Until about 30 years ago, atmospheric scientists believed that all of the ozone in the lower atmosphere (troposphere) intruded from the upper atmosphere (stratosphere), where it formed by the action of sunlight on oxygen molecules. The work of atmospheric chemists during the 1970s dramatically altered that view. Now we understand that more than half of the ozone in the troposphere comes from chemical interactions within the troposphere itself.
In the stratosphere, ozone shields us from the Sun’s deadly ultraviolet radiation. But in the troposphere, this same gas impairs lung capacity and reduces agricultural productivity. Both human activities and natural processes generate the chemical compounds that serve as “precursors” to the formation of ozone. Currently human activities generate about as much ozone as natural processes do (Fishman 2002), creating a public health hazard.
To solve the ozone pollution problem, governments have established ozone standards, revising them as new knowledge comes to light. The U.S. Environmental Protection Agency (EPA) asserts that accumulated knowledge from 3,000 new studies of the last 15 years proves that breathing ozone in smaller amounts than previously thought causes significant harm to human lungs. (U.S. Environmental Protection Agency, 1997). National standards of many nations may need adjustment.
Governments enact legislation to enforce adherence to pollution standards. They also support ozone-monitoring activities in key parts of the world, and fund research to enhance our ability to forecast ozone levels. But in spite of standards, regulations, monitoring and research, the ozone problem persists. We are not only creating more ozone in some populated areas, but our ozone pollution crosses political boundaries and often reaches across the Earth’s widest oceans. To regulate air quality, we need international cooperation and a strong scientific basis on which to make legally binding agreements. Solving the problem also requires a shift in the ways we get and use energy. Everyone has a role to play in regaining healthy levels of ozone in the air we breathe.
|Ozone in the Troposphere|
Ozone forms naturally through the atmosphere’s interaction with sunlight. A complex series of reactions takes place over several hours (and sometimes over several days) involving nitrogen oxides (NOx) and volatile organic compounds. (“Volatile” refers to an extreme readiness to vaporize.) Both sets of these precursor compounds exist naturally in the atmosphere. They also occur as by-products of fossil fuel combustion at high temperatures. NOx come from coal-fired power plants, automobiles, diesel trucks and buses, farming and construction equipment, boats, and trains. Volatile organic compounds come from gasoline combustion and the evaporation of liquid fuels, solvents and organic chemicals, such as those in some paints and cleaners.
Because ozone reacts easily with biological tissues, it tends to be destructive to them. Its effect on human lungs resembles a slow burn. Medical researchers have discovered that breathing ozone over several months to years at levels that are now common causes irreversible damage to the lungs of mammals in laboratories. Being mammals ourselves, we can expect respiratory impairment from overexposure to ozone. Children, seniors and all adults who exercise regularly outdoors in summer are particularly vulnerable. Plant species and ecosystems suffer from ozone as well. Productivity drops significantly in several species of important agricultural crops when ozone concentrations reach levels that are now common during the growing season in several parts of the country. To learn more about the health effects of ozone at the Earth’s surface, see the article, The Ozone We Breathe.
How much more ozone are we breathing because of our fossil fuel consumption? Researchers began measuring ozone in Europe about 125 years ago,
when ozone concentrations peaked at around 10 parts per billion in a given volume of air (ppb). Although the technique of ozone measurement then
was to observe ozone’s oxidation of potassium iodide on paper (Schonbein paper) and was less accurate than current techniques, modern studies
have reexamined these procedures and concluded that we can depend on them. (Fishman, 2002) An assessment of the global rate of increase of
tropospheric ozone is difficult to determine from measurements at only a few locations, but we know that the global level has increased
significantly over the last 100 years.
Ozone concentrations vary widely over space and time. Generally values are highest where intense sunlight combines with extensive industrial and motor vehicle activity, and they reach especially high levels in conditions of hot, stagnant air. In the United States, high ozone concentrations occur most frequently in California, eastern Texas, the industrial Midwest, and most of the eastern states. The length of the "ozone season" (when ozone concentrations far exceed national standards) varies from one area of the United States to another. In the southern and southeastern states, it sometimes lasts nearly the entire year.
