Surface-based ozone

Near the earth's surface, very little ozone is present -- less than 1 ozone molecule per 10,000,000 molecules. However, when ozone forms near the earth's surface it can actually become a health hazard. How? According to the EPA, in its publication, Smog -- Who does it hurt?, "In the Earth's lower atmosphere, near ground level, ozone is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources react chemically in the presence of sunlight."⁵ Elevated ozone concentrations tend to irritate the human respiratory system, and asthmatics are especially susceptible to such effects.

Ozone concentrations can reach high levels in many U.S. communities, especially during the summer months when pollutants tend to collect most easily near the ground.

Smoggy Atlanta. (Photo courtesy of Department of Health and Human Services, Region 4)

However, ozone is not just a problem in metropolitan areas. The Great Smoky Mountains, in the eastern United States, also have occassional ozone problems. There's even a webpage set up by the National Park Service to provide real-time ozone monitoring information for the Great Smoky Mountains National Park.

Of course, just because it's a hazy day doesn't necessarily mean there is an ozone problem. Indeed, there is an important difference between haze and smog. Haze, which is composed of tiny dust particles and liquid droplets, occurs naturally and is not typically considered to involve health problems. It can often be seen on a daily basis just about anywhere in rural America during the summertime; appearing as a bluish cast against a distant background. Persistent haze is something that originally gave the Smoky Mountains its name. Only when the haze mixes with pollutants (like ozone) do health problems start to increase. Sometimes these pollutants can travel for many miles, from urban areas, into more remote areas -- such as the Smokies.

According to the EPA, in its publication Ozone Monitoring, Mapping and Public Outreach, "Ground-level ozone is regulated under the Clean Air Act, the comprehensive federal law that regulates air emissions in the United States."⁶ Each state is required to develop a network of monitors that will measure ambient concentrations of ozone. State and Local Air Monitoring Stations (SLAMS), National Air Monitoring Stations (NAMS) and Photochemical Assessment Monitoring Stations (PAMS) are all part of this network. The equipment used to measure ground-based ozone is called an "ozone analyzer," and measures the concentration of ozone in a sample of ambient air.