the first week or so of this course we'll introduce and discuss several
important properties or characteristics of the atmosphere.
We'll look at the present composition of the atmosphere
and how that has evolved over time. We'll see how and try to
understand why air
temperature, air density, and air pressure change with altitude.
throw in a little material on air pollutants. Many of
these concepts will be used throughout the
remainder of the course.
first lecture we'll mostly be concerned with the composition of our
atmosphere. Here are some questions to think about.
1. Can you see air?
2. Can you smell or taste air?
3. Can you feel or touch air?
4. What is air made of?
look like pretty simple questions at first glance. Air as
clear, transparent, and invisible. But sometimes the air looks
foggy or hazy. This is often because you are seeing the effects
small water droplets (fog) or small particles of some material
(haze) suspended in the air. The droplets and particles
themselves may be too small to
be seen with
the naked eye but are visible because they scatter (redirect)
light. We will learn more about the scattering of light
soon. The sky itself is blue. This is a little more
complicated form of scattering of sunlight by air molecules.
You normally can't smell or taste the main constituents of air. We can
however smell certain air pollutants and trace gases even when
their concentrations are very small.
It is harder to answer Question 3 because we are always in contact with
air. At sea level we will learn that air pressure is pressing on
inch of our bodies with almost 15 pounds of force. If that were
change suddenly we'd feel it; and it would probably be very
painful. The air that surrounds us also affects the transport of
energy into and out of our bodies. You can appreciate this by
comparing the feeling you have standing in 70 F air and in 70 F
Lets answer Question #4 by first listing the two most abundant
gases in air. The most abundant gas is nitrogen (N2).
Liquid nitrogen is relatively inexpensive and is readily available at a
research university like the
University of Arizona. You can see liquid
nitrogen if you were to pour it into a styrofoam cup (liquid nitrogen
is very cold and would probably cause a glass or plastic cup to crack
or shatter). Liquid nitrogen is clear (not blue as shown in
the figure above) and looks
water (though you certainly wouldn't want to drink it). At room
temperature the liquid nitrogen evaporates and turns into invisible
nitrogen gas. The cloud that you see surrounding the cup is made
up of water droplets and/or ice crystals. Nitrogen was
discovered in 1772 by Daniel Rutherford (a Scottish
botanist). Atmospheric nitrogen is relatively unreactive
and is sometimes used to replace air in packaged foods to preserve
freshness. We'll use liquid nitrogen in several of the class
demonstration in this course.
the second most abundant gas in the atmosphere. Oxygen is the
most abundant element (by mass) in the earth's crust, in ocean water,
and in the human body. Here's
photograph of liquid oxygen.
It actually does have a (very faint) blue
color (I was disappointed when I saw the picture the first
time because I had imagined that liquid oxygen might be a deep vivid
Here are the 5 most abundant gases in the earth's atmosphere (you can
find a much longer, more complete list at a variety of online
sources). The percentage concentrations will vary somewhat
depending on how moist the air is.
Water vapor and argon are the 3rd and 4th most abundant
gases in the
atmosphere. The concentration of water vapor can vary from near
0% to as high as 3% or 4%. Water vapor is, in many locations, the
most abundant gas in air. Most of the year in desert regions like
Arizona, argon is in 3rd position and water vapor is 4th.
Water vapor, a gas, is
invisible. Clouds are visible because they are made up of small
drops of liquid
water or solid crystals of ice. Water is the only compound that
naturally in solid, liquid, and gaseous phases in the atmosphere.
Argon is an unreactive noble gas (helium, neon, krypton, xenon, and radon are also noble gases).
Noble gases are often used in "neon
Each of the noble gases has a distinctive color (from
left to right: argon, helium, krypton, neon, and xenon).
Here's a little more
explanation (from Wikipedia)
worry about all these
additional details. The noble gases have full valence electron shells. Valence electrons
are the outermost electrons of an atom and are normally
the only electrons that participate in chemical bonding.
Atoms with full valence electron shells are extremely stable, have less
of a tendency to gain or lose electrons, and therefore do not readily
form chemical bonds.
plays an important role in the formation of
and weather. Meteorologists are very interested in knowing and
keeping track of how
much water vapor is in the air at a particular place and time.
One of the variables they use is the dew point temperature. The
actually in the air. The
higher the dew
point value, the more water vapor the higher the water vapor
The chart below gives a rough equivalence between dew point
temperature and percentage concentration of water vapor in the air.
Air temperature will always be equal to or warmer than
the dew point
temperature. Experiencing 80o dew points would be very
unpleasant (and possibly life threatening because your body might not
be able to cool itself). Click
to see current dew point temperatures across the U.S.
The dew point has another "job."
We can use the cup of liquid nitrogen to show this.
