In this section, we will explore what is ozone and what is ultraviolet radiation. We then will explore the relationship between ozone and ultraviolet radiation from the sun. It is here that ozone plays its essential role in shielding the surface from harmful ultraviolet radiation. By screening out genetically destructive ultraviolet radiation from the Sun, ozone protects life on the surface of Earth. It is for this reason that ozone acquires an enormous importance. It is why we study it so extensively.
About 90% of the ozone in our atmosphere is contained in the stratosphere, the region from about 10 to 50-km (32,000 to 164,000 feet) above Earth's surface. Ten percent of the ozone is contained in the troposphere, the lowest part of our atmosphere where all of our weather takes place (see Chapter 2). Measurements taken from instruments on the ground, flown on balloons, and operating in space show that ozone concentrations are greatest between about 15 and 30 km. The yellow curve in Figure 1.01 shows how ozone amount varies with altitude. This sort of plot is called a vertical profile. (See Chapter 3 for a more complete description of the vertical distribution of ozone.)
The ozone concentrations shown in Figure 1.01 are very small, typically only a few molecules O3 per million molecules of air. But these ozone molecules are vitally important to life because they absorb the biologically harmful ultraviolet radiation from the Sun. There are three different types of ultraviolet (UV) radiation, based on the wavelength of the radiation, as explained below in section 2.4. These are referred to as UV-a, UV-b, and UV-c. Figure 1.01 also shows how far into the atmosphere each of these three types of UV radiation penetrates. We see that UV-c (red) is entirely screened out by ozone around 35 km altitude. On the other hand, we see that most UV-a (blue) reaches the surface, but it is not as genetically damaging, so we don't worry about it too much. It is the UV-b (green) radiation that can cause sunburn and that can also cause genetic damage, resulting in things like skin cancer, if exposure to it is prolonged. Ozone screens out most UV-b, but some reaches the surface. Were the ozone layer to decrease, more UV-b radiation would reach the surface, causing increased genetic damage to living things.
Because most of the ozone in our atmosphere is contained in the stratosphere, we refer to this region as the stratospheric ozone layer. In contrast to beneficial stratospheric ozone, tropospheric ozone is a pollutant found in high concentrations in smog. Though it too absorbs UV radiation, breathing it in high levels is unhealthy, even toxic. The high reactivity of ozone results in damage to the living tissue of plants and animals. This damage by heavy tropospheric ozone pollution is often manifested as eye and lung irritation. Tropospheric ozone is mainly produced during the daytime in polluted regions such as urban areas. Significant government efforts are underway to regulate the gases and emissions that lead to this harmful pollution, and smog alerts are regular occurrences in polluted urban areas.
To appreciate the importance of stratospheric ozone, we need to understand something of the Sun's output and how it impacts living systems. The Sun produces radiation at many different wavelengths. These are part of what is known as the electromagnetic (EM) spectrum. EM radiation includes everything from radio waves (very long wavelengths) to X-rays and gamma rays (very tiny wavelengths). EM radiation is classified by wavelength, which is a measure of how energetic is the radiation. The energy of a a tiny piece or "packet" of radiation (which we call a photon) is inversely proportional to its wavelength.
The human eye can detect wavelengths in the region of the spectrum from about 400 nm (nanometers or billionths of a meter) to about 700 nm. Not surprisingly, this is called the visible region of the spectrum. All the colors of light (red, orange, yellow, green, blue, and violet) fall inside a small wavelength band. Whereas radio waves have wavelengths on the order of meters, visible light waves have wavelengths on the order of billionths of a meter. Such a tiny unit is called a nanometer (1 nm= 10-9 m). At one end of the visible "color" spectrum is red light. Red light has a wavelength of about 630 nm. Near the opposite end of the color spectrum is blue light, and at the very opposite end is violet light. Blue light has a wavelength of about 430 nm. Violet light has a wavelength of about 410 nm. Therefore, blue light is more energetic than red light because of its shorter wavelength, but it is less energetic than violet light, which has an even shorter wavelength. Radiation with wavelengths shorter than those of violet light is called ultraviolet radiation.
The Sun produces radiation that is mainly in the visible part of the electromagnetic spectrum. However, the Sun also generates radiation in ultraviolet (UV) part of the spectrum. UV wavelengths range from 1 to 400 nm. We are concerned about ultraviolet radiation because these rays are energetic enough to break the bonds of DNA molecules (the molecular carriers of our genetic coding), and thereby damage cells. While most plants and animals are able to either repair or destroy damaged cells, on occasion, these damaged DNA molecules are not repaired, and can replicate, leading to dangerous forms of skin cancer (basal, squamous, and melanoma).
