved was thought to have created the Moon. Remnants of sufficiently large,nuclear fusion—a process that early solar system planetesimals are present today converted lighter elements (principally hydrogen) in the vast asteroid field between Mars and Jupiter. into heavier elements (primarily helium)—began. The Oort cloud—a collection of icy comets and Such reactions released amazing quantities of dust that surrounds the outer edges of our solar radiant energy while creating (fusing) all matter system—also acts as a relic of conditions present in heavier than hydrogen. the early stages of solar system formation. Occasionally, a star exploded, sending heavier In these early times Earth’s atmosphere is be- elements outward through the galaxy. Vast clouds lieved to have consisted of light and inert (noble) of dust, called nebula (Figure 2.1), formed, and gases such as hydrogen, helium, neon, and argon. the original gravitational accumulation process These gases were effectively swept away as the began anew. Our solar system is believed to have solar wind—radioactive particles from the Sun formed from a nebula approximately 5 billion moving through space at nearly the speed of years ago. The Sun gravitationally attracted the light—developed. Today, Earth is largely devoid of bulk of the elements that composed the nebula. noble gases as a result. So how did the atmosphere Planets formed as balls of dust gravitationally col- that we know today form? lected over various orbits about the primitive Sun. The composition of the atmosphere can be ex- Earth was one such ball of dust. As it grew the ele- plained by looking at volcanic activity, which is ments fused together and collapsed under their rather limited over Earth’s surface today but was own weight and gravity. As Earth grew in size, its apparently widespread billions of years ago as the gravity increased proportionately. Friction caused early Earth cooled slowly from its primordial mol- the Earth materials to melt. Melting was also en- ten state. As volcanic material cooled, gases were released through the process of outgassing, which

primarily involved diatomic nitrogen (N2) and car- bon dioxide (CO2), with lesser amounts of water va- por, methane (CH4), and sulfur. The condensation of water vapor into liquid water in the cool atmo- sphere formed clouds and precipitation. Precipita- tion collected in low-elevation areas of the planet and accumulated over time to form the oceans. Our planet is unique in the solar system because of the presence of liquid water. This is a conse- quence of many related, and interacting, factors— some of which include distance to the Sun and atmospheric composition. Because water is essen- tial to life, it is not surprising that Earth is the only planet known to support life.

■■Atmospheric Composition

Today, the dry atmosphere consists primarily of N and diatomic oxygen (O ). Diatomic nitrogen is Figure 2.1 The Orion Nebula. 2 2 a stable gas that comprises 78% of the present-day Courtesy of NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team. atmospheric volume (Table 2.1). The abundance

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Atmospheric Composition 13

Table 2.1 Composition of the Dry Atmosphere to have evolved from the further development of eukaryotes. Prokaryotes and eukaryotes would Gas Percentage of Air have had to develop in the oceans, however, be- Nitrogen (N ) 78.08 cause without oxygen in the atmosphere the pro- 2 99.03 Diatomic oxygen (O2) 20.95 tective ozone (O3) layer could not have formed to Argon (Ar) 0.93 protect terrestrial life from the harmful ultraviolet

Carbon dioxide (CO2) 0.039 (UV) radiation emitted by the Sun. Over time, CO2 All others 0.003 continued to accumulate, as it became a larger and larger component of the atmospheric volume. By about 3 billion years ago another major de-

of N2 has increased as a percentage of the total velopment in the history of life on Earth apparently atmospheric volume primarily because it is not caused another major change to the atmospheric removed as effectively from the atmosphere as composition. The early evolution of eubacteria, are most other atmospheric gases. The residence and later, protists, and eventually aquatic green

time—the mean length of time that an individual plants led to a significant extraction of CO2 from

molecule remains in the atmosphere—of N2 is the atmosphere in photosynthesis—the process believed to be approximately 16.25 million years. by which green plants derive energy through the The next-most abundant gas in the present-day breakdown of food—and in the accumulating bio-

dry atmosphere is oxygen, comprising approxi- mass of those plants. O2 is released into the atmo- mately 21% of the atmospheric volume. About sphere as a byproduct of photosynthesis. As green 0.93% of the remaining 1% of the dry atmosphere plants began to populate Earth, first in the oceans

is composed mostly of argon (Ar), and a wide array and later on land after the presence of O2 gradually of atmospheric trace gases constitute the remain- led to the formation of ozone (O3) and the O3

der. Of these, CO2 is the fourth-most abundant gas layer—atmospheric CO2 decreased in concentra- in the dry atmosphere, representing 0.039% of the tion while atmospheric O2 simultaneously in- dry atmosphere, or 390 parts per million (ppm). It creased. Today most of the atmospheric CO2 is plays an especially important role in maintaining stored in vast quantities of sedimentary rock, orig- the temperature of the planet at a level comfort- inally extracted from the atmosphere by living

