Ozone catalytic destruction cycles

These reactions emphasize the importance of solar radiation in NO/NO2 catalytic destruction cycle of ozone. One can immediately see that to provide any reliable observational basis for importance of NO/NO2 in ozone balance, we must have not only NO/NO2 concentration but also its diurnal variation which provides proper check on the time constants for the reactions described in Eqs. (7)-(9). 

These reactions constitute a source of OH in upper stratosphere, which ultimately controls the upper boundary of the ozone layer through the OH/HO2 catalytic destruction cycles [5] getting rid of about 10% of the existing ozone [6]. The decrease of the ozone layer is a well known problem, see e,g, [3,7-9]. 

It is interesting to compare the rate constants of the oxygen-only ozone destruction reaction with those of the catalytic ozone destruction cycle. The rate constants for reactions 4-6 at 30 km are given below in units of cm molecules s . 

In 1974, Cicerone and Stolarski suggested that if there were sources of atomic chlorine in the stratosphere, the following catalytic ozone destruction cycle... 

The atmospheric chemistry of the organobromides is similar to that of the organochlo-rides degradation ultimately produces bromine atoms which may participate in catalytic destruction of ozone through a BrOx catalytic cycle (reactions 12 and 13). 

Similar chain reactions can be written for reactions involving R02- In contrast, when relatively little NO is present, as in the remote atmosphere, the following cycle can dominate over ozone production leading to the catalytic destruction of ozone, viz ... 

Potentially, the most important effect of reactive halogen species maybe that their chemistry may lead to the catalytic destruction of ozone via two distinct cycles Cycle I ... 

Other cycles including more than two reactions are known to destroy ozone efficiently (see Chapter 5). Catalytic cycles can be very efficient, even if the concentration of the catalyst X is several orders of magnitude smaller than that of ozone. The efficiency of such a cycle regarding the destruction of ozone is given by the number of times the cycle repeats itself before the catalyst is eventually lost, usually by conversion to a stable reservoir species (e.g., NO and NO2 are converted into HNO3). The rate limiting step for the destruction of ozone in a cycle like (2.40) is provided by the reaction that ultimately determines the rate at which ozone is destroyed. Such a step can be identified, provided that all reactions involved are known. 

The dominant removal process for FCO radicals is also the major formation reaction of FC(0)0j radicals, that is, reaction (101). It has been suggested that the interconversion reaction of FC(0)Oj radicals with ozone could lead to the possible involvement of FC(0)02 and FC(0)0 radicals in catalytic ozone destruction cycles. 

The solubility of HBr in sulfuric acid has been studied as well [37]. Bromine radicals contribute to ozone destruction through a catalytic reaction cycle involving BrO and CIO. Thus heterogeneous chemistry of bromine containing species merits some attention, even though most of the stratospheric bromine is already present in active species. Table 1 shows these results in a manner analogous to the HCl and HNO3 results. 

Towards the end of the CIAP programme some researchers had turned their interest to the potential input of reactive chlorine radicals on stratospheric ozone. In the most thorough of these studies, Stolarski and Cicerone [50] calculated substantial ozone depletions if inorganic chlorine were present in the stratosphere at a volume of mixing ratio of 1 nmol/mol of air. Odd oxygen destruction would take place by the catalytic reaction cycle (21) + (22). This reaction sequence is very similar to the... 

Halogen oxide radicals such as CIO and BrO are important reactive intermediates in the catalytic cycles of ozone destruction in the middle and upper stratosphere. The first absorption band CIO(/l211 <— X2 I) starts from 318 nm and has a series of vibronic bands that converge to a broad continuum at wavelengths shorter than 264nm (Fig. 8).98-101 In this continuum region four dissociation pathways are thermodynamically possible,33... 

In the late 1960s, direct observations of substantial amounts (3ppb) of nitric acid vapor in the stratosphere were reported. Crutzen [118] reasoned that if HN03 vapor is present in the stratosphere, it could be broken down to a degree to the active oxides of nitrogen NO (NO and N02) and that these oxides could form a catalytic cycle (or the destruction of the ozone). Johnston and Whitten [119] first realized that if this were so, then supersonic aircraft flying in the stratosphere could wreak harm to the ozone balance in the stratosphere. Much of what appears in this section is drawn from an excellent review by Johnston and Whitten [119]. The most pertinent of the possible NO reactions in the atmosphere are... 

It is possible to similarly estimate the effect of the various cycles upon ozone destruction. The results can be summarized as follows between 15 and 20 km, the N03 catalytic cycle dominates between 20 and 40 km, the N02 cycle dominates between 40 and 45km, the N02, HO, and O mechanisms are about equal and above 45 km, the HO reactions are the controlling reactions. 

Approximately one-half of the total reaction of CIO and BrO results in the destruction of ozone. Other mechanisms exist, such as a catalytic cycles that are rate-limited by the reaction between CIO and O and between CIO and H02 (25), but the contribution from these reactions is small in the polar regions. 

The destruction of ozone by another catalytic cycle (an HO cycle) is. n mated to be about 10% of the NO cycle... 

Thus a number of catalytic reactions are associated with the re-formation of 02 already indicated in equation (5). In other words, a halogen, or any other atom which attacks ozone, always reappears as a result of the reaction between its oxide and atomic oxygen, and hence there is a permanent cycle leading to the destruction of ozone. In atmospheric chemistry, therefore, it is important to find out how these constituents appear, and to assess their importance. 

Johnston (543) and others have proposed that the most important catalytic cycle responsible for ozone destruction is a N0-N02 cycle... 

Molina and Rowland (711, 843) were the first to predict the possible j destruction of the stratospheric ozone by a catalytic cycle involving Cl atoms released from the photolysis of chlorofluoromethanes by sunlight. Since chlorofluoromethanes are unreactive with atoms and radicals in the troposphere, they eventually reach the stratosphere where they are photodis-sociated into Cl atoms by solar radiation below 2300 A. 

The oxidation scheme for halomethanes not containing a hydrogen atom is similar to that for those which do, except that it is not initiated by tropospheric reaction with hydroxyl radicals, since the fully halogenated methanes are unreactive. Consequently, substantial amounts of CFCs and halons are transported intact up into the stratosphere, where they absorb UV radiation of short wavelength and undergo photodissociation (equation 36) to a halogen atom and a trihalomethyl radical. The halogen atom Y may enter into catalytic cycles for ozone destruction, as discussed in the introduction. 

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