The EPA reports an overall decrease in ozone concentrations of 11 percent from 1982 to 2001 (U.S. Environmental Protection Agency, 2002), largely due to regulation of industrial and vehicular emissions. But ozone levels in some parts of the country have risen, and breathing unhealthy levels of ozone is a real concern. In State of the Air 2002, the American Lung Association reports, “Three-quarters of the nation’s population who reside in areas with ozone monitors ... are breathing in unhealthy amounts of ozone pollution” (American Lung Association, 2002). According to the National Institute of Environmental Health Sciences (NIEHS), as the 20th century ended there were still 32 “non-attainment areas” in the United States, where people commonly breathe concentrations of ozone in excess of the air quality standards. (NIEHS 2000) The EPA maintains a Web site called AirNow where the public can learn about ozone levels where they live.
|Ozone Pollution Standards|
As part of their efforts to protect human health and the environment, governments assume the responsibility for setting pollution standards. Authorities consider the effects of ozone exposure on children and other people with sensitive respiratory systems, and also decide on acceptable concentrations of ozone (and other toxic chemicals) on the basis of acceptable risk. A given risk is acceptable if severe ill effects remain at a reasonable level. For example an acceptable risk might be one chance in 500,000 of one person contracting lung cancer from breathing polluted air of a specific composition for one year. However determining a specific health effect of any given atmospheric condition on any particular population is impossible. Individual people are too different physiologically from one another, environmental conditions vary too widely, and the atmosphere is too dynamic and complex. Factors such as high and low temperature, humidity, altitude, concentrations of other pollutants, and nutritional status can influence peoples’ health outcomes due to overexposure to air pollution.
Since 1971, the EPA has established national air quality standards for ozone. Set in 1997, the current national air quality standard for ozone is 0.08 parts per million (ppm), or 80 parts per billion (ppb), averaged over 8 hours. For a given geographic area to be in compliance, its fourth highest 8-hour concentration in a year, averaged over three years, must be equal to or less than that amount. However implementation of the 8-hour standard is taking years to get underway because of legal challenges by industry leaders who feel that compliance will prove too expensive. In February 2001, the U.S. Supreme Court unanimously upheld the principle that national air quality standards must be based on health considerations alone, and need not take into account the cost to businesses of meeting the standards. That major question is resolved, but minor legal issues remain unresolved. In the meantime, a less healthful standard of 0.12 ppm (120 ppb) for one hour, with an average of exceeding that standard only once over three consecutive years, still guides state governments in the U.S.
The World Health Organization (WHO) recommends stricter guidelines than the EPA standards, 60 ppb over 8 hours not to be exceeded more than 20 days in one year. Such guidelines do not carry the legal “teeth” that national standards do, but many countries choose to follow them or even more stringent ones when setting standards. In Canada, the Air Quality Standard for ozone is 50 ppb over 8 hours.
In addition to establishing national standards for ozone pollution, governments enact clean air legislation to control the emissions of ozone’s precursor chemicals. In the absence of government controls, the United States experienced an increase of 690 percent NOx emissions and 260 percent volatile organic compound emissions between 1900 and 1970. The Clean Air Act became law in 1970, with amendments in 1977 and 1990, and has subsequently helped improve air quality. Of the six principal air pollutants—carbon monoxide, lead, nitrogen oxides, particulate matter, sulfur dioxide, and volatile organic compounds—all but nitrogen oxides have decreased significantly.
In 1999, the EPA revised the format of its Air Quality Index (AQI) to make it useful to the public. This index depicts ozone levels in five categories of increasing health concern, each with an associated color. If the air is Code Orange, Red, or Purple, an alert is in effect, and the public can take action to avoid overexposure to ozone. Federal, state, and local agency, newspapers, broadcast news, and Internet publications use the index when reporting on air quality information.
In order to solve the ozone problem, we need a firm research basis from which to make decisions. Many people hold the misconception that scientists fully understand atmospheric chemistry and dynamics. In fact much remains unknown. For example, some estimates of ozone precursors released into the air by various human activities disagree by orders of magnitude. We don’t yet know as much as we need to know about ozone chemistry at night, nor about the transport of gases between the lower and upper reaches of the atmosphere. We have a great deal to learn about connections between air quality and climate, and we still cannot predict our chemical weather.
