Geology Reference
In-Depth Information
reformation of the ClO
x
reservoir species (e.g. via reaction (2.96)). The
ClO self-reaction ozone destruction cycle is supplemented by a coupled
ClO/BrO cycle.
ClO
þ
BrO
-
Cl
þ
Br
þ
O
2
(2.109)
or
ClO
þ
BrO
!
BrCl
þ
O
2
BrCl
þ
hv
!
Cl
þ
Br
ð
2
:
110
;
2
:
111
Þ
Cl
þ
O
3
-
ClO
þ
O
2
(2.92)
Br
þ
O
3
-
BrO
þ
O
2
(2.76)
The stratospheric bromine comes from a range of source gases including
CH
3
Br.
58
The contribution of bromine to ozone loss in the polar regions
has increased faster than that of chlorine because abundances of bro-
mine continue to increase at a time when those of chlorine are levelling
off (see Figure 27).
52
Cycles (2.105, 2.92, 2.106-2.108) and (2.76, 2.92,
2.109-2.111) account for most of the ozone loss observed in the Ant-
arctic stratosphere in the late winter/spring season. At high ClO abun-
dances, the rate of ozone destruction can reach 2-3% day
1
in late
winter/spring.
Figure 34 shows a summary of the photochemistry and dynamics in
the polar stratosphere, illustrating the time profiles of the key chlorine
species coupled to the temperature requirements in the vortex. Figure 35
shows measurements of a range of chemical species and temperature in
the 2004 ozone hole. The data reflects the main chemical and physical
features on the ozone hole.
In summary, the key features of Antarctic ozone loss are
(i) The circulating winds in the polar regions enable the formation of
a stable vortex which provides a gigantic ''reaction vessel'' for
ozone depletion to occur within.
(ii) The low temperatures in the vortex encourage the formation of
PSCs, which enhance the production of active chlorine species.
(iii) Pre-conditioning of the atmosphere takes place during the polar
winter releasing chlorine from reservoir molecules.
(iv) At sunrise, molecular chlorine is dissociated into free atoms that
can destroy the ozone.
(v) Chlorine atoms are regenerated by the dimer reactions.
