Environmental Engineering Reference
In-Depth Information
13.3 Volcanic gas emissions
Water vapour (H 2 O) and carbon dioxide (CO 2 ) are the most abundant volatile
species released during a volcanic eruption, but in the short term their effect on
atmospheric composition is negligible because of the insignificant relative contri-
bution to the high atmospheric background concentrations of H 2 O and CO 2 .
However, on the timescale of the age of Earth, volcanic
'
outgassing
'
has been
the source of our current atmosphere (see also Chapter 14 ).
Compared to 35,000 Teragrams (Tg) of anthropogenic CO 2 emissions per year,
estimates of the CO 2
flux from present-day subaerial and submarine volcanism
range between 130 and 440 Tg (Gerlach, 2011 , and references therein). Self et al .
( 2008 ) estimated that the Deccan Traps released up to 14 Tg of CO 2 per km 3 of
lava erupted (assuming a degassing ef
ciency of 80%). For a typical decade-long
CFB eruptive phase producing a total lava volume of 1,000 km 3 this equates
to 1400 Tg of CO 2 per year, which is one order of magnitude smaller than the
current anthropogenic
ux.
Volcanic eruptions also release halogen species (mainly bromine oxide, hydro-
gen chloride, hydrogen bromide and hydrogen
fluoride), which play an important
role in volcanic plume chemistry (von Glasow, 2010 ). Over the industrial
era, anthropogenic chloro
uorocarbon (CFC) emissions have resulted in high
stratospheric chlorine and bromine concentrations. These halogens destroy strato-
spheric O 3 ef
'
'
.
Stratospheric O 3 depletion was also observed after the eruptions of El Chichón
in 1982 and Mount Pinatubo in 1991 because volcanic aerosol particles serve
as surfaces for heterogeneous reactions promoting conversion of less reactive
(anthropogenic) chlorine/bromine species into more reactive forms (Solomon
et al ., 1998 ; Solomon, 1999 for a review). Therefore, in general, the chemical
impact of volcanic eruptions is to exacerbate anthropogenic-driven O 3 destruction.
Anthropogenic chlorine concentrations are steadily declining; hence, future erup-
tions will not deplete stratospheric O 3 except if an eruption itself injects suf
ciently, leading to phenomena such as the Antarctic
ozone hole
cient
chlorine (in the form of hydrochloric acid) into the stratosphere; this is a matter
of debate (e.g. Tabazadeh and Turco, 1993 ; Kutterolf et al ., 2013 ; see also
Chapter 16 ). Chapter 20 discusses how halogen-bearing species emitted by
ood
basalt eruptions could have resulted in global-scale O 3 depletion during the end-
Permian (252 Ma) emplacement of the Siberian Trap province.
To date, sulfur dioxide (SO 2 ) is the sole volcanic volatile species that has been
observed to alter the radiative balance of the atmosphere, because of its conversion
to volcanic sulfuric acid aerosol. Sulfur species contribute between 2% and 35% by
volume of the gas phase, and SO 2 and hydrogen sul
de (H 2 S) are most abundant.
In the troposphere, H 2 S rapidly oxidizes to SO 2 , which commonly has a chemical
 
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