Earth & its
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.
of 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.
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.
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.
7: Layers of the Earth's atmosphere. (adapted from G.W. Vantoon
S.J. Duffy, Environmental Chemistry: A Global Perspective,
Oxford University Press, 2000.)
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.
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
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).
Composition of the Atmosphere
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.
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).
(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).
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.
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.
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).
is produced in natural processes of decay and natural combustion processes
such as forest fires and volcanoes.
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.
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
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.
2: 1990 Composition of Clean, Dry Air.
(fraction by volume)