Environmental Engineering Reference
In-Depth Information
Global estimates of contemporary volcanic halogen emissions were made by, e.g.,
Pyle and Mather (
2009
) who state that arc-related emissions dominate the global
halogen degassing budget with 4.3 (
0.2) Tg/a for HF and
5
-
15 Gg/a for HBr; other authors find somewhat higher values (e.g. Shinohara,
2013
). Data on the global volcanic iodine (hydrogen iodide, HI) flux are very
uncertain; estimates are in the range of 0.5
1) Tg/a for HCl, 0.5 (
-
2 Gg/a (Pyle and Mather,
2009
),
0.2
7.7 Gg/a (Snyder and Fehn,
2002
) or 0.11 Gg/a (Aiuppa
et al
.,
2005
). In
the geological past there were periods for which much higher volcanic halogen
emissions are estimated, e.g. during Siberian Traps volcanism (see also
Chapter 20
).
-
8.2.2 Chemical speciation of halogens in volcanic plumes
Halogens released to the atmosphere by volcanoes are thought to be predominantly
degassed as hydrogen halides (see, e.g., Symonds
et al
.,
1994
; Schwandner
et al
.,
2004
; Carroll and Holloway,
1994
), which should be thermodynamically favoured
over other halogen species, as suggested by equilibrium model data (e.g. Gerlach,
2004
; Martin
et al
.,
2006
,
2008
). Nevertheless, there may be some conversion to
molecular halogens, halogen atoms and even halogen oxides once the plume
begins to mix with air and is still at high temperature (in the
source region
'
).
However, measurements and laboratory kinetic data show that the transformation
'
-
at least of HBr to other bromine molecules
-
can be relatively fast in dilute plumes.
Already at plume ages
2 minutes the contribution of HBr to total bromine might
fall below 50% (von Glasow,
2010
). Also, recent measurements of ClO and OClO
indicate oxidation of chloride in the plume.
In comparison to bromine and chlorine measurements, volcanic iodine chemis-
try has been given even less consideration, owing to difficulties in detection. Iodine
is probably also released as HI (e.g. Honda,
1970
), but no thermodynamic model
studies exist. Volcanic iodine could eventually form the reactive halogen species
IO that affects plume and atmospheric chemistry. However, to date iodine oxides
have not been detected in volcanic plumes.
<
8.2.3 Chemical transformation in plumes
In the following we use the term
reactive halogen species
'
(RHS), for X, X
2
, XO,
OXO, HOX, XNO
x
where X denotes chlorine, bromine or iodine atoms. Thermo-
dynamic (equilibrium) models of the source region only predict that a very small
fraction of Br is initially present as BrO, followed by a period of bromide to BrO
conversion. Both predictions are supported by a series of studies: Oppenheimer
et al
. (2006) and Bobrowski
et al
.(
2007
) found initially low BrO/SO
2
ratios in
'