able for life in its present form. Earth’s early atmo- things. The amount of atmospheric O2 present

sphere apparently contained far more CO2 than today probably represents a similar percentage to today’s and little or no O2. So where did most of that of CO2 in the early atmosphere. the CO2 go after outgassing in the primitive atmo- sphere, and how did O2 come to replace it? Carbon Cycle The evolution of Earth’s atmospheric composi-

tion (including O2) involves significant interactions The process described above essentially repre- with the biosphere, hydrosphere, and lithosphere. sents the atmospheric component of the carbon About 3.5 billion years ago an interesting develop- cycle, the continuous movement of carbon ment occurred in the extensive waters of primor- through the Earth–ocean–atmosphere system. dial Earth that profoundly affected the evolution of Carbon can exist in various Earth–ocean– the atmosphere. Single-celled organisms, called atmosphere reservoirs—the components of a sys- prokaryotes, began to appear. These simple ances- tem that effectively store matter and/or energy for tors of bacteria and green algae absorbed nutrients a certain period of time, after which they allow for directly from the surrounding environment. Pro- the movement (flux) of that matter and/or energy

karyotes released CO2 to the atmosphere as a by- to another component of the system. Carbon res- product of fermentation, the process by which ervoirs include the atmosphere, which houses

simple organisms acquire energy through the CO2; the biosphere, which comprises all living breakdown of food. The evolution of prokaryotes matter; and the oceans, which include dissolved led to more complex, often multicellular, organ- carbonates (Figure 2.2). Over time carbon cycles isms called eukaryotes, which contain more com- among these various reservoirs. The carbon resi-

plex internal structures and release even more CO2 dence time is different for each reservoir, with the into the atmosphere. Most life on Earth is believed rates of exchange directly related to the size of the

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14 Chapter 2 — Atmospheric Structure and Composition

CO2 in atmosphere Combustion of fuels Diffusion Respiration

Photosynthesis

Dissolved CO2 Photosynthesis

Respiration Death and decay Death and decay Fossil fuels Carbonates in sediment

Figure 2.2 The global carbon cycle.

reservoir. The largest carbon reservoir, compris- matter over time. Some of the decaying carbon ing the vast majority of carbon in the Earth–ocean– ­dissolves directly into water, becoming part of the atmosphere system, is sedimentary rock, which oceanic carbon reservoir, and some reenters the may store carbon for billions of years. atmosphere through diffusion. Much oceanic bio- The second-largest reservoir is the oceans, which mass eventually solidifies into sedimentary rock.

act as a sink for atmospheric CO2 by absorbing many Finally, the atmosphere represents the smallest gigatons of CO2 from the atmosphere each year. carbon sink, and the carbon exchange rate to this Over time CO2 is transported from the atmosphere reservoir is rapid—only about 100 years. into the deep ocean layers in a very slow process. The natural carbon cycle is being disrupted by

Once the CO2 is transported into the deep ocean, it human activities. Since the dawn of the Industrial may remain there for many thousands of years. Revolution in the late 1700s, people have been The biosphere (including soil) is also a reservoir burning ever-increasing quantities of fossil fuels—

that acts as a sink for CO2 from the atmosphere. It deposits of carbon primarily in the form of coal, is then transported and stored elsewhere in the oil, and natural gas. Fossil fuels contain carbon system. For example, the vast majority of carbon in that was removed from the atmosphere long ago the rock reservoir was extracted from the atmo- through natural processes and stored in the vast sphere through biological processes. Residence rock reservoir. Normally, this carbon would be re- time associated with the biosphere may be exam- released back to the atmosphere over millions of ined on a number of levels. Carbon is held directly years. However, humans are extracting and releas- in the biosphere as long as the living organism re- ing this material back to the atmosphere in very mains alive. Once the organism dies, carbon exits short periods of time. The atmospheric quantity of

this sink as the remains of the organism decay. CO2 increased from 270 ppm in 1800 to 390 ppm Some of this carbon may become buried naturally today, with the bulk of the increase occurring since and transported into the rock reservoir. This 1950 (Figure 2.3). process is very effective in marine environments Whereas clues of the atmospheric concentra-

where an abundance of organic matter filters to tion of CO2 in the distant past come from chemical the ocean floor, building huge layers of organic analysis of air bubbles trapped in ice, CO2 concen-

