What is the ozone hole and where is it in the atmosphere?
Ozone is a gas that is naturally present in our atmosphere. Each ozone molecule contains three atoms of oxygen and is denoted chemically as O3. Ozone is found primarily in two regions of the atmosphere. About 10% of atmospheric ozone is in the troposphere, the region closest to Earth (from the surface to about 10-16 kilometers (6-10 miles)). The remaining ozone (90%) resides in the stratosphere, primarily between the top of the troposphere and about 50 kilometers (31 miles) altitude. The stratosphere is often referred to as the “ozone layer."
Effects of Ozone Depletion
Calculations indicate that for each one percent decrease in atmospheric ozone, the amount of solar ultraviolet radiation reaching the ground will increase by 2 percent. It is estimated that a 2 percent increase in solar ultraviolet radiation could increase future skin cancer cases by 3 to 6 percent each year. There are presently about 500,000 cases of skin cancer diagnosed each year just in the United States alone.
It has also been suggested that solar ultraviolet radiation can damage the human immune system, cause billion of dollars worth of crop damage, and adversely affect plankton in the ocean, the base of the marine food chain.
When did the Antarctic ozone hole first appear?
The Springtime Antarctic ozone hole is a new phenomenon that appeared in the early 1980s. The observed average amount of ozone during September, October, and November over the British Antarctic Survey station at Halley, Antarctica, first revealed notable decreases in the early 1980s, compared with the preceding data obtained starting in 1957.
The ozone hole is formed each year when there is a sharp decline (currently up to 60%) in the total ozone over most of Antarctica for a period of about three months (September-November) during spring in the Southern Hemisphere. Late-summer (January-March) ozone amounts show no such sharp decline in the 1980s and 1990s. Observations from three other stations in Antarctica and from satellite-based instruments reveal similar decreases in springtime amount of ozone overhead.
Balloon-borne ozone instruments show dramatic changes in the way ozone is distributed with altitude. The ozone hole has been shown to result from destruction of stratospheric ozone by gases containing chlorine and bromine, whose sources are mainly human-produced halo carbon gases.
Before the stratosphere was affected by human-produced chlorine and bromine, the naturally occurring springtime ozone levels over Antarctica were about 30-40% lower than springtime ozone levels over the Arctic. This natural difference between Antarctic and Arctic conditions was first observed in the late 1950s by Dobson. It stems from the exceptionally cold temperatures and different winter wind patterns within the Antarctic stratosphere as compared with the Arctic. This is not at all the same phenomenon as the marked downward trend in total ozone in recent years.
Why do we care about atmospheric ozone?
Ozone in the stratosphere absorbs some of the Sun’s biologically harmful ultraviolet radiation. Because of this beneficial role, stratospheric ozone is considered “good ozone." In contrast, ozone at Earth’s surface that is formed from pollutants is considered “bad ozone" because it can be harmful to humans and plant and animal life. Some ozone occurs naturally in the lower atmosphere where it is beneficial because ozone helps remove pollutants from the atmosphere.
What are the principal steps in stratospheric ozone depletion caused by human activities?
The initial step in the depletion of stratospheric ozone by human activities is the emission of ozone-depleting gases containing chlorine and bromine at Earth’s surface. Most of these gases accumulate in the lower atmosphere because they are unreactive and do not dissolve readily in rain or snow. Eventually, the emitted gases are transported to the stratosphere where they are converted to more reactive gases containing chlorine and bromine. These more reactive gases then participate in reactions that destroy ozone. Finally, when air returns to the lower atmosphere, these reactive chlorine and bromine gases are removed from Earth’s atmosphere by rain and snow.
What emissions from human activities lead to ozone depletion?
Certain industrial processes and consumer products result in the atmospheric emission of “halogen source gases." These gases contain chlorine and bromine atoms, which are known to be harmful to the ozone layer. For example, the chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), once used in almost all refrigeration and air conditioning systems, eventually reach the stratosphere where they are broken apart to release ozone-depleting chlorine atoms. Other examples of human-produced ozone-depleting gases are the “halons," which are used in fire extinguishers and which contain ozone-depleting bromine atoms. The production and consumption of all principal halogen source gases by human activities are now regulated worldwide under the Montreal Protocol.
A new adjustment to protect stratospheric ozone and mitigate climate change under the Montreal Protocol entered into force May 14 2008. As agreed by all 191 Parties to the Montreal Protocol at the September 2007 meeting, the adjustment to accelerate the phase-out of hydrochlorofluorocarbons (HCFCs) will reduce climate emissions by 16 billion metric tonnes of carbon dioxide-equivalent through 2040, which equals the emissions from 70 million US households for 30 years, according to EPA.
The HCFC targeted in the accelerated phase-out can be 2,000 times more potent in contributing to climate change than CO2. The adjustment also provides significant health benefits, avoiding 1,000 deaths from skin cancer in the US alone.