Ozone destruction mechanism

This simple oxygen-only mechanism consistently overestimates the O3 concentration in the stratosphere as compared to measured values. This implies that there must be a mechanism for ozone destruction that the Chapman model does not account for. A series of catalytic ozone-destroying reactions causes the discrepancy. Shown below is an ozone-destroying mechanism with NO/NO2 serving as a catalyst ... 

The chemical reactions in the oxygen-only mechanism. Sections 5.4.3 and 10.4 substantially underestimate the ozone destruction rate ... 

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. 

An example of great environmental interest is the catalytic mechanism for ozone destruction by the hydroxyl radical, which is believed to be... 

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. 

In remote tropospheric air, where NO concentrations can be quite low (17), the HO + CO oxidation mechanism can follow other pathways, leading to net ozone destruction rather than formation (18, 19). Reactions 1 through 5 typify the more complex catalytic reactivity of HO with hydrocarbons, which produce a complex array of oxidation products while generating ozone pho-tochemically (11-13). 

Johnston, Crutzen, and others have also recognized that the natural ozone balance in the stratosphere cannot be explained on the basis of the Chapman mechanism and air motions. Johnston (542) has concluded that the calculated ozone destruction rate based on the Chapman reactions and air motions can explain only 20% of the natural destruction rate. About 80% of ozone produced by sunlight must be destroyed by a mechanism other than (VIII-43) and (VII1-44). 

Catalytic Destruction There are several different mechanisms of ozone destruction that predominate... 

Chlorine atoms can also act as a catalyst for the destruction of ozone. The first step of a proposed mechanism for chlorine-catalyzed ozone destruction is... 

Nitric oxide, NO-, is another radical also thought to cause ozone destruction by a similar mechanism. One source of NO- in the stratosphere is supersonic aircraft whose jet engines convert small amounts of Ng and Og to NO-. Write the propagation steps for the reaction of Og with NO. 

A Radical Chain Mechanism Leading to Ozone Destruction 550... 

The ozone destruction is mainly caused by its own photolysis (Chapman-mechanism,13)) ... 

In these reactions NO is not consumed while it destroys ozone. Rather, NO acts as a catalyst to ozone destruction in a pure oxygen atmosphere. Because it is faster, the catalytic cycle proceeds several times during the same time interval in which the 03 loss reaction of the Chapman mechanism occurs once. 

Until about 1964, the Chapman mechanism was thought to be the principal set of reactions governing ozone formation and destruction in the stratosphere. First, improved measurement of the rate constant of reaction 4 (above) indicated that the reaction is considerably slower than previously thought, leading to larger abundances of 03 as predicted by (5.10)—(5.12). Then, measurements indicated that the actual amount of ozone in the stratosphere is a factor of 2 less than what is predicted by the Chapman mechanism with the more accurate rate constant of reaction 4 (Figure 5.5). It was concluded that significant additional ozone destruction pathways must be present beyond reaction 4. 

Ozone is formed by the photolytic decomposition of NO2 yielding oxygen radicals and by the reaction sequence NO2 — HNO3 — NO3 O3. In particular, the reaction of NO2 with hydroxyl radicals to form HNO3 increases ozone concentration because two radicals, NO2- and -OH, which catalyze ozone decomposition, are removed. Other radicals are also important for ozone destruction in the stratosphere, especially chlorine oxides see Chlorine, Bromine, Iodine, Astatine Inorganic Chemistif). The mechanism of ozone destruction is complicated as there many compounds involved. Chlorine nitrate and dinitrogen pentoxide can act as reservoir species for CIO-, NO2-, and NO3- radicals. 

The ozone destruction processes that must be added to the Chapman mechanism take the form of a catalytic cycle ... 

The mechanism of ozone destruction in the polar stratosphere is thus as follows. Two ingredients are necessary cold temperatures and sunlight. The absence of either one of these... 

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