Gaseous Constituents Of The Atmosphere

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Table 2-1 shows the constituents of clean, dry air near sea level. Usually, the atmosphere also contains water vapor and dust, but these occur in amounts that vary widely from place to place and from time to time. Carbon dioxide, the fourth constituent on the list, has, as we shall see, been increasing in concentration during this century, mostly through the burning of fossil fuels such as coal and petroleum products. The concentrations given in Table 2-1 are estimated global averages for 1989; some of the trace constituents, like methane, have also been increasing in concentration, as noted later (see Section 3.3.3 and Figure 3-14).

Trace constituents may vary in concentration in different parts of the atmosphere and in different parts of the globe. This is because many of these trace constituents are produced by different human and natural sources in different locations and are then removed from the atmosphere in different ways and by different means. For example, nitrogen oxides may be produced not only from human activities such as combustion in air but also from natural occurrences such as lightning discharges through the nitrogen and oxygen of the atmosphere and some photochemical reactions in the upper atmosphere (see Chapter 5, Section 5.2.1). Sulfur compounds arise not only from the combustion of sulfur-containing fuels but also from volcanic action. Methane

TABLE 2-1

Some Constituents of Particulate-Free Dry Air near Sea Level

TABLE 2-1

Some Constituents of Particulate-Free Dry Air near Sea Level

Component

Volume

(%)

n2

78.084

o2

20.948

Ar

0.934

co2

0.0350

Ne

0.00182

CH4

0.00017

Kr

0.00011

H2

0.00005

N20

0.00003

H2

0.00005

Co

0.00001

Xe

9 x

10-6

03

1-10 x

:10-6

no2

2x

10-6

nh3

6 x

10-7

so2

2x

10-7

CH3C1

5x

10-8

CF2C12

4x

10-8

CFC13

2x

10-8

C2H4

2x

10-8

H2S

1 x

10-8

CC14

1x

10-8

CH3CCI3

1x

10-8

generally comes from decaying organic matter while ammonia, H2S, and some of the N20 arise from decomposition of proteins by bacteria. The concentration of ozone, given in Table 2-1 at sea level, varies a great deal with altitude and somewhat with latitude. Section 2.5 discusses ozone concentration in greater detail.

Although oxygen is a very reactive gas, its concentration is presently constant in our atmosphere; this implies a dynamic equilibrium involving atmospheric oxygen. It is well known that photosynthesis in green plants adds large amounts of oxygen to the atmosphere during daylight hours. Although oxygen may also be produced in other ways, the amounts are much smaller than those produced by photosynthesis. It seems reasonable to suppose that the oxygen produced by photosynthesis makes up for the equally vast amounts of oxygen used up by the respiration of plants and animals, by weathering of rocks, by burning of fossil fuels, and by other oxidative processes that take place in the atmosphere. This balance, however, requires green plants that were

280-

1700

1750

1800

1850 Year

1900

1950

2000

FIGURE 2-2 Historical variation of carbon dioxide in the atmosphere from 1744 through 1999. Data from the U.S. National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring and Diagnosis Laboratory (CMDL), Carbon Cycle-Greenhouse Gases Branch; antarctic ice core samples and atmospheric measurements. http://www.cmdl.noaa.gov.

not present in the very distant past. The various theories that describe the earth as evolving from a primordial, lifeless state to its present condition, and the physical evidence that supports these theories, suggest that little or no oxygen was present on the primordial earth.

Carbon dioxide has been released into the atmosphere in significant amounts since the beginning of the industrial age through the burning of fossil fuels.1 Furthermore, the destruction of forests in the tropics and temperate regions of the earth has reduced the removal of CO2 from the atmosphere by green plants. Studies of carbon dioxide in ice cores from both Greenland and Antarctica have shown a large variability in the concentration of CO2 in the atmosphere over the past 160,000 years (see Figure 3-13), but in more recent times it was about 280ppm from 500 b.c. to about a.d. 1790. Figure 2-2 shows the average CO2 concentration in the atmosphere from 1744 to 2000 as estimated from measurements made on Antarctic ice cores and in the atmosphere. Measurements made in the atmosphere at Mauna Loa, Hawaii, indicate that the average concentration of CO2 was 315 ppm by 1958 and about 370 ppm by 2000. Figure 2-3 shows the data obtained at Mauna Loa between

1The carbon cycle, which includes carbon dioxide, is discussed in greater detail in Section 10.2.

Carbon Dioxide Past 600 000 Years Graph

Year

FIGURE 2-3 Monthly variation of atmospheric concentrations of carbon dioxide at Mauna Loa Observatory. Data prior to May 1974 are from the Scripps Institution of Oceanography; data since May 1974 are from the National Oceanic and Atmospheric Administration, Climate Monitoring and Diagnosis Laboratory Carbon Cycle-Greenhouse Gases Branch. http://www.cmdl.noaa.gov.

Year

FIGURE 2-3 Monthly variation of atmospheric concentrations of carbon dioxide at Mauna Loa Observatory. Data prior to May 1974 are from the Scripps Institution of Oceanography; data since May 1974 are from the National Oceanic and Atmospheric Administration, Climate Monitoring and Diagnosis Laboratory Carbon Cycle-Greenhouse Gases Branch. http://www.cmdl.noaa.gov.

