All processes in the Earth's atmosphere, and most processes on the Earth's surface, are powered by the energy the Earth receives from the Sun in the form of electromagnetic radiaiton.
As the radiation passes through the atmosphere, some is absorbed, some is scattered, and some passes right on through to the surface.
Changes in the composition of the atmosphere (e.g. the amount of ozone) change the amount of radiation that arrives at the surface.
Some of the radiation that enters the atmosphere is scattered back into space. Using space-based instrumentation, we can make measurements that we can use to determine the physical state of the atmosphere.
The primary source of energy that drives all processes that take place in the Earth's atmosphere and on its surface is the Sun. The Sun's energy comes to us mostly in the form of electromagnetic radiation, or light. To understand climate, weather, biological processes, and the interactions among these, requires an understanding of the nature of electromagnetic radiation, and how it interacts with Earth's atmosphere, surface and biota.
In this lecture we will describe the essential features of light and the characteristics of its interaction with constituents of the atmosphere: atoms, molecules, and relatively large, semitransparent particles such as water droplets, ice crystals, and other aerosols. We will describe the light that comes from the sun and reaches the top of the atmosphere, how light is changed by its interactions with constituents in the atmosphere (and how those constituents are changed in turn) and describe the light that falls on the surface of the Earth. We will also discuss the radiation that leaves the Earth.
The Sun emits electromagnetic radiation of all sorts; not only visible light, but also radio waves, infrared, ultraviolet light, and X-rays. The spectrum of the Sun's radiation (see the figure below) is complicated, but it is generally dominated by the characteristics of blackbody radiation: a broad distribution of intensity with respect to wavelength, whose maximum intensity is in the visible region. Superimposed on this broad distribution is a multitude of peaks and valleys that are due to the chemical constituents of the sun's atmosphere, shown in part in the inset in Figure 4-1.
This figure shows a very wide region (Extreme ultraviolet to the left, far infrared to the right) of the spectrum of the solar light reaching the top of the Earth's atmosphere.
Light can also be deflected by individual molecules in a process
called Rayleigh scattering. This
process is different from Mie scattering in a number of ways. First,
if there are enough particles, Mie scattering results in exiting
radiation whose intensity is pretty much the same, regardless of what
angle you look at it from. However, Rayleigh scattering has a very
strong dependence on the viewing angle,
and the degree of this angular dependence
depends upon the wavelength; the shorter the wavelength (in the
visible region, this means the bluer the light), the stronger the
scattering. This is why the sky appears blue. When the sun is low in
the sky, light that passes over our heads would not be seen by us at
all if it were not scattered by molecules in the atmosphere. The fact
that we look up and see blue light is due to the fact that the
blue-violet end of the visible part of the spectrum is
Rayleigh-scattered more strongly than light in the red-orange part of
the spectrum. (If you look at a picture of astronauts on the moon,
you see there is no visible "sky" beyond them, since there is no
atmosphere to scatter the sun's light.)
There are many kinds of processes in which matter absorbs or scatters light, and a few in which both of these occur. In some of these the scattering or absorbing particle (atom, molecule, or larger assembly of molecules) is essentially unchanged by the interaction with the light, but in others, the particle is very much changed. For example, radiation in the extreme ultraviolet and x-ray regions of the spectrum would be extremely harmful to life on this planet (and is of great concern for astronauts and spacecraft that travel outside the atmosphere). However, oxygen molecules (O2) high in the atmosphere, absorb this very energetic radiation and, in the process, split into individual oxygen atoms. Since there is a lot of oxygen in the atmosphere, a high-altitude layer that is rich in oxygen atoms forms, and below this layer there is very little of this very harmful radiation.
Lower in the atmosphere, there is still a small amount of this
oxygen-dissociating radiation. However, because there is less of it,
not as much of the oxygen is dissociated. In this part of the
atmosphere, individual oxygen atoms can combine with oxygen molecules
to form ozone, O3. Ozone absorbs radiation in the middle of the
ultraviolet region of the spectrum, protecting us from that radiation
which is not absorbed by the oxygen molecules higher in the
atmosphere. When ozone does this, it dissociates back into an oxygen
atom and an oxygen molecule, but the atom will quickly recombine with
another oxygen molecule to make another molecule of ozone.
The top panel of this figure shows the efficiencies of oxygen (O2) and ozone (O3) at absorbing photons, as a function of the photon wavelengths. Note that the O2 curve has been multiplied by 104 to put it on the scale of the graph. Molecule for molecule, ozone is much more efficient at absorbing light than oxygen. However, there is much more oxygen than ozone (by a factor of about 105).
The bottom panel shows the fluxes of radiation, as a function of wavelength, at the top of the atmosphere, at an altitude of 30 km, and at sea level. At 30 km, from 200 nm to 225, about as much radiation has been absorbed by oxygen as by ozone. However, at the surface, the radiation has been effectively completely absorbed, and mostly by ozone as the radiation passes through the ozone layer.
