Ozone photodecomposition

There is some controversy on the mechanism of ozone photodecomposition by UV light [367, 414, 643] but McGrath and Norrish [410, 455] identified 0(1D) atoms in the flash photolysis of Oa. 

Chlorine atoms and other chlorine species formed by photodecomposition of carbon tetrachloride in the stratosphere can catalyze reactions that destroy ozone. As the manufacture of carbon tetrachloride for use in chlorofluorocarbons is phased out according to a recent international agreement (EPA 1987e), the impact of carbon tetrachloride on atmospheric ozone is likely to decrease. 

Salthammer et al. (1999) examined emissions from commonly available coatings used on furniture and identified numerous oxidation products. These were observed without the addition of oxidants such as ozone, indicating that oxidation in air (perhaps including photodecomposition for some compounds) under typical conditions is sufficient to generate such products. For example, emissions of 2-ethylhexanol were identified from di-2-ethylhexyl phthalate, used as a plasticizer in many coatings. 

The advanced oxidation process with ozone and UV-radiation is initiated by the photolysis of ozone. The photodecomposition of ozone leads to hydrogen peroxide (Peyton, 1988). Ultraviolet lamps must have a maximum radiation output at 254 nm for an efficient ozone photolysis. 

Oxidation of 4-chlorophenol can be brought about by single photodecomposition by hydroxy radicals generated from Fenton s reagent (H2O2 plus Fe ions) . Irradiation in the 320-400 nm range with Fenton s reagent is also effective in the oxidation of 4-chlorophenol . Continuous irradiation at 365 nm has identified two different reaction pathways with formation of the 4-chlorodihydroxycyclohexadienyl radical and also of the chlorophenoxyl radical. The quantum yields of these processes have been determined to be 0.056 and 0.015, respectively . Reaction of 4-chlorophenol with ozone leads to the formation of 4-chloro-l,3-dihydroxybenzene and 4-chloro-l,2-dihydroxybenzene. The latter product is produced in quantity in the presence of hydroxyl radicals . ... 

Fig. 2-13. Quantum yields for the formation of O( D) atoms in the photodecomposition of ozone as a function of wavelength. The threshold behavior near 310 nm is of considerable importance to tropospheric chemistry. Note the temperature dependence of the quantum yield in the threshold region.

The atmosphere contains many trace gases that are photochemically active. Few, however, attain a significance similar to oxygen or ozone in driving atmospheric chemistry. Table 2-6 gives an overview on the photochemical behavior of atmospheric constituents. The last column indicates whether the photodecomposition of the substance is important to atmospheric chemistry or not. The molecules listed may be subdivided into three groups. (1) For a number of gases such as methane or ammonia that enter... 

The carbonyl halides thus formed also are subject to photodecomposition whereby further chlorine (or fluorine) atoms are released. Their interaction with ozone results in the catalytic cycle... 

In this reaction sequence ozone is reconstituted so that there is no net loss. Yet another link between the CIO and NO families is the formation of chlorine nitrate and its photodecomposition ... 

The photodecomposition of CH3I generates iodine atoms. Chameides and Davies (1980) have discussed possible subsequent reactions. The most probable pathway is reaction with ozone, I + 03— 02+IO, but the fate of the IO radical is uncertain and the consequences cannot yet be assessed. 

Atmospheric N20 was discovered in 1938 by Adel via infrared absorption features in the solar spectrum. For the next 30 yr, N20 aroused little interest, presumably because it is neither a hazardous pollutant nor does it display any particular chemical activity. In fact, there are no gas-phase reactions that remove it from the troposphere as far as we know. In the stratosphere (see Chapter 3), N20 undergoes photodecomposition, and it reacts with O( D). The second reaction is the major source of higher nitrogen oxides in that region, and since these reduce ozone catalytically via chain reactions, N20 is an important agent in controlling the stratospheric ozone balance. The recognition of this relationship by Crutzen (1970, 1971) and McElroy... 

CFCs do not decompose in the lower atmosphere. Photodecomposition occurs in the stratosphere via absorption of uv radiation and subsequent release of atomic chlo -rine which can catalyze ozone breakdown. CFC-ozone depletion hypothesis M J. Molina, F. S. Rowland, Nature 249, 810 (1974). Reviews focusing on atmospheric chemistry of CFCs, uses, potential hydrogen-substituted replace-... 

The photodecomposition of ozone may form electronically excited oxygen atoms, 0( D), and excited molecular oxygen with absorption of the 193-nm radiation (Reaction [13.35]). The 0( D) species formed in Reaction [13.35] is much more reactive than the ground-state oxygen atoms [0( P)] ... 

A correlation of these results with ambient ozone levels (about 30 ppb) was suggested, but photodecomposition of BaP was also shown... 

The photodecomposition of ozone reverses the reaction that forms it. We thus have a cyclic process of ozone formation and decomposition, summarized as follows ... 

The steady-state ozone concentration at any point is determined by the balance of O2 photodissociation (5.1) and the removal mechanisms represented by reactions (5.22) and (5.23), and also the rates of transport in and out of the gas volume of interest, all of which vary depending on the conditions. The chemical lifetime (see Sect. 5.7.1 for a formal definition) of O3 is very short at the top of the stratosphere, because of the high rate of its photodecomposition (5.22), and steady-state levels are relatively low, but it increases towards lower altimdes and higher latitudes, where it can be several months. 

In the troposphere it is produced by a variety of reactions, but principally the photodecomposition of ozone. About 10 % of tropospheric O3 results from its downward transport across the tropopause. Residual UV radiation at A < 310 nm, penetrating the troposphere, causes photolysis, by reaction (5.52) to produce the singlet O atom. 

Photodissociation, photolysis or photodecomposition is a chemical reaction in which a chemical compound is broken down by photons . Studies on molecular photodissociation reactions aim at understanding the chemistry of bond cleavage induced by irradiation with light. More than 500 papers with photodissociation (or photolysis, or photodecomposition) in the title have been published in 2012 and 2013. Theoretical calculations play an important role in most of these publications. The studied systems are carbonyl compounds, aromatic compounds, water, aliphatic and aryl halides, ozone and so on. The theoretical study on the photodissociation of NO3 is impressive, and it is selected here as an example. 

In the atmosphere, the hydroxyl radical forms when excited oxygen atoms—formed from the photodecomposition of ozone—react with water vapor. 

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