Ozone depletion state

In this sequence the Cl also acts as a catalyst and two molecules are destroyed. It is estimated that before the Cl is finally removed from the atmosphere in 1—2 yr by precipitation, each Cl atom will have destroyed approximately 100,000 molecules (60). The estimated O -depletion potential of some common CFCs, hydrofluorocarbons, HFCs, and hydrochlorofluorocarbons, HCFCs, are presented in Table 10. The O -depletion potential is defined as the ratio of the emission rate of a compound required to produce a steady-state depletion of 1% to the amount of CFC-11 required to produce the 1% depletion. The halons, bromochlorofluorocarbons or bromofluorocarbons that are widely used in fire extinguishers, are also ozone-depleting compounds. Although halon emissions, and thus the atmospheric concentrations, are much lower than the most common CFCs, halons are of concern because they are from three to ten times more destmctive to O, than the CFCs. 

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."... 

Montreal Protocol on Substances that Deplete the Ozone Layer. The phase-out of CFCs began on Jnly 1, 1989, and by 1997, a hydroflnorcarbon, HFC134a, with zero ozone depletion potential, became the dominant refrigerant in the United States. The phase-out of CFCs in developing countries is on a slower schedule. 

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. 

Mario Molina United States, b. Mexico stratospheric ozone depletion... 

The history of ozone depletion took a dramatic turn in 1985 when J. C. Farman at the BAS Halley Bay station announced that ozone levels over the Antarctic had decreased by more than 40 percent between 1977 and 1984. Farman explained that ozone levels had fallen so low that one could say that a "hole had formed in the ozone layer above the South Pole. In 1984, that "hole covered an area of more than 15 million square miles (40 million square kilometers), equal to the size of the continental United States. Clearly, ozone depletion was not a long-term problem about which scientists could debate for the next century or so. It was an issue that demanded quick attention and action. 

The contribution of individual compounds to ozone depletion is characterized by the ozone depletion potential (ODP). The ODP of a compound as normally defined is the ratio of the global loss of ozone (i.e., integrated over latitude, altitude, and time) from that compound at steady state per unit mass emitted relative to the loss of ozone due to emission of unit mass of a reference compound, usually taken as CFC-11 (CFC13) (Wuebbles, 1983 Fisher et al., 1990 Solomon et al., 1992). The ODP thus provides a relative measure of the overall impact of a compound on ozone destruction over the long term. Values for ODPs have been derived using a variety of models (e.g., see World Meteorological Organization, 1995, 1999). 

TABLE 13.3 Atmospheric Lifetimes and Steady-State Ozone Depletion Potentials (ODP) Predicted Using Either a Two-Dimensional Model or a Semiempirical Method b... 

Potential trace gas Atmospheric lifetime (years) Steady-state Model ozone depletion Semiempirical"... 

ODPs and the final, steady-state values can be used to assess the relative importance of various compounds to stratospheric ozone depletion on both short and long time scales. For a more detailed discussion of ODPs, see Ko et al. (1994a) and the World Meteorological Organization Report (1995, 1999). 

Because one of the major impacts of increased UV at the earth s surface is expected to be an increase in skin cancer, several countries now include UV forecasts as part of their weather reports (e.g., see Kerr, 1994), and estimates of skin cancer increases due to ozone depletion have been made. Figure 13.20, for example, shows the estimated number of excess cases of skin cancers for the United States and for northwestern Europe if no controls had been imposed on CFCs and halons and those expected with the Copenhagen Amendments (Slaper et al., 1996). Clearly, there is expected to be a major impact of the control strategies on the incidence of skin cancer, although the number of excess cases will still be about 33,000 per year in the United States and 14,000 per year in northwestern Europe at the projected worst-case year of 2050. 

Trichloroethane was a major solvent, particularly for cold and vapor degreasing. It was phased out for emissive uses in the United States in 1996 because of its ozone depletion potential. The only application left is as chemical precursor for HCFC-141b and HCFC-142b. However, both are subject to phaseout schedule of the Montreal Protocol,... 

At steady-state, as we noted before the downward revision of a (O2) causes the ozone depletion — AOs to diminish, (Table 6) no doubt due to the smaller peak value of ClY as a result of the shorter lifetime of the chlorof luorocarbons. 

---