Ozone

Subject: Environment

Context:

The levels of ozone have increased across the Northern Hemisphere in the last 20 years, a new study by the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder in the United States has showed.

Concept:

• Ozone is a gas made up of three oxygen atoms (O3). It occurs naturally in small (trace) amounts in the upper atmosphere (the stratosphere).
• Ozone protects life on Earth from the Sun’s ultraviolet (UV) radiation.
• In the lower atmosphere (the troposphere) near the Earth’s surface, ozone is created by chemical reactions between air pollutants from vehicle exhaust, gasoline vapors, and other emissions. At ground level, high concentrations of ozone are toxic to people and plants.
• Ninety percent of the ozone in the atmosphere sits in the stratosphere, the layer of atmosphere between about 10 and 50 kilometers altitude.
• The natural level of ozone in the stratosphere is a result of a balance between sunlight that creates ozone and chemical reactions that destroy it.
• Ozone is destroyed when it reacts with molecules containing nitrogen, hydrogen, chlorine, or bromine. 
• Increased levels of human-produced gases such as CFCs (chlorofluorocarbons) have led to increased rates of ozone destruction, upsetting the natural balance of ozone and leading to reduced stratospheric ozone levels. These reduced ozone levels have increased the amount of harmful ultraviolet radiation reaching the Earth’s surface.
• The Montreal Protocol, an international agreement to address the global problem of ozone destruction, was signed by more than 70 countries in 1986.
• It set goals of reducing CFC production 20% by 1993 and 50% by 1998. Since the agreement was signed, these targets have been strengthened to call for the elimination of the most dangerous CFCs by 1996 and for regulation of other ozone-depleting chemicals.