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Atmospheric Composition 15

390 forests are located. The relative lack of photo­ 380 synthetic activity during the dormant northern 370 winter months causes a buildup of atmospheric 360 CO2 into March. Likewise, minimum atmospheric 350 CO2 in northern hemisphere autumn results from 340 the buildup of biomass throughout the northern 330 hemisphere’s spring and summer months. ppm 2 320 The long-term exponential growth in the atmo- CO 310 spheric CO2 concentration concerns most climatol- 300 ogists and environmental scientists. Carbon dioxide 290 is an integral component of Earth’s energy balance 280 because it absorbs energy that is radiated from the 270 Earth and then reemits energy back downward to 260 the Earth, thereby keeping the surface warmer than it would be if the CO were not present. This phe- 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2 Year nomenon is known as the greenhouse effect. The rapid increase in the quantity of atmospheric CO2 Figure 2.3 Exponential rise in atmospheric carbon di- is a likely culprit for the observed increases in oxide over the past 1000 years. temperature of Earth’s surface in the last several decades. However, some recent studies question whether the CO is a cause or an effect of the warm- tration has been measured directly since 1957 atop 2 ing; as oceans warm, their capacity to hold CO in Mauna Loa in Hawaii—a site as far removed as 2 dissolved form decreases, which would thereby in- possible from local sources of pollution. The time crease the diffusion of CO to the atmosphere. Re- series of atmospheric CO since 1957 is known as 2 2 gardless of whether the input of atmospheric CO is the Keeling curve (Figure 2.4), named for Charles 2 a cause or effect of warming, however, the slow car- Keeling, the climatologist who showed that CO re- 2 bon cycle and unknown capacity for the biosphere leased from fossil fuel combustion would accumu- and oceans to absorb additional atmospheric CO late significantly in the atmosphere. 2 has concerned most scientists about the ultimate The Keeling curve not only verifies the rapid impacts of fossil fuel consumption. increase since 1957, but it also reveals the seasonal

cycle of CO2. Maximum concentrations occur in early spring in the northern hemisphere, where Constant and Variable Gases most of the world’s middle- and high-latitude Constant gases are those that have relatively long residence times in the atmosphere and that occur in 400 uniform proportions across the globe and upward through the bulk of the atmosphere. These gases include nitrogen, oxygen, argon, neon, helium, 380 krypton, and xenon. Variable gases are those that change in quantity from place to place or over time 360 (Table 2.2). They generally have shorter residence times than constant gases, as various processes

rts per Million 340 combine to cycle the gases through reservoirs. The Pa most notable variable gas is water vapor, which can 320 occupy as much as 4% of the lower atmosphere by volume. Higher percentages of water vapor are im- 1960 1970 1980 1990 2000 2010 possible because atmospheric processes (cloud for- Year mation and precipitation) limit the amount that may be present in the atmosphere for any location. Figure 2.4 The Keeling curve. Data from: Scripps Insti- tution of Oceanography and NOAA Earth System Re- The atmosphere is very efficient in ridding itself of search Laboratory. excess water vapor.

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16 Chapter 2 — Atmospheric Structure and Composition

Table 2.2 Concentrations of Variable Gases actually reach zero because there is always at least of the ­Atmosphere some water vapor present in the lower atmosphere, but the total reaches about 0.00001% of the atmo- Gas ppm of Air spheric volume in central Antarctica.

Water vapor (H2Ov) 0.1–40,000 Several other variable gases are important. Carbon dioxide (CO ) ~390 2 Among these, CO2 is most abundant. As stated ear- Methane (CH ) ~1.8 4 lier, the variable nature of CO2 stems from its in- Hydrogen (H ) 2 ~0.6 creasing quantity over time, since the late 1700s. Nitrous oxide (N O) 0.31 2 ~ The rate of increase is about 0.4% per year or by Carbon monoxide (CO) ~0.09 about 35% since 1800. Other notable variable gases

