Stratospheric ozone depletion

Therefore even ozone is a very important player in stratospheric chemistry, it is a mistake to consider this species as a major component of this part of the atmosphere. In reality, the major species in terms of numbers of molecules are still nitrogen (78%), oxygen (21%), and argon (1%). 

Each oxygen atom can then combine with an oxygen molecule to form ozone  

Ozone is then destroyed when it reacts with some other compound, e.g., with NO  

Addition of either chlorine or bromine atoms leads to extra, and very efficient, pathways for ozone destruction. The free chlorine (or bromine) atom reacts with ozone, and the product of that reaction removes a free oxygen atom  

Removing a free oxygen atoms also reduce ozone since one less ozone molecule will be formed via reaction 17.4.2. Thus, the chlorine atom reactions effectively remove two ozone molecules by destroying one and preventing the formation of another. Additionally, the original chlorine atom is regenerated to catalytically destroy more ozone. This reaction cycle can proceed thousands of times, destroying up to 100,000 molecules of O3 before the chlorine is removed from the system (e.g., by the formation of HCl). 

Reduction of ozone is greatly enhanced over the poles by a combination of extremely low temperatures, decreased transport and mixing, and the presence of polar stratospheric clouds that provide heterogeneous chemical pathways for the regeneration of atomic chlorine. The resulting rate of O3 destruction is much greater than the rate at which it can be naturally replenished. 

In the stratosphere, molecular oxygen, O2, absorbs solar radiation of wavelengths shorter than 242 nm and dissociates into two oxygen atoms. These oxygen atoms combine with two oxygen molecules to form two ozone molecules as follows  

This formation mechanism is quite different from that described previously for the troposphere and summarized in cycles Cl and C2 of Box 5.2. Whereas oxides of nitrogen promote ozone formation in the troposphere, in the stratosphere, where the chemical composition and light spectmm are quite different, the effect of oxides of nitrogen is to catalyze ozone destmction via the reactions  

It is now recognized that this cycle is the principal means by which ozone is limited in the natural stratosphere [48]. Also, whereas ozone is an undesirable pollutant in the troposphere, in the stratosphere ozone performs the necessary function of shielding the earth s surface from biologically damaging ultraviolet radiation. 

Whitten et al. [2] considered total bomb yields in the range of 5000-10,000 Mt. They distributed the weapon yields either equally between 1 and 5-Mt weapons or equally between 1 and 3-Mt weapons. They also considered that the NOx was either uniformly distributed throughout the Northern Hemisphere or spread uniformly between 30° and 70°N. Maximum depletion of the ozone column occurred two to three months following the NOx injection and ranged from 35-70 %. The 35 % 

The NAS report [1] reaches similar conclusions. A 10,000 Mt war, confined to the Northern Hemisphere, is projected to result in a 30-70 % ozone column reduction in the Northern Hemisphere and a 20-40 % reduction in the Southern Hemisphere. Again, the characteristic recovery time was found to be approximately 3 years. Within 10 years the ozone column depletions were estimated to have decreased to 1-2 %. 

The Antarctic ozone hole is one of the most dramatic indications of anthropogenic environmental change. Depletion of stratospheric ozone via the catalytic mechanisms described in Chapters 5, 10, and 12 was first detected in a surprising fashion by British observers in Antarctica (Farman et al., 1985). They measured increases of springtime solar UV radiation penetrating the atmosphere at wavelengths that normally are absorbed by O3. The results showed almost a factor of 2 depletion 

The chemical reactions in the oxygen-only mechanism (see Sections 5.3 and 10.4) substantially underestimate the ozone destruction rate  

Crutzen (1971) and Molina and Rowland (1974) showed that a second class of catalytic processes exist that result in the destruction of ozone  

CFC-12 (CCI2F2) is the main known catalyst currently acting in the so-called ozone hole in the Antarctic spring. 

It now appears that both the extreme magnitude and geographic limitations of the Antarctic ozone depletion are due to meteorological patterns peculiar to the South Pole. The decrease, which exceeds the small reduction in the rest of the stratosphere apparently involves the circulation of the polar vortex, a complex interaction of Cl with oxides of nitrogen, their physical trapping in extremely cold (T — 80° C) clouds, and preferential removal of some species by precipitation. 

The energy of a photon of electromagnetic radiation is given by Planck s equation  

Oxygen atoms produced by the ultraviolet dissociation of can take part in several reactions that create and destroy ozone. One set of reactions involving atomic and molecular oxygen is known as the Chapman reactions. The reactions are named after Sydney Chapman (1888-1970) who first proposed them in 1930. In one reaction, an oxygen atom combines with O to form ozone. Alternately, it can recombine with another oxygen atom to produce an oxygen molecule or react with ozone to produce two oxygen molecules. These reactions are summarized as 

Ozone formed in the stratosphere is unstable. It can be broken down by ultraviolet light shorter than 320 nm (UV-C and UV-B) according to the reaction  

The XO compound formed in this reaction then reacts with oxygen to regenerate X and molecular oxygen  

The regenerated S can destroy another O3 molecule, and the process can continue indefinitely. Thus a single atom or molecule can destroy many ozone molecules. 

