Stratospheric Ozone Depletion - The Changing Arctic Environment: The Arctic Messenger - page 96

Why Is Stratospheric Ozone Important?
Stratospheric ozone absorbs solar ultraviolet radiation (UV), which warms the stratosphere.
More importantly, this absorption acts as a sort of filter. The ozone allows only a relatively
small proportion of the highly energetic UV radiation to reach the troposphere below. It is
within the troposphere that we and every other living organism play out our lives. Oxygen
was absent from the earliest atmosphere, but that changed with the evolution ofthe first pho-
tosynthetic bacteria about3.5billionyearsago.Sincethen,lifeonEarthhasevolvedinways
that depend on the protective stratospheric ozone filter.
The UV radiation arriving from the sun is classified according to its wavelength and,
therefore, its energy. UV-A has the lowest wavelength, is the least energetic and is the least
biologically active form. It is only partially filtered out by the ozone layer and thus it is the
most common form to reach Earth's surface. UV-C has the shortest wavelength, is the most
energetic and is potentially the most biologically active form. Putting it dramatically, UV-C
is capable of ripping apart the chemical bonds that hold biological chemicals together. For-
tunately for us, it is very rapidly mopped up in the atmosphere by molecular oxygen and
nitrogen gas. We rely on well water in our home. Before it reaches our taps, however, the
water passes by an enclosed UV-C lamp that kills any bacteria that may have been lurking
in the well.
That leaves us with the intermediate form UV-B, whose properties lie between the ex-
tremes of UV-A and UV-C. It is partially absorbed by stratospheric ozone, but the residual
amount that reaches Earth's surface is sufficiently energetic to cause effects in plants, an-
imals (including humans) and even materials. (Think about what happens to unprotected
plastics and fibreglass when left for some time in the sun.) The stratospheric level with the
highest concentrations of UV-B lies at an average altitude of about 25 kilometres and is
known as the ozone layer . UV-induced ozone production is on average highest over the trop-
ics, but this inventory is then partially moved by circulatory patterns within the stratosphere