ERTH 535:
Planetary Climate Change
(Spring 2017) 		Atmospheric
Composition 		Dr. Dave Dempsey
Dept. of Earth & Climate Sci.,
SFSU

The atmosphere consists of:

  1. a mixture of different gases; and
  2. tiny liquid and solid particles suspended in the gas mixture.

1. Mixture of gases

In clean, dry air (that is, a sample of air without any water vapor, and no suspended liquid or solid particles), the proportions of different types of molecules are:

Type
of
Molecule 		Percentage
of all Molecules
in the Sample 		Meteorological Importance
nitrogen
(N2) 		78% Well, it's what air is mostly made out of, and so when air moves around—that is, "wind"!—it's mostly nitrogen gas that we feel. Otherwise, not terribly important meteorologically (though it's biologically important).
oxygen
(O2) 		21% A distant second to nitrogen as a constituent of air; most of the air sample that isn't nitrogen is oxygen. Absorbs some ultraviolet radiation from the sun, starting a process responsible for creating ozone in the stratosphere. Otherwise not terribly important meteorologically. (Of course, it's very important biologically.)
argon
(Ar) 		1% Zilch. It's a "couch potato" gas; it doesn't interact with other substances chemically and doesn't absorb or emit radiation—doesn't do much of anything. (It's not even biologically important.)
carbon dioxide
(CO2) 		0.038% Affects the transfer of longwave infrared radiation through the atmosphere, contributing significantly to the greenhouse effect, which is responsible for keeping the earth's surface warm enough to be habitable by living organisms such as us. CO2 concentrations in the atmosphere have been increasing since the industrial revolution in the 19th century, at increasingly faster rates. There is an international consensus among scientists that higher concentrations of CO2 will enhance the greenhouse effect and cause global warming (which is already happening), though to an uncertain degree and with uncertain effects.

The proportions of the four gases above are essentially the same everywhere in the atmosphere (except for small local differences in carbon dioxide due to differences in photosynthetic activity by green plants). Another gas present in clean, dry air:

ozone
(O3) 		Variable:
average is 0.000001%;
typical maximum 0.0001%
(in ozone layer) 		Absorbs destructive ultraviolet radiation from the sun, making it possible for life to exist on earth's surface.

There are other gases present besides those in clean, dry air, of course. Water vapor is by far the most important meteorologically.

water
vapor
(H2O) 		Variable,
from almost 0%*
to about 4%**
  1. It is the source of water for cloud droplets and ice crystals (and hence, ultimately, precipitation).

  2. It affects the transfer of longwave infrared radiation through the atmosphere—in fact, it's the most significant contributor to the greenhouse effect (but not to global warming, initially); and

  3. It absorbs or releases heat when it changes between gas, liquid and solid phases or states. Water (that is, H2O, regardless of phase or state) is the only substance that exists in all three phases within the normal range of atmospheric temperatures and pressures.
*At very high altitudes; in very cold polar regions; and often in deserts.
**At the earth's surface over tropical oceans.




2. Tiny liquid and solid particles suspended in the air

Particle
Type 		Comments
Water droplets
and/or
ice crystals
  • These particles average perhaps 10-20 microns (a micron is one millionth of one meter, or one thousandth of a millimeter) across. Individually these are too small to see, but together they can scatter enough light so that their collective presence is visible as a cloud.

  • Clouds are simply aggregates of very large numbers of them. They reflect visible light very well—on the average, clouds reflect back to space about 20% of the solar radiation striking the earth (about 2/3 of all solar radiation reflected by the planet), helping keep the earth much cooler than it would be without them.

  • Clouds absorb and emit all wavelengths of longwave infrared radiation very well. This makes them a significant contributor to the greenhouse effect.

  • If enough of cloud droplets and/or ice crystals can collide and coalesce and become big enough, the resulting precipitation particles (rain drop, snow flake, hail stone, etc.) will fall out of the cloud and reach the earth's surface. The typical size of a precipitation particle is about 100 times the diameter of a cloud droplet or ice crystal (though they can certainly get much bigger than that), so it contains about 100x100x100 = 1,000,000 of them (since the volume is proportional to the diameter cubed). Getting that many cloud droplets or crystals together is not easy, which is why most clouds don't produce precipitation.
Dust, ash,
smoke, pollen,
microorganisms, salt
  • Near the earth's surface, these typically are present in concentrations ranging from 500 particles per cubic centimeter over the open ocean, to 50,000 particles per cubic cm well inland, though the variability is great. (A cubic centimeter is approximately equal to the volume occupied by the end of your finger including the fingernail.)

  • They can serve as tiny, solid surface onto which water vapor can condense to form cloud droplets or ice crystals—clouds would be very rare without suspended solid particles of dust, salt, etc.

  • They affect the transfer of radiation (especially visible light, often by reflecting it back to space, which causes the earth's surface to cool, as during the period following the eruption of Mt. Pinatubo in the Philippines in the early 1990's; or by absorbing it, which warms the atmosphere but cools the surface.)

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