Environmental Engineering Reference
In-Depth Information
Composition
Earth's atmosphere is composed of 16 dry gases and water, usually in the form of water vapor.
Atmospheric Gases
Nitrogen makes up approximately 78 percent of the total composition of the atmosphere. It is in
equilibrium with Earth's abiotic and biotic systems. Nitrogen enters the biotic system either through
nitrogen fixation or lightning, which turns nitrogen gas into usable forms of nitrogen for plants. In or-
der to be converted back to atmospheric nitrogen, nitrogen in the lithosphere undergoes denitrifica-
tion. The combustion of biomass can also release nitrogen into the atmosphere.
Nitrogen
(N
2
)
Oxygen represents approximately 21 percent of the total composition of the atmosphere. Oxygen is
released to the atmosphere during photosynthesis and is used by plants and animals during cellular
respiration.
Oxygen
(O
2
)
Natural Greenhouse Gases
The concentration of water vapor in the atmosphere varies greatly depending on location, but it is
about trace to 4 percent of the total composition of the atmosphere. Above the world's oceans, near
the equator, and in the tropical regions, the water vapor percentage is higher than it is in the atmo-
sphere over the poles and the world's deserts, where it can be very low.
Water
vapor
(H
2
O)
Since the beginning of the last ice age and prior to the Industrial Revolution, carbon dioxide has
been in equilibrium between the atmosphere and living organisms. CO
2
is important for photosyn-
thesis and for helping to maintain the natural greenhouse effect on Earth. However, since the begin-
ning of the Industrial Revolution, the volume of CO
2
has increased approximately 25 percent
(though it is still less than 1 percent of the atmosphere). Chapter 7 will discuss how humans are al-
tering the concentration of CO
2
in the atmosphere.
Carbon
dioxide
(CO
2
)
Methane is a naturally occurring component of the atmosphere, making up less than 1 percent of
the total gases. Methane is a principal component (approximately 87 percent) of natural gas and is
used for heating and cooking in many countries. It is one of the fossil fuels that is tapped by drilling
into the Earth. Natural gas may seep from the Earth's crust, contributing to methane in the atmo-
sphere, but it's more likely to be leaking from the gas pipes used to transport it or from equipment
that burns it. Humans have increased the concentration of methane in the atmosphere approxim-
ately 150 percent since the beginning of the Industrial Revolution, which will be discussed further in
Chapter 7.
Methane
(CH
4
)
Naturally occurring nitrous oxide is a greenhouse gas that is produced by bacteria in solids and
from the oceans. It has the ability to remain in the atmosphere for over 100 years and makes up
less than 1 percent of the atmosphere. The major sources of nitrous oxide are agricultural practices,
industrial activities, burning of solid waste, and fossil fuels. This is further explored in Chapter 7.
Nitrous
oxide
(N
2
O)
Ozone, which is less than 1 percent of the total atmosphere, is very important for life on Earth. The
majority (over 90 percent) of ozone is found in the stratosphere 8 to 30 miles above the Earth. The
ozone layer absorbs UV radiation from the sun, thereby protecting life on Earth from harmful rays.
Ozone (O
3
) is formed by a naturally occurring reaction in the atmosphere. The chemical reaction for
the formation of ozone is:
O
2
+ UV → O + O
O + O
2
→ O
3
Troposphere ozone is considered a pollutant. Ozone in the stratosphere was once in equilibrium.
These issues are further discussed in Chapter 7.
Ozone
(O
3
)
