Chemical reactions ultraviolet radiation

As discussed in greater detail in Chapter 7 with respect to atmospheric reactions of air pollutants, hydroxyl radical, HO-, is the most important reactive intermediate in atmospheric chemical reactions. Ultraviolet radiation is very much involved in the formation of hydroxyl radical through various photochemical reactions, and depletion of UV-absorbing stratospheric ozone may increase the levels of this species. The HO- radical is very active in determining the fates of atmospheric methane, carbon monoxide, hydrochlorofluorocarbons and hydrofluorocarbons that are substitutes for ozone-depleting CFCs, and other gases relevant to climate and ozone levels, and it is very much involved in the formation and dissipation of photochemical smog (see Chapter 7, Section 7.8). 

Related Work on Photochemical Smog Modeling. Models for photochemical air pollution require extensions of earlier methods. Coupled chemical reactions and radiation attenuation in the ultraviolet introduce nonlinearities into the analysis. Consequently, the superposition of linear solutions from collections of point, line, or finite-area sources may inaccurately describe the chemical interactions with meteorological conditions in the air basin. Chemical evolution of pollutants, therefore, demands a step-by-step description to refiect the cumulative effects of the processes occurring. 

Sinks, chemical species, or method OH, reaction with OH radical S, sedimentation P, precipitation scavenging NO, reaction with NO radical uv, photolysis by ultraviolet radiation Sr, destmction at surfaces O, adsorption or destmction at oceanic surface. 

Such structural changes are a consequence of chemical reactions of which the most common are oxidation, ozone attack, dehydrochlorination and ultraviolet attack. (Reactions due to high-energy radiation or to high temperature are not considered here as causing natural aging.) Over the years many materials have been introduced as antioxidants, antiozonants, dehydrochlorination stabilisers and ultraviolet absorbers—originally on an empirical basis but today more and more as the result of fundamental studies. Each of these additive types will be eonsidered in turn. 

In photo CVD, the chemical reaction is activated by the action of photons, specifically ultraviolet (UV) radiation, which have sufficient energy to break the chemical bonds in the reactant molecules. In many cases, these molecules have a broad electronic absorption band and they are readily excited by UV radiation. Although UV lamps have been used, more energy can be obtained from UV lasers, such as the excimer lasers, which have photon energy ranging from 3.4 eV (XeF laser) to 6.4 eV (ArF laser). A typical photo-laser CVD system is shown schematically in Fig. 5.14.117]... 

Reactions 1 to 4 are known collectively as the Chapman mechanism (first outlined by Sidney Chapman (1) in 1930. They basically explain how ozone can exist in the stratosphere in a dynamic balance it is continuously being produced by the action of solar ultraviolet radiation on oxygen molecules and destroyed by several natural chemical processes in the atmosphere. 

Possibilities exist for the involvement of halo-genated species such as CCI2F2 (CFC-12) or CCI3F (CFC-11) inasmuch as they can influence the column amounts of stratospheric O3 which is both a strong absorber of solar ultraviolet radiation and an absorber and emitter of infrared radiation. (Refer back to Fig. 7-11 for a survey of the chemical reactions that are involved.)... 

From comparison of the data presented in Table 2.2 [8], it is obvious that the energy of the microwave photon at a frequency of 2.45 GHz (0.0016 eV) is too low to cleave molecular bonds and is also lower than Brownian motion. It is therefore clear that microwaves cannot induce chemical reactions by direct absorption of electromagnetic energy, as opposed to ultraviolet and visible radiation (photochemistry). 

It is well known that y or X photons have energies suitable for excitation of inner electrons. We can use ultraviolet and visible radiation to initiate chemical reactions (photochemistry). Infrared radiation excites bond vibrations only whereas hyperfrequencies excite molecular rotation. In Tab. 1.1 the energies associated with chemical bonds and Brownian motion are compared with the microwave photon corresponding to the frequency used in microwave heating systems such as domestic and industrial ovens (2.45 GHz, 12.22 cm). 

Chemiluminescence (CL) is the emission of the electromagnetic (ultraviolet, visible, or near infrared) radiation by molecules or atoms resulting from a transition from an electronically excited state to a lower state (usually the ground state) in which the excited state is produced in a chemical reaction. The CL phenomenon is relatively uncommon because, in most chemical reactions, excited molecules... 

Finally, and apart from the importance of micelles in the solubilization of chemical species, mention should also be made of their intervention in the displacement of equilibria and in the modification of kinetics of reactions, as well as in the alteration of physicochemical parameters of certain ions and molecules that affect electrochemical measurements, processes of visible-ultraviolet radiation, fluorescence and phosphorescence emission, flame emission, and plasma spectroscopy, or in processes of extraction, thin-layer chromatography, or high-performance liquid chromatography [2-4, 29-33],... 

The thermal plasma is a source of high energy density with temperature of a few thousand degrees and high ultraviolet radiation. These result in fast reaction rates, high throughput in smaller reactors, heat generation independent of the chemical composition, avoidance of dioxins and furans... 

A chemical reaction induced by the absorption of a photon having energy corresponding to electromagnetic radiation in the ultraviolet or visible range. 

Chemical reaction that is caused by the absorption of ultraviolet, visible, or infrared radiation ([2], p. 302). 

Ozone (O3) is formed in the tropical stratosphere, around 12 to 30 miles above the ground, where solar radiation is intense it then migrates to the polar regions. The O3 concentration can be as high as 10 ppm in the stratosphere there, it absorbs a large part of the harmfirl ultraviolet radiation from the sun, thereby protecting life on Earth. CFCs are volatile and persist in the lower atmosphere (the troposphere) because of their inert nature they resist chemical degradation reactions. It is estimated that... 

Above the troposphere is the stratosphere, which extends many kilometers above the earth. Chemicals slowly migrate into the stratosphere, where there are few reactions to remove them. The stratosphere is also irradiated by high intensities of ultraviolet Hght which induces photochemical reactions such as production of O3 from O2. [In the troposphere O3 is bad, but in the stratosphere it is good because it absorbs ultraviolet radiation from the sun and prevents it from reaching the earth s surface.]... 

Materials exposed to the out-of-doors are particularly vulnerable since they are exposed to a higher concentration of chemicals (such as SOj, HC1, and Oj) ultraviolet radiation, which promotes many chemical reactions and moisture, which can ead to increased crazing and fracture of surface as the seasons change. Further, surface temperatures can reach almost 100 C on a hot day, increasing the susceptibility to attack as well as the rate of reactions already proceeding. 

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