The cloud came from moisture in the
air. The cloud was not made of nitrogen gas (which is
invisible). Note again that a certain amount of "artistic"
was used in the figure above; liquid nitrogen is not blue and water
clouds are not green.
atmosphere, which was composed mostly of hydrogen (H) and helium (He),
was very different from the atmosphere that we have today. This
original atmosphere either escaped into space (the earth was hot and
were moving around with enough
speed that they could overcome the pull of the earth's gravity) or was
swept into space by the solar wind.
Our present atmosphere is though to have come from volcanic eruptions.
Volcanoes emit a lot of water vapor and carbon
the earth began to cool the water vapor condensed and began to create
oceans. Carbon dioxide dissolved in the oceans and was slowly
turned into rock. Smaller amounts of nitrogen (N2) are emitted by
volcanoes. Nitrogen is relatively inert and remained in the
air. Nitrogen concentration built up over time.
Volcanoes didn't add any of the oxygen that is the
Where did that come from?
The oxygen is thought to have first come from
water vapor and carbon dioxide by the ultraviolet light in
sunlight. The O and OH react
to form O2 and H.
Here's a comment from the anonymous survey conducted in one of the
classroom versions of this course:
It is sometimes easier and clearer to show or explain a reaction
in formulas instead of words. I wouldn't expect you to remember
the chemical formulas in the example above (though you should be able
to interpret the equations if they are written down). You won't
have to balance chemical equations in this course either. You
remember that the earth's original oxygen came from oxygen in water and
carbon dioxide. It's probably also good to remember that
ultraviolet light is capable of breaking molecules apart.
Once molecular oxygen (O2)
started to accumulate in the
air it began to react
with atomic oxygen (O)
Once formed, ozone began to absorb ultraviolet
light and life forms were able to safely move from the oceans (which
absorb UV light in the
absence of ozone) onto land. Eventually plants and photosynthesis
would become the main source of atmospheric oxygen.
Note that combustion (and respiration) is really just the opposite
of photosynthesis. We burn fossil fuels to generate energy.
Water vapor and carbon dioxide are by products.
Here's a slightly different look at the origin
and buildup of oxygen in the earth atmosphere.
This somewhat confusing
figure is included in a photocopied set of ClassNotes used in the
classroom version of this course. It shows some of the important
events in the history of the earth
and evolution of the atmosphere. The numbered points were
Note at Point 1: the
is thought to be between 4.5
4.6 billion years old.
The iron catastrophe was an important event in the earth's
history. The accumulation and circulation of liquid metal in the
earth's core gave the earth a magnetic field. The magnetic field
then began to deflect and thereby protect the earth from the solar
wind. Remember the solar wind
may have swept away the earth's original atmosphere.
2) are column-shaped
up of layers of sedimentary rock, that are created by microorganisms
(cyanobacteria = blue green algae) living at the top of the
stromatolite. Fossils as old as 2.7 billion years of the very
small microbes have been found in stromatolites and are some of
the earliest records of life on earth. Much older (3.5 to 3.8
B years old) stromatolites presumably also produced by microbes, but
without fossils, have been also been found.
We're learning about stromatolites
because the cyanobacteria were able to produce oxygen using
Living stromatolites are found
few locations today. The picture above is from Coral Bay Australia, located on
western tip of the continent. The picture was probably taken at
low tide, the stromatolites would normally be covered with ocean water.
Once cyanobacteria began to produce
oxygen in ocean water, the oxygen reacted with dissolved iron (iron
ions in the figure below) to form hematite or magnetite. These
two minerals precipitated out of the water to form a layer on the sea
Periodically the oxygen production would decrease or stop (rising
oxygen levels might have killed the cyanobacteria or seasonal changes
might have slowed the photosynthesis). During these times of low
dissolved oxygen concentrations, layers of a different mineral, jasper,
would form on the
ocean bottom. Eventually the cyanobacteria would recover, begin
producing oxygen again, and a new layer of hematite or magnetite would
form. The rocks that resulted, containing alternating layers of
black hematite or magnetite and red layers of jasper are known as the
iron formation (Point 3).
samples are fairly impressive because they are fairly heavy
(they contain a lot of iron) and are also 2 - 3 billion years old!
Eventually the dissolved iron in
the ocean was used up (Point 4
in the timeline figure above).
Oxygen produced by cyanobacteria no longer reacted with iron and was
free to diffuse from the ocean into the
atmosphere. Once in the air, the oxygen could react with iron in
sediments on the earth's surface. This produced red colored
(rust colored) sedimentary rock. None of these so-called red beds
are older than
about 2 B years old. Thus it appears that a real buildup up
oxygen began around 2 B years ago. Oxygen concentrations reached levels
that are about the same as today around 500 to 600 years ago (Point 5
in the figure).