Environmental Engineering Reference
In-Depth Information
halogens and gaseous mercury. The study of plume chemistry will ultimately
allow a precise assessment of these in
uences.
(3) The composition of volcanic gases gives hints on processes occurring in the Earth
'
s
interior, in particular on magma composition and degassing processes. For as long
as half a century we have known that halogen/sulfur ratios measured in fumaroles
indicate changes in volcanic activity (e.g. Cl/S: Noguchi and Kamiya,
1963
).
Today, advances in technology (see
Section 8.3
), in particular in spectroscopic tech-
niques, allow remote analysis of many species in volcanic plumes (see also
Chapters 7
and
9
) and even continuous monitoring (Galle
et al
.,
2010
). Among other technologies,
differential optical absorption spectroscopy (DOAS) and Fourier-transform infra-
red (FTIR) spectroscopy are popular remote-sensing techniques. The data gained using
these techniques provide new possibilities to investigate volcanic volatile compositions
with high temporal resolution and also during explosive eruptions. Besides relatively
long-lived molecules (such as SO
2
), transient species, in particular halogen radicals
(e.g. bromine monoxide, BrO), can also be routinely detected. Frequently, the
relatively stable gases (e.g. CO
2
,SO
2
) are simultaneously measured as indicators for
dilution of the reactive species by ambient air entrained into the plume.
8.2 Composition and halogen chemistry of volcanic plumes
The chemical composition of volcanic plumes is in
uenced not only by the relative
emission source strengths of the individual species, but is also modi
ed by the chemical
conversion of volcanic gases in the atmosphere to secondary emission products.
For a long time it was assumed that halogens in volcanic plumes would behave
rather passively, being mainly important for the acidity budget of the atmosphere (e.g.
acid rain) and, possibly, for stratospheric chemistry. In general, the chemistry in
volcanic plumes was believed to be restricted to the oxidation of sulfur. This view
changed drastically with the detection of bromine monoxide (BrO) in the plume of
Soufrière Hills (Montserrat) by Bobrowski
et al
.(
2003
) using a passive optical
absorption technique (Multi-Axis
DOAS or MAX-DOAS, see
Section 8.3
). Since
then, BrO has been detected at many other volcanoes, including Ambrym, Masaya,
Mount Etna, Nyiragongo, Stromboli, Villarica, Popocateptl, Kasatochi, and many
et al
.,
2011
;Hörmann
et al
.,
2013
;Kelly
et al
.,
2013
;see
Figure 8.1
). Today chlorine
radicals (chlorine monoxide, ClO; chlorine dioxide, OClO) are also measured by
DOAS and many other halogen species (bromine dioxide, OBrO; iodine monoxide,
IO; iodine dioxide, OIO; molecular bromine, Br
2;
molecular iodine, I
2
)canin
principle be measured by DOAS, which has become a standard measurement tech-
nique for volcanic emissions monitoring. This chapter will focus on bromine emis-
sions and chemistry, including some discussion on chlorine and iodine.
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