1. Ozone Depletion

The Sun's invisible rays make ozone

The most intense radiation from the Sun is emitted at visible wavelengths, and our atmosphere permits it to reach the ground. That is the colored sunlight that our eyes respond to. The Sun emits lesser amounts of invisible, short-wavelength radiation, which is partially or totally absorbed in the atmosphere.

Even though the total amount of invisible solar radiation is substantially less than the visible emission, the individual short-wavelength rays are more energetic. That is why we get sunburns from the ultraviolet radiation that manages to get through the atmosphere, and you need to be protected from the Sun when climbing at high altitudes where the air is thinner and more ultraviolet penetrates the atmosphere. The greater energy of radiation at shorter wavelengths also explains why X-rays, generated by machines, can see through your skin and muscles to detect your bones.

When absorbed in our air, the invisible short-wavelength radiation from the Sun transfers its energy to the atoms and molecules there. The solar ultraviolet radiation is largely absorbed in the stratosphere, located between 10 thousand and 50 thousand meters above the Earthís surface. This is where the ozone layer is continuously replenished and destroyed.

When ultraviolet rays strike a molecule of the ordinary diatomic oxygen that we breathe, denoted by O2, they split it into its two component oxygen atoms, or two O. Some of the freed oxygen atoms then bump into, and become attached with, an oxygen molecule, creating an ozone molecule, abbreviated O3, that has three oxygen atoms instead of two. The Sunís ultraviolet rays thereby produce a globe-circling layer of ozone in the stratosphere.

Although the ozone is present to the extent of only about 10 parts per million, the ozone layer is critical to life below. It protects us by absorbing most of the Sun's ultraviolet emission and keeping its destructive rays from reaching the ground. If there were no ozone shield, plants, animals and humans could not even exist on land.

The amount of ozone in the stratosphere resembles the level of water in a leaky bucket. When water is poured into the bucket, it rises until the amount of water poured in each minute equals the amount leaking out. A steady state has then been reached, and the amount of water in the bucket stops rising and it will stay at the same as long as you keep pouring water in at the same rate. However, if you pour the water in at a different rate, or punch a few more holes in the bucket, the steady-state level of water in the bucket changes.

Solar ultraviolet radiation supplies ozone to the stratosphere from above, like pouring water into a bucket, at a rate that depends on the varying ultraviolet output of the Sun. We have recently been punching holes in the ozone layer from below, with chemicals used in our everyday lives.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University