Ozone (O3) ~0.4 include CH4, nitrous oxide (N2O), carbon monox- Fluorocarbon 12 (CCl2F2) ~0.0005 ide (CO), tropospheric ozone (O3), and a family of chemicals known as chlorofluoro­carbons (CFCs). Humans have had at least some influence in the The amount of water vapor in the atmosphere concentration of all these gases, and CFCs are varies widely across space because water and en- entirely human derived. Collectively, these gases ergy must be available at the surface for evapora- make up only a small amount of the atmosphere, tion to occur. Water vapor content is maximized but they can have important implications for some over locations with abundant energy and surface processes. water, so the wettest atmospheres occur over tropi- cal waters and rain forest regions. In addition, wa- ■■Faint Young Sun Paradox ter vapor is largely limited to the lower atmosphere because as height increases, atmospheric water Evidence contained in sedimentary rocks and sedi- vapor is increasingly likely to condense to liquid ments, ice sheets, and fossils reveals that Earth’s water in the cooler, high-altitude conditions. average temperature has remained within a range Surprisingly, some locations that experience of perhaps 15 C° (27 F°) for most, if not all, of its geo- little precipitation may have abundant water va- logical history. This implies that even global-scale por in the atmosphere. The maximum amount of shifts in mean environmental conditions, from ice water vapor that may exist in the atmosphere is ages to ice-free conditions on Earth, have occurred directly related to air temperature. When high within a range of temperature variability that is temperatures combine with a nearby surface smaller than the summer to winter temperature water source, high amounts of water vapor are difference at most locations outside the tropics. usually present. For example, the Red Sea region This fact has caused considerable consternation tends to have high quantities of atmospheric wa- for many climate scientists because of an apparent ter vapor despite the lack of precipitation. This re- contradiction between what is known about en- gion is dry not because water vapor is unavailable ergy released during the evolution of stars, such as but because the region lacks a means by which the our Sun, and evidence of Earth’s temperature precipitation process can occur easily. through geological time. Stars obtain energy As implied by the Red Sea example, deserts are through the constant nuclear fusion of hydrogen generally not the regions of lowest atmospheric into heavier elements. These reactions cause stars water vapor content. Instead, polar regions are to expand gradually and grow hotter and brighter normally the driest locations on Earth from an at- over time. Eventually, stars expend their sources of mospheric moisture perspective. This is because energy and extinguish themselves. We can assume little energy is present in cold air to evaporate wa- that energy emitted from the early Sun was about ter. Furthermore, as air cools, water vapor readily 25% to 30% percent less than that emitted today, condenses to form clouds and perhaps precipita- because this pattern is observed throughout the tion, thereby minimizing the mass of water vapor life cycle of other stars like the Sun. We also know in the atmosphere. So the regions with the least that even small changes in solar output can induce water vapor tend to be the coldest locations on drastic climatic changes on Earth. If solar output Earth. Over such locations in winter, the water va- today were decreased by 25% to 30%, temperatures por content approaches zero. The total will never would quickly plummet to a point whereby Earth

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Faint Young Sun Paradox 17

would be entirely frozen. Because the young Sun ture to remain relatively stable via the greenhouse must have been weak, at first glance it would ap- effect. That argument suggests that during times pear that Earth must have been frozen for the first in Earth history when the Sun was relatively weak, 3 billion years of its history. But little evidence has such as in Earth’s early atmosphere, the high con-

been found to support the notion that Earth was centrations of CO2 may have effectively absorbed ever below freezing on a global annual average ba- large amounts of radiation emitted from Earth sis. Furthermore, little credible evidence has been and reemitted much of that radiation back down- found for widespread glaciation during the first ward in the greenhouse effect. Just as the solar half of Earth’s existence. This apparent contradic- output began to increase, the onset of plant evolu- tion between a weak Sun but relatively warm global tion and proliferation would have been removing

conditions is the faint young Sun paradox. enough atmospheric CO2 to keep the insolation How could temperatures have been above and temperatures on Earth fairly constant.

freezing during the early times of geological his- But if excessive levels of CO2 indeed caused tory? How could Earth maintain a small variance Earth to remain warm despite a weak Sun, such in temperature over time if the Sun were much concentrations probably would have been too high weaker in the early history of the planet? How could to allow the generation of organic molecules, so temperatures maintain themselves over time as life could not have existed easily. Furthermore, no the Sun grew hotter? geological evidence has been found to suggest that

The most logical answer to these questions is CO2 concentrations were ever large enough to have that the Earth–ocean–atmosphere system must created such a strong greenhouse effect. Specifi- have some type of internal regulator that keeps cally, in an oxygen-free atmosphere such as early

temperatures within a reasonable range regardless Earth would have had, CO2 levels of about eight of changes to solar output over time. This regulator times today’s concentrations would have produced

must have been present in the early atmosphere. the mineral siderite (FeCO3) in the top layers of the How would this have happened? Examining the ra- soil as iron reacted with the CO2, but not enough diation balance of Earth today reveals that surface FeCO3 has been found in relic soils to support this temperatures are only indirectly caused by incom- hypothesis. Better explanations for the faint young ing solar radiation—insolation—being absorbed Sun paradox were sought.

