by Jeannie Allen January 27, 2002
The sun’s awesome power drives a multitude of chemical reactions that are critical for life on Earth. In the atmosphere, ultraviolet radiation at wavelengths smaller than 242 nanometers splits molecular oxygen (two atoms bonded together) into atomic oxygen (individual atoms). Then when some energetically excited individual oxygen atoms encounter molecular oxygen, they can bond to form three-oxygen molecules, ozone.
Ozone packs a punch in our lives that’s out of proportion to its small concentrations in the atmosphere. Its influence can be for good or ill depending on where it is. Ozone far above us in the upper atmosphere (stratosphere) absorbs and protects us from deadly ultraviolet radiation. Ozone in the lower atmosphere (troposphere) where we live is toxic. It reacts easily with biological tissue, donating oxygen atoms in the process known as oxidation. Breathing too much ozone over time impairs human lung capacity and causes illness and, for a few, premature death. Other animals and some plants also suffer from ozone overexposure. Several important crop plants such as soybeans and tobacco respond to currently common concentrations of ozone with lower rates of photosynthesis and reduced productivity.
Ozone formation occurs naturally throughout the atmosphere. From a few percent to as much as 50 percent of the ozone in the troposphere intrudes from the stratosphere. (The exact amount depends on location and time of year.) The rest forms from two groups of chemical compounds that occur both naturally and as by-products of fossil fuel combustion: nitrogen oxides (NOx) and volatile organic compounds (VOCs), which are carbon-containing gases and vapors such as gasoline fumes. Carbon monoxide also plays a critical role in some ozone formation reactions. Sunlight must be present for ozone to form, hence the term, “photochemical” smog. Without sunlight, no ozone forms.
As the human population has risen sharply and we have rapidly industrialized our economies over the last century, our consumption of fossil fuels has risen dramatically. Amounts of the byproducts of fossil fuel combustion emitted into the atmosphere have risen just as dramatically. Some of these byproducts contribute to ozone formation, therefore ozone concentrations in the air we breathe have risen as well. Ozone concentrations in the mid-1880s peaked somewhere around 10-15 parts per billion (ppb) in a given volume of air (Finlayson-Pitts and Pitts, 1999). Ozone levels in the troposphere now average 35-40 ppb around the globe in even the most remote regions (Finlayson-Pitts and Pitts, 1999; Fishman 1999; Madronich 1993). Typically in some suburban and rural areas in summer, ozone levels range from 80 to 150 ppb for several days at a time, far exceeding the U.S. National Ambient Air Quality Standard of 80 ppb averaged over an eight-hour period. According to the American Lung Association’s State of the Air 2002> report, unhealthy levels of ozone reached fully half of the American public during each of the last three years. In the most polluted urban areas of the world, ozone concentrations occasionally reach 500 ppb. (Finlayson-Pitts and Pitts, 1999
Of all our common air pollutants, ozone has proven the most elusive to control. It forms through a highly complex series of reactions that take place over several hours, and that shift according to the presence of a multitude of other chemical species. Some governments have attempted to reduce ozone levels by mandating the reduction of hydrocarbons in motor vehicle emissions. But in recent years, it has become apparent that we must control NOx as well if we are to breathe healthy levels of ozone.
Because tropospheric ozone forms over time, controlling it entails understanding the physical dynamics of chemical transport through the atmosphere: winds, humidity, atmospheric pressure, and so on. Since ozone and the chemicals that participate in its formation (precursors) can travel several hundred kilometers or farther on the wind, controlling ozone also requires monitoring and other research all around the globe. Since 1978, NASA and the European Space Agency (ESA), with the participation of Japanese and Canadian science organizations, have monitored tropospheric ozone using increasingly sophisticated satellite instruments. The Aura satellite will go into orbit in 2004 to make daily global maps based on high precision measurements of tropospheric ozone and its precursors.
next: Ozone, Space, and Time
Ozone, Space, and Time
Wind directions and speeds, high or low concentrations of NOx and VOCs, precipitation, and air temperatures influence ozone concentrations throughout the troposphere. Because ozone formation takes place over time, and winds can carry air parcels far downwind of NOx and VOC sources, people in some rural areas breathe more ozone than people in some urban areas.
