Earth & its Atmosphere

The Earth's mass exerts a gravitational force that holds to the Earth a large amount of gases, known as the atmosphere. In this section, we will look at the layers and general composition of the atmosphere and discuss its major roles in maintaining life on Earth. We will not discuss in detail how the atmosphere has evolved over the history of the Earth, nor how living systems on Earth affect the atmosphere. Note briefly, however, that James Lovelock, author of Gaia, proposes that the atmosphere owes its current composition to feedback from living systems. He remarks that life on Earth requires a particular atmospheric composition, and this composition is in turn maintained by the interaction between biological systems and the atmospheric system.

Layers of the Atmosphere

The atmosphere consists of five layers: the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The thickness of these layers is slightly different around the globe, and also varies according to temperature and season. In this discussion, we will focus primarily on the troposphere and the stratosphere because they are the most affected by anthropogenic (or man-made) pollutants.

The troposphere is the layer closest to the Earth's surface. It is a layer of air approximately 10 to 15 kilometers thick that is constantly in motion. The conditions in this layer determine practically all of the Earth's weather patterns. It derives its name from the Greek word "Tropos," meaning "turning" or "mixing." The constant motion in this layer is significant in discussing air quality because it results in the dispersion of pollutants. In one respect this dispersion is considered beneficial because it has the effect of diluting pollutants, which can reduce harmful impacts on a local level. On the other hand, this dispersion also results in the movement of air pollutants (and therefore air pollution problems) from areas of high pollution production to areas of lower production. For example, pollutants produced in an industrialized and heavily populated city often adversely impact smaller communities and ecosystems in a large surrounding area.

The stratosphere is the layer just above the troposphere. It is approximately 40 kilometers thick and is composed mostly of dry stable air. In contrast to the troposphere, pollutants in the stratosphere do not disperse, and tend to remain in the atmosphere for long periods of time.

Figure 7: Layers of the Earth's atmosphere. (adapted from G.W. Vantoon and S.J. Duffy, Environmental Chemistry: A Global Perspective, Oxford University Press, 2000.)

As electromagnetic radiation travels through the atmosphere, shorter wavelengths are absorbed by the molecules in the first few miles. This high frequency radiation is capable of stripping the electrons from the molecules and dissociating the O₂ and N₂ molecules into O and N atoms, and ions or charged units such as O₂+, N₂+ (O₂ and N₂ molecules with one electron missing), etc. Thus the upper layers of the atmosphere are also called the ionosphere because they contain ions (or charged atoms and molecules). Only radiation of wavelength 220 nm or longer penetrates deeper into the atmosphere, reaching the stratosphere.

A stream of charged particles from the sun and the galaxy in general also falls on the upper layers of the atmosphere. Because they are charged, they are affected by the Earth's magnetic field--and depending upon their charge (+ or -) spiral toward the North or South pole. These concentrated streams of particles (often referred to as "cosmic rays") falling on the poles are visible as the Northern and Southern Lights (Auroral Lights).

Figure 2, Table 1, and Figure 4 all show that the spectral region from about 10 nm to about 350 nm is the ultraviolet region. The absorption spectrum of the ozone molecule is from 240 to 300 nm, while the O₂ molecule absorbs wavelengths shorter than 175 nm (splitting into O atoms). This absorption of the O₃ and O₂ molecules is the basis of the ozone layer (more later).

Chemical Composition of the Atmosphere

The Earth's atmosphere is composed primarily of nitrogen and oxygen, as well as some argon. There are also several other trace gases, meaning they occur in very small amounts. The proportion of molecules that naturally occur in the troposphere is described in Table 2. It is important to note the concentrations of these chemical compounds compared to the anthropogenically generated chemicals that may enter the atmosphere.

The major constituents are oxygen (O₂) and nitrogen (N₂). Other components such as argon, CO₂, NO, and O₃ are produced in minute quantities in natural processes. However, industrial and other technological human activities (such as automobile traffic) have begun to increase the amounts of materials such as CO₂ by amounts that are beginning to make a difference in the balance of circulation and radiation absorption in the troposphere. Effects of these changes range from local atmospheric problems, like smog, to problems of much greater scale, such as global climate change (more later).

Chlorofluorocarbons (CF₂Cl₂, CFCl₃) are a family of chemicals that do not occur in nature, but were produced in large quantities in the last century. These chemically inert compounds rise into the stratosphere and cause disruptions in the ozone layer (more later).

Numerous other gases circulate particularly in the troposphere in small quantities. The rare gases Argon (Ar), Neon (Ne), and Krypton (Kr) slowly drift up released from various processes on the ground, and remain non-reactive. Water vapor (H₂O), carbon dioxide (CO₂) and methane (CH₄) also arise from natural processes.

Water is the most highly variable gas in the atmosphere. The water fraction in the atmosphere (measured by the relative humidity) varies from place to place and day to day.

The water cycle described in the Materials System unit is, of course, vital to life on Earth, as is a certain level of CO₂. H₂O and CO₂ are essential molecules for photosynthesis. H₂O and CO₂ are also central in moderating the temperature of the atmosphere as the Earth rotates (more later).

CO₂ is produced in natural processes of decay and natural combustion processes such as forest fires and volcanoes.

Methane arises from natural processes such as cows and paddy fields. It is also produced in numerous underground processes of decay in the soil in the absence of oxygen, especially in marshes. Another source of the release of methane into the atmosphere is during the extraction and transportation of natural gas.

Carbon monoxide, oxides of nitrogen, NO, NO₂, and more complex nitrogen compounds are formed as a byproduct of the operation of the internal combustion engine and other fossil fuel-based technologies. Thus in countries with high levels of transportation, these gases also exist in local regions of the atmosphere. Oxides of sulfur are also released in coal burning (more later).

The troposphere is therefore a highly varying mixture of gases. Note that compared to the amounts of oxygen and nitrogen, the other gases are in small quantities measured in units of parts per million (or ppm) meaning one molecule of the gas in every million molecules of air (approximately 780,000 N₂ and 21,000 O₂). These small ppm-level imbalances in the composition of the atmosphere are enough to cause disruption in local and global atmospheric conditions and affect temperature and weather patterns.

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