Page 259 - Materials Chemistry, Second Edition

4.5 Impact Categories, Impact Indicators and Characterisation Factors 243

bromine) atoms in the stratosphere by photolysis of volatile and persistent sub-
stances with chlorine or bromine as substituents. This definition of the impact
indicator concerns both the ozone depletion in the homogeneous gaseous phase of
the stratosphere predicted by Rowland and Molina (though it caught little public
attention), as well as a spectacular but temporally and spatially limited depletion
which relates to the ozone hole.
An ODP value was introduced for the quantification of a relative scale of
ozone-harmful activities of substances. It literally reads 194) :
The ODP represents the amount of ozone destroyed by emission of a gas over the
entire atmospheric lifetime (i.e. at steady state) relative to that due to emission of
the same mass of CFC-11.
Formally it is similar to the GWP and it is also handled alike. Numerical
values originate from relatively simple model calculations. Values increase with
persistence of a substance in the troposphere (i.e. with growing probability of
entering the stratosphere) and with an increase of chlorine atoms per mass unit in
the stratosphere. In the case of brominated halons an approximate 10-fold catalytic
activity of bromine compared to that of chlorine is included. The reference factor
is the ODP of CFC-11 or R11 which is arbitrarily set to one – in complete analogy
to the GWP of carbon dioxide.
The ODP characterising factors for some important ozone-depleting substances
are shown in Table 4.10.
The highest values (ODP ≫ 1) are due to brominated halons applied as fire-
extinguishing agents. Other perhalogenated carbon compounds (molecules, only
exhibiting chlorine and fluorine, but no H as substituent) figure between ODP = 0.5
and 1.1 R11-equivalents.
Dinitrogenmonoxide (N O) is a well-known ozone-depleting gas with a different
2
impact mechanism: the persistent gas enters the stratosphere and is then converted
by reaction with oxygen atoms into NO (NO + NO ). However this reaction in a
x 2
complicated way depends on the site of the reaction (particularly in the proximity
of the tropopause). 195) More recently, the potent GHG N O has been proposed
2
to become the most important ozone-depleting gas of this century (given the
declining emissions of the CFC). 196) According to this recent paper, N O reacts
2
most efficiently in the mid stratosphere where the highest concentration of ozone
is also found. The removal of O contributes to the general decay (as originally
3
proposed by Roland and Molina) but not to the formation of the yearly ‘ozone hole’.
Ravishankara, Daniel and Portmann (2009, loc. cit.) as well as Wuebbles (2009, loc.
cit.) argue that this sink should not be neglected. Furthermore, an interim ODP for
N O has been calculated as 0.017 kg CFC 11 equivalents per kilogram N O emitted.
2
2

194) WMO (1999).
195) Kl¨ opffer and Meilinger (2001b).
196) Ravishankara, Daniel and Portmann (2009) and Wuebbles (2009).