Solar radiation photodissociation

The photodissociation of S02 into SO and O atoms is markedly different from the photodissociation of N02. The bond to be broken in the sulfur compound requires about 560kJ/mol. Thus, wavelengths greater than 2180 A do not have sufficient energy to initiate dissociation. This fact is significant in that only solar radiation greater than 2900 A reaches the lower atmosphere. If a photochemical effect is to occur in the S02-02 atmospheric system, it must be that the radiation electronically excites the S02 molecule but does not dissociate it. 

CC12FCC1F2. These compounds are non-toxic and non-flammable, and their thermodynamic properties are ideally suited for the compression/ expansion cycle in cooling and heat pump appliances. However, CFCs are chemically very inert, so when they are vented into the atmosphere, they do not react with atmospheric constituents. They diffuse unscathed first into the troposphere, then penetrate slowly into the stratosphere. There, the solar UV radiation photodissociates these compounds, liberating free chlorine atoms (the C-Cl bond is weaker than the C-F bond). The chlorine atoms react with atmospheric O3 to form chlorine oxide, which in turn reacts with atmospheric atomic oxygen regenerating chlorine atoms ... 

Nitric oxide is also present in the upper atmosphere its role has been reviewed by Nicolet.326-328 Because of solar radiation, important processes are photoionization, photodissociation, and the formation of electronically excited levels. The continuum seen in the night airglow has often been ascribed to reaction (4). However, both the y and / bands of NO are absent in the night airglow. Since the / and y emissions arise from... 

Concentration profiles of H2, He, CH4, and NH3 at 150 K and with the < tldy diffusion coefficient K = 3 x I05 cm2 sec-1 have been calculated by Strobel (943) and are given in Fig. VIII—15. Because of the presence of CH4 ibove the layer of NH3, solar radiation of wavelengths only above about 1600 A is effective in photodissociating NHj. 

The ionization of NO by the Lyman-a line is the main source of ions in the D region. The photodissociation of NO in the upper atmosphere occurs from the /t2Z + (F > 4), B2n (c > 7), and C2n (F > 0). The dissociation rate of NO by the solar radiation is proportional to the integrated absorption coefficient of various bands (that is, the oscillator strength). From Table V 4 it can be seen that absorption by the /if (12,0) and 6 bands is most important in leading to photodissociation. 

Photochemistry of the Polluted Stratosphere. The intensity of solar ladi.i tion reaching the stratosphere is attenuated by oxygen and ozone. Sim < < < s is transparent to radiation of wavelengths above 1800 A, while 03 absi.il, light weakly in the region 1900 to 2100 A [see Figs. VI-12b and 12c. 11,< effective wavelengths of solar radiation for photodissociation are 18(>o n. 2200 and above 2900 A in the stratosphere (843). 

The peak of the O2 photodissociation occurs in the stratosphere (near 35 km for an overhead sun) where the total number of 02 molecules pho-todissociated is of the order of 107 cm-3 sec-1. Below the ozone peak (<25 km) the photodissociation rate decreases rapidly, particularly when the solar zenith angle increases. Below 20 km, the atomic oxygen production becomes very small and there is no atomic oxygen production in the troposphere by the 02 photodissociation. The ozone photodissociation is the result of the absorption of solar radiation in the visible and the ultraviolet ... 

This cycle would lead to considerable destruction of ozone if it were not for the moderating influence of the very rapid photodissociation of N02 by solar radiation at wavelengths less than 400 nm ... 

Above the mesopause, Tg increases rapidly. In this region, termed the thermosphere (Fig. 2), absorption of short wavelength solar radiation is occurring (Fig. 3) which results in the efficient photodissociation of molecular oxygen, and the photoionization of the O atoms so produced and of the 02 and N2 molecules. Thus, Tg increases beyond 1000 K, approaching 2000 K at times. Whereas below 100 km the neutral gas particles, the ions and the electrons in the plasma all possess the same kinetic temperature, above 100 km, due to the lower pressure and the subsequent reduced electron/heavy particle collision frequency and the large amount of energy imparted to the photoelectrons, the electron temperature, Te increases above Tg (and Tj the ion temperature, which is Tg, see Fig. 2). 

