Revised: 01/13/00
I. The Atmosphere's Composition & Structure
A. Evolution of Earth’s Atmosphere
-
Earth’s atmosphere has changed significantly since Earth was first
formed
1) Primordial Atmosphere - derived from the solar nebula (existed
for a short time frame)
2) Evolutionary Atmosphere - gases derived from the earth’s interior
- outgasing; "purple sulfur bacteria" produced organic materials
from inorganic elements (3.6 billion years ago)
3) Living Atmosphere - first photosynthesis by "blue-green algae"
(3.3 billion years ago)
4) Modern Atmosphere - an abundance of life as a consequence
of high levels of oxygen
B. Earth’s Present Day Atmosphere: Composition
Defn - Atmosphere - a thin envelope of gases that surrounds
the earth. It is held to the earth by the force of gravity and moves with
the earth as the earth rotates.
-
Total mass of the atmosphere = 5.1x1021 grams
-
Total mass of the earth = 6.0x1027 grams
-
Mass of the atmosphere ~ one millionth mass of the earth
-
Natural Air = clean air + pollutants
-
Clean Air = dry air + water vapor
Gas % by Volume
Dry Air:
nitrogen (N2) 78.08%
oxygen (O2) 20.95%
argon (Ar): 0.93%
Trace Gases: He, Ne, Kr, Xe, Rn, H2
Water Vapor - a variable constituent of clean air
-
~ 4% in warm, moist tropical air
-
~ 0% in cold, dry, polar air
Other variable constituents of air (trace amounts):
-
ozone (O3), carbon monoxide (CO), sulfur oxides (SOx),
nitrogen oxides (Nox,), carbon dioxide (CO2)
C. Vertical Structure of the Atmosphere
(1) Pressure (weight of the overlying atmos.) changes
with height
-
pressure (force per unit area) of the atmosphere; it decreases with
height above the ground
-
pressure is often used as a vertical coordinate in meteorological applications
(2) Density - [mass per unit volume (m/v)] of the atmosphere
decreases
with height above the ground.
ht. above the ground; fraction
of atmosphere below
5.5 km (3.4 mi)
1/2
11 km (6.8 mi)
3/4
16.5 km (10.2 mi)
9/10
32 km (19.9 mi)
99/100
(3) Temperature - the atmos. can be classified by layers based
on the average vertical temperature profile
Troposphere:
-
the layer of the atmosphere nearest the earth to ~ 10 km) where temperature
generally decreases with height
-
the location of most of the atmosphere’s moisture (and consequently weather)
-
its upper boundary is the tropopause
Stratosphere:
-
the layer above the troposphere (~10-50 km) where temperature generally
increases
w/height
-
very stable (warmer air over cooler air); thus vertical motions are weak
-
has a low moisture content; thus few clouds
-
upper boundary is the stratopause
-
most of the atmosphere’s ozone (O3) is found here (@ ~ 25 km)
-
ozone in the stratosphere is destroyed when it absorbs ultraviolet radiation
from the sun (O3 molecule becomes atomic (O) and molecular (O2)
oxygen)
-
some of the absorbed energy warms the stratospherethe
ozone layer protects biological life below from dangerous UV radiation
-
top of the stratosphere is the stratopause
Mesosphere:
-
the layer above the stratosphere (~ 50-80 km) where
temperature decreases w/height
-
upper boundary is the mesopause
Thermosphere:
-
the layer above the mesosphere (above ~ 80 km) where
temperature increases w/height again
-
O2 (molecular oxygen) is photodissociated
here
O2 + UV > O + O
-
O2 absorbs UV radiation from the sun, and
in the process, heating of the atmosphere occurs
-
because of the low density of molecules and atoms, a
small amount of UV energy absorption produces a large temperature increase
(3) Vertical Structure by Composition:
-
"homosphere" (0-80 km)- a well mixed region where
the composition is uniform (78% N2, 21% O2, etc.)
due to turbulent mixing
-
"heterosphere"(80 km and higher) - gases are
layered by atomic weight; because there's less mixing