Because of the scattering of light by the atmosphere, some of the light that falls on the top of the atmosphere ends up being redirected upward, and goes into space. In addition to this backscattering from the atmosphere, some of the light that reaches the surface of the Earth is reflected back upward and leaves the atmosphere. When you look at the Earth from space (from the Space Shuttle, for example), you see the surface of the earth and clouds. If you look at the edge (or limb) of the Earth, you see what almost appears to be a sliver of bluish or reddish glow. This is the light backscattered from the atmosphere. By measuring this light from space, either from the Space Shuttle or from permanently orbiting satellites, we can measure the concentrations of many of the chemicals that are found in the atmosphere, which play an important role in controlling the radiation at the surface (including ozone and other species).
There are no atmospheric constituents that absorb significant
amounts of light in the near ultraviolet and visible regions of the
spectrum. Plants and animals on the Earth have adapted themselves to
be able to make use of this radiation; plants use the Sun's energy
to turn carbon dioxide into organic molecules via photosynthesis, and
animals use this radiation to see.
Electromagnetic radiation carries energy, and the amount of energy is related to the wavelength of the radiation. For convenience of discussion, the spectrum (that is, the range of all possible wavelengths) is divided into a number of major regions: x-ray, ultraviolet, visible light, infrared, and radio. Radiation in the ultraviolet and x-ray regions is much more energetic than light in the visible region. As a result, radiation in those regions can have a greater impact on the atoms and molecules it encounters, causing such phenomena as the loss of electrons and dissociation into smaller molecules and atoms. And just as these phenomena can occur when such energetic light encounters molecules in the atmosphere, so too can they occur when that light encounters molecules that make up animal, vegetable, and mineral at the Earth's surface. Thus, the shielding effect of the Earth's atmosphere is important to protect us from the harmful effects of far-ultraviolet and x-rays. Electromagnetic radiation in the infrared and radio regions is much less energetic than in the x-ray and ultraviolet regions, and so generally it does not have as great an effect on the molecules it encounters. Although there are parts of the infrared region where the atmosphere is quite strongly absorbing (this is very important for understanding the Greenhouse Effect), the infrared radiation that does get through has a very small biological effect. The main biological effect of infrared radiation is indirect, via the heating of the biosphere.
The lower curve (marked "Surface") in the figure above, for wavelengths longer than 290 nm, shows the spectrum of the light that penetrates through the atmosphere and falls on the surface of the Earth. Note that there is very little radiation that has wavelength shorter than about 315 nm in the uvb region, or the regions where the energy the light is carrying is greater than this (shorter wavelengths). This light has been filtered out by the atmosphere, the more energetic having been absorbed by oxygen molecules (which thereby dissociate to form oxygen atoms), and the somewhat less energetic (up to about 315 nm) having been absorbed by ozone. Though there are atmospheric constituents that absorb some of the light in the visible region of the spectrum (even ozone does this) they do not absorb very much light. In the infrared region, there is a great deal of absorption by atmospheric constituents, though there are some regions where the atmosphere is essentially transparent. The intensity of the Sun's light falls off rapidly into the far infrared. The Sun is also a powerful source of radio waves (wavelengths from millimeters to meters), but most of the radio emissions are associated with storms on the surface of the sun, and vary considerably with the number of sunspots and other indicators of solar activity. In contrast to this, the radiation output of the Sun in the uv region (from 290 nm to 360 nm) is relatively constant as the solar storms come and go.
Because the atmosphere scatters and absorbs light, by studying the light that reaches a place on the Earth's surface, we can learn much about the chemical composition of the atmosphere above that place. But the radiation that is sent back toward space, from atmospheric scattering, by reflection from the Earth's surface, and by thermal radiation from both the surface and the atmosphere, is also affected by the composition of the atmosphere. So we can also find out about the state of the Earth's atmosphere by measuring the radiation from space using satellite-borne instruments. This can be done over most any location on the Earth.
To summarize, the Sun is the energy source that drives all processes in the atmosphere and the surface of the Earth. This energy comes in the form of electromagnetic radiation (gamma rays, x-rays, ultraviolet, visible light, infrared, and radio waves). Electromagnetic radiation interacts with atoms and molecules in a wide variety of ways, both in the atmosphere and in the biosphere. The interactions of the Sun's light with the constituents of the atmosphere causes much of the higher-energy radiation, which can be harmful to living organisms, to be absorbed so that little or none of this radiation reaches the surface of the Earth. Anything that alters the chemical composition of the atmosphere has the potential to affect the amount of light, or other electromagnetic radiation, that falls on the Earth's surface. We can measure the radiation that is scattered into space, and use those measurements to determine the concentrations of many important chemicals in the Earth's atmosphere.
This chapter will address the following specific questions:
4.1.1 What is
light?
4.1.2 What is the nature of light
radiated by the Sun?
4.1.3 How does light
move through the atmosphere?