Stratosphere reactions involving atomic oxygen

OCS, N20 and even CH4 have long residence times. The CFCs (chlorofluoro-carbons, Fig. 3.4b refrigerants and aerosol propellants) also have very limited reactivity with OH. Gases like these build up in the atmosphere and eventually leak across the tropopause into the stratosphere. Here a very different chemistry takes place, no longer dominated by OH but by reactions which involve atomic oxygen (i.e. O). Gases that react with atomic oxygen in the stratosphere can interfere with the production of 03 ... 

This process dominates the odd oxygen destruction near the tropopause because it is the most effective catalytic cycle involving only ozone as the reactive odd oxygen species. Most of the other HOx, NOx, and C10x cycles (see below) also require reaction with atomic oxygen, which is present only in very small amounts at low altitudes. For example, the following cycles are important in the middle and upper stratosphere, and in the mesosphere... 

The actual destruction of ozone in the stratosphere actually involves hundreds of different reactions. Besides the Chapman reactions and destruction by CFCs, many other chemical species can destroy ozone. In 1970, Paul Crutzen (193 3-) showed that nitrogen oxides could destroy ozone. Nitric oxide can remove an oxygen atom from ozone and be regenerated according to the following reactions ... 

However, in the stratosphere, where H20 is not photodissociated, it is necessary to consider a reaction involving the ozone photodissociation. This is a process in which an excited oxygen atom O( D) reacts with a molecule to produce H and OH (Fig. 4) as follows ... 

On the basis of ratios of C and C present in carbon dioxide, Weinstock (250) estimated a carbon monoxide lifetime of 0.1 year. This was more than an order of magnitude less than previous estimates of Bates and Witherspoon (12) and Robinson and Robbins (214), which were based on calculations of the anthropogenic source of carbon monoxide. Weinstock (250) suggested that if a sufficient concentration of hydroxyl radical were available, the oxidation of carbon monoxide by hydroxyl radical, first proposed by Bates and Witherspoon (12) for the stratosphere, would provide the rapid loss mechanism for carbon monoxide that appeared necessary. By extension of previous stratospheric models of Hunt (104), Leovy (150), Nicolet (180), and others, Levy (152) demonstrated that a large source of hydroxyl radical, the oxidation of water by metastable atomic oxygen, which was itself produced by the photolysis of ozone, existed in the troposphere and that a chain reaction involving the hydroxyl and hydroperoxyl radicals would rapidly oxidize both carbon monoxide and methane. It was then pointed out that all the loss paths for the formaldehyde produced in the methane oxidation led to the production of carbon monoxide [McConnell, McElroy, and Wofsy (171) and Levy (153)1-Similar chain mechanisms were shown to provide tropospheric... 

In searching for mechanisms involving excited oxygen atoms have you discovered any processes involving the vibrationally excited states of either the ground electronic state or low-lying excited states of O2 which would drive these chemical cycles backward Have you considered candidate reactions other than those already identified for the production of odd oxygen in the stratosphere ... 

Reactions involving these species sum in such a way as to destroy 03 and atomic oxygen while restoring the OH or NO molecules. They can thus be regarded as catalysts for 03 destruction. In this case the catalysts are chemical species that facilitate a reaction, but undergo no net consumption or production in the reaction (see also Box 4.4). The important point of these catalytic reaction chains in the chemistry of stratospheric 03 is that a single pollutant molecule can be responsible for the destruction of a large number of 03 molecules. 

In the lower stratosphere, because of the relative scarcity of oxygen atoms, the effective HO, cycles involve direct reactions of HO , and O3, HO, Cycles 2 and 4. NO, Cycles 1 and 2 account for less than 20% of the total O3 loss between 16 and 20 km. The halogen cycles account for about one-third of the total O3 loss. As in the case of the NO, cycles, the effective halogen cycles in the lower stratosphere do not explicitly involve the oxygen atom. The general halogen cycle is ... 

Fig. 2. General reaction scheme in the atmosphere in which are simultaneously involved the chlorine,. nitrogen, and hydrogen radicals (atoms or molecules) related to the production or loss of odd oxygen in the stratosphere.

It is known that measured ozone concentrations are lower than can be accounted for by the simple Chapman cycle. This has led scientists to look for other influences on the concentration of ozone. First, let s briefly consider one of the natural reactions that destroys ozone. UV radiation can break the oxygen-hydrogen bond of a water molecule in the stratosphere to generate hydrogen atoms and hydroxyl radicals ( OH). These two species are involved in many reactions, some of which actually convert Og to Og. However, this process, which scientists now believe is an efficient process above 50 km, has been occurring since the ozone layer developed, and there is little, if anything, that humans can do about it. The system has obviously attained a steady state that includes this perturbation. 

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