Environmental Engineering Reference
In-Depth Information
lifetime of hours to days. If released into the stratosphere, the lifetime of SO
2
increases to about 3 weeks due to slower removal processes than in the
troposphere. SO
2
is removed from the atmosphere via dry and wet deposition
(
'
acid rain/snow
'
) or oxidization to form sulfuric acid aerosol. Gas-phase oxidation
of SO
2
by the hydroxyl radical (OH
) forms sulfuric acid (H
2
SO
4
) vapour, which is
a low volatility compound that forms new particles (
) and/or rapidly
condenses to grow existing particles to larger sizes. Within clouds, SO
2
undergoes
aqueous-phase oxidation via reactions with