Figure 1.02 shows plots of the Sun's energy, a quantity referred to as solar flux. It refers to the amount of solar energy in watts falling perpendicularly on a surface one square centimeter, and the units are watts per cm2 per nm. The plots represent the Sun's energy (or flux) at four different altitudes in the atmosphere: the surface, 20 km, 30 km, and the "top" of the atmosphere (over 100 km). Because of the strong absorption of UV radiation by ozone in the stratosphere, the intensity decreases at lower altitudes in the atmosphere. In addition, while the energy of an individual photon is greater if it has a shorter wavelength, there are fewer photons at the shorter wavelengths, so the Sun's total energy output is less at the shorter wavelengths. Because of ozone, it is virtually impossible for solar ultraviolet to penetrate to Earth's surface. For radiation with a wavelength of 290 nm, the intensity at Earth's surface is 350 million times weaker than at the top of the atmosphere. If our eyes detected light at less than 290 nm instead of in the visible range, the world would be very dark because of the ozone absorption!
To appreciate how important this ultraviolet radiation screening is, we can consider a characteristic of radiation damage called an action spectrum. An action spectrum gives us a measure of the relative effectiveness of radiation in generating a certain biological response over a range of wavelengths. This response might be erythema (sunburn), changes in plant growth, or changes in molecular DNA. The blue line superimposed on Figure 1.02 shows the action spectrum for DNA. It represents the probability of DNA damage by UV radiation at various wavelengths. Fortunately, where DNA is easily damaged (where there is a high probability), ozone strongly absorbs UV. At the longer wavelengths where ozone absorbs weakly, DNA damage is less likely. The red line in Figure 1.02 shows the calculated UV spectrum at Earth's surface if there was a 10% decrease in ozone. In response to this decrease in protective ozone, the amount of DNA damaging UV increases, in this case, by about 22%. Considering that DNA damage can lead to maladies like skin cancer, it is clear that this absorption of the Sun's ultraviolet radiation by ozone is critical for our well being.
While most of the ultraviolet radiation is absorbed by ozone, some does make it to Earth's surface. Typically, we classify ultraviolet radiation into three parts, UV-a (320-400 nm), UV-b (280-320 nm), and UV-c (200-280 nm). Sun screens have been developed by commercial manufacturers to protect human skin from UV radiation. The labels of these sun screens usually note that they screen both UV-a and UV-b. Why not also screen for UV-c radiation? From Figure l.01, we can see that when UV-c encounters ozone in the mid-stratosphere, it is quickly absorbed so that none reaches Earth's surface. UV-b is partially absorbed and UV-a is barely absorbed by ozone. Ozone is so effective at absorbing the extremely harmful UV-c, that sun screen manufacturers don't need to worry about UV-c. Manufacturers only need to eliminate skin absorption of damaging UV-b and less damaging UV-a radiation.
The screening of ultraviolet radiation by ozone depends on other factors, such as time of day and season. The angle of the Sun in the sky has a large effect on the UV radiation. When the Sun is directly overhead, the UV radiation comes straight down through our atmosphere and is only absorbed by overhead ozone. When the Sun is just slightly above the horizon at dawn and dusk, the UV radiation must pass through the atmosphere at an angle. Because the UV passes through a longer distance in the atmosphere, it encounters more ozone molecules and there is greater absorption and, consequently, less UV radiation striking the surface. Figure 1.03 shows the varying intensities of UV radiation responsible for sunburn (erythemal exposure) during July 1988 over the globe.
The image in Figure 1.03 is derived from satellite observations of the solar UV reflected off Earth's atmosphere detected by the NASA Total Ozone Mapping Spectrometer (TOMS). Intense sunburn-causing UV is shown by the red-orange colors, while lesser values are shown by the blue-purple colors (as indicated by the color scale at bottom of the figure). Tropical regions with low ozone and the Sun high overhead have very intense exposure. High latitude regions with higher ozone amounts and the Sun lower in the sky have rather weak exposures. Higher altitudes in the Rockies and Himalayas also have higher exposure because the column of air that radiation passes through is shorter. Clouds also decrease the UV that is incident on Earth's surface (so cloudiness does have its benefits), though it is possible to get sunburned on a cloudy day.