Stratospheric Ozone Depletion

Ozone, denoted chemically as O3, is a relatively unstable gas with each molecule containing three atoms of oxygen. In a thermally stratified atmosphere, ozone occurs in the lowest two layers, the troposphere and the stratosphere, where it performs different roles. In the stratosphere, ozone occurs in large amounts over a deep vertical extent, forming the ozone layer. Stratospheric ozone is sometimes called “good ozone” because it absorbs some of the potentially harmful ultraviolet (UV) radiation from the sun, which can cause skin cancer and can damage vegetation, among other things. Ozone in the troposphere, particularly near the surface, is a secondary air pollutant, which, as a principal component of photochemical smog, is known to have adverse effects on the environment and human health. Stratospheric ozone accounts for about 90% of atmospheric ozone.

The amount of ozone in the stratosphere at any given time largely depends on the balance between the rates of production and destruction of ozone, which are controlled by factors including sunlight intensity, temperature, and the quantity of other gases involved in the ozone formation-destruction process. Ozone forms naturally in the stratosphere whenever UV sunlight is present. When high-energy UV radiation from the sun strikes an oxygen molecule, it causes the molecule to split into two free atoms, each of which combines with an oxygen molecule to produce an ozone molecule. Destruction of ozone in the stratosphere takes place as [Page 2699]quickly as its formation because the chemical is so reactive. As ozone absorbs UV radiation, the molecule splits into one free oxygen atom and one molecule of oxygen. The free oxygen atom then collides with an ozone molecule to form two molecules of oxygen. These reactions occur continually wherever UV sunlight is present in the stratosphere. The net result is a reservoir of ozone that peaks at about 10 parts per million in the midstratosphere, between 12 and 25 kilometers in altitude.

Figure 1 EP/TOMS corrected total ozone for September 1, 2005

Source: National Aeronautics and Space Administration. (2007). Total ozone mapping spectrometer. Retrieved December 15, 2008, from http://jwocky.gsfc.nasa.gov/eptoms/ep_v8.html.

The amount of ozone in a total column of the atmosphere is expressed in Dobson units (DU). A typical value for the ozone column is 300 DU. In the mid 1970s, data collected in the upper atmosphere first showed that the amount of ozone in the stratosphere had diminished. In recent years, the Ozone Monitoring Instrument (OMI) onboard National Aeronautics and Space Administration's (NASA's) Aura satellite and the EP/TOMS (Earth Probe/Total Ozone Mapping Spectrometer) have been used to get a global picture of ozone levels. Analyses of data from these instruments show that except over tropical latitudes, stratospheric ozone depletion occurs across all latitudes, with the greatest depletion, representing the disappearance of up to 60% of stratospheric ozone, occurring above Antarctica in September, creating what is popularly called the “ozone hole.” An example of global ozone data derived from EP/TOMS for September 1, 2005, is shown in Figure 1. Apparent from the image is the ozone hole over Antarctica, represented by a decreased ozone amount [Page 2700]ranging from about 250 to <120 DU. More detailed information about the ozone hole can be obtained from Figure 2, which is a plot of the daily values of the total column ozone derived from different instruments for several years over Antarctica (40° S to 90° S).

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