The net result (2O3 à 3O2) is that one chlorine atom destroys two ozone molecules and is regenerated in the process, allowing it to react over and over again. So, a single chlorine atom can destroy thousands of ozone molecules. And because CFCs are removed very slowly from the atmosphere, their ozone-depleting power persists for decades after they are emitted.
As a result of the widespread use of CFCs in consumer products, the ozone layer became depleted, particularly through a phenomenon that most of us know as "the ozone hole." The word "hole" should not be taken literally; there is no hole in the sky. But the area has significantly less—as much as two-thirds less—protective ozone to shield the Earth's surface.
In the past decade, ozone holes have stretched as much as 29 million square kilometers across the skies over Antarctica. Residents of New Zealand and Australia are warned each year to take precautions from the sun, as the ozone hole can occasionally stretch from the South Pole toward middle latitudes.
How do you find and study a hole in the atmosphere?
Atmospheric ozone can be measured both directly and remotely. Scientists make direct measurements by launching instruments on balloons or by carrying them up on research airplanes. The instruments use chemical reactions or pass UV light through a sample of air to measure the presence and amount of ozone.
Remote measurements of ozone are made by instruments on the ground and on satellites; in either case, the instrument of choice is usually a spectrometer or photometer—a device that measures how gases absorb or emit light. In the 1920s and 1930s, British scientist Gordon Dobson first used photographic plates and then a spectrophotometer—a device that consists of a spectrometer and a photometer—to measure differences in ultraviolet light wavelengths reaching the Earth’s surface. Although the instruments have been modernized, the basic premise of Dobson's work continues to this day. The most common unit of measurement for atmospheric ozone levels—known as the Dobson unit (DU)—was named in his honor.
Scientists first became aware of the depletion of the ozone layer in 1985, when British researchers in Antarctica were measuring the amount of ozone in the skies above. They found that ozone levels seemed lower in September, October, and November than at other times of the year. They compared their measurements to data collected every year since 1957 and found less ozone in the 1980s compared to earlier years.
At the same time, scientists at NASA's Goddard Space Flight Center were examining satellite measurements of unusually low ozone over Antarctica. After the British scientists published their results, the NASA scientists published the first images of an area of ozone depletion over Antarctica that was as large as the continent. The term "ozone hole" was soon coined to describe the seasonal decrease in the UV-blocking gas.
Satellite measurements of the ozone layer have been collected for nearly 35 years. Starting with the Total Ozone Mapping Spectrometer, NASA engineers and scientists have developed successive instruments onboard satellites, airplanes, the space shuttle, and the International Space Station to determine the amount of ozone in the air. For the past decade, the workhorse for ozone measurements has been NASA's Aura satellite and its Ozone Monitoring Instrument (OMI), which has extended and improved upon more than 30 years of observations. The recently launched Suomi National Polar-Orbiting Partnership satellite carries a successor to the OMI instrument that is continuing the record of observations.
How will our vast ozone experiment turn out?
Piecing together clues from the laboratory and real-world observations, scientists eventually explained to politicians and chemical manufacturers why we needed to stop the production of ozone-depleting substances such as CFCs. The conversation led to an international agreement in 1987 known as the Montreal Protocol on Substances that Deplete the Ozone. Leaders with different political ideologies came together to create a model for global solutions to environmental problems. Many experts have called it the most successful treaty in the United Nations’ history.
The 1987 treaty set timelines for countries to reduce and eventually phase out the manufacture and sale of CFCs and other ozone-depleting chemicals. Motivated by the protocol, chemists, engineers, and manufacturers eliminated chlorine from most refrigerants and developed new chemicals that break down faster and lower in the atmosphere. The economic and environmental impact of the change from CFCs has been minimal so far.
While nations agreed to stop depleting ozone, nature will need time to catch up. CFCs are stable and long-lived compounds, and it will take a significant amount of time—estimated at 50–100 years—for the chemicals released decades ago to break down in the atmosphere. In fact, the worst global ozone losses and largest ozone holes occurred more than 15 years after the Montreal Protocol was signed.
The numbers tell the story of a changing atmosphere. In 1979, when scientists were just beginning to understand that ozone could be destroyed, the hole over Antarctica was 1.1 million square kilometers, with a minimum ozone concentration of 194 Dobson Units (DU). In 1987, the area of the hole reached 22.4 million square kilometers and ozone concentrations dropped to 109 DU. By 2006, the worst year for ozone depletion, the numbers were 29.6 million square kilometers and just 84 DU. By September 2012, the hole spanned 21.2 million square kilometers and measured 124 DU—the smallest hole since 2002.
As the Antarctic ozone hole has stabilized, there have been other promising signs. In the past decade, researchers found that the amount of ozone-depleting chemicals in both the lower and upper atmosphere reached a peak around the year 2000 and has been slowly declining. From the 1980s to the early 2000s, global levels of stratospheric ozone also dipped by 5% to 6%, but they have been rebounding slightly in the past few years.