Atmospheric chemistry solar radiation absorption

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. 

The central role of hydroxyl radicals in atmospheric chemistry is well illustrated by examining the atmospheric cycles of methane and carbon monoxide. A quantitative assessment of both of these species was carried out in the 1920s in Belgium by Marcell Migeotte, who detected their absorption lines in the spectrum of infrared solar radiation reaching Earth s surface. 

Photochemical processes play a critical role in the chemistry of the atmosphere, since they control the daytime production of reactive free radicals, which initiate chemical transformations of many trace compounds. The photodissociation of atmospheric molecules occurs by absorption of solar ultraviolet (UV) and/or visible (VIS) radiation. The rate of photolysis is determined by the absorption cross-sections of the dissociated molecule and by the quantum yield for the various products channels at the absorbing wavelengths. 

Ozone plays important roles in atmospheric chemistry and physics. Up to 5 x 10 molecules cm or 4-5 ppmv (10 mixing ratio by volume) of ozone is found in the stratosphere where the solar radiation intensity of shorter wavelengths is high. Ozone in the stratosphere has made it possible for animals to live on land by preventing harmful 200-300 nm UV radiation from reaching the Earth s surface. The absorption of UV radiation by ozone, however, heats the air, disturbing air convection and creating a layered structure in the stratosphere. As some useful and stable chemicals like chlorofluorocarbons (Fre-ons) and other substances were found to decompose in the stratospheric environment and to destroy the... 

FIGURE 1 Photodissociation of ozone in the near ultraviolet spectral region. Overlap of solar radiation actinic intensity, ozone absorption cross section, and 0( D) quantum yield to derive the 0( D) production frequency as a function of wavelength. [From Zellner, R., ed. (1999). Global Aspects of Atmospheric Chemistry, Steinkopff/Springer, Darmstadt, Germany.]... 

The examples discussed above illustrate the utility of vibrational overtone excitation by red sunlight in atmospheric photochemistry. The low absorption cross-section of vibrational overtones limits the importance of such light-initiated chemistry. However, when reactive electronic states are high in energy (as is the case with most alcohols and acids) or when UV radiation is suppressed at high solar zenith angles, vibrational overtone initiated photochemistry has been used to explain discrepancies between measurements and model results. 

Photolytic reaction of oxygen O2 is the most fundamental reaction of stratosphere chemistry. In the first place, the formation of stratosphere in the earth s atmosphere has been brought on by the temperature inversion of the atmosphere due to the absorption of solar radiation by O3 molecules that were produced by the reaction of 0( P) atoms from the photolysis of O2 with another O2 molecule. In this sense the photolytic reaction of O2 is the basic of basics of stratospheric chemistry. 

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. 

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