Robert j. Charlson, Cordon H. Orians, and Cordon V. Wolfe 

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Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

The other global environmental problem, stratospheric ozone depletion, was less controversial and more imminent. The U.S. Senate Committee Report supporting the Clean Air Act Amendments of 1990 states, Destruction of the ozone layer is caused primarily by the release into the atmosphere of chlorofluorocarbons (CFCs) and similar manufactured substances—persistent chemicals that rise into the stratosphere where they catalyze the destruction of stratospheric ozone. A decrease in stratospheric ozone will allow more ultraviolet (UV) radiation to reach Earth, resulting in increased rates of disease in humans, including increased incidence of skin cancer, cataracts, and, potentially, suppression of the immune system. Increased UV radiation has also been shown to damage crops and marine resources."... 

An important effect of air pollution on the atmosphere is change in spectral transmission. The spectral regions of greatest concern are the ultraviolet and the visible. Changes in ultraviolet radiation have demonstrable adverse effects e.g., a decrease in the stratospheric ozone layer permits harmful UV radiation to penetrate to the surface of the earth. Excessive exposure to UV radiation results in increases in skin cancer and cataracts. The worldwide effort to reduce the release of stratospheric ozone-depleting chemicals such as chlorofluorocarbons is directed toward reducing this increased risk of skin cancer and cataracts for future generations. 

Chlororocarbon (CFG) refrigerants are inherently safer with respect to fire, explosion, and acute toxic hazards when compared to alternative refrigerants such as ammonia, propane, and sulfur dioxide. However, they are believed to cause long term environmental damage because of stratospheric ozone depletion. 

Deals with issues that affect the quality of our air and protection from exposure to harmful radiation. OAR de >el-ops national programs, technical policies, and regulations for controlling air pollution and radiation exposure. Areas of concern to OAR include indoor and outdoor air quality, stationaiy and mobile sources of air pollution, radon, acid rain, stratospheric ozone depletion, radiation protection, and pollution prevention. 

Problems that rank relatively high in duee of the four typos, or at least medium in all four, include criteria air pollutiuits, stratospheric ozone depletion, pesticide residues on food, and other pesticide risks (runoff and air deposition of pesticides)... 

Recognition of the threat of stratospheric ozone depletion posed by chlorofluorocarbons and chloro-fltiorohydrocarbons led 131 countries to sign the Montreal Protocol in 1987. Production of chlorofluorocarbons was banned as of January 1, 1996, because of their potential to further deplete stratospheric ozone. Chlorofluorohydrocarboiis will be... 

Certainly, photochemical air pollution is not merely a local problem. Indeed, spread of anthropogenic smog plumes away from urban centers results in regional scale oxidant problems, such as found in the NE United States and many southern States. Ozone production has also been connected with biomass burning in the tropics (79,80,81). Transport of large-scale tropospheric ozone plumes over large distances has been documented from satellite measurements of total atmospheric ozone (82,83,84), originally taken to study stratospheric ozone depletion. 

In summary, biomass burning is a major source of many trace gasses, especially the emissions of CO2, CH4, NMHC, NO,, HCN, CH3 CN, and CH3 Cl (73). In the tropics, these emissions lead to local increases in the production of O3. Biomass burning may also be responsible for as much as one-third of the total ozone produced in the troposphere (74). However, CH3 Cl from biomass burning is a significant source for active Cl in the stratosphere and plays a significant role in stratospheric ozone depletion (73). 

Global Global climate change Stratospheric ozone depletion... 

F. S. Rowland. Stratospheric Ozone Depletion by Chlorofluorocarbons. 1995 Nobel Lecture. Angewandte Chemie. International English edition. 35 (Sept. 6, 1996) 1786-1798. 

Whisnant et al. has developed an online project in computation chemistry that can be used to investigate the role of Cl20 in stratospheric ozone depletion. 

This is probably the key heterogeneous reaction for Antarctic stratospheric ozone depletion, and serves as a useful focus for the discussion of the theoretical challenges that must be addressed in dealing with fairly complex chemistry in a complex environment, challenges enlivened — as will be seen below — by the evident chemical involvement of the ice surface environment. 

We conclude by illustrating how our understanding of these chemical processes in our clean and polluted troposphere and stratosphere plays a crucial role in generating the exposure portions of scientific health risk assessments. Such assessments provide the foundation for sound, health-protective and cost-effective strategies for the control of tropospheric ozone, particles, acids, and a spectrum of hazardous air pollutants (including carcinogens and pesticides)—as well as for the mitigation of stratospheric ozone depletion. 

Because of the gaseous nature of many of the important primary and secondary pollutants, the emphasis in kinetic studies of atmospheric reactions historically has been on gas-phase systems. However, it is now clear that reactions that occur in the liquid phase and on the surfaces of solids and liquids play important roles in such problems as stratospheric ozone depletion (Chapters 12 and 13), acid rain, and fogs (Chapters 7 and 8) and in the growth and properties of aerosol particles (Chapter 9). We therefore briefly discuss reaction kinetics in solution in this section and heterogeneous kinetics in Section E. 

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