Geography 140
Introduction to Physical Geography
Lecture: Gaseous Composition of the Atmosphere
I. In this lecture, I introduce a completely new major section of the
course: the earth's atmosphere.
A. Basic definition: The earth's atmosphere is a mixture of various
gases and other "stuff," which is held to it by gravity.
B. It is about 10,000 km thick (~6,000 mi. or so), but it is densest at
sea level (well, denser yet at spots below sea level, such as Badwater
in Death Valley, the Salton Sea, and the Dead Sea) and thins out very
quickly as you move upwards.
1. Some 97 percent of it is found in the first 30 km of the earth's
surface.
2. The remaining 3 percent of it thins out from there to the top of
the atmosphere.
3. There's no sharp and definite boundary to the planet's atmosphere:
By the time you get to 10,000 km "out there," the density of
hydrogen atoms per unit of volume drops to the level of
interplanetary space.
II. The chemical composition of the lower atmosphere (out to about 100 km,
the homosphere) is relatively uniform.
A. Gaseous constituents of the homosphere:
1. The overwhelming bulk of the atmosphere's constituent chemicals
exist in a gaseous state, as "air." Almost all of it is nitrogen
and oxygen, with a teensy amount of trace gases.
2. Nitrogen is far and away the most common gas in our atmosphere. It
makes up 78.1 percent of clean (as IF), dry air by volume (since
nitrogen is a fairly light molecule, it makes up about 75.5 percent
of the mass of the atmosphere).
a. Atmospheric nitrogen exists in the form, N2, or molecular nitrogen.
b. This particular molecule is very stable: It does not easily
unite with other chemicals because two nitrogen atoms form a
powerful covalent bond, sharing three electron pairs between
them.
i. Geotrivia for you. Nitrogen will combine with other
elements only when strongly heated. Nitrogen atoms in
other compounds are very unstable, explosively dropping
their bonds with the other atoms in order to pair up in
their favorite match: one another, to form nitrogen gas.
This is why nitrogen atoms are found in explosives, such as
NITROglycerin and triNITROtoluene (TNT).
ii. But, when nitrogen is happily pair-bonded as nitrogen gas,
it is very stable. As such, it is neutral filler stuff in
the atmosphere (kind of like soy powder in hot dogs?).
iii. Actually, the nitrogen gas in the atmosphere serves to
limit the combustibility of many substances in the presence
of oxygen (the second most abundant gas in the atmosphere):
If there were more oxygen gas in the atmosphere, forest
fires and other combustion events would be even more,
pardon the pun, explosive.
c. An interesting irony: Plants and animals need nitrogen in order
to make proteins and nucleic acids (such as the four in DNA).
Unfortunately, nitrogen in its happiest state, N2, is
too stable for most living creatures to use, so nutrient
nitrogen is a scarce and limiting resource for life, even though
we're sitting at the bottom of an atmosphere filled with
nitrogen! Plants and, ultimately, animals depend on bacteria to
"fix" nitrogen into a useful form, nitrate (NO3 that
they can absorb and use.
3. Oxygen in the form of O2 is the second most common gas
in clean (ha!), dry air. It makes up 21.0 percent of the
homosphere by volume (because oxygen is a somewhat heavier
molecule than nitrogen, it makes up 23.1 percent of the mass of the
atmosphere).
a. Superficially, oxygen gas resembles nitrogen gas in that they
both consist of two atoms of their respective elements.
b. That's where the resemblance ends: Oxygen gas is HIGHLY
reactive, highly prone to combine with many other chemicals in a
general process called "oxidation." The stuff is downright
promiscuous!
c. Familiar forms of oxidation (which entails a loss of electrons
from the oxidized substance and the release of energy) include:
i. Respiration. Animals and plants take in oxygen and use it
to oxidize sugar into carbon dioxide and water, releasing
energy for movement (animals), growth, repair, heat, what
have you.
ii. Combustion is the rapid oxidation of compounds, such as the
cellulose in wood producing fire.
iii. Some kinds of mineral weathering represent very slow
oxidative processes, such as rust of iron compounds in
rocks.
