advanced

Ozone in the Atmosphere

This diagram shows the physical phenomena and observing systems present at various heights in the atmosphere. At left is the height axis (kilometers on the left, miles on the right). At right is the temperature at various heights (Celsius on the left, Fahrenheit on the right). The color of the vertical bar shows cooling as one ascends through the troposphere and warming in an ascent through the stratosphere. High-flying planes are found near the tropopause, the cold, dry boundary region between the troposphere and stratosphere. Ozone is most concentrated in the lower stratosphere (bottom left). Without ozone, life on Earth wouldn’t exist as we know it. A naturally occurring form of oxygen with three atoms instead of the two in atmospheric oxygen, ozone can be found in the lower stratosphere (about 15–25 miles, or 25–40 kilometers, above Earth). There it intercepts ultraviolet light from the Sun that could otherwise prove deadly to people, animals, and plants.

Unfortunately, pollution is creating two ozone-related problems:

1. Certain pollutants deplete ozone in the stratosphere, exposing us to unhealthy levels of ultraviolet radiation.
2. Human-related emissions are spurring the formation of harmful ground-level ozone, which is a major contributor to smog.

Concerns over the first of these problems crystallized in the mid-1980s when a team of British scientists found an “ozone hole” in the stratosphere that occurred over the Antarctic during the Southern Hemisphere spring. As much as 70% of the continent’s stratospheric ozone was depleted for weeks at a time. Subsequent research has shown significant loss of stratospheric ozone over the high latitudes in the Northern Hemisphere. While less regular and dramatic than the Antarctic depletion, the Arctic ozone loss is still a cause for concern.

The main culprit behind ozone depletion is the emission of chlorofluorocarbons, which are used for refrigeration and a variety of industrial applications. Beginning in 1987, the international community has taken steps to reduce and eventually end the use of CFCs, which should lead to a restoration of stratospheric ozone levels by 2100.

What about ground-level ozone? This pollutant is difficult to regulate, partly because it is not emitted directly into the air. Instead, it forms in the atmosphere when nitrogen oxides and volatile organic compounds react in the presence of sunlight. Thousands of sources contribute to ground-level ozone, including motor vehicle exhaust and chemical solvents. Emissions from natural sources also play a role. NCAR scientists study how emissions from such natural sources as trees and lightning interact and contribute to ozone levels in the lower atmosphere.

Ground-level ozone is one of the most harmful forms of pollution, causing respiratory problems and damaging plants. An important research focus at NCAR is determining how atmospheric processes lead to the formation of ozone, even in relatively pristine regions. NCAR scientists investigating ground-level ozone fluctuations in the Arctic have determined that photochemical reactions—chemical changes in the presence of sunlight—are largely responsible for springtime peaks of the gas, although seasonal intrusions downward of ozone-rich air from the stratosphere also play a role. Such information will lead to better understanding of this important pollutant and potentially assist in efforts to reduce it.