Environmental Engineering Reference
In-Depth Information
source region). It is likely that the rate-limiting step of the BrO formation is the
mixing of O
3
and free radicals into the plume across the
'
boundary
'
of the plume to
'
the ambient
background
'
atmosphere. The plume diameter grows according to the
square root of the plume age (for constant eddy diffusivity) and therefore the plume
surface area grows proportionally to the plume age. Oxidation of halogenides
should be more effective at the plume edge, which is indeed seen in the measure-
ments of Bobrowski
et al
.(
2007
), Louban
et al
.(
2009
) and General
et al
.(
2014
).
Chlorine oxides (ClO, OClO) and Cl
2
have also been observed in volcanic
plumes and fumaroles, though only on few occasions
e.g. Sakurajima, Japan (Lee
et al
.,
2014
), Puyehue
-
Cordón Caulle, Chile (Theys
et al
.,
2014
) and at the
Tolbachik volcanic complex, Russia (Zelenski and Taran,
2012
). It should be
noted that the direct detection of ClO by UV spectroscopy (DOAS, see
Section
8.3.2
, below) is dif
-
cult and the abundance of this species may have been consid-
erably overestimated. In fact, model simulations by Bobrowski
et al
.(
2007
) and
Roberts
et al
.(
2009
) found ClO levels about a factor of 40 lower than the early
ClO measurements. Recent DOAS measurements of OClO, which is in a photo-
stationary state with ClO via:
BrO
þ
ClO
!
OClO
þ
Br
(8.8)
and:
þ
ν !
þ
OClO
h
ClO
O
(8.9)
do indicate much lower ClO levels, which are more in line with the model
is very different from that of bromine. As (in contrast to Br atoms) Cl atoms react
with ubiquitous CH
4
(forming HCl) there is no
chlorine explosion
'
; thus, most of
the emitted HCl stays in the gas phase. This difference in the reactivity of hydrogen
halides is probably also the reason why the ratio of HBr/SO
2
dramatically
decreases downwind of volcanoes, whereas the ratio HCl/SO
2
appears to stay
fairly constant (e.g. Voigt
et al
.,
2014
).
'
uence of volcanic reactive halogens on the atmosphere
From the above it becomes clear that volcanic plumes are an interesting environ-
ment for homogeneous and heterogeneous chemical processes, which in particular
modify the in
8.2.4 In
uence of volcanic gases on the atmosphere. For instance, the
conversion of (very water-soluble) hydrogen halides to (much less soluble) halo-
gen oxides probably leads to longer atmospheric residence times of RHS and thus
wider spatial distribution of these species.
