Ozone-depletion potential

The flux of ozone from the stratosphere to the troposphere is taken as that across the 380 K potential temperature surface at mid and high latitudes. This ozone is of stratospheric origin, because the flux of air from the upper tropical troposphere into the lowermost stratosphere contains very little ozone. The ozone flux from the stratosphere to the troposphere is estimated by general circulation models. 

The stratospheric ozone-depleting potential of a compound emitted at the Earth s surface depends on how much of it is destroyed in the troposphere before it gets to the stratosphere, the altitude at which it is broken down in the stratosphere, and chemistry subsequent to its dissociation. Halocarbons containing hydrogen in place of halogens or containing double bonds are susceptible to attack by OH in the troposphere. (We will consider the mechanisms of such reactions in Chapter 6.) The more effective the tropospheric removal processes, the less of the compound that will survive to reach the stratosphere. Once halocarbons reach the stratosphere their relative importance in ozone depletion depends on the altitude at which they are photolyzed and the distribution of halogen atoms, Cl, Br, and F, contained within the molecule. 

The effect of a compound on stratospheric ozone depletion is usually assessed by simulating transport and diffusion, chemistry, and photochemistry in a one-dimensional vertical column of air from the Earth s surface to the top of the stratosphere at a given 

TABLE 5.3 Steady-State Ozone Depletion Potentials (ODPs) Derived from Two-Dimensional Models  

1a The rate coefficients for 0( D)- -03— 202 and O + O3— 202 are given below. In the middle stratosphere, at about 30 km, T = 230 K, typical noontime concentrations of 0( D) and O are 50 molecules cm-3 and 7.5 x 107 molecules cm 3, respectively. Evaluate the rates of both 03 loss processes at this altitude. Which loss process dominates  

Chlorofluorocarbons (CFC)s had been used as blowing agents for rigid, closed cell insulation foams for many years because they offer outstanding fire resistance in addition to good thermal insulation, since the CFCs are non flammable. Flowever, CFCs have been phased out because they are said to be detrimental to the ozone layer (10). 

Hydrochlorofluorocarbons (FICFC)s, such as 1,1-dichloro-l-flu-oroethane with low ozone depletion potential have been alternatives for CFCs. Flowever, FlCFCs are also being phased out under the Montreal Protocol. The Montreal Protocol on Substances That Deplete the Ozone Layer is an international treaty designed in order to protect the ozone layer by phasing out the production of a number of substances that are believed to bear an ozone depletion potential. 

The use of single component fluids or azeotropic mixtures, which are mixtures that do not fractionate on boiling and evaporation, is desirable for FlFC s. 

Finding suitable substitutes for FICFC is complex, as any substitute should possess certain properties, such as chemical stability, low toxicity, inflammability, as well as the same efficiency of the re- 

72iIT/(controiiable) — ( R134a — alternative) ( R134 alternative) (9.2) 

Source Goodbane, M. SAE Paper 1999-01-084, 1999. With permission. a Kilograms of C02 per year. 

Stratospheric ozone depletion by CFCs is shown in the following equations  

the chlorine atoms act cataly tically and reduce the natural balance of02/03 in the atmosphere. Each chlorine atom can react with as many as 1,000 ozone molecules. 

---

Compound CAS Registry Number Relative ozone depletion potential... 

Perfluorinated ethers and perfluorinated tertiary amines do not contribute to the formation of ground level ozone and are exempt from VOC regulations (32). The commercial compounds discussed above have an ozone depletion potential of zero because they do not contain either chlorine or bromine which take part in catalytic cycles that destroy stratospheric ozone (33). 

The cap is the percentage of the calculated level of chlorofluorocarbons consumed in the base year plus the calculated level of hydrofluorocarbons consumed the same base year It applies only to HCFCs Calculated in the context means that the amount of each substance is adjusted by its ozone depletion potential (ODP), a measure of its potential to deplete stratosphenc ozone relative to that of CFC 11... 

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. 

Ozone Depletion Potential (ODP) Global Warming Potential (GWP) ... 

Table 1 lA presents tabulations of the safety of important refrigerants, but this list does not include aU available refrigerants. Table 11-5 summarizes a limited list of comparative hazards to life of refrigerant gas and vapor. The current more applicable refrigerants from the m or manufacturers of the CFC and HCFC refrigerants and their azeotropes/ blends/mrxtures are included, but the list excludes the pure hydrocarbons such as propane, chlorinated hydrocarbons such as methyl chloride and others, inorganics, ammonia, carbon dioxide, etc. See Table 11-6. The CFC compounds have a longer and more serious ozone depletion potential than the HCFC compounds, because these decompose at a much lower atmospheric level and have relatively short atmospheric lifetimes therefore, they do less damage to the ozone layer. Table 11-7 summarizes alternate refrigerants of the same classes as discussed previously. Table 11-8 correlates DuPont s SUVA refrigerant numbers to the corresponding ASHRAE numbers.

SUVA 9000 (R-407C) HCFC-22 An equivalent pressure replacement for HCFC-22 with 0 ozone depletion potential for use in commercial and residential air conditioners and heat pumps. Suva 9000 provides the closest match to HCFC-22 performance in existing HCFC equipment design. 

Ozone Depletion Potential. This is calculated in a similar manner to global warming potential and is expressed relative to CFC-11. Factors for all common gases having significant effects on the ozone layer have been calculated. 

Haseley [2.4] differentiates between two categories of refrigerants The first group can be exchanged in compressor systems without changing the compressor itself, but changing the injection valves. The second group with no Ozone Depletion Potential (ODP). 

Methyl bromide, 4 345-347 12 62 chiral derivatizing agent, 6 96t ozone depleting potential, l 809t... 

Propellant 225cb, ozone depleting potential, l 809t... 

Hydrochlorofluorocarbon-141b, or 1,1-dichloro-l-fluoroethane (HCFC141b), has been developed as a replacement for fully halogenated chlorofluorocarbons because its residence time in the atmosphere is shorter, and its ozone depleting potential is lower than that of presently used chlorofluoro... 

It is desired to find a replacement for Freon-12 as an alternative refrigerant [Duvedi Achenie (1996)]. The objective is to find replacements that have the same refrigerant related properties as Freon-12 but without the harmfull environmental properties (such as the Ozone depletion potential) - The new refrigerant must absorb heat at temperatures as low as -1.1°C and reject heat at temperatures as high as 313 K. It must also respect the following characteristics ... 

Kayukawa et al. [17] studied the PVT properties of triiluoromethyl methyl ether, because it is a possible refrigerant with zero ozone depletion potential and low global-warming potential. One series of their data is shown in Table 5.6. Calculate Z, the compressibihty factor, and the molar volume in mol m from the given data, and fit the data for Z as a function of 1 /Pm to both a linear and a quadratic equation to see whether a third virial coefficient is warranted by the data. 

Nimitz, J.S. and Skaggs, S.R. Estimating tropospheric lifetimes and ozone depletion potentials of one- and two-carbon hydrofluorocarbons and hydrochlorofluorocarbons, fJnvzron. Sci. TechnoL, 26(4) 739-744, 1992. 

---