Oxygen from photolysis

Nitrogen oxides also play a significant role in regulating the chemistry of the stratosphere. In the stratosphere, ozone is formed by the same reaction as in the troposphere, the reaction of O2 with an oxygen atom. However, since the concentration of O atoms in the stratosphere is much higher (O is produced from photolysis of O2 at wavelengths less than 242 nm), the concentration of O3 in the stratosphere is much higher. 

Washida, N., Y. Mori, and I. Tanaka. Quantum yield of ozone formation from photolysis of the oxygen molecule at 1849 and 1931 A. J. Chem. Phys. 54 1119-1122, 1971. 

The reaction of 01D above can be shown to be important as the ultraviolet photolysis of N02, which gives rise to 01D,62 results in the production of IO in the presence of CF3I.26 The abstraction of atomic oxygen from N02 by I(52Ph) would be an endothermic process. Thus although reaction (99) appears to be established, its contribution following the photolysis of CF3/03 mixtures may not be quantitatively assessed. The importance of reaction (98) therefore remains in doubt. 

Table II. Product Rate Data from Photolysis of 1,1 -Azoisobutane— Oxygen Mixtures Using a Full Mercury Arc...

Fig. 2 a Molar absorption coefficients for ferric species of interest (Fe3+ and FeOH2+), and b HO quantum yields obtained from photolysis of pH 3 ferric solutions in the wavelength range 280-370 nm in the presence and absence of oxygen (data from [10])... 

Fig. 5 Major reactions in a system involving photolysis of Fe(III) oxalate in the presence of oxygen (from [53])...

The mechanism of the photochromic reaction has been explained on the basis of results of flash photolysis experiments on 2-methylacetophenone [31] in solution (Scheme 2) (Haag et al., 1977). The photo-enolization occurs in the triplet state through hydrogen abstraction by the carbonyl oxygen from the methyl group, followed by intersystem crossing. The enol form is isoraerized back to the starting material thermally. 

Stratospheric CO2 is known to posses an unusual isotopic composition, enriched in both 0 and (Gamo et al., 1989 Thiemens, 1999 Thiemens et al., 1991 Wen and Thiemens, 1993 Yung et al., 1991). 0 enrichment of 2%c is observed in CO2 at altitudes of 20-25 km, and of more than 10%o at altitudes of 30-35 km (Gamo et al., 1989 Thiemens and Jackson, 1990). This enrichment is likely to pass on from ozone 0 enrichment. It is based on the formation of a short-lived CO3 transition state, which is created by the interaction of CO2 with electronically exited 0( D), derived from photolysis of O3 in the stratosphere. The CO3 intermediate breaks apart to CO2 randomly losing oxygen, which is processed back to O2. This mechanism therefore effectively transfers the 0 (and O) enrichment from ozone to CO2. 

Most of the releases of carbonyl sulfide to the environment are to air, where it is believed to have a long residence time. The half-life of carbonyl sulfide in the atmosphere is estimated to be 2 years. It may be degraded in the atmosphere via a reaction with photochemically produced hydroxyl radicals or oxygen, direct photolysis, and other unknown processes related to the sulfur cycle. Sulfur dioxide, a greenhouse gas, is ultimately produced from these reactions. Carbonyl sulfide is relatively unreactive in the troposphere, but direct photolysis may occur in the stratosphere. Also, plants and soil microorganisms have been reported to remove carbonyl sulfide directly from the atmosphere. Plants are not expected to store carbonyl sulfide. 

Ga ago is the discovery of sulfate minerals in deposits of that time (Walter et ah, 1980). Although small amounts of oxygen from abiotic photolysis of water could have resulted in the oxidation of reduced sulfur compounds to form sulfates, it is also possible that part or most of the sulfate was derived from anaerobic photosynthesis according to reaction (2) above. 

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