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UV-B (Dangerous)

Figure L2, The altitude to which solar radiation penetrates into the atmosphere is a function of wavelength. Radiation shorter than 195 nm is absorbed in the mesosphcre above 50 km The longer-wavelength ultraviolet is mainly absorbed in the stratosphere by ozone.

In the troposphere there is very little ozone. Until about 20 years ago it was thought that the troposphere contained only ozone that had been transported down from the stratosphere. At that time tropospheric ozone was considered to be interesting only in the study of atmospheric transport, and its enormous importance for the chemistry of the troposphere was not recognized. Tropospheric ozone makes up only about 10% of all ozone in the atmosphere, with an average volume mixing ratio of about 40 nmol/mol (nanomole per mole, n = nano — 10~9). However, as we discuss in this chapter, in the absence of tropospheric ozone the chemical composition of the atmosphere would be totally different,

[f we look at the altitude to which solar radiation penetrates into the atmosphere, we see that the very short wavelengths - shorter than 200 nm - are to a large degree removed by 50 km (see Figure 1,2), This happens primarily through the absorption of the radiation by atomic and molecular forms of oxygen (O) and nitrogen (N). But these main gases do not absorb beyond about 240 nm. Fortunately, ozone does so very strongly in the 200-300 nm wavelength range. H ere it not for atmospheric ozone, this radiation would penetrate to the Earth's surface. For the Earth's current biosphere, this would have had catastrophic consequences. Only during the past one-third of the Earth's age has the atmosphere contained comparable amounts of ozone (and oxygen) as at present. The Earth has thus been without the protective shield of oxygen and ozone during most of its existence. This must have forced primitive life to develop only in dark hideaways shielded from sun's damaging ultraviolet rays. The average concentration of ozone in the atmosphere amounts to only about 0.3 per million air molecules, but it nevertheless suffices to absorb the main part of the dangerous L Y radiation.

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Bevond 300 nmt the absorption ability of ozone becomes weaker so that UV radiation at longer wavelengths can penetrate to the Earth's surface- It is the radiation up To 320 nm, also called UV-B radiation {B stands for "biologically active") that still poses a problem for life on Earth. Light-skinned people are all familiar with the fact that when they stay too long unprotected in ihe sun, they get sunburned, and from frequent exposures skin cancer may develop. Plants can also be affected by this radiation - On the other hand, we know from research conducted during the past 25 years that this same radiation is also very important for keeping our atmospheric environment clean, The reason is the following: Up to wavelengths of about 335 nm, U V radiation is capable of splitting an ozone molecule into an oxygen molecule and an excited oxygen atom (O*), The latter has enough energy to react with atmospheric water vapor to produce hydroxy! radicals, with the chemical formula OH (Levy, 1971).

Rla 05 + hi/ 0* +02(<335nm) R2 O* + H20 — 2 OH

The hv in reaction Rla, and elsewhere in this chapter, symbolizes a photon with frequency v and energy h, where h is Planck's constant,

The OH radical can be called the "detergent" of the atmosphere, because it is the main species, that reacts with almost all gases, thus removing them from the atmosphere. Without OH radicals, the chemical composition of the atmosphere would be totally different.

There are three factors that are important for the formation of the OH radical: ozone, water vapor, and UV-B radiation. The average concentration of OH amounts to only about 4 out of 10H air molecules (Prinn et al, 1995); negligibly few; one might say; but without this highly reactive radical the chemical composition of the atmosphere would be totally different. Molecular oxygen, which makes up almost 21% of the atmosphere, is not capable of oxidizing any of the atmospheric gases; their oxidation requires initial attack by OH radicals. Ozone in the troposphere is thus not at all the inert gas it was taken for until about 25 years ago, but rather it plays a key role in atmospheric chemistry, Although the role of ozone and hydroxy! in tropospheric chemistry is a fascinating subject, in this chapter we concentrate on stratospheric ozone,

1.2 The Ozone Hole

The main part of this chapter concentrates on the topic of the stratospheric ozone and the dramatic development of the so-called "ozone hole." To explain what has happened, I must start with a short overview of ozone layer chemistry

Stratospheric ozone is formed through the photolysis of oxygen by solar ultraviolet radiation of wavelengths less than 240 nm, a process that humans cannot influence. The photolysis of O2 produces two oxygen atoms, each of which combines with oxygen molecules to form ozone (Chapman, 1930).

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