Ozone stratospheric chemistry

The amount of ozone in the stratosphere is determined by a dynamic balance between the processes of production and destruction. Ozone is formed by a two stage process that begins with photodissociation of oxygen by wavelengths 242 nm 

Ozone loss occurs through a number of different reactions 

The catalytic cycles involving M leave M unchanged and able to go around the cycle many more times destroying further ozone molecules. The most important molecules that can take the place of M are OH, NO and Cl, though there are other examples and more complex chemical reactions that can also destroy ozone. The active species, M, are lost through conversion into less active species 

The production and destruction processes above occur naturally to produce the dynamic photochemical equilibrium of ozone distribution described earlier. This equilibrium has been upset by the addition of extra species of M to the stratosphere, most notably chlorine (Cl), from human sources. 

In the stratosphere, ozone is produced when ultraviolet light with wavelengths between 200 and 240 nm breaks the bond between two oxygen atoms 

In turn, the highly reactive oxygen atom reacts with other molecules of oxygen—as in Eq. [4-34]—to form 03  

in turn, can be destroyed by interaction with another photon that breaks it into an oxygen molecule (02) and an oxygen atom (O). Stratospheric ozone also can be destroyed by reaction with other species, such as nitric oxide (NO) — as in Eq. [4-35], and chlorine atoms (from CFCs). The net concentration of ozone is established by the rates of both the production and destruction reactions. 

Stratospheric ozone is produced at maximum rates in equatorial regions, where solar radiation is most intense. Ozone does not really occur as a layer, but instead as a broadly distributed gas whose peak concentration occurs in midstratosphere. The total amount of ozone present in the atmosphere is small, typically between 200 and 400 Dobson units. A Dobson unit is the amount of ozone that, if gathered together in a thin layer covering Earth s surface at a pressure of 1 atm, would occupy a thickness of 1/100 of a millimeter (10 gm). The entire ozone shield, which protects life on Earth from damage by the UV-B radiation of the Sun (ultraviolet radiation in the 280-320 nm range), is equivalent to a layer of ozone only 2 to 4 mm thick at sea level pressure. 

Concern about the diminishment of stratospheric ozone began more than two decades ago. Molina and Rowland (1974) proposed that the release of CFCs through human activity played a major role in 03 depletion. In 1978, the United States banned the use of CFCs as propellants in aerosol sprays. In 1987, the Montreal Protocol on Substances that Deplete the Ozone Layer was established to halt the production by industrialized countries of most 03-destroying CFCs by 2000. The Montreal Protocol was subsequently amended to change the date to 1996 (Ham, 1993). 

---

Understand how photolysis produces radicals by bond cleavage and account for the importance of radical species in photochemical chain reactions, stratospheric ozone chemistry and the photochemistry of the polluted troposphere. 

Recent discussions of stratospheric chemistry have dealt with the effect of freons on ozone balance through a Cl/ClO catalytic destruction of ozone. The fundamental absorption band of CIO is measured to be at 11 /xm. Isotopically substituted CO2 laser based OA absorption measurement technique should allow us to carry out fundamental measurements on CIO and its diurnal variation in the stratosphere to provide yet another important parameter (in addition to NO above) in the stratospheric ozone chemistry. 

For those more inclined to use environmental topics to enrich thermodynamics and kinetics parts of the physical chemistry curriculum, Modeling Stratospheric Ozone Chemistry and the Contrail projects are two examples. 

Water droplets and particulate matter often influence the rates of chemical transformations in the atmosphere. Whereas homogeneous reactions involve only gaseous chemical species in the atmosphere, reactions involving a liquid phase or a solid surface in conjunction with the gas phase are called heterogeneous reactions. Reactions that occur much more rapidly in water than in air may occur primarily in droplets, even though the droplets constitute only a small fraction of the total atmospheric volume. Solid surfaces also can catalyze reactions that would otherwise occur at negligible rates specific examples are discussed in the following sections on acid deposition and stratospheric ozone chemistry. 

The HO, NO, and CIO cycles are all coupled to one another, and their interrelationships strongly govern stratospheric ozone chemistry. The NO and CIO cycles are coupled... 

It turns out that the HO NO, and CIO, cycles are all coupled to one another, and their interrelationships strongly govern stratospheric ozone chemistry. The NO, and CIO, cycles are coupled by reactions 4.34 and 4.35. For example, increased emissions of N2O would lead to increased stratospheric concentrations of NO and hence increased ozone depletion by the NO, catalytic cycle. Likewise, increasing CFC levels will lead to increased ozone depletion by the CIO, cycle. However, increased NO, will lead to an increased level of the CIONO2 reservoir and a mitigation of the chlorine cycle. Thus the net effect on ozone of si-... 

Summary of Midlatitude and Tropical Stratospheric Ozone Chemistry... 

Methane levels in the atmosphere are increasing (see Figure 2.7). In the stratosphere CH4 is involved in stratospheric ozone chemistry in multiple ways. Reaction of CH4 with 0( D) is a source of stratospheric OH, and that oxidation is a. source of a portion of stratospheric H2O, which, itself, is a source of OH through reaction with 0( D). Hydroxyl radi-... 

In view of this, it has been proposed that hydrated electrons generated on the surface of stratospheric ice crystals, via cosmic rays, could contribute to Cl formation via DEA of adsorbed CFCs. " Photodetachment of the chloride ions might then provide a mechanism to generate the Cl radicals that lead to ozone destruction. However, attempts to link these laboratory observations directly to stratospheric ozone chemistry have been strongly criticized, " although modeling does leave open the possibility that, at the very least, HCl destruction on ice crystals might be important for stratospheric chlorine chemistry. More work is evidently needed to resolve this controversy. 

With respect to stratospheric ozone chemistry, I discarded the theory of Hampson and Hunt and concluded ... at least part of the solution of the problem of the ozone distribution might be the introduction of photochemical processes other than those treated here. The influence of nitrogen compounds on the photochemistry of the ozone layer should be investigated. ... 

By combining models of meteorology and ozone, Paul pioneered the field of atmospheric chemistry, and showed how local emissions can have a global effect, even though the substances in question occur in minute, i.e., trace amounts. With his work, that has had an impact well beyond his own field, he followed in the footsteps of pioneers in chemistry in the past centuries such as Scheele, Priestley, Lavoisier, and Laplace. Like Paul, they were also intrigued by the chemical composition of air, what controls it, and tried to unravel its importance for life on Earth. The central role of nitrogen oxides in stratospheric ozone chemistry was the first of Paul s impressive series of discoveries. 

---