4. The remaining 1 percent of clean, dry air by volume is made up of a
variety of trace gases.
a. Argon, A, an inactive, neutral gas, makes up about 0.93 percent
(or 1.3 percent by mass), which just about takes care of that
last piddling one percent. You might think that there's nothing
else worth discussing in the remaining 0.06 percent. Guess
again!
b. Carbon dioxide gas, CO2, makes up only about 0.03
percent of clean, dry air by volume. This vanishingly small
amount is very important to the earth's radiation balance and is
the subject of much controversy today.
i. It is very important in weather processes, because it traps
radiant energy from the sun and reradiated energy from the
earth in certain infrared wavelengths. By so doing, it
delays the departure of energy from the earth system into
outer space, and this delay permits the lower atmosphere to
maintain a reasonably stable and warm temperature.
ii. If the earth were a chunk of black rock with no atmosphere,
its blackbody temperature would average about -18° C:
the temperature set by the intensity of incoming solar
radiation, if that radiation were instantly reradiated into
outer space.
iii. The earth's average temperature (the Sahara and Antartica
averaged in together) is actually about 15° C, some
33° above its blackbody temperature: The difference
reflects the absorption of roughly half the infrared energy
being reradiated by Earth, energy that is, therefore, held
up in its departure.
iv. The tiny amount of carbon dioxide gas is a major player in
absorbing this reradiated energy. Carbon dioxide is an
important part of the earth's thermostat, if you will: Its
absorbtion capacity times its abundance means that carbon
dioxide accounts for about 54 percent of the greenhouse
effect that keeps Earth 33° C warmer than blackbody
levels.
v. Plot complication: The amount of carbon dioxide is
increasing.
a. At the present, it is increasing about 0.4 percent a
year.
b. This increase is associated with human activities, with
combustion, such as combustion of petroleum fuels in
vehicles, residential furnaces, power plants, industrial
processes, and in human-set fires.
c. Even perfect, clean, non-polluting combustion releases
water vapor and carbon dioxide as its byproducts.
d. Whether clean or dirty, human-induced combustion has
been accelerating over the last couple of centuries, as
we are releasing hydrocarbon fossil fuels (that took
millions of years to accumulate) in a geological
"instant."
e. Deforestation, especially in the tropics, is often done
through torching (slash and burn agriculture), which
then combusts plant material and adds to the atmospheric
carbon dioxide. The death of all these plants
drastically reduces photosynthesis, by which carbon
dioxide is combined with water to create sugar and
oxygen (thereby removing carbon dioxide gas from the
atmosphere).
f. Carbon dioxide levels in the earth's atmosphere have
about doubled in the last century, going up about 15
percent just in the last 40 years.
vi. The reason this is of concern is that one consequence of
the build up of carbon dioxide gas is global warming, as
the carbon dioxide raises the earth's thermostat.
a. Global average temperatures have risen about 0.5° C
in the last century (~1° F), coïnciding with
the increase in carbon dioxide.
b. They are predicted to rise another 3° C (~6° F)
over the course of the next century.
c. This may have a variety of unpleasant side effects:
1. Accelerated melting of polar ice, which could raise
sea levels and flood coastal plains and cities (where
a large fraction of the human population lives and
works).
2. The increased energy lingering in the earth system
could result in more storms (such as hurricanes) and
more violent storms at that.
3. Increased warming could exaggerate temperature
extremes, floods, and droughts.
4. Migration of plant and animal species poleward,
including tropical diseases moving into the mid-
latitudes (e.g., dengue, West Nile virus in New York
City).
d. It may have a few nicer effects, including accelerated
photosynthesis and crop productivity.
viii. The picture isn't as simple as a direct, linear
relationship between carbon dioxide levels and global
temperatures, however.
a. Increased temperatures of ocean water promotes the
accelerated dissolution of atmopheric carbon dioxide
into carbonic acid, which can then combine with other
elements and precipitate to the ocean floor as carbonate
rock (e.g., limestone). This ocean sump for carbon
dioxide will slow the build up of carbon dioxide, but it
is not known with how much efficiency.
