7.1: The Atmosphere

Gases are highly compressible. Even small changes in pressure produce measurable changes in volume. As pressure increases, the gas molecules are forced closer together and the volume is reduced. In the atmosphere, pressure decreases rapidly with altitude (Fig. 7-2). Consequently, the density of the atmosphere decreases progressively with increasing height above the Earth’s surface. As a result, the atmosphere is stratified throughout its more than 100-km height (CC1).

Figure 7-2. Structure of the atmosphere. The atmospheric circulation that determines the Earth’s climate and weather takes place within the troposphere. Movements of gases between the troposphere and the stratosphere are limited and slow. Ozone is present throughout the stratosphere, but its concentration peaks at an altitude of about 22 km.

Temperature and vapor pressure (water vapor concentration) also affect the density of air, but much less than the changes in pressure with altitude. The atmosphere is steeply stratified near the Earth’s surface because of the rapid pressure change with altitude. As a result, density driven vertical motions of air masses in the troposphere are limited to only a few kilometers above the Earth’s surface.

Atmospheric Structure

The atmosphere is separated vertically into three distinct zones: troposphere, stratosphere, and mesosphere. Beyond the mesosphere is a zone called the thermosphere which is a transition zone between the atmosphere and outer space. The troposphere lies between the Earth’s surface and an altitude of about 16 to 18 km near the equator and less than 10 km over the poles (jet airplanes fly at an altitude of about 10 to 12 km). Vertical movements of air masses are caused by density changes and occur mainly in the troposphere. Although temperature generally decreases with altitude within the troposphere, density also decreases because the reduction in atmospheric pressure with altitude more than offsets the effect of lowered temperature (Fig. 7-3). This chapter pertains primarily to the troposphere because the atmospheric motions created by heat transfer from oceans to atmosphere are restricted to this zone.

Figure 7-3. Average total amount of ozone above the Earth’s surface over Antarctica during the month of October, as measured by a series of satellite instruments. The small circular area that has no color directly over the pole in the 1970 data is an area where no data were obtained. Nevertheless, it is clear that there is a broad region of very low ozone levels over Antarctica during the 1990s that was not present during the 1970s. This is the Antarctic ozone hole. The extent of the ozone hole has declined in recent years due to a global ban on the use of CFCs but other factors are likely involved. For example, the 2022 Hunga Tonga volcanic eruption may have contributed to an increase in ozone depletion in that year.

Between the top of the troposphere and an altitude of approximately 50 km, is the stratosphere. The air in the stratosphere is “thin” (that is, molecules are much farther apart than they are at ground level, because pressure is low), vertical air movements are primarily very slow diffusion, and temperature increases with altitude (Fig. 7-3). Within the stratosphere is a region called the ozone layer, where oxygen gas (O₂) is partially converted to ozone (O₃) by reactions driven by the sun’s radiated energy.

Ozone Depletion

The concentration of ozone in the Earth’s ozone layer decreased progressively during the 1980s and 1990s, particularly in the region around the South Pole, where the depletion was so great that it has become known as the “ozone hole” (Fig. 7-3). The decrease varies seasonally for both poles and is greatest during the spring, which is in October for the Southern Hemisphere.

The ozone layer absorbs much of the sun’s ultraviolet light, so depletion of the ozone layer could have severe adverse consequences for people and the environment. Ultraviolet light causes sunburn, eye cataracts, and skin cancers in humans; inhibits the growth of phytoplankton and possibly land plants; and may have harmful effects on other species.

The effects of ozone depletion on the environment are largely unknown. Studies have shown that the growth rate of phytoplankton in the Southern Ocean around Antarctica is significantly reduced (by 6 to 10%) in areas under the ozone hole. All life in the Antarctic, including whales, penguins, and seals, is ultimately dependent on phytoplankton for food (Chaps. 12, 13). Consequently, besides the direct effects of the increased ultraviolet radiation, ozone depletion may adversely affect species by reducing their food supply.

There is still some uncertainty about the causes of ozone depletion, but the accepted view is that the primary cause is gases called chlorofluorocarbons (CFCs) released to the atmosphere. CFCs are synthetic chemicals used in many industrial applications. Until close to the end of the last century, most spray cans used CFCs to produce the aerosol and almost all refrigerators and air conditioners used substantial quantities of a mixture of CFCs called “freon.”

CFCs are highly resistant to decomposition and may remain in the atmosphere for decades. The long residence time (CC8) allows the CFCs to diffuse slowly upward until they reach the ozone layer. There they are decomposed by ultraviolet light, and their chlorine is released. The chlorine atoms take part in a complicated series of chemical reactions with ozone molecules whereby the ozone molecule is destroyed but the chlorine atom is not. Because it is not destroyed, the chlorine atom can destroy another ozone molecule and then another. In fact, one chlorine atom, on average, can destroy thousands of ozone molecules.

Ozone depletion in polar regions is greatest in spring because stratospheric clouds of supercold ice crystals are formed at that time. These clouds remove nitrogen compounds from the ozone layer. Thus, in spring, chlorine that would normally react with the nitrogen compounds reacts with the ozone instead.

CFC use as an aerosol gas in spray cans was banned and eliminated virtually worldwide in the early 1980s. Later, a global international treaty, the Montreal Protocol of 1987, was established that called for total elimination of CFC manufacture and replacement of these compounds by other chemicals by the year 2000. Although these measures will probably eliminate ozone depletion eventually, most scientists believe that the depletion will continue until at least the middle of this century, if not longer, until the CFCs already released have diffused slowly upward through the atmosphere and been decomposed. Also, the compounds used to replace CFCs destroy ozone in much the same manner as CFCs but far less effectively. The number of refrigeration units has rapidly increased and continues to increase as the standard of living rises globally. This has substantially delayed recovery of the ozone layer. Indeed, ozone depletion continues to be observed in both Northern and Southern Hemispheres although it appears to be decreasing slowly. However, other factors may interfere. For example, the 2022 Antarctic ozone hole was one of the largest on record, possibly partially due to the effects of the Hunga-Tonga volcanic eruption in January of that year. However, the year to year variability in the magnitude of the ozone hole is large so Hunga Tonga contribution is uncertain. 

Depletion of ozone in the ozone layer in the stratosphere should not be confused with the problem of elevated ozone concentrations in smog in the troposphere (lower atmosphere). Ozone is released into the troposphere by various human activities and is created by photochemical reactions of other gases in the troposphere. Ozone is one of the principal harmful components of smog. However, this ozone does not contribute to the ozone layer because it does not last long enough in the atmosphere to be transported up through the troposphere and stratosphere to the ozone layer.