Stratospheric oxygen chemistry

Chemistry in the stratosphere (which goes up to about 50 km in altitude) differs from that in the troposphere for three reasons (i) radiation below 300 nm is available for photodissociations that do not occur in the troposphere (Fig. 5.20), (ii) not all trace gases (those with a short lifetime) are either available or found in very 

Evidently, O3 formation and reaction follow (5.145), which have no importance in the troposphere but do form the natural O3 removal pathway in the stratosphere. 

There are other important differences between the stratosphere and troposphere. Whereas the troposphere is heated from the bottom (i.e. the earth s surface), the stratosphere heats from the top (i. e. by incoming solar radiation). This positive temperature gradient results in an extremely stable layering. Therefore, mixing and transport are much weaker than in the troposphere. At the bottom of the stratosphere, close to the tropopause, the lowest temperatures are found (200-220 K and at the poles down to 180 K), whereas temperature rises up to 270 K can be found at 50 km altitude. At each point of the stratosphere the temperature is determined by adiabatic radiation processes O3 absorbs UV radiation and heats the air and CO2 absorbs IR radiation and cools the air. The chemical composition (mixing 

The photolysis of oxygen was described many years ago by Chapman (1930). The attention to the stratospheric ozone was drawn by Bates and Nicolet (1950), who presented the idea of catalytic O3 decay. However, only through the implications of manmade influences on the stratospheric ozone cyele by Crutzen (1971), Johnston (1971), Molina and Rowland (1974) as well as Stolarski and Cicerone (1974) was our attention to the stratospheric ozone layer drawn. 

Two of the above listed reactions comprise the O3 catalytic decomposition  

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One of the key distinguishing features between the chemistry of the troposphere and that of the stratosphere is that ultraviolet photons sufficiently energetic to break an oxygen-oxygen bond are present in the stratosphere but not in the troposphere. As first noted by William J. Humphreys in 1910, this photodissociation of stratospheric oxygen molecules leads to the generation of ozone and, as a result, much higher concentrations of ozone are found in the stratosphere than in the troposphere. 

Figure 5.2. Stratospheric chemical cycle affecting odd oxygen species in the stratosphere. The numbers in boxes represent concentrations (cm-3) calculated at 25 km altitude while the numbers associated with the arrows account for the reaction fluxes (cm-3s-1) between different compounds (24 hour global average conditions). Note that the figure extends beyond the simple pure oxygen chemistry case and that NO2 is identified as an odd-oxygen reservoir. (See following sections for more details from Zellner, 1999).

It was first observed by Thiemens et al.4 that stratospheric C02 possesses a large and variable mass-independent isotopic composition. This composition was suggested as deriving from isotopic exchange with Of11)), the product of ozone photolysis.5 6 As later confirmed by rocket-borne collection of stratospheric and mesospheric air, this unique isotopic signature provides an ideal tracer of odd oxygen chemistry of the Earth s upper atmosphere, one of the most important upper atmospheric processes. There are, however, several features that require further measurement (laboratory and atmospheric) and theoretical considerations. 

The chemistry of the stratospheric ozone will be sketched with a very broad brush in order to illustrate some of the characteristics of catalytic reactions. A model for the formation of ozone in the atmosphere was proposed by Chapman and may be represented by the following "oxygen only" mechanism (other aspects of... 

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. 

The primary sources that are responsible for the presence of this family of compounds in the atmosphere emit NH3, N20, and NO to the troposphere, the lowest level of the atmosphere, which extends to approximately 10 km from the earth s surface. NH3 seems to undergo very little chemistry in the atmosphere except for the formation of aerosols, including ammonium nitrate and sulfates. NH3 and the aerosols are highly soluble and are thus rapidly removed by precipitation and deposition to surfaces. N20 is unreactive in the troposphere. On a time scale of decades it is transported to the stratosphere, the next higher atmospheric layer, which extends to about 50 km. Here N20 either is photodissociated or reacts with excited oxygen atoms, O (lD). The final products from these processes are primarily unreactive N2 and 02, but about 10% NO is also produced. The product NO is the principal source of reactive oxidized nitrogen species in the stratosphere. 

Thus, when studying atmospheric chemistry, it is necessary always to take into account the vertical and horizontal movements in the atmosphere, as well as the conditions controlling those chemical reactions that do not spontaneously lead to photochemical equilibrium. These conditions are applicable not only to ozone in the lower stratosphere, but also to atomic oxygen in the upper mesosphere above 75 km. In fact, equation (4) shows that, with increasing height, the formation of O3 becomes less and less important because of the decrease in the concentration of 02 and N2. Above 60 km the concentration of atomic oxygen exceeds that of ozone, but it is still in photochemical equilibrium up to 70 km. However, at the mesospause (85 km), it is subject to atmospheric movements, and its local concentration depends more on transport than on the rate of production. 

The importance for stratospheric chemistry of reactions taking place in an atmosphere containing oxygen and hydrogen has been known for about 30 years.6 The presence of atomic hydrogen gives rise to the following reactions ... 

In the 1930s, S. Chapman proposed the following set of reactions to explain the chemistry of oxygen species in the stratosphere ... 

In some respects, the oxygen cycle in the atmosphere is most concerned with ozone chemistry which will be discussed later today. The effect of nitrogen fertilizers on the atmosphere would be most strongly felt through perturbation of stratospheric ozone. The general consensus now seems to be that the effect of nitrogen fertilizers upon the atmosphere will be small compared to the magnitude of other sources. 

Greshake A., Hoppe P., and Bischoff A. (1996) Mineralogy, chemistry, and oxygen isotopes of refractory inclusions from stratospheric interplanetary dust particles and micrometeorites. Meteorit. Planet. Sci. 31, 739-748. 

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 ... 

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. 

The process of dissimilatory dentrification occurs anaerobically and is mediated by bacteria that use nitrate in place of oxygen as an acceptor of electrons during respiration. The result is the formation of molecular nitrogen and nitrous oxide. The nitrous oxide plays a role in the chemistry of stratospheric ozone and is, therefore, extremely important bio-geochemically. These bacteria are heterotrophic and derive energy from the anaerobic oxidation of organic compounds. 

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