Part F: A Closer Look at....

Ultraviolet Radiation in Our Environment

The activity we call science is one of the most popular ways of making sense out of what our senses tell us. We usually think of science as a collection of "the sciences": physics, chemistry, biology, astronomy, geology, and so on. But outside the science departments and the textbooks, these categories all blend together. When we try to understand nature we find that everything is connected. Nature is usually too complicated for us to analyze from the viewpoint of more than one of the science disciplines at a time, except at a superficial level. For this reason, "the ozone problem" provides an uncommon opportunity to explore the physical, chemical, geological, and biological interactions that govern the biological effects of solar ultraviolet radiation.

A chain of causes and effects:

Sunlight is the energy source of most life forces; life as we know it evolved because of sunlight and depends on sunlight for its continued existence. But these life-giving forces also have the potential for destroying life. Our biosphere is the result of a balance between these opposing potentials.
Sunlight is delivered in small packets of energy called photons. When an ultraviolet photon leaves the sun heading in our general direction, there is a small but real chance that it will collide with a DNA molecule in someone's skin cell and increase the likelihood that he or she will develop a skin cancer. The chance is small, but the number of photons is extremely large, so thousands of people develop skin cancer every year. We can trace the journey of those photons using the methods of "the sciences."
Ultraviolet photons are produced whenever something is heated to a high enough temperature. This can be hydrogen on the surface of the sun or a piece of tungsten wire in a bright light bulb. To understand this step you must explore some basic physics, which will describe how the photons travel through space (at the speed of light, of course, because light is made of photons). Photons from the sun must pass through the earth's atmosphere before they can have any effect on us. This is a hazardous step for an ultraviolet photon, as there are many ways it can be absorbed or deflected. Ozone molecules in the upper part of the atmosphere are particularly effective in absorbing them. Dust and other small particles remove photons. The chance of a photon making it through also depends on its energy. Ironically, the less energy it has--down to a point--the greater its chances of getting through. To understand this you must explore some chemistry. The photons that make it through the atmosphere run another gauntlet in penetrating the outer layer of your skin, especially if your skin contains much natural pigment or has been smeared with a sunscreen lotion. Most of the photons will be intercepted in their journey, but many will reach the nucleus of one of the growing cells at the base of your skin, and when they do it is very likely that they will damage one of the DNA molecules. Now your exploration takes you into biology.
If DNA damage was the end of the story, we would not be here. In the early stages of evolution of life on this planet, the atmosphere did not contain much oxygen so there was no ozone layer. The organisms that survived and evolved all have the ability to repair the damage that ultraviolet photons inflict on their DNA molecules. So a biochemical reaction is the final step in determining whether a solar ultraviolet photon will increase your chance of getting skin cancer.

What is Sunlight?

Light is a form of the energy we call electromagnetic radiation. Other forms are called radio and TV waves, microwaves, infrared (IR), ultraviolet (UV), x-rays, and gamma rays. Physicists think of all of these as examples of the same phenomenon with different energies. In some ways these forms of energy act like waves, so they are traditionally described in terms of their wavelengths. In other ways they act like particles and then it is more useful to describe them in terms of their photon energies. Since we are interested in understanding how they transfer their energy to molecules, damaging the molecules in the process, we will use the photon energy idea. Wavelengths are commonly labeled in nanometers (nm), where 1 nm = 1 10-9 meters, and photon energies are conveniently expressed in units of electron volts (eV).
Having two different ways to describe the same thing seems confusing, but we simplify it somewhat by using more common names for ranges of wavelengths or photon energies. With visible light these names are the colors. In the case of ultraviolet, which we can't see, the names are less colorful. Table 1 lists some examples of energies, wavelengths, and common names of some of the photons we will be talking about.

Table 1

Names Used for Photons of different energies

< td> UV-B
Name Energy Range(eV) Wavelength Range (nm)
Infrared less than 1.6 greater than 760
Visible Light 1.6 - 3.1 760 - 400
Red 1.6-2.0 760-610
Orange 2.0-2.1610-590
Ultraviolet greater than 3.1less than 400

To Learn More...
Read Describing Amounts of Work or Energy and Light and Energy.

Energy Spectra

A common way to visualize quantities that depend on the photon energy of radiation is to plot the quantities against the energy. For visible light, this gives a picture that has the colors lined up in the same order as they are in the rainbow or the color spectrum produced by passing white light through a glass prism (Red, Orange, Yellow, Green, Blue, Violet). We will plot a number of different kinds of spectra on an energy scale, but we will also label some of the important cases as wavelengths, always keeping in mind that shorter wavelengths correspond to higher photon energies. Figure 1 shows the spectrum of radiation from the sun that reaches the top of our atmosphere. The number of photons of each energy falling on each square meter in each second (irradiance) is plotted against the energy. Notice that most of the photons are in the visible light region and that the UV makes up a small fraction of the spectrum. Figure 2 is an expanded view of the UV region, showing the three commonly-used subdivisions, UV-A, UV-B, and UV-C. Notice here how much more radiation exists at the low energy end of the UV (near the visible). The upper curve in Figure 2 represents the spectrum of sunlight at the top of the earth's atmosphere and the lower curve shows it after being filtered through the atmosphere. This shows the dramatic effect the atmosphere--primarily the ozone--has in filtering out the most energetic photons in the UV-B and UV-C regions.

Some important points to observe in Figure 2 are the following:
1. Only UV-A and UV-B reach the surface of the earth; the ozone layer at its present concentration absorbs all of the UV-C.
2. The ozone also absorbs most of the UV-B, especially the more energetic photons (those with wavelengths less than 300 nm).
3. The ozone has very little effect on the amount of UV-A that reaches the surface.

What is Ozone?

The oxygen in the atmosphere exists in three chemical forms. Most of it, but not quite all, is O2 , which is sometimes called molecular oxygen (or more often "oh-two"). A very small amount of oxygen in the upper part of the atmosphere exists as single oxygen atoms, O, or atomic oxygen. At pressures found at lower altitudes, atomic oxygen reacts very rapidly to form O2, so there isn't any O to speak of where we live. The third form, O3, with three oxygen atoms bonded together, is ozone (or "oh-three). It is found in the upper part of the atmosphere (stratosphere) and also in cities as a form of air pollution. The greatest concentrations of stratospheric ozone are found at altitudes between 15 and 25 km. This ozone in the stratosphere filters out harmful ultraviolet radiation, so it benefits all living things. Ozone in the air we breathe still filters out some UV, but it is quite toxic to most organisms because it is very reactive and does more harm than good.

To Learn More...

Read Stratospheric Ozone.

Figure 1: Energy spectrum of solar radiation.

Figure 2: Spectrum of ultraviolet radiation at top of atmosphere and at the surface of earth for 300 Dobson Units of ozone.

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Last updated Friday July 11 1997