Ozone and some of its precursors are intercontinental travelers. Some air pollution from North American reaches Europe, and pollution from Asia reaches western North America. Extensive biomass burning in South America raises ozone levels in Australia, and the same activity in Africa degrades air quality over the Pacific Ocean.
Ozone concentrations also vary through time, throughout the day and through the year. The highest ozone concentrations of the year generally occur during summer, when sunlight is most intense. On a daily cycle, as industrial and motor vehicle activity rises throughout the morning, concentrations of NOx and VOCs also rise. Ozone concentrations consequently reach maximum shortly after the peak in vehicle traffic, about noon or soon thereafter. Downwind from urban areas, ozone may peak later in the afternoon or even after dark. After sunset, when no more sunlight initiates ozone formation, ozone concentrations fall as ozone reacts with other chemicals and rapidly settles onto various surfaces. NOx and VOC concentrations drop as they too participate in other reactions.
Chemistry of Ozone
In the language of a simplified chemical formula,
In the troposphere near the Earth’s surface, ozone forms
through the splitting of molecules by sunlight as it does in the
stratosphere. However in the troposphere, nitrogen dioxide, not
molecular oxygen, provides the primary source of the oxygen atoms
required for ozone formation. Sunlight splits nitrogen dioxide into
nitric oxide and an oxygen atom.
Nitrogen oxides (NOx) Nitric oxide and nitrogen dioxide are together known as NOx and often pronounced “nox.” Sources of NOx include lightning, chemical processes in soils, forest fires, and the intentional burning of vegetation to make way for new crops (biomass burning). NOx also come from smokestack and tailpipe emissions as by-products of the combustion of fossil fuels (coal, oil, and natural gas) at high temperatures. Coal-fired power plants are the primary sources of NOx in the United States. Automobiles, diesel trucks and buses, and non-road engines (farming and construction equipment, boats, and trains) also produce NOx.
Volatile organic compounds (VOCs) such as hydrocarbons. “Volatile” refers to an extreme readiness to vaporize. Some plants and bacterial processes in soils emit VOCs. (The smell of a pine forest comes from a hydrocarbon called alpha-pinene.) VOCs also come from gasoline combustion and from the evaporation of liquid fuels, solvents and organic chemicals, such as those in some paints, cleaners, barbecue starter, and nail polish remover.
Ozone formation in the troposphere requires both NOx and VOCs. In a
highly simplified version of tropospheric ozone-forming reactions,
Ozone formation with the hydrocarbon methane provides a useful example of the general pattern that most such reactions follow. The methane example is somewhat simpler and easier to follow than the others, described in steps.
Most ozone formations in the troposphere involve non-methane hydrocarbons. The chemistry of ozone formation from non-methane hydrocarbons follows the general pattern described above but is much more complex. NOx and VOCs together include about 120 different chemical compounds, and hundreds of chemical reactions can take place. Some of the participating chemicals may be intercepted part of the way through the process by reactions with other chemicals in the atmosphere, and may form intermediate compounds that act as temporary reservoirs for varying amounts of time.
An additional challenge arising for anyone tracking tropospheric ozone-forming reactions is that they entail interactions between different phases of matter (gas, liquid, and particles known as aerosols), and can occur on various kinds of aerosol surfaces in the atmosphere. Changing environmental conditions such as air temperature and humidity also affect ozone chemistry. Furthermore, many of the chemicals involved have very short lifetimes before they react with other chemicals to form new compounds. Scientists face myriad challenges in their pursuit of understanding tropospheric ozone chemistry.
Fishman, Jack. 1990. Global Alert: The Ozone Pollution Crisis. (New York and London: Plenum Press)
Fishman, Jack, et al., NASA Langley Research Center, Hampton, VA. 1999. Surface Ozone Measurements: Exciting Science for All Seasons All the Time.
Madronich, Sacha. 1993. Tropospheric photochemistry and its response to UV changes. In The Role of the Stratosphere in Global Change. Vol. 18. NATO-ASI Series, Editor M-L. Chanin. (Amsterdam: Springer-Verlag) Pp. 437-61
Turco, Richard P. 1996. Earth Under Siege. (New York: Oxford University Press)
Step 1. Sunlight splits ozone into an oxygen molecule and an oxygen atom.
Some Chemicals Involved in Ozone Formation: