Sprinkled throughout the atmosphere are pale blue molecules of a toxic gas that are essential to most life on Earth. This gas is ozone.
Ozone is toxic because it is highly reactive. This is why it can sterilize drinking water, eliminate odors, bleach colors, and decompose rubber. Fortunately, the amount of ozone at ground level is usually too low for these effects to be observed. However, high concentrations of various air pollutants and sunlight can increase ozone levels near the ground from a few tens of molecules per billion molecules of air (ppb) to a few hundred ppb. These levels of ozone can damage plants, cause eye irritation, inflame mucous membranes and impair the performance of athletes.
Ozone is essential to life because it shields the Earth from the damaging, even lethal, ultraviolet radiation emitted by the sun. This filtering ability is particularly remarkable when you consider the relative scarcity of ozone molecules. For every billion molecules in the atmosphere, only around 300 are ozone.
Imagine you could poke a tube through the entire atmosphere over your head and bring all the ozone molecules in the tube down to the surface. If they were then subjected to the same temperature and atmospheric pressure (standard temperature and pressure or STP) as you are, they would form a layer only about 3-millimeters thick.
Although they may be formed in many different ways, all the ozone molecules in the stratosphere are identical. An oxygen molecule (O2) is composed of two oxygen atoms (O). Ultraviolet radiation can split an O2 molecule and leave behind two free O atoms. Various chemical reactions leave O atoms as a byproduct. In either case, the free O atom can merge with an O2 molecule to form triatomic oxygen (O3), more commonly known as ozone.
Figure 1: Formation of ozone molecules
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The term "ozone layer" generally refers to a relatively high concentration of ozone in the stratosphere, a layer of very dry air around 15 to 35 kilometers (9 to 22 miles) above the Earth's surface. However, about 10 percent of the total ozone is found in the troposphere, the lowest portion of the atmosphere. The troposphere contains 90 percent of the atmosphere and nearly all of the atmosphere's water vapor. Storms form in the troposphere and usually stay there. But the tops of especially powerful thunderstorms occasionally poke into the lower stratosphere.
The tropopause, the border between the troposphere and the stratosphere, ranges from around 10 to 15 kilometers above the surface. The heat of summer increases the height of the tropopause; the height is reduced in winter.Other things being equal, when the tropopause is low, the amount of stratospheric ozone is high and vice versa.
Figure 2: Atmospheric distribution
The ozone between the surface and the tropopause forms only a fraction of the ozone over most locations. Nevertheless, it absorbs solar UV more efficiently than an equal amount of stratospheric ozone. This is because scattering caused by dust and aerosols increases the distance that rays of sunlight travel on their way to the surface. In spite of this benefit, tropospheric ozone is often referred to as "bad" ozone because of its adverse effects in high concentrations. If the same ozone were somehow to drift into the stratosphere, it would be called "good" ozone.
Forming and Destroying Tropospheric Ozone
Tropospheric ozone is produced in many ways. Some is formed by lightning or by UV radiation from the sun. Most is formed by chemical reactions which take place in the presence of sunlight.
One such reaction is the conversion of nitrogen dioxide (NO2) into nitrogen oxide (NO) in the presence of solar UV. The O atom left over from this reaction combines with an O2 molecule to form an ozone molecule. Various other gases in the atmosphere can combine with NO to form more NO2, which then can cause a buildup of ozone. Gaseous organic chemicals, in the presence of nitrogen oxides and sunlight, can also contribute to ozone production.
The same photo-chemistry that forms ozone can also destroy it. Indeed, some photochemical processes in the atmosphere are called "do-nothing" reactions since they destroy as much ozone as they create. Ultraviolet radiation leaking through the ozone high in the stratosphere can also create and destroy ozone in the troposphere.
Measuring Tropospheric Ozone
There are several ways to measure tropospheric ozone. Since ozone is a strong oxidizer, it changes the color of some chemical compounds and solutions. For example, paper soaked in a mixture of starch and potassium iodide will change color when exposed to ozone.
The reaction of ozone with various chemicals, gases, and even some lubricating oils causes a faint luminescence that can be detected by a sensitive photomultiplier tube. Detection systems such as this are known as chemiluminescence detectors.
Since ozone absorbs ultraviolet radiation so effectively, many kinds of ozone detectors incorporate a UV lamp and a detector. Air is passed through a chamber, and any attenuation is assumed to have been caused by ozone. A problem with this method is that attenuation can also be caused by dust. Therefore, it's common practice to use two chambers, one of which receives air from which any ozone has been scrubbed by a catalytic converter. Alternatively, scrubbed and unscrubbed air can be passed in sequence through the same chamber. Either way, the error caused by dust and other contaminants in the ozone-free sample can then be determined by subtraction.