Photochemistry. Both CFC13 and CF2C12 arc used as aerosol propellants and refrigerants in large quantities. They arc chemically inert in the troposphere. However, when they diffuse into the stratosphere they are photodissociated by solar radiation to produce Cl atoms. The Cl atoms so formed would catalytically destroy 03 in the stratosphere (see Section VIII 2, p. 350). 

All these compounds are active participants in environmental photochemistry. Nitrate radicals absorb solar radiation at approximately 600-700 nm and in the excited state dissociate to NOx and oxygen. There are two possible sets of the photodissociation products ... 

Photodissociation of atmospheric molecules by solar radiation plays a fundamental role in the chemistry of the atmosphere. The photodissociation of trace species such as ozone and formaldehyde contributes to their removal from the atmosphere, but probably the most important role played by these photoprocesses is the generation of highly reactive atoms and radicals. Photodissociation of trace species and the subsequent reaction of the photoproducts with other molecules is the prime initiator and driver for the bulk of atmospheric chemistry. 

Numerical method for modelling atmospheric O2 photodissociation in the wavelength range 175—200 nm Br atom laser action from photodissociation of IBr by solar radiation. Theoretical and experimental study t Cross-section for Ij photodissociation in seeded supersonic beams by CW Ar laser radiation at 514.5 nm... 

Rate of atmospheric NO, photodissociation by solar radiation 458 NO, photodissociation coefficient as a function of solar zenith 459... 

NO by the solar radiation is proportional to the integrated absorption coefficient of various bands (that is, the oscillator strength). From Table V 4 it can be seen that absorption by the [i (12,0) and 6 bands is most important in leading to photodissociation. 

Photodissociation Photodissociation is a process in which high-energy ultraviolet solar radiation is absorbed by molecules, causing their chemical bonds to break. In the upper atmosphere, the photodissociation of oxygen absorbs much of the high-energy UV radiation and produces atomic oxygen. 

Photodissociation is far more likely to occur than photoionization, however, because the energy needed to bring about dissociation is much less than that required for photoionization. In fact, once a water molecule escapes from the comet nucleus into the coma, the average time in which it is likely to he broken apart by solar radiation is about 25 seconds. By contrast, the average amount of time during which a water molecule is ionized by solar radiation is likely to be a few hours. Such reactions are more likely to occur when water molecules have trailed off into the comet s tail at distances of about 1 AU. Consequently, the coma of a comet can he expected to consist largely of neutral remnants of the photolysis of water molecules, such as H and OH, while ionized species, such as H+ and free electrons, are more likely to be found in the comet s tail. 

At wavelengths longer than 100 nm, solar ultraviolet radiation photodissociates atmospheric molecules. The solar Lyman-o line at... 

As shown by Johnston and Selwyn (1975), the cross section of N2O varies strongly with temperature. The quantum yield for photodissociation is unity, and the products are N2 and 0(1D). The atmospheric photolysis rate comes predominantly from the absorption of solar radiation in the 02 Herzberg continuum and Schumann Runge bands. Jn2o 9 x 10-7s-1 for A > 175 nm. 

As mentioned earlier, ozone in the stratosphere prevents UV radiation emitted by the snn from reaching Earth s surface. The formation of ozone in this region begins with the photodissociation of oxygen molecules by solar radiation at wavelengths below 240 nm ... 

The behavior of the total 02 photodissociation coefficient in the stratosphere is shown in Fig. 2-12. The strong decrease of the value toward lower altitudes by more than five orders of magnitude is due to the attenuation of solar radiation by both oxygen and ozone. In fact, j 02) depends sensitively on the amount and the distribution of ozone assumed in the calculation. Uncertainties arise also from computational problems at high degrees of attenuation in the lower stratosphere. Figure 2-12 thus indicates a range of values. 

Table 2-7. Some Photochemically Active Trace Constituents of the Atmosphere and Associated Photodissociation Coefficients Calculated from the Solar Radiation Flux Outside the Earth Atmosphere (Nicolet, 1978) ...

The formation of the stratospheric ozone layer can be understood most simply on the basis of a reaction model composed of a minimum set of four elementary processes (a) the dissociation of oxygen molecules by solar radiation in the wavelength region 180-240 nm (b) the attachment of oxygen atoms to molecular oxygen, leading to the formation of ozone (c) the photodissociation of ozone in the Hartley band between 200 and 300 nm and (d) the destruction of ozone by its reaction with oxygen atoms. The reactions may be written... 

---