Photodissociation in the Upper Atmosphere

Write balanced chemical equations for each of the following Teactions (a) The nitric oxide molecule undergoes photodissociation in the upper atmosphere, (b) The nitric oxide molecule undergoes photoionization in the upper atmosphere. (c) Nitric oxide undergoes oxidation by ozone in the stratosphere, (d) Nitrogen dioxide dissolves in water to form nitric acid and nitric oxide. 

The production of NOz, with NO as a possible precursor to NOz, has been observed when synthetic air or 02/N2 mixtures are photolyzed using a deuterium lamp, an argon flash lamp, or a 185-nm mercury line (Zipf and Prasad, 1998a). They proposed that this occurs from the reaction of electronically excited 02(B%) with N2, or photodissociation of 02 N2 dimer, and that the rate of NOx production from this process could be comparable to that from reaction (13b) (Zipf and Prasad, 1998a Prasad, 1998). If this proves to be the case, there must be some unidentified NOx sinks to be consistent with the measured NOx concentrations in the upper atmosphere. 

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... 

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. 

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. 

Ultraviolet radiation with the very highest energy is absorbed during photodissociation and photoionization in the upper atmosphere. Because most of this harmful radiation does not reach Earth s surface, life can exist. 

Photodissociation and photoionization are important processes that absorb high-energy ultraviolet radiation in the upper atmosphere. 

Interpreting Data Why are photodissociation and photoionization reactions more common in the upper atmosphere than in the lower atmosphere ... 

Methane only photodissociates in the upper part of the middle atmosphere because its absorption cross section becomes very weak at wavelengths longer than 145 nm (Figure 4.39). The most intense part of the spectrum is located below 130 nm, where the cross section is about 1.9 xl0 17cm2. The Lyman a line dominates the photolysis rate with... 

Gijs, A., A. Koppers, and D.P. Murtagh, Model studies of the influence of O2 photodissociation parameterizations in the Schumann-Runge bands on ozone related photolysis in the upper atmosphere. Ann Geophys If, 68, 1997. 

Describe the processes of photodissociation and photoionization and their role in the upper atmosphere. (Section 18.1)... 

The electrons in the upper atmosphere result mainly from photoionization, which occurs when a molecule in the upper atmosphere absorbs solar radiation and the absorbed energy causes an electron to be ejected from the molecule. The molecule then becomes a positively charged ion. For photoionization to occur, therefore, a molecule must absorb a photon, and the photon must have enough energy to remove an electron. 000 (Section 7.4) Notice that this is a very different process from photodissociation. 

The photodissociation of ozone by ultraviolet light in the upper atmosphere is a first-order reaction with a rate constant of 1.0 x 10 s at 10 km above the planet s surface. 

The half-life of a reactant is the time it takes for its concentration to fall to one-half its original value. Although this quantity can be defined for any reaction, it is particularly meaningful for first-order reactions. To see why, let s remrn to the system we considered in Example Problem 11.5, the photodissociation of ozone by UV light in the upper atmosphere. 

The ionosphere of Venus is the most explored and best understood one in our solar system besides that of the earth. The atmosphere at the surface of Venus consists of approximately 96.5% CO2 and 3.5% N2. Photodissociation of CO2 results in atomic oxygen becoming the major atmospheric constituent above about 150 km. The behavior of the ionosphere of Venus is controlled by chemical processes below an altitude of about 180 km. This region of the ionosphere is analogous to the terrestrial E-region fi om the point of view that the main ion is molecular and is under chemical control. However, unlike the earth the maximum plasma density peaks near an altitude of 140 km (see Fig. 14), and is the result of a peak in the photoionization rate. Venus is an excellent example of the importance of chemical processes in establishing the nature of some of the important aspects of an ionosphere. The ion with the largest density is Oj, and yet there is practically no neutral O2 in the upper atmosphere. As mentioned earlier, the major neutral gas constituents in the upper atmosphere are CO2 and O. The photoionization of these neutral gas species is followed by the reactions indicated below, which very effectively turn these initial ions into 02" ... 

Section 18.3 Ozone is produced in the upper atmosphere from the reaction of atomic oxygen wilh O2. Ozone is itself decomposed by absorption of a photon or by reaction with an active species such as NO. Chlorofluoro-carbons can undergo photodissociation in Ihe stratosphere, introducing atomic chlorine, which is capable of catalyti-cally destroying ozone. A marked reduction in the ozone level in Ihe upper atmosphere would have serious adverse consequences because e ozone layer filters out certain wavelenglhs of ultraviolet light that are not removed by any other atmospheric component. 

In this reaction, an ozone molecule collides with an oxygen atom to form two oxygen molecules in a single elementary step. The reason that Earth has a protective ozone layer in the upper atmosphere is that the activation energy for this reaction is fairly high and the reaction, therefore, proceeds at a fairly slow rate the ozone layer does not rapidly decompose into O2. However, the addition of Cl atoms (which come from the photodissociation of man-made chlorofluorocarlxms) to the upper atmosphere makes available another pathway by which O3 can be destroyed. The first step in this pathway—called the catalytic destruction of ozone—is the reaction of Cl with O3 to form CIO and O2 ... 

Sulfuric acid is produced in the upper atmosphere of Venus by the Sun s photochemical action on carbon dioxide, sulfur dioxide, and water vapor. Ultraviolet photons of wavelengths less than 169 nm can photodissociate carbon dioxide into carbon monoxide and atomic oxygen. Atomic oxygen is highly reactive. When it reacts with sulfur dioxide, a trace component of the Venusian atmosphere, the result is sulfur trioxide, which can combine with water vapor, another trace component of Venus s atmosphere, to yield sulfuric acid. In the upper, cooler portions of Venus s atmosphere, sulfuric acid exists as a liquid, and thick sulfuric acid clouds completely obscure the planet s surface when viewed from above. The main cloud layer extends from 45-70 km above the planet s surface, with thinner hazes extending as low as 30 km and as high as 90 km above the surface. The permanent Venusian clouds produce a concentrated acid rain, as the clouds in the atmosphere of Earth produce water rain. 

There is a distinct possibility that nitrogen oxides are of great importance in ozone photochemistry. In the first place we urgently need observations on their concentrations in the stratosphere. Investigations about the photodissociation products of N2O and its origin (see Bates/Hays 1967) should be continued and extended in order to establish if N2O is an important source for odd nitrogen in the upper atmosphere. If, however, most of the stratospheric NO and NO2 is produced at very high levels by other processes some solar cycle influence on the ozone layer will be possible. 

Although inert in the lower atmosphere (troposphere), the hilly halogenated CFCs and Halons diffuse into the upper stratosphere where they are photodissociated, ie, photolyzed, by the intense ultraviolet radiation. 

The atmospheric conditions and the rates of the various reactions for which measured laboratory values are available indicate that the maximum rate of photodissociation of oxygen occurs in the stratosphere, and results in a maximum concentration of ozone also in the stratosphere at about 25 km.3 Detailed studies show that the ozone is in photochemical equilibrium at levels above that of its maximum concentration. In other words, the production of ozone by solar ultraviolet radiation is automatically compensated by the re-formation of oxygen molecules in the upper stratosphere where equations (8) and (9) can be applied. 

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