Environmental Engineering Reference
In-Depth Information
plumes, which generally increase with distance from the emission source and thus
atmospheric processing time.
These observations and kinetic considerations, as well as atmospheric chemistry
model calculations (e.g. Bobrowski
et al
.,
2007
; Roberts
et al
.,
2009
; von Glasow,
2010
) qualitatively explain BrO formation via self-accelerating, autocatalytic
photo-chemical reactions involving multi-phase chemistry. The temporal evolution
of BrO in the plume is, however, probably in
uenced by the bromine speciation at
the vent and possibly by the presence of other trace species (e.g. NO
x
). Interest-
ingly, the currently assumed bromide to BrO conversion mechanism in volcanic
plumes was found to be the same as the
bromine explosion
'
mechanism (Platt and
Lehrer,
1997
) occurring in polar regions (see, e.g., von Glasow and Crutzen,
2007
). A
'
first step is the uptake of gas phase HOBr and HBr into aerosol particles:
HOBr
(gas)
!
HOBr
(aq)
(8.1)
Br
(aq)
H
þ
(aq)
!
!
þ
HBr
(gas)
HBr
(aq)
(8.2)
and a pH-dependent reaction in the aqueous phase:
Br
-
(aq)
þ
H
þ
(aq)
HOBr
(aq)
þ
!
Br
2 (aq)
þ
H
2
O
(8.3)
In a volcanic plume, acidic aerosol particles (e.g. sulfuric acid aerosol) are
abundant making
reaction (8.3)
very ef
cient. Subsequently Br
2
is released in
the gas phase, where it is rapidly photolysed during daytime:
Br
2
þ
h
ν !
2Br
(8.4)
The resulting Br radicals react with O
3
producing BrO:
Br
þ
O
3
!
BrO
þ
O
2
(8.5)
rmed by observations of the
plume composition. In addition, the presence of BrO in the plume leads to rapid
ozone destruction via self reaction:
The BrO resulting from
reaction (8.5)
has been con
BrO
þ
BrO
!
2Br
þ
O
2
(8.6a)
BrO
þ
BrO
!
Br
2
þ
O
2
(8.6b)
The key ozone destruction steps in the reaction scheme above are
reactions (8.6a)
(and
reaction (8.6b)
followed by
reaction (8.4)
) and
reaction (8.5)
. In addition, the
reaction of BrO with free radicals (HO
2
) leads to the formation of HOBr:
BrO
þ
HO
2
!
HOBr
þ
O
2
(8.7)
The required atmospheric oxidants O
3
and HO
2
will be mixed into the plume
while it expands into the ambient atmosphere (HO
2
may be partly produced in the