1958 and 2000. Mauna Loa is situated at a latitude of approximately 19°N. Note that there are seasonal variations in the carbon dioxide content of the atmosphere over Mauna Loa: less C02 is found in the summer, the growing season in the Northern Hemisphere. Hawaii may seem rather far south to feel the effects of this; more northerly areas like Scandinavia and northern Alaska show seasonal variations that have twice the amplitude of those over Mauna Loa. Seasonal variations in C02 concentration in the Southern hemisphere are much smaller.

At present, about 1.9 x 1010 metric tons of "new'' C02 are released into the atmosphere every year, mostly from the burning of fossil fuels; this is estimated to be about 10% of the amount used by green plants in the same time. About 47% of this new C02 is absorbed by the oceans rather quickly; given enough time, it is possible that the oceans would absorb much more. More details on the distribution of C02 in nature and on the carbon cycle in general are given in Section 10.2. At this time, the atmospheric concentration of C02 appears to be increasing at the rate of 1.3 ppm per year. The possible effects of this increase on the earth's climate are discussed in Chapter 3.

Carbon monoxide as well as C02 is released to the atmosphere by the burning of fossil fuels, and also by volcanic and biological activity. It is possible that carbon monoxide is accumulating in the atmosphere above the Northern Hemisphere at this time, but the data are too variable for definitive conclusions. It is known, however, that carbon monoxide cannot be accumulating as rapidly as it is emitted into the atmosphere, and thus there has been much interest in the origin and fate of atmospheric C0. It appears that about 50% of the carbon monoxide entering the atmosphere each year comes from natural processes. Estimates of carbon monoxide entering the atmosphere each year made since 1990 are as follows: over 109 metric tons from natural causes such as the oxidation of methane arising from the decay of organic matter, the biosynthesis and degradation of chlorophyll, and release from the oceans, and over 109 metric tons from human activities. These figures indicate that removal of C0 from the atmosphere must be occurring with great efficiency. Carbon monoxide reacts with hydroxyl radicals in the lower atmosphere (see Section 5.3.2), and there is evidence that fungi and some bacteria in the soil can utilize CO, converting it to C02. However, because of its toxicity, CO can be a major problem in areas in which its concentration is elevated. For example, although the worldwide concentration of C0 varies between 0.1 and 0.5 ppm, its concentration in the air over large cities can be 50-100 times as great, up to 50 ppm. It has been difficult to quantify the toxic effects on humans due to the C0 in the atmosphere above large cities because the C0 concentration in automobiles in heavy traffic can be much higher and because smokers inhale large quantities of CO. However, in one study2 it was shown that 20.2 ppm of CO above Los Angeles County in 1962-1965 resulted in 11 more deaths per day than occurred when the CO concentration was 7.3 ppm.

Sulfur and nitrogen compounds in the atmosphere are discussed at greater length in Chapter 10. At this point, we shall simply say that the sulfur compounds (mostly SO2) that arise from the burning of fossil fuels come back to earth in the form of sulfuric acid dissolved in rainwater after undergoing further oxidation in the atmosphere (see Chapter 5). Some of the nitrogen oxides also dissolve in rainwater to form acids. Normal rainwater, which always contains CO2 as well as some naturally occurring acids, may have a pH as low as 5.0 (see Section 11.4). In the last 50 years, however, rain and snow of much lower pH, that is, higher acidity, has been reported from many parts of the world. Rainfall in the northeastern United States now usually has a pH between 4.0 and 4.2, but individual rainstorms may have a pH as low as 2.1. Major industrial sources of atmospheric sulfur compounds are generally situated upwind of areas that have acid rain, and individual storms that have rain with very low pH have usually passed over large sources

2A. C. Hexter and J. R. Goldsmith, Science, 172, 265 (1971).

of these compounds. Tracer experiments, in which a marker gas such as sulfur hexafluoride has been injected into a waste gas stream from a chimney and followed by an aircraft containing appropriate instrumentation, have shown that such gases do not disperse where they first appear but move with the prevailing winds. For example, gases released from the east coast of England have been followed to Scandinavia. The Scandinavian countries, which are downwind not only from England but also from Germany's heavily industrialized Ruhr Valley, have had rain with pH as low as 2.8. Even in South America's Amazon Basin, with very little nearby industrial activity, pH values as low as 4 have been recorded. Acid rains in Europe and elsewhere have led to rapid weathering of ancient structures and sculptures during the latter half of this century; some of these structures, such as the Parthenon in Athens, had stood virtually unchanged for hundreds and, in some cases thousands of years. Some of the chemistry involved is discussed in Section 9.2.2. In Germany and elsewhere, ancient forests are dying, presumably because of acid rain. The effects of acid rain on lakes, running waters, and fish are described in Section 11.4.

Methane is a trace gas that has received considerable attention in recent years. Studies of Antarctic ice cores indicate that the atmospheric methane concentration was as variable as the carbon dioxide concentration over the past 160,000 years (see Figure 3-13). However, from 25,000 b.c. to a.d. 1580, the methane content of the atmosphere was approximately constant; it began increasing at that time up to the present value of about 1.75 ppm. This increase was as large as 0.016 ppm/year in 1991; with some large oscillations, this rate of increase has dropped between 1991 and 1999. Natural sources of methane include anaerobic bacterial fermentation in wetlands and intestinal fermentation in mammals and insects such as termites, and these account for most of the methane entering the atmosphere. The "sinks'' for atmospheric methane are not well understood, and the impact of human activities on methane formation and destruction has not been ascertained. The increase in methane content of the atmosphere can have important consequences for the earth's climate (see Section 3.3.3).

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