Sulfur dioxide and other chemicals can interfere with the chemical and UV detection of ozone. When scientists at the Montsouris observatory near Paris became aware of this problem in 1905, they built a second chemical ozone detector. The air inlet for the new detector was fitted with a 4-meter (13-feet) hose of natural rubber, which completely destroyed any ozone passing through it. In this way any errors in the original detector caused by gases other than ozone could be eliminated.
Several times my son Eric and I have measured the amount of tropospheric ozone between the bottom and top of mountains in New Mexico. We do this by measuring the total amount of ozone in the atmosphere with a UV-sensitive instrument that is pointed at the sun. One of us goes to the top of a mountain while the other stays at the base. We then make a series of observations at prearranged times. Later, we subtract the ozone measured at the mountaintop from the measurements made at the base to determine the amount of ozone in between. So far our results (a few Dobson Units per vertical kilometer; see Figure 5 in printed article) have agreed with measurements made from balloons by the National Oceanic and Atmospheric Administration (NOAA).
Tropospheric Ozone Cycles
The amount of ozone above any given spot of Earth is rarely constant. Consider the diurnal or daily ozone cycle over Albuquerque, New Mexico.
Early on a July morning at the base of the Sandia Mountain aerial tramway just northeast of the city, ground-level ozone concentration might be, say, 20-30 ppb. As the sun rises high in the sky, photochemical ozone production increases, especially when the wind is from the southwest and the clean mountain air is spiked with nitrogen oxides and hydrocarbons from automobile exhaust. Although little or no photochemical smog may be visible, the ozone concentrations might reach 40-60 ppb by late afternoon. As the sun sinks behind the volcano cinder cones west of Albuquerque, the ozone level also falls. Late that evening, the ozone returns to its normal "background" level.
Tropospheric Ozone Trends and Effects
The ozone measured at the ground near Paris, France, from 1876-86 was only around a third to a half of what is usual in unpolluted areas today. The increase since then is generally believed to be caused by human activities. At least two-thirds of the nitrogen oxides are believed to come from the burning of fossil fuels, wood, forests and agricultural wastes. Nitrogen oxides are also produced naturally by lightning, forest fires, and soil. Organic chemicals, such as methane and hydrocarbons, can be byproducts of plants, animals, and human activity.
It's interesting to compare the amount of ozone in a vertical column of air adjacent to a mountain with that over a city. In the summer of 1989, Eric and I measured 5.8 DU (Dobson Units) of ozone between the base and crest of Sandia Mountain, an altitude difference of 1,164 meters (3,819 feet). A few weeks earlier I had measured about 5 DU of ozone between street level and an observation deck atop the 110th floor of the World Trade Center in New York City (420 meters or 1,377 feet). There was much more ozone in the air above New York City than in the air near Sandia Mountain.
Surprisingly, however, much of the air at street level has less ozone than you might expect. A government official who makes ozone measurements at fixed sites told me that ozone levels can be much higher in the air over New York and in the surrounding regions than at ground level in the city. Apparently the high number of air pollutants, people, rubber tires, and the like, suppresses ozone concentrations at street level.
Figure 3: New York City ozone levels
When California forced a significant reduction of hydrocarbon emissions from automobiles, the ozone in downtown Los Angeles fell. Ozone levels downwind, however, continued to rise. The scientists who puzzled over this dilemma noticed that there is considerably more vegetation down-wind from the central city. They concluded that ozone is reduced only when both hydro-carbons and nitrogen oxides are reduced.
The relationship of trees and ozone is particularly interesting. Too much ozone can damage or even kill trees. Ironic-ally, trees emit hydrocarbons that participate in chemical re-actions that produce ozone. Several years ago William Chameides of the Georgia Institute of Tech-nology studied satellite images of Atlanta and found that 57 percent of the city was wooded. He and his co-workers concluded that Atlanta's trees emitted at least as many hydrocarbons as the city's cars, trucks, buses and factories.
Most references to the ozone layer mean the ozone found in the stratosphere. There it forms a vaporous shield that protects life on Earth from the lethal effects of the sun's UV radiation. If you've flown in the Concorde, then you have probably travelled through the bottom of the stratospheric ozone layer.
Forming and Destroying Stratospheric Ozone
Ozone in the stratosphere is formed by a natural photochemical process when ultraviolet radiation from the sun splits molecules of oxygen into the individual oxygen atoms from which they are formed. The free O atoms soon react with O2 molecules to form O3.