b. Tropical forest combustion builds up carbon dioxide in
the short run (a byproduct of combustion is carbon
dioxide and the loss of photosynthesis prevents the
reuptake of carbon dioxide), but it may cause an
intermediate-term increase in photosynthesis: The
secondary successional vegetation that moves into a
fired area is often hyperactive in its photosynthetic
activities (sun-loving, fast-growing plants
photosynthesize like crazy). Again, we don't have the
data to predict how much offset to global warming this
provides.
c. On top of all else, the earth's climate is not stable:
There are centuries-long trends in temperature that are
independent of human activity (e.g., the Little Ice Age
of the 18th century, from which we may still be
warming). Again, big surprise, we are not sure how much
of the global warming is anthropogenic (human-created)
and how much reflects secular (centuries-long) trends in
the planet's climate.
ix. The growing consensus in the scientific community is that
anthropogenic increases in carbon dioxide are a serious
concern and may account for at least part of the global
warming of the last couple of centuries and that it would
be better to err on the side of caution and do everything
we can to reduce carbon dioxide emissions.
x. The costs of doing so, however, may be extremely high and a
lot of people (not just oil companies) are worried about
the opportunity costs of responding to the carbon dioxide
and global warming problem with insufficient data. They
would prefer to err on the side of continuing economic
growth pending the resolution of every single scientific
question.
xi. The dilemma for decision-makers is, of course, that, if
carbon dioxide buildup is creating global warming, waiting
for all the scientific dotting of the i's and crossing of
the t's may mean we respond when it's way too late to
prevent ecological catastrophe (and the attendant economic
costs).
xii. This is an area that can use as many new scientists as our
educational institutions can pump out, scientists in
disciplines as diverse as geography, geology, oceanography,
meteorology, biology, public health and medicine, and
economics. Hopefully, some of you might be among them?
There is so much we don't know, and there are such huge
stakes involved.
c. Ozone (O3 is a special form of oxygen. Instead of
being made up of two oxygen atoms, it's made up of three.
i. Two of the atoms have a double covalent bond, while the
third bonds to one of them with a single bond.
ii. It is created by the action of ultraviolet radiation from
lightning and from the sun (high energy radiation with
wavelengths shorter than visible light, i.e., from 0.4
microns to 0.04 microns). Really shortwave UV (UV-C, 0.04-
0.29 microns, and UV-B, 0.29-0.32 microns) has the energy
to pop the double covalent bonds of O2 when it
is absorbed by them. The freed oxygen radicals (O) then
quickly bond to other O2 to form ozone
(O3).
iii. This absorbtion blocks the UV radiation from making it all
the way through the earth's atmosphere to the surface, and
a good thing, too, since UV is quite hostile to life,
penetrating tissues and dinging DNA:
a. Skin cancer is one expression of too much exposure to
UV.
b. A sunburn is a kinder and gentler expression.
c. A suntan is your skin's statement that it has been
injured by the penetration of UV: Melanin granules
absorb a lot of it and darken, producing the bronzed
look. The more melanin you have, the more UV can be
absorbed, and the darkened melanin granules confer a bit
of protection from the further penetration of
ultraviolet rays. Most of the damage is done in the
process of acquiring the tan. The darker you are
naturally, the better off you are in coping with UV
penetration. The lighter you are, the more vulnerable
you are to skin cancer.
iv. A secondary process of ozone construction entails the
knocking off of the third oxygen atom from the ozone
molecule: The single bond is weaker than the double bond
in regular oxygen and in one pair of the ozone molecule.
The weaker bond can be broken by less energetic rays, by
UV-A, with wavelengths from 0.32-0.40 microns. The freed
oxygen radical then finds another oxygen molecule to look
for a place to bond, again absorbing some UV radiation and
keeping it from making it to the earth's surface.
v. Ozone is a teensy component of the homosphere overall:
0.001 percent by volume, varying from 0 to 12 parts per
million, depending on elevation, latitude, and time.
vi. Ozone is not evenly distributed in the earth's atmosphere:
About 90 percent of it is concentrated about 20 to 50 km up
(roughly 12 to 30 mi. up).
