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18 What is an "Ozone Depletion Potential?"


This article is from the Ozone Depletion FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.

18 What is an "Ozone Depletion Potential?"

The ozone depletion potential (ODP) of a compound is a simple measure of
its ability to destroy stratospheric ozone. It is a relative measure:
the ODP of CFC-11 is defined to be 1.0, and the ODP's of other compounds
are calculated with respect to this reference point. Thus a compound with
an ODP of 0.2 is, roughly speaking, one-fifth as "bad" as CFC-11.

More precisely, the ODP of a compound "x" is defined as the ratio of
the total amount of ozone destroyed by a fixed amount of compound x to
the amount of ozone destroyed by the same mass of CFC-11:

                        Global loss of Ozone due to x
        ODP(x) ==       ---------------------------------
                        Global loss of ozone due to CFC-11.

Thus the ODP of CFC-11 is 1.0 by definition. The right-hand side of
the equation is calculated by combining information from laboratory
and field measurements with atmospheric chemistry and tranport models.
Since the ODP is a relative measure, it is fairly "robust", not overly
sensitive to changes in the input data or to the details of the model
calculations. That is, there are many uncertainties in calculating the
numerator or the denominator of the expression, but most of these
cancel out when the ratio is calculated.

The ODP of a compound will be affected by:

The nature of the halogen (bromine-containing halocarbons usually
have much higher ODPs than chlorocarbons, because atom for atom Br
is a more effective ozone-destruction catalyst than Cl.)

The number of chlorine or bromine atoms in a molecule.

Molecular Mass (since ODP is defined by comparing equal masses
rather than equal numbers of moles.)

Atmospheric lifetime (CH3CCl3 has a lower ODP than CFC-11, because
much of the CH3CCl3 is destroyed in the troposphere.)

The ODP as defined above is a steady-state or long-term property. As
such it can be misleading when one considers the possible effects of CFC
replacements. Many of the proposed replacements have short atmospheric
lifetimes, which in general is good; however, if a compound has a short
_stratospheric_ lifetime, it will release its chlorine or bromine atoms
more quickly than a compound with a longer stratospheric lifetime. Thus
the short term effect of such a compound on the ozone layer is larger
than would be predicted from the ODP alone (and the long-term effect
correspondingly smaller.)(The ideal combination would be a short
tropospheric lifetime, since those molecules which are destroyed in the
troposphere don't get a chance to destroy any stratospheric ozone,
combined with a long stratospheric lifetime.) To get around this, the
concept of a Time-Dependent Ozone Depletion Potential has been
introduced [Solomon and Albritton] [WMO 1991]:

                 Loss of ozone due to X over time period T
 ODP(x,T) ==     ----------------------------------------------
                 Loss of ozone due to CFC-11 over time period T

As T->infinity, this converges to the steady-state ODP defined previously.

The following table lists time-dependent and steady-state ODP's for
a few halocarbons [Solomon and Albritton] [WMO 1991]

Compound               Formula         Ozone Depletion Potential  
                                     10 yr  30 yr   100 yr   Steady State
CFC-113               CF2ClCFCl2      0.56    0.62    0.78    1.10
carbon tetrachloride  CCl4            1.25    1.22    1.14    1.08
methyl chloroform     CH3CCl3         0.75    0.32    0.15    0.12
HCFC-22               CHF2Cl          0.17    0.12    0.07    0.05
Halon - 1301          CF3Br          10.4    10.7    11.5    12.5


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