This process works both ways: O3 molecules that are unlucky enough to be struck by UV radiation are split back to an O2 molecule and a free O atom. The free O atom can merge with an O2 molecule to once again form an O3 molecule.
Ozone molecules that drift lower down in the stratosphere are protected from the destructive effects of UV radiation by the overlying blanket of ozone molecules. But even molecules of ozone deep in the ozone layer are not entirely safe, for they can be destroyed by reactions involving sunlight and oxides of nitrogen, hydrogen, chlorine, and bromine. Although all these chemicals can arise from natural sources, they can also arise from human activity. For example, manufactured chlorofluorocarbons (CFCs) have been a concern since 1970 when James E. Lovelock detected their presence in air. In 1974, Sherwood Rowland and Mario Molina proposed that CFC molecules could eventually drift into the stratosphere, where UV radiation would break them down into chlorine monoxide (ClO) and other ozone-destroying compounds.
In recent years, evidence has been accumulating that CFCs may indeed be causing a gradual reduction of ozone, particularly in regions near the poles during early spring. The question now being researched is how much ozone they might ultimately destroy. Fortunately solar UV is constantly creating new ozone. Therefore, CFCs cannot destroy all the ozone.
Measuring Stratospheric Ozone
Ozone in the stratosphere can be measured directly using instruments on aircraft, rockets, and–especially–balloons. Many of the same kinds of sensing systems used for measuring ozone at the surface have been modified for these roles.
Thanks to ozone's well-known ability to absorb ultraviolet radiation, the total amount of ozone (troposphere plus stratosphere) can be measured indirectly from the surface or from space. Several kinds of optical instruments have been developed for measuring ozone from the surface, including the Dobson spectrophotometer and various instruments that use filters or diffraction gratings to measure narrow bands of ultraviolet.
The Dobson spectrophotometer plays a key role in ground-based ozone monitoring efforts. Invented in the late 1920's by G. M. B. Dobson, this instrument divides sunlight into a spectrum with a prism and measures the ratio of two UV wavelengths about 20 nanometers (nm) apart. Dust and aerosols can cause errors in ozone observations by scattering one wavelength more than another. Dobson observations are usually made at two pairs of wavelengths to cancel out this error.
The Dobson instrument is expensive, nearly 2 meters (6 ft) long, and heavy–about 40 kilograms (85 pounds). Since it measures the ratio of two or more ultraviolet signals, it provides no informa-tion about the amount of solar ultraviolet.
The Brewer ozonometer uses a diffraction grating to separate the sun's ultraviolet wavelengths. (A diffraction grating consists of several parallel grooves that split light up into several wavelengths. A compact disk produces an effect much like a diffraction grating–except that the rainbow colors are produced by parallel rows of pits instead of grooves.) This expensive instrument is smaller than the Dobson and well-suited for automated data taking. It also measures sulfur dioxide, a gas that can interfere with ozone measurements. Some scientists believe that the Brewer measures ozone with higher precision than the Dobson. Indeed, Canada has switched from Dobsons to Brewers.
A third kind of instrument uses optical filters to measure two or more UV wavelengths. Such instruments are cheaper, smaller, and easier to use than the Dobson and the Brewer. They also provide information about the sun's direct ultraviolet. For these reasons, I selected the filter approach when designing an instrument to measure total ozone several years ago.
Some sensors on satellites can measure the amount of ozone at various altitudes by observing the sun as it rises and sets through the atmosphere above the Earth's limb (an astronomical term referring to the edge of a planetary body's disk). Other satellite sensors measure the amount of the sun's ultraviolet that is scattered back into space from the atmosphere below. Since these wavelengths are absorbed by ozone, processing the backscatter from one or more pairs of wavelengths permits one to estimate the amount of ozone between the satellite and the ground.
It's important to note the latter kind of instrument cannot measure all the ozone between the top of the atmosphere and the surface. Clouds can get in the way, and little or none of the UV backscattered from the lowest few kilometers above the surface can penetrate back through the ozone layer. An estimate of lower tropospheric ozone is added to the satellite ozone equation to correct this problem. The estimate is based on measurements made from the ground during annual calibration checks and measurements made by balloon sensors.
Stratospheric Ozone Cycles
Superimposed on the daily ozone cycle near the ground are seasonal changes in the amount of stratospheric ozone. In the northern hemisphere, the total amount of ozone is lowest during winter. The amount of ozone begins to rise rapidly during spring and gradually diminishes in the summer and fall.