at the surface. If surface receipt of solar radiation A second explanation is that ammonia (NH3) alone determined temperatures, average Earth caused the early greenhouse effect. This hypothesis temperatures would be about -18°C (0°F). Instead, was proposed by Carl Sagan and George Mullen of the transfer of energy (either from the Sun or Earth) Cornell University in the late 1970s and was based

absorbed in the atmosphere down to the surface on the observation that NH3 behaves as a very augments radiation received at the surface directly strong greenhouse gas. The problem with this argu­

from the Sun. Certain gases in the atmosphere, ment is that experiments have shown that NH3 is such as H2O and CO2, are known to be efficient ab- easily broken up by UV radiation in oxygen-free sorbers of energy escaping the surface. Much of this conditions, such as in the atmosphere before the

energy is then reemitted back down to reheat the arrival of photosynthesis. Nevertheless, the NH3 surface. This process—the greenhouse effect—is explanation is still plausible because shielding by

responsible for the life-supporting temperatures other gases may have allowed NH3 to accumulate we enjoy today. The net result is that the average in the lower atmosphere and be an important temperature of Earth is raised from 18°C (0°F) to a greenhouse gas. more comfortable 15°C (59°F). A third explanation, proposed by Harvard sci- So which greenhouse gases could have helped entists in the early 2000s, is that in the pre- the early Earth to remain relatively warm? The photosynthesis Earth, the planet’s oxygen-devoid two most abundant greenhouse gases in today’s atmosphere made conditions ideal for oxygen-

atmosphere are water vapor and CO2. As far as we intolerant microbes called methanogens. These can tell, the quantity of water vapor has remained methanogens (so-named because they release

relatively constant since primordial times. Until CH4 as a waste product) may have allowed CH4 to very recently it was assumed that the wide fluctu- produce a very strong greenhouse effect, which

ations of CO2 over time caused Earth’s tempera- would have warmed Earth even though the Sun

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18 Chapter 2 — Atmospheric Structure and Composition

emitted less radiation at the time. It is well-known ■■Atmospheric Structure that CH4 is a very effective greenhouse gas. Once oxygen entered the atmosphere from photo­ The atmosphere may be divided into a series of synthesis, the methanogens began to die off and layers based on thermal qualities (Figure 2.5).

CH4 became less important as a greenhouse gas. The lowest layer of the atmosphere is called the In today’s oxygen-rich atmosphere the concentra- troposphere. This is a very thin zone confined to

tion of CH4 is extremely minute—an average of the first 8 to 20 km (5 to 12 mi) above Earth’s sur- only 1.8 ppm in the atmosphere. Some geoscien- face, yet this atmospheric layer contains approxi- tists believe that the demise of the methanogens mately 75% of the mass of the atmosphere. The caused Earth’s first global ice age and perhaps compressibility of air allows its weight to exert a contributed to subsequent ice ages. downward force on, and compress, the lower at- Regardless of the explanation, most of Earth’s mosphere. This layer therefore also contains air of history is believed to have been dominated by the greatest mass per unit volume: density. somewhat higher temperatures than exist today, The term “troposphere” is derived from the or at least temperatures that are not far below Greek word meaning “to turn.” This indicates that those now. The debate over the explanation of the the troposphere is a region in which mass is con- faint young Sun paradox lingers on. stantly overturning, largely as a result of thermo-

Thermosphere 100 60

90 Heterosphere

80 Mesopause 50

70 40 60 Mesosphere

50 Altitude (miles) Stratopause 30 Altitude (kilometers) 40

Mean temperature

20 Homosphere 30 Stratosphere

Ozone (O3) layer 20 Isothermal 10 Tropopause layer 10 Mt. Everest Troposphere 0 0 0 80 20 32 40 60 Degrees F –80 –60 –40 –20 –140 –120 –100 0

10 20 30 Degrees C –90 –80 –70 –60 –50 –40 –30 –20 –10

Temperature

Figure 2.5 The vertical structure of the atmosphere.

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Atmospheric Structure 19

dynamic (heat-driven) processes. Most insolation increase of temperature with height—a tempera- passes through the atmosphere before being ab- ture ­inversion—is caused by the absorption of UV

