Chapman mechanism

Ozone formation occurs in the stratosphere above 30km altitude, where solar UV radiation of wavelengths less than 242 nm slowly dissociates molecular oxygen  

The oxygen atoms react with 02 in the presence of a third molecule M (N2 or O2) to produce O3  

Reaction 2 is, for all practical purposes, the only reaction that produces ozone in the atmosphere. The O3 molecule formed in that reaction itself strongly absorbs radiation in the wavelength range of 240-320 nm (recall Chapter 4) to decompose back to 02 and O 

Consequently, 0(1D) is effectively converted instantaeously to ground-state O, and the photodissociation of 03 by both reactions 3 and 3 (above) can be considered to produce entirely ground-state O atoms. 

The mechanism (reactions 1-4 above) for production of ozone in the stratosphere was proposed by Chapman (1930) and bears his name. 

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Reactions 1 to 4 are known collectively as the Chapman mechanism (first outlined by Sidney Chapman (1) in 1930. They basically explain how ozone can exist in the stratosphere in a dynamic balance it is continuously being produced by the action of solar ultraviolet radiation on oxygen molecules and destroyed by several natural chemical processes in the atmosphere. 

The rate of photolysis, J, depends on the absorption cross-section, a, the number density, the scale height and the angle, all of which are unique properties of a planetary atmosphere. For the Earth and the Chapman mechanism for ozone the O3 concentration maximum is 5 x 1012 molecules cm-3 and this occurs at 25 km, shown in Figure 7.12, and forms the Chapman layer structure. 

The reality of using thermodynamics is somewhat simpler than the preceding derivations imply. Consider the reaction we have been using in the formation of ozone in the Chapman mechanism ... 

Although the Chapman mechanism models the general shape of the profile of stratospheric ozone correctly, it seriously overestimates its concentration. To account for this discrepancy, a catalytic process which increases the rate of step 4 occurs (Figure 7.12). Trace amounts of radicals present in the stratosphere reduce natural ozone levels below those predicted by the Chapman scheme ... 

Reactions 2-167 to 2-170 constitute the Chapman mechanism for the creation and destruction of ozone in the unpolluted stratosphere. 

Based on the Chapman mechanism, Pitts (1983) proposed that 6-N02-BaP, with two peri hydrogens and the N02 out of plane, should be less stable photo-chemically than the 1- and 3-NOz isomers with only one peri hydrogen. This proved to be the case. Thus, in solution-phase irradiations of these isomers, 6-NOz-BaP decomposed rapidly whereas the 1- and 3-isomers were much more stable (Zielinska, 1985). A key question then was whether or not these results could be extrapolated to give their relative photodecomposition rates when irradiated as particle-bound species on the surfaces of primary combustion products and ambient aerosols (vide infra see also Feilberg and Nielsen, 1999b). 

Subsequently, Benson (1985) reported 1-nitropyrene deposited on glass photodecomposed in sunlight with a half-life of 14 h. The reaction was accompanied by loss of the nitro group, formation of a phenolic derivative and possibly quinones, and a significant reduction in mutagenicity, consistent with the Chapman mechanism and previous results on nitro-BaP isomers (Finlayson-Pitts and Pitts, 1986). 

Figure 6. Schematic of the major gas-phase cycles in the stratosphere. The Chapman mechanism (oxygen reactions) is indicated by the top four arrows.

These four reactions constitute the Chapman mechanism for establishing the abundance of ozone in the stratosphere. However, the abundance of ozone is dictated also by other loss mechanisms that mimic reaction 4. 

The Chapman mechanism. The mechanism of ozone formation and destruction in the stratosphere was first formulated by Chapman (205) in 1930. He did not consider the effects of minor constituents and physical transport processes that have since been recognized as important factors to explain the discrepancy between the calculated results and the actual observation. According to his mechanism, ozone is formed by the photolysis... 

Deviation from the Chapman mechanism. It was recognized by Nicolet (740) that the observed Oa concentrations were much less than the calculated values even near the stratopause where the physical transport processes are not important (see Fig. VIII 10). He suggested that to explain the observed ozone concentration, the effective value of k44, the rate constant for the destruction of Oj, must be much larger than that given in (VIII-44g) for a pure 02- N2 atmosphere. 

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). 

On the simplest possible level, the ozone in the stratosphere is maintained by the Chapman mechanism, (13), (16), (22-24) plus the catalytic cycles which in cryptic form are shown as (25)-(30)... 

The end result of the Chapman mechanism, and of the modified Chapman processes as elucidated by later investigators" "" is that sodium and other metals (if Chapman-like mechanisms hold for the other metals) are abundant in atomic form. The kinetic studies show that molecular compounds are not present in large quantities above 85 km. Further work " showed that following the formation of dense atomic trails during the vaporization process, molecular recombination in the wake occurs to form smoke or dust that can then act as a delayed source of sodium (and other) atoms. It was then suggested that NaO reacts with atmospheric H2O to form gaseous NaOH, with the latter reacting with atmospheric CO2 in a three-body reaction to form NaHCOs" ... 

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

Laboratory experience had convinced chemists earlier that the Chapman mechanism needed a supplement of additional reactions. In 1960, McGrath and Norrish discovered the formation of OH radicals in the reaction of water vapor with 0( D) atoms generated by the photolysis of ozone, and they proposed a chain decomposition of ozone by water radicals. Meinel (1950) had previously demonstrated the existence of OH in the upper... 

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. 

Above the troposphere, in the stratosphere and mesosphere, the gas density becomes increasingly more pronounced, and the thermal conditions become subject to complex variations, both temporal and spatial. At a height of 25-35 km the oxygen molecules are split by the influence of solar UV radiation to form ozone in accordance with the Chapman mechanism, which is responsible for tbe absorption of 97% of harmful ultraviolet solarradiation. Devoidof this shield, life on the terrestrial surface would have been doomed to extinction (see Chapter 6). 

The principal reactions in the photochemical process of formation and destruction of ozone are reactions (l)-(4), collectively known as the Chapman mechanisms... 

Describe the Chapman mechanism for ozone formation-destrnction processes in the stratosphere. 

Does the Chapman mechanism present the correct description of ozone chemistry in the stratosphere and what are the limitations of this mechanism ... 

Again, we caution that rg3 is not the overall lifetime of an 03 molecule rather, this quantity is the characteristic time required for the Chapman mechanism to achieve a steady-state balance after some perturbation. When rg3 is short relative to other... 

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