![[ ozone concentration in atmosphere, courtesy of
NASA-Goddard ]](https://home.csulb.edu/~rodrigue/jpegs/ozoneconcentration.jpg)
a. This is the famous ozone layer in the stratosphere.
b. It's just as well ozone hangs out in the stratosphere,
absorbing UV radiation for us down here, since ozone
down here is a rather nasty corrosive pollutant (a
component of smog and, like carbon monoxide, the product
of faulty combustion), which attacks plastics, rubber,
and living tissue (skin and lungs) with great gusto.
vii. It varies in concentration horizontally, too, particularly
when we examine it through time.
a. To get at this, we need to imagine all the ozone
molecules in a column of air stretching from the ground
to the top of the atmosphere. If we took all the ozone
molecules and pushed them to the very bottom of the
atmosphere (someplace at sea level, calibrating the
temperature to 0° C), we would create a thin layer
of "nuthin' but us ozone molecules." Every millimeter
of thickness is called 100 Dobson Units. 1 mm = 100 DU
(a millimeter, by the way, is one thousandth of a meter,
which is a tad more than a yard, so a millimeter is a
teense more than one thousandth of a yardstick). Now,
let us visualize the horizontal distribution of ozone in
Dobson Units all over the Earth at one point in time,
let's say about now:
![[ NASA TOMS image of today's ozone distribution ]](ftp://jwocky.gsfc.nasa.gov/pub/eptoms/images/global/Y2005/FULLDAY_GLOB.PNG)
b. For an explanation of the map, click here
c. So, ozone levels are the lowest over Antarctica,
particularly in Antarctica's spring.
d. Very disturbingly, there has been a substantial decline
in minimum ozone levels over Antarctica over the last
couple of decades and the area covered by less than 220
DU has been increasing through that time (the ozone hole
is defined as any area with less than 220 DU).
e. You can see for yourself in this lab,
in which you will make two X-Y graphs showing, first,
the trends in ozone minima through time and, then, the
trends in size of the ozone hole through time.
viii. The depletion of the ozone layer is an artifact of human
activity.
a. One element that is extremely effective in breaking up
ozone molecules is chlorine.
1. Chlorine often enters the stratosphere in
chlorofluorocarbons, molecules combining carbon,
fluorine, and chlorine (CFCs). CFCs are very stable,
inert compounds that, because of their benign impacts
on the environment here at the surface, were and are
widely manufactured for all sorts of purposes,
including air conditioning and refrigeration.
2. Very sadly, while stable down here, they may become
very unstable and reactive under the special
circumstances of the stratosphere over Antarctica.
3. Released down here, they join the air in the
atmosphere and, mixed in thoroughly, they get carried
eventually into the tropics, where the heat lifts air
(including the CFCs) way above ground. Up there,
some of them drift into the stratosphere and, when
some of these wander into the Antarctic region, the
extreme cold and unique stratospheric clouds of
Antarctica cause the chlorine atoms to dissociate
from the rest of the CFC molecules. Then, when the
Antarctic spring begins, sunlight lets the chlorine
atoms disrupt ozone molecules to create regular
oxygen and ClO, which can combine with NO to create
ClONO, which can then break into NO2 and
Cl (which can then repeat the cycle). ClO can also
interact with O3 to form two O2
and .... another independent Cl atom, which can
repeat the cycle ad infinitum. Because Cl
atoms can repeatedly go through chemical cycles that
break up an ozone molecule, it is estimated that one
measly chlorine atom in the stratosphere over
Antarctica in the spring can destroy 100,000 ozone
molecules before it drifts out of the region or gets
bound up in some other, less reactive compound.
Nasty stuff.
b. Another nasty beast is bromine.
1. Bromine is found in the form of bromocarbons
(commonly used in fumigants, such as methyl bromide,
and fire extinguishers).
2. They, too, get carried to the Antarctican
stratosphere in the winter.