sorbed by the Earth’s surface. The heated surface radiation by the triatomic form of oxygen (O3), or then warms the air directly above it through the ozone. process of conduction. This gives the lowest layers The so-called ozone layer in the stratosphere oc- of the atmosphere buoyancy and causes the air to curs because of several processes that have impor- rise in a process known as convection. Eventually tant implications for terrestrial life on the planet. this air cools and sinks. Because this vertical To understand the workings of the ozone layer we movement is integral to the development of most must first review the nature of radiation reaching weather-related processes, the troposphere is Earth from the Sun. Insolation arrives in Earth’s sometimes referred to as the “weather sphere.” atmosphere in a wide range of wavelengths, which Because the atmosphere is heated primarily are measured in micrometers (millionths of a from Earth’s surface and because the compression meter [µm]). Shorter wavelengths are associated of atmospheric gas decreases with height (as the with more intense (and therefore more harmful to weight of the atmosphere above it decreases), a living things) energy than longer wavelengths. The decrease of temperature usually occurs with in- Sun emits more energy at a wavelength of about creasing height through the troposphere. This de- 0.5 µm than at any other wavelength, with succes- crease is known as the environmental lapse rate. sively less energy emitted at successively shorter Through the troposphere, air cools at an average and longer wavelengths (Figure 2.6). By conven- rate of 6.5 C°/km (or 3.5 F°/1000 ft), although this tion, energy from solar origin shorter than 4.0 µm is value may vary widely from place to place and usually referred to as shortwave radiation in the from day to day, and even on an hourly basis. atmospheric sciences. Wavelengths of the peak According to one form of Charles’ law, in an amounts of shortwave radiation occur in the visi- ideal gas (which the atmosphere approximates), ble part (0.4 to 0.7 µm) of the electromagnetic density decreases as temperature increases if pres- spectrum—the full assemblage of all possible sure remains constant. Air that is warmer than sur- wavelengths of electromagnetic energy. rounding air thus rises. Because Earth is not heated Any insolation with wavelengths less than equally and relatively warm air rises, the tropo- 0.4 µm is too intense to allow terrestrial life to exist. sphere does not have a uniform depth. Instead, the UV radiation falls between wavelengths of 0.01 and troposphere is thicker near the equator than near 0.40 µm, making UV radiation harmful to living or-

the poles. Near the equator the layer is approxi- ganisms. Fortunately, N2 absorbs electromagnetic mately 20 km (12 mi) thick, whereas near the poles radiation of wavelengths below 0.12 µm. But how in winter the thickness is only about 8 km (5 mi). does the ozone layer protect us from UV radiation Roughly the same amount of atmospheric mass ex- at wavelengths between 0.12 and 0.40 µm?

ists over the two locations, but the density of that air Some of the diatomic oxygen (O2) that enters the is less over the equator and greater over the poles. atmosphere from photosynthesis near the surface The top of the troposphere is called thetropo - pause. This feature marks the boundary between the troposphere below and the next layer of the at- For 6000 K mosphere above (Figure 2.5). The average tempera- For 6000 K ture at the tropopause is about -57°C (-70°F), which represents quite a decrease from the 15°C (59°F) average temperature of the surface. 10 µ m t Intensity The layer above the troposphere is the strato- For 300 K sphere. Temperatures remain somewhat constant Radian from the tropopause upward into the stratosphere for about 10 km (6 mi). Any zone of relatively 0.5 µ m ­constant temperature with height, such as this Wavelength one, is called an isothermal layer. Above the iso- Figure 2.6 Emitted energy by wavelength for the Sun thermal layer temperatures actually increase with and Earth, assuming a surface temperature of 6000 K for height through the rest of the stratosphere. This the Sun and 300 K for the Earth.

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20 Chapter 2 — Atmospheric Structure and Composition

may reach the stratosphere over time. Because occur. Although O3 is found throughout the strato- O2 molecules effectively absorb UV radiation at sphere, if all stratospheric ozone were compressed wavelengths between 0.12 and 0.18 µm, the O2 to the surface it would create a layer only 3 mm reaching the stratosphere is exposed to incoming thick. Since the U.S. ban on CFC production took harmful radiation. When this radiant energy effect on January 1, 1996, the ozone layer has shown

strikes O2 ­molecules, a chemical reaction in the some signs of recovery. presence of light that splits the molecular bonds— The ozone formation process is responsible for photodissociation—is triggered and two mon­ the temperature inversion in the stratosphere. As

atomic oxygen (O) atoms are liberated. Because O is O3 molecules absorb UV radiation, they acquire en- inherently unstable, it bonds quickly and easily ergy. The 3O at the top of the stratosphere has the with other atoms and molecules. Some of these first opportunity to gain energy (and, therefore,

O atoms chemically bond with an O2 molecule to temperature, because temperature is a measure of form an O3 molecule that effectively absorbs UV ra- the energy or heat content of matter) from incom- diation at wavelengths between 0.18 and 0.34 µm. ing UV radiation. Its temperature, therefore, is

But in the absorption process the O3 becomes photo- higher than for molecules lower in the stratosphere. dissociated into O and O2, and the O then bonds The process of 3O production and dissociation hap- with another O2 to form O3. The process then repeats pens in the stratosphere because this is the upper- endlessly, ensuring that oxygen is continuously be- most layer for which atmospheric density is high

ing reworked into O3 in the stratosphere. UV radia- enough to allow O and O2 to meet and bond quickly tion at wavelengths between 0.18 and 0.34 µm is enough so that incoming UV radiation is absorbed effectively “absorbed”—actually used in chemical effectively. Temperatures rise to approximately processes—such that only the UV radiation at wave- -18°C (0°F) at the stratopause, which is about lengths between 0.34 and 0.40 µm filters to Earth’s 48 km (29 mi) above the surface. The stratopause is surface. This harmful radiation can cause skin can- the boundary between the stratosphere and the cer, cataracts, and other problems if we are exposed layer above it. The layer above the stratosphere is to it in large doses, but at least we are protected from the mesosphere, from the Greek prefix meso-, the much more harmful shorter UV wavelengths. which means “middle.” Although this layer does sit Early in geological history, the first organisms near the middle of the atmosphere from an altitude

must have formed in murky waters because no O2 perspective—in the region between the strato- (and, therefore, no O3) existed to protect them pause and about 80 km (50 mi) above the surface— from UV radiation. By the time life evolved into the low-density mesosphere does not represent the

shallow water areas and onto land surfaces, O2 re- middle of the atmosphere by density or volume. leased from photosynthesis had built a strato- Because of the compressibility of gases, the middle

spheric O3 layer. Life has never been exposed to of the atmosphere by density and volume is only excessive amounts of UV radiation and has never about 5.5 km (3.4 mi) above the surface—well adapted to it. Increasing exposure to UV radiation within the troposphere. is damaging to terrestrial life. Similar to the troposphere, temperatures in the Humans have contributed to thinning the very mesosphere decrease with height. The tempera-

fragile O3 layer over the past half-century by pro- ture inversion characteristic of the stratosphere is ducing CFCs. Most general uses for CFCs involved not present in the mesosphere because it is too

refrigeration both as a gas (Freon) and as an insu- high for photodissociated O2 to encounter other lating substance (foam and Styrofoam). CFCs oxygen atoms or molecules to bond with quickly were also used as a propellant for aerosol sprays. enough to absorb the incoming UV radiation. In- When chlorine from CFCs and bromine are re- stead, the increased density and proximity to the leased to the atmosphere, they can make their way surface and stratospheric heat sources below the upward to the stratosphere where they readily mesosphere make the lower mesosphere warmer bond with monatomic oxygen atoms. Such a bond than the top of this layer. Temperatures at the

does not allow the O to bond with O2 to produce mesopause average approximately -84°C (-120°F). the O3 that would absorb UV radiation. The result Few processes of consequence to weather and is that increased amounts of UV radiation reach climate are known to occur in the mesosphere in the surface, where adverse effects on organisms part because so little atmospheric mass exists in

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© Roman Krochuk/ShutterStock, Inc. - - - mean Atmospheric Structure Atmospheric DISTRIBUTION OR OR molecules have thefirst opportunity to of a molecule—the of distance an individ 2 SALE SALE FOR FOR The number The of thosemolecules with very high and O tainextraordinarily high temperatures because 2 NOT NOT perature inversion occurs because the uppermost N heat that even if you could somehow survive for thanmore a fraction a second of those at heights, wouldyou freeze to death instantly even tem at peratures (2000°F)! above 1100°C absorb insolation. Their position allows them to ­at Earth’s magnetic field captures charged high-energy particles from the Sun. temperatures is miniscule, because however, of thesparseness the of atmosphere such at heights. The total massof the thermosphereaccounts for only percent the of about 0.01 total atmospheric mass.decrease The density,of mass, and volume the of atmosphere can be expressed by the free path ual molecule must travel before encountering an other molecule.mean The free pathat the surface is the on a micrometer. of order By contrast, in the thermosphere the mean free path is the on order kilometera of Despite more. or the fact that the individual molecules havevery high amounts of energy, there are so few molecules to contain the

. 3 ------LLC. LLC. , which Learning, Learning, thermosphere . Bartlett Bartlett & & homosphere

) andsouthern lights Jones Jones © © heterosphere Figure 2.7 ). But even). these processes have ; in the stratosphere. The first three aurora borealis. 3 he T

2.7 From the surface up to the mesosphere, the The heterosphere The corresponds to the finalther aurora australis aurora aurora borealis Figure absorptionof insolation, the thermospheric tem ( minimal effecton Earth’s weather and climate. proportion atmospheric of gases is about the same as that the at surface, except the for greater con centrationof O this zone. Charged particles from the Sun that are captured by Earth’s magnetic field in meso the sphere can disrupt telecommunications during their release energy. of These same charged parti cles are also responsible the for northern lights ( mal layer the of atmosphere, the Likethe stratosphere, the thermosphere is charac terized by temperatures that increase with height— a temperature inversion. Unlike the stratosphere, wherehowever, the inversion exists because O of “spheres” of the of “spheres” atmosphere are thus sometimes collectively known as the means Above the sphere.” “same mesosphere gases stratify into layers according to their atomic weights because there is so little mass to “stir That them up.” region is termed the 9781284028775_CH02_0011.indd 21 2ND PAGES © Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION

22 Chapter 2 — Atmospheric Structure and Composition

A thermopause does not exist; instead, the by evidence suggesting that methane-emitting mi- atmosphere simply merges slowly into inter­ crobes may have played a greater role than previ- planetary space. Individual gas molecules may ously believed in Earth’s early history by absorbing be gravitationally attracted to the planet for quite energy emitted by Earth via the greenhouse effect. a distance into space. Most agree, however, that This methane may have kept Earth’s temperatures the atmosphere extends no higher than about within a narrow range of variability, despite a 1000 km (600 mi) above the Earth’s surface. weaker solar output. The troposphere is the lowest layer of the ■■Summary ­atmosphere and is where nearly all weather and climate processes of importance occur. Tempera- The atmosphere is a fragile and complex collec- tures in the troposphere usually decrease with tion of gases gravitationally attracted to Earth. It height because of the increased density in the originated early in the history of the planet as vol- most compressed part of the atmosphere—the canic materials cooled and outgassed. The early part nearest to the surface—and because of prox- atmosphere is believed to have been composed imity to the surface, which absorbs insolation

primarily of N2 and CO2, but O2 gradually replaced more effectively than the air above it. The strato- CO2 as the second-most abundant gas since the sphere is the second layer from the surface and is evolution and proliferation of simple organisms characterized by increases in temperature with and green plants, which have stored carbon in height—a temperature inversion—because of

their biomass and released O2 into the atmosphere ozone absorption. In the mesosphere tempera- over the past 3 billion years. tures decrease with height for the same reason Carbon is stored for certain periods of time they do in the troposphere. The final thermal layer within a number of reservoirs such as rock layers, of the atmosphere is the thermosphere, which is the ocean, biomass, and the atmosphere. This cycle characterized by a temperature inversion because of carbon is important in the history of Earth. Its of the direct absorption of incoming radiation by

importance has been challenged recently, however, N2 and O2.

XXKey Terms

Aerosol Greenhouse gas Photodissociation Atmosphere Heterosphere Photosynthesis Biosphere Homosphere Planetesimal Carbon cycle Hydrosphere Prokaryote Charles’ law Hydrostatic equilibrium Reservoir Chlorofluorocarbon (CFC) Insolation Residence time Condensation Isothermal layer Second law of thermodynamics Conduction Keeling curve Shortwave radiation Constant gas Lithosphere Sink Convection Mean free path Solar wind Density Mesopause Stratopause Diffusion Mesosphere Stratosphere

Electromagnetic spectrum Methane (CH4) Temperature inversion Environmental lapse rate Methanogen Thermosphere Eukaryote Micrometer Tropopause Evaporation Nebula Troposphere Faint young Sun paradox Newton’s second law of motion Ultraviolet (UV) radiation Fermentation Nuclear fusion Variable gas Flux Oort cloud Wavelength Fossil fuel Outgassing

Greenhouse effect Ozone (O3) Terms in italics also appeared in Chapter 1.

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Summary 23

XXReview Questions XXQuestions for Thought

1. Explain how Earth and its atmosphere formed. 1. Give as many examples of the second law of 2. How is today’s atmosphere similar to and dif- thermodynamics as you can think of, both re- ferent from early Earth’s atmosphere? lated to and not related to the atmosphere. 3. Describe how oxygen came to compose al- 2. Why do the aurora borealis and aurora austra- most 21% of the atmosphere today. lis occur in the mesosphere and not elsewhere 4. Given that solar output had increased over the in the atmosphere? past 4.6 billion years, how have Earth’s tem- 3. Why are the auroras not visible in the tropical peratures remained fairly constant over that parts of the Earth? same time? 5. What is residence time and why is it important? 6. What is the carbon cycle and how does it operate? go.jblearning.com/Climatology3eCW 7. Describe the thermal structure of the Connect to this book’s website: go.jblearning.com/ atmosphere. Climatology3eCW. The site provides chapter 8. What causes the thermal characteristics outlines, further readings, and other tools to help associated with each thermal layer of the you study for your class. You can also follow useful atmosphere? links for additional information on the topics 9. Compare and contrast the heterosphere and covered in this chapter. the homosphere. 10. Why is there no defined top to the atmosphere?

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