Geology Reference
In-Depth Information
Bromine oxide species can be formed in the polar boundary layer
38,39
and areas with high salt levels such as the Dead Sea.
40
The major source
of gas-phase bromine in the lower troposphere is thought to be the
release of species such as IBr, ICl, Br
2
and BrCl from sea-salt aerosol,
following the uptake from the gas phase and subsequent reactions of
hypohalous acids (HOX, where X
¼
Br, Cl, I).
41
HOBr
þ
(Br
)
aq
þ
H
1
-
Br
2
þ
H
2
O
(2.74)
The halogen release mechanism is autocatalytic
38
and has become
known as the ''Bromine explosion''. The Br
2
produced in reaction
(2.74) is rapidly photolysed, producing bromine atoms that can be
oxidised to BrO by O
3
(reaction (2.76))
BrX
þ
hn
-
Br
þ
X
(2.75)
Br
þ
O
3
-
BrO
þ
O
2
(2.76)
the resultant BrO reacting with HO
2
(reaction (2.77)) to reform HOBr.
BrO
þ
HO
2
-
HOBr
þ
O
2
(2.77)
Thus, the complete cycle has the form
surface
;
HO
x
BrO
þ
O
3
þð
Br
Þ
aq
þð
H
þ
Þ
aq
!
2BrO
þ
products
ð
2
:
78
Þ
where effectively one BrO molecule is converted to two by oxidation of
bromide from a suitable surface. Figure 21 shows a schematic represen-
tation of the Bromine-explosion mechanism.
37
It is worth noting that the
bromine explosion mechanism only occurs from sea salt with a pH
o
6.5
therefore requiring acidification of the aerosol potentially caused by the
uptake of strong acids likely to be of anthropogenic origins or naturally
occurring acids.
42,43
The wide-spread occurrence of these heterogeneous
mechanisms are supported by the observed depletion of bromide ions in
sea-salt aerosol.
42,43
Case Study IV - BrO in the springtime Arctic - In the spring time in
both the Arctic and Antarctic large clouds of BrO-enriched air masses
are observable from space (see Figure 22).
44,45
These clouds cover
several thousand square kilometres over the polar sea ice with BrO
levels up to 30 pptv. The BrO is always coincident with low levels of
ozone in the MBL.
38
In order to observe these events, there is a
requirement for meteorological conditions that stop mixing between
