Ultraviolet radiation atmospheric absorption

Much of environmental chemistry and water chemistry are concerned with electrons in atoms. In the atmosphere, the absorption of electromagnetic radiation, primarily ultraviolet radiation, promotes electrons to higher energy levels, forming reactive excited species and reactive free radicals with unpaired electrons. These phenomena can result in photochemical reactions such as the formation of stratospheric ozone, which is an essential filter for solar ultraviolet radiation. Atomic absorption and emission methods of elemental analysis, important in the study of pollutants, involve transitions of electrons between energy levels. 

Measurements of ozone (O3) concentrations in the atmosphere are of particular importance. Ozone absorbs strongly in the ultraviolet region and it is this absorption which protects us from a dangerously high dose of ultraviolet radiation from the sun. The vitally important ozone layer lies in the stratosphere and is typically about 10 km thick with a maximum concentration about 25 km above the surface of the earth. Extreme depletion of ozone in a localised part of the atmosphere creates what is known as an ozone hole. 

Spectrum of ozone, showing maximum absorption of ultraviolet radiation at a wavelength near 260 nm. [Adapted from R. P Wayne, Chemistry of Atmospheres (Oxford Clarendon Press, 1991).]... 

The necessary starting point for any study of the chemistry of a planetary atmosphere is the dissociation of molecules, which results from the absorption of solar ultraviolet radiation. This atmospheric chemistry must take into account not only the general characteristics of the atmosphere (constitution), but also its particular chemical constituents (composition). The absorption of solar radiation can be attributed to carbon dioxide (C02) for Mars and Venus, to molecular oxygen (02) for the Earth, and to methane (CH4) and ammonia (NH3) for Jupiter and the outer planets. 

Since the atmosphere shields us from most deep ultraviolet radiation and from infrared radiation, the bulk of visible light (the solar spectrum) ranges from 350 to 750 nm. The 25,000 Frauenhofer15 "dark" lines are interruptions (in the range 295 to 1000 nm) in the continuous solar emission spectrum, due to absorption by the chemical elements present in the sun s atmosphere. Ultraviolet radiation was discovered by Ritter16 in 1801. Some radio waves do penetrate the earth s atmosphere, and they are most intense during solar storms. Infrared radiation also penetrates to some extent. 

The concerns for changes in atmospheric ozone can be divided into two major categories changes in total column of ozone, and changes in the concentrations at particular altitudes. The penetration of ultraviolet radiation to the surface of the earth is determined almost entirely by the total amount of ozone in the atmospheric column, with very litde dependence on the altitude distribution of this ozone. However, if the prime concern is with processes such as the conversion of ultraviolet energy into heat after absorption by ozone (i.e. with the temperature structure of the stratosphere), then a redistribution of ozone to different altitudes is extremely important. 

Absorption of ultraviolet radiation by O3 causes it to decompose to O2. In the upper atmosphere, therefore, a steady-state concentration of ozone is achieved, a concentration ordinarily sufficient to provide significant ultraviolet protection of the Earth s surface. However, pollutants in the upper atmosphere such as nitrogen oxides (some of which occur in trace amounts naturally) from high-flying aircraft and chlorine atoms from photolytic decomposition of chlorofluorocarbons (from aerosols, refrigerants, and other sources) catalyze the decomposition of ozone. The overall processes governing the concentration of ozone in the atmosphere are extremely complex. The following reactions can be studied in the laboratory and are examples of the processes believed to be involved in the atmosphere ... 

As was mentioned in Chapter 2, the major source of heat in the middle atmosphere is provided by absorption of ultraviolet radiation, particularly by ozone and to a lesser extent by molecular oxygen... 

Figure 3.3. Net radiative heating rate associated with (1) absorption of ultraviolet radiation by molecular oxygen in the upper mesosphere and thermosphere, and by ozone in the stratosphere and mesosphere, and (2) emission of infrared radiation by atmospheric CO2, O3, and H2O. Values given in K/day and positive in the summer hemisphere (net diabatic heating) and negative (net diabatic cooling) in the winter hemisphere. Prom London (1980).

The theoretical framework just presented was first suggested by Chapman (1931). This theory provides an explanation for the behavior of the layers of ionization in the thermosphere or of photodissociation in the middle atmosphere. The production of ozone through photodissociation of molecular oxygen exhibits a maximum near 45 km dependent on the insolation. The rate of heating through absorption of ultraviolet radiation by ozone similarly leads to a maximum near the stratopause. Numerous examples of such layers can be found in the neutral and ionized atmosphere. 

Convection ceases at the tropopause level, and the temperature in the stratosphere and mesosphere is determined strictly by radiation balance. At altitudes above 20 km the absorption of solar ultraviolet radiation becomes increasingly important. The temperature peak at the stratopause has its origin in the absorption of near-ultraviolet radiation by stratospheric ozone. In fact, the existence of the ozone layer is in itself a consequence of the ultraviolet (UV) irradiation of the atmosphere. The enormous temperature increase in the thermosphere is due to the absorption of extremely shortwaved and thus energetic radiation coupled with the tenuity of the atmosphere, which prevents an effective removal of heat by thermal radiation. Instead, the heat must be carried downward by conduction toward denser layers of the atmosphere, where H20 and C02 are sufficiently abundant to permit the excess energy to be radiated into space. 

Ozone O3 has a different molecular structure, and therefore a different set of energy levels from diatomic oxygen, O2. The absorption spectrum of O2 is therefore not the same. O2 absorbs ultraviolet radiation of much shorter wavelengths. Diatomic ox n and other components of the atmosphere filter out harmful radiation from the sun, in the ultraviolet region, up to 220 nm. This radiation would damage our eyes and skin if it reached the surface of the Earth. 

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