3. Unlike chlorine, they don't readily form inert
compounds that can be carried out of Antarctica, so
they recycle longer and are estimated to take out
from 1 million to 10 million ozone molecules.
c. CFCs and bromocarbons are being phased out by
international agreement: We can't replace the ozone,
but we can gradually reduce the key elements that attack
the ozone.
ix. In the meanwhile, the depletion of stratospheric ozone and
the spatial extension of the Antarctican (and a smaller
Arctic) ozone hole means that more ultraviolet radiation,
especially UV-B, is making it to the surface of the planet.
a. For humans, this translates into a greater risk for skin
cancer and sunburn: We have seen a doubling of skin
cancer reports in the last two decades!
b. Interestingly, even people with loads of melanin in
their skins are reporting sunburns!
c. This may reflect:
1. Ozone depletion.
2. Culture: More people run around in nearly nothing
these days, exposing lots more skin than was the norm
100 years ago.
3. Social geography: More people of northern and western
European extraction are living in the Sunbelt, to
which they are not evolutionarily adapted. Type I
(albino and near-albino) and II (pale skin and eyes
and red or light blonde hair) skins were favored by
natural selection at high latitudes to permit Vitamin
D production in low sun-angle climates; they are not
adapted to live half-nekkid in the Sunbelt, where
Type III (light skin that tans fast and almost never
burns), IV (olive or brown complexion), and V (very
dark or black) skin is better adapted.
d. "Slip, slap, slop," as they say in Australia (which is
uncomfortably close to the Antarctican ozone hole):
Slip into more clothes, slap on a hat, and slop on some
SPF.
1. If you're a Type I or II, put on SPF 15+ each day.
2. If you're a Type III or IV, you might get along with
SPF 8-10.
3. Even if you're a Type V, play it safe and use an SPF
4-6.
x. Ozone is a minor greenhouse gas, accounting for about 7
percent of the warming of the earth above its expected
blackbody temperature.
d. Some more greenhouse gases:
i. Methane: CH4
a. It makes up about 0.0002 percent of the atmosphere (2
parts per million).
b. It accounts for about 12 percent of the greenhouse
effect.
c. It is, however, much more efficient a greenhouse gas
than carbon dioxide, about 40 times as effective in
absorbing radiation.
d. It is created, among other ways, by agricultural
production, such as rice production and, er, how to put
this delicately, ummmmm, cattle flatulence and belching.
e. It is also increasing like crazy: about 1 percent a
year.
ii. Nitrous oxide: N2O
a. It makes up about 0.00003 percent of the earth's
atmosphere (300 parts per billion).
b. It's even more efficient a greenhouse gas than methane,
about 200 to 310 times more absorbtive than carbon
dioxide.
c. It currently accounts for about 6 percent of the
greenhouse effect.
d. Yep, it's increasing, too, being produced by natural
processes in soil and water AND by those pesky human
activities, such as fossil fuel combustion, agricultural
soil management, and animal manure production and
management, about 0.02 percent a year, half the rate of
increase in carbon dioxide.
iii. CFCs (mentioned above in the section on ozone depletion)
figure in here, too.
a. Currently they are about 0.0000003 percent of the
atmosphere (3 parts per billion).
b. They are unbelievably efficient absorbers of radiation:
20,000 times more efficient than carbon dioxide.
c. They presently account for about 21 percent of the
greenhouse effect.
d. They are increasing at the rate of 5 percent a year,
hopefully a rate being brought down by the international
accords to reduce ozone depletion.
iv. The point of detailing these very minor trace gases is that
many of them are increasing much faster than carbon dioxide
and are much more efficient radiation absorbers. This
means that it is plausible that these may become much
bigger culprits in global warming than carbon dioxide, when
they are considered together.
e. Other gases present:
i. Hydrogen
ii. Neon
iii. Krypton (not to be confused with Superman's kryptonite )
iv. Helium
Now, I don't expect you to memorize each of these gases and all of their
characteristics and percentages. What I want is for you to know the major
players, why they're important (nitrogen, oxygen, carbon dioxide, and ozone),
and their relative abundances and for you to be aware that other, really minor
gases may become more significant in the greenhouse effect through time.
In the next lecture, I'll summarize the solids in the atmosphere and the
special case of atmospheric water.
Document and © maintained by Dr.
Rodrigue
First placed on web: 10/06/00
Last revised: 06/07/05