Ross J. Salawitch, a senior research scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., explains.

Ozone (O3) in the troposphere—the atmosphere's lowest layer—is termed "bad ozone" because elevated levels lead to human health problems as well as damage to crops and forests. Ozone one layer up, in the stratosphere, is termed "good ozone" because it provides a shield from harmful, long-wavelength ultraviolet radiation from the sun.

Atmospheric ozone levels are regulated primarily by local chemical processes: In the stratosphere, ozone is produced following the breakdown of molecular oxygen (O2) by short-wavelength ultraviolet sunlight--the separate oxygen atoms then recombine with another molecule of oxygen gas to form ozone. The abundance of stratospheric ozone is maintained like liquid flowing into a leaky bucket. Ozone is continuously produced and removed by natural processes, but industrial pollutants add "new leaks to the bucket," further reducing stratospheric ozone levels.

CLICK THE IMAGE to view a more detailed version of this schematic of ozone chemistry in stratosphere and troposphere.

Chlorofluorocarbons (CFCs) are responsible for the majority of observed stratospheric ozone depletion. These gases had been used as refrigerants and solvents as well as propellants in aerosol cans. Although CFCs are nonreactive in the troposphere, they can be slowly transported to the stratosphere where they break down into molecules such as chlorine monoxide (ClO), which depletes ozone by transforming it back into oxygen gas.

Typical stratospheric ozone levels are about 400 Dobson units (DU)—named in honor of British scientist Gordon Dobson who pioneered the measurement of atmospheric ozone. Every spring over Antarctica, extremely cold conditions enable chemical reactions that produce very high levels of ClO, resulting in rapid ozone removal. Within the so-called "ozone hole," levels can drop to 85 DU.

Typical tropospheric ozone levels are about 25 DU, but vary substantially depending on local conditions. Ozone production in the troposphere is much less efficient than in the stratosphere because the intensity of ultraviolet sunlight is greatly reduced. Human activities such as fossil fuel combustion and biomass burning lead to elevated levels of tropospheric carbon monoxide, nitrogen oxides and hydrogen oxides. These gases participate in a series of chemical reactions that produce ozone.

In the troposphere, temperature decreases with increasing altitude, allowing convection—the rapid vertical mixing of air parcels—to occur. At its border with the stratosphere, temperature begins to rise and convection essentially stops.

In the absence of convection, a slow exchange of air occurs across the tropospheric-stratospheric border. CFCs are transported to the stratosphere by this process, which also allows ozone exchange between the atmospheric levels. The net transfer of ozone is from the stratosphere to the troposphere, because of the higher levels of ozone in the upper level. This exchange, however, plays only a minor role in determining ozone abundances in both the stratosphere and troposphere.

The Montreal Protocol has banned production of CFCs throughout the world, and the stratospheric ozone layer is expected to fully recover over the next 50 to 100 years. Major efforts are being undertaken to implement emission control strategies that will limit tropospheric ozone to less than prescribed levels. These initiatives are challenged by global industrialization and the fact that tropospheric ozone is affected by pollutants emitted both locally and from distant upwind sources, sometimes from other countries or continents.