Environmental Engineering Reference
In-Depth Information
TABLE 8.2
Composition of the Atmosphere
Mean Molecular
Weight (m/mol)
Concentration Parts
per Million by Volume
Gas
Chemical Symbol
Nitrogen
N
2
28.013
780,840
Oxygen
O
2
31.999
209,460
Argon
Ar
39.948
9,340
Carbon dioxide
CO
2
44.010
384
Neon
Ne
20.180
18.18
Helium
He
4.003
5.24
Methane
CH
4
16.043
1.774
Krypton
Kr
83.798
1.14
Hydrogen
H
2
2.106
0.56
Nitrous oxide
N
2
O
44.012
0.32
Xenon
Xe
131.293
0.09
Ozone
O
3
47.998
0.01-0.10
to be picked up later by the wind and sent airborne again. Factors controlling whether a
contaminant remains in the atmosphere include
• Physical and chemical factors of the contaminants including vapor pressure,
molecular weight, solubility, and reactivity
• Geography and local topography
• Climate and weather conditions
The average person inhales approximately 20,000 L of air per day (USEPA 2008a). Each
year, the World Health Organization (2009) estimates that 2.4 million people die from
causes directly attributable to air pollution, with the elderly and young children at the
most risk. Specific diseases caused from prolonged exposure to air contaminants are
chronic and often do not immediately appear after exposure. These diseases include heart
disease, lung cancer, and bronchitis. The burning of fossil fuels in power plants and auto-
mobile, truck, and bus exhaust account for 90% of all air pollution in the United States
(USEPA 2008a). Figure 8.15 shows some of the significant sources, methods of transport,
and removal of air pollutants in the atmosphere (USEPA 2008a).
Deposition of contaminants onto the land from the atmosphere occurs in two different
ways:
1. Dry deposition. Dry deposition is typically dust or particulate matter settling out
of the air. The amount of dry deposition depends upon the amount of suspended
particles, wind speed and duration, and particle size. Figure 8.16 shows a dust
storm potentially depositing a significant amount of dry material. Contaminants
are often present within dry deposition events, especially in urban areas where
they may be sorbed onto the surfaces of particulate matter in the air (USEPA
2008a).
2. Wet deposition. Wet deposition occurs when snow, fog, or a rain droplet
forms and then dissolves or carries a contaminant to the surface. Acid rain
is a good example of wet deposition. Figure 8.17 shows the wet deposition of
contaminants.
Search WWH ::
Custom Search