Atmospheric chemists measure ozone at ground level, in the air with balloons and aircraft, and from space with Earth-orbiting satellites. Each vantage point offers a different perspective, a different geographic scope, and usually a different degree of spatial resolution.
Measurements on the ground contribute chiefly to understanding specific localities because ozone levels change so much from one locality to another and from one day to the next. The EPA, along with states and local air agencies, has established a number of monitoring networks to collect air quality data. One network of Photochemical Assessment Monitoring Stations (PAMS) collects and reports detailed data for NOx, volatile organic compounds, ozone, and meteorological conditions for areas in the United States that have the most severe and persistent ozone problems. The State and Local Air Monitoring Network (SLAMS) and the National Air Monitoring Network (NAMS) also track ozone air quality across the country.
Balloons and aircraft give us a regional view, though still rather limited in space and time. They make virtual lines of data points through the atmosphere. Much of the perspective needed for inter-regional and global studies comes from satellites. Space-based instruments on satellites help us understand how ozone travels from one region to the next, and from one continent to another. Satellites collect data over the entire globe at far less cost than a network of ground-based systems could achieve over a fraction of the area. We are just beginning to explore the capacity of satellites to identify sources of air pollution and where polluted air travels. NASA’s Aura satellite, scheduled to launch in 2004, will make a big leap in that capability.
Satellite mission teams are improving their capability to pinpoint ozone precursor sources and to track their movements, particularly as scientific teams collaborate on corresponding field campaigns with balloons and aircraft. Integrating and comparing ground-based, balloon, aircraft, and satellite measurements is a crucial step in achieving an accurate and complete story of ozone chemistry, and it is a major focal point for current research.
Long-term, consistent monitoring is essential, because some changes in the atmosphere happen very slowly and trends are often obscured by the wide variability of measurements and climate. Several satellites are in orbit or will launch in the near future to look at complex tropospheric ozone chemistry. In 2004, the Aura satellite will carry a payload of four instruments, each of which will measure ozone or its precursors in different and overlapping ways. Taken together, data from the instruments on Aura will provide rich new sources of information on the horizontal and vertical distribution of key atmospheric pollutants and greenhouse gases and how their distributions evolve and change over time. Further, combining Aura data with data from other missions will reveal even more information on these and other important issues.
|Solving the Problem|
Given the effort and resources that governments devote to controlling surface ozone, why are we still experiencing ozone overexposure, and how can we solve the problem? Reducing ozone concentrations means changing the way we get and use energy. That change entails shifts in thinking, and short-term spending for long-term gain in public health. In the past, considerations about public health have not driven most business and private decisions about energy sources and use. Since 1970, the Clean Air Act has exempted the oldest, dirtiest coal-burning power plants from complying with modern emissions standards. These older power plants emit as much as ten times more nitrogen oxides and other pollutants than modern coal plants (American Lung Association 2002). However change is in the wind, so to speak. Based on a 1998 rule by the EPA (the Ozone Transport Reduction Rule), many states have adopted legislation for controlling NOx from power plants.
Another reason for the continuing problem with ozone stems from the choices we make about transportation. Individual cars emit much less pollution than they did fifty years ago, but a lot more cars are on the road today, and they are traveling a lot more miles—in fact, four times as many miles. In 1970, Americans traveled 1 trillion miles in motor vehicles, and in 2000 they traveled about 4 trillion miles (EPA 1993). Another element in the pollution mix is that buses, trucks, and sport utility vehicles (SUVs) legally exceed the standards applied to other vehicles.
Government activities alone cannot achieve healthy levels of ozone. Individuals can reduce ozone pollution by using energy efficiently and by increasing the use of renewable energy sources such as wind, sun, water, and geothermal heat. A multitude of resources on the Worldwide Web offer suggestions for individual actions to improve air quality.
Clearly the tropospheric ozone problem challenges our capacity to change, but it’s a problem we can solve. With international cooperation, a strong scientific basis on which to make legally binding agreements, and activities at the individual level, we can regain a healthy level of ozone in the air we breathe.