This gradual seasonal variation in ozone is marked by sharp spikes and dips associated with weather systems. Passage of a cold front, for example, may cause the amount of ozone to increase 20 percent or more for a day or two. A warm front may cause an comparable decrease. Meterologists refer to regions of diminished ozone as ozone minimums, and regions of high ozone as ozone maximums.
I'll never forget the giant ozone maximum that passed over South Texas on March 16, 1990 (See Figure 6, page 8 of printed version). That morning I made a few ozone observations around 11:00 a.m. and was surprised to find the highest amount of ozone I had ever measured, around 360 DU. Since the ozone amount kept climbing as noon approached, I assumed something was wrong with the instrument. But both filters were clean, no wires were dangling in front of them, and the battery was fresh.
After 1:00 p.m. the ozone amount climbed even faster than it had before noon. By 1:30 p.m. it was more than 440 DU and shortly before 2:00 p.m. it reached 460 DU. The ozone amount then began a sharp slide to pre-noon levels.
Several months later, data from the TOMS (Total Ozone Mapping Spectrometer) instrument aboard the Nimbus-7 satellite confirmed the extraordinarily high ozone levels of March 16. My son Eric wrote a Pascal program that transformed the TOMS data into a color-coded ozone map of the United States. The map disclosed an enormous tongue of high ozone reaching down from Canada and ending in South Texas. A check of weather records revealed that this ozone maximum was associated with a giant weather system that moved in from the Pacific and crossed the United States over a 3-day period.
Figure 4: How ozone is measured
Figure 5: Annual cycles over Texas
Figure 6: Unusually high levels over Texas
Stratospheric Ozone
Trends and Effects
Recall that most ground observations of the ozone layer measure the total amount of ozone in a column between the instrument and the top of the atmosphere. Therefore, these measurements include the total amount of both tropospheric and stratospheric ozone.
Daily measurements of the amount of total column ozone have been made at Arosa, Switzerland since 1926. The total amount of ozone back to 1912 has been determined by a careful analysis of solar measurements made by the Smithsonian Institution. Since 1957, more than 70 Dobson spectrophotometers have made regular measurements of ozone.
These measurements show that the total amount of ozone varies in cycles that may last a decade or more. For example, during the 1960's, scientists at NOAA found that ozone over North America increased by about 5 percent. Since 1970, however, ozone over the northern hemisphere has declined around 5 percent. Since this decline is seen in both measurements from the ground and from satellites, there is little disagreement that it is real. But there is consider-able disagreement concerning the reason for the decline, and its significance.
Some scientists believe the decline is primarily a byproduct of natural metero-logical cycles, a changing climate and possibly the solar cycle. They support their case by pointing to ozone cycles over the past 50 or more years. Others believe the decline is in large part caused by contami-nation of the atmosphere by pollutants that contribute to the destruction of ozone–particularly CFCs. Some believe the eruption of major volcanoes such as El Chichon in 1982 and Pinatubo in 1991 exacerbate the problem. Still others believe ozone is impacted both by natural meteorological trends and ozone-destroying chemicals.
These issues are being studied and debated by the scientists who study ozone. Meanwhile, as has been widely reported in the press, there has been considerable political fallout over the issue of declining ozone.
That's one reason why more than 70 nations have signed the Montreal Protocol, an agreement to eventually ban production of most CFCs. Because it is believed that CFSs can remain in the atmosphere for decades, even a total ban will not restore the ozone to its pre-1970 condition–if indeed CFC's are the principal culprit. Instead, ozone will continue to decline at, perhaps, a few percent or so per decade until CFCs are no longer present.
If the spectral sensitivity of a honey bee's eyes could somehow be added to yours, rainbows would have an additional streak of color adjacent to the violet band. This invisible band of "black light" is known as ultraviolet radiation.
Ultraviolet radiation is divided into three bands. The wave-lengths below 290 nm are referred to as UV-C. The wavelengths between 290 and 320 nm are referred to as UV-B. And the wavelengths between 320 and 340 nm are known as UV-A. The ozone layer blocks all UV-C. The UV-B that leaks through is what causes sunburn.
The Ultraviolet Sky
The sky is blue because most of its molecules are just the right size to scatter the blue wavelengths of sunlight. These molecules also scatter UV wavelengths. This means that the entire daytime sky is a gigantic source of UV-A and UV-B (remember that all the UV-C is absorbed by ozone).
The UV radiation from the sky can be described as direct, diffuse, or global. Direct radiation is that which comes directly from the sun. Diffuse radiation is that scattered from clouds and molecules of air. Global is the sum of direct and diffuse UV radiation. Most natural materials reflect UV rather poorly. But snow is an excellent UV reflector; and so is water. In short, exposed parts of your body can receive a signifi-cant does of UV even when shaded from the direct sun.
Ultraviolet Hazards
The energy of electromagnetic radiation is inversely related to its wavelength. In other words, radiation with short wavelengths–like x-rays–has much higher energy than radiation with longer wavelengths, such as visible light. For this reason, ultraviolet radiation is more energetic than visible light.
That's why the sun's UV-B radiation readily causes sunburn while UV-A and visible sunlight do not, even though much more UV-A and visible light reach the earth. (Of course, even visible sunlight will cause sunburn if it is concentrated with a lens or reflector.) The sun's UV-B can also damage the chromosomes in human skin cells–which can eventually lead to skin cancer. Excessive UV-B can also cause cataracts and alter the immune system.
Plants can be damaged by excessive UV-B. So can organisms that live at least part of their life cycle near the surfaces of lakes, rivers and oceans. More research is needed to better understand the nature of such damage and the amount and wave-lengths of UV that are responsible.
Ultraviolet Benefits
The public has been frequently reminded about the dangers of UV-B exposure. It's important to realize, however, that solar UV also plays an essential role in human health.
Perhaps the most important contribution of solar UV is its ability to stimulate the production of vitamin D in the outer layers of the skin. This gives UV the ability to prevent and to cure rickets and to maintain a healthy skeleton. Both natural and artificial UV radiation are also used to treat psoriasis.
Measuring Ultraviolet Radiation
If you've ever retrieved a rolled up newspaper which has lain in the summer sun for a few hours, you know that newsprint darkens when exposed to solar UV. So does freshly cut or sanded pine and other woods. Colored construction paper and fabrics are bleached by solar UV.
A few years ago during a field trip to New Mexico, my son and I tacked a strip of freshly sanded pine atop the crate bolted in the back of our pickup that carried our instruments and supplies. We covered the wood with a strip of tape, several centimeters of which we removed each day. After 10 days on the UV-drenched highways of Texas and New Mexico, the strip of wood was divided into 10 segments, each slightly darker than the next.
Some chemicals will also change color when exposed to solar UV. But an electronic instru-ment is necessary to quantitatively measure the intensity of UV. The two principle kinds of UV sensors used with such instruments are photo-tubes and solid-state detectors. Specialized phototubes known as photomultipliers can be made exceptionally sensitive to UV. But they are expensive, fragile, and require a high operating voltage. Solid-state detectors are not as sensitive. In addition, they are are cheaper, sturdier, and much easier to use.
An optical filter can be placed between a UV detector and the sun to enable the detector to respond only to specific regions of the UV spectrum. Alternatively, a prism or grating can be used to select specific wavelengths.
Various methods are used to measure direct, diffuse and global UV. Direct UV is easily measured by placing the detector inside a collimator tube that limits its field of view. Ideally, the field of view of the collimator should not exceed a few degrees.
Global UV is measured by placing a diffuser plate (such as ground silica or diffuse UV-transmitting plastic) over the detector and its filter.
Diffuse UV can be measured with a global detector by placing a small disk so that its shadow completely covers the detector. This blocks direct sunlight so that the detector signal is entirely from the diffuse sky. Subtracting the diffuse amount from the global value gives the direct UV.
Ozone absorbs some of the sun's infrared radiation. It even weakly absorbs the wavelengths around 600 nm, which appear orange to the human eye. Its most important absorption, of course, occurs at UV wavelengths below around 340 nm. The absorption increases so rapidly below 320 nm that little or no measurable radiation below 295 nm reaches the surface at sea level.
As noted, it's important to understand that the scattering caused by dust and aerosols can cause ozone near the ground to absorb more UV than an equal amount of ozone in the stratosphere. This helps explain why some scientific studies have found a slight decrease in UV-B over the past decade, a time during which the amount of stratos-pheric ozone has decreased and the amount of tropospheric ozone has increased.
Evaluating claims about the ozone layer requires basic knowledge not only about ozone and its effects on ultraviolet, but about the research methods that have been used to understand the ozone layer's dynamics. Hopefully, this article has provided some of that knowledge–and perhaps even sparked some interest in doing some independent investigating. If so, be sure to check out some of the resources mentioned on the Readings about Ozone and Ultraviolet page. Also, be sure to read the other articles in this issue, on the ozone hole and on the health effects of UV.
Copyright (c) 1993 by Forrest M. Mims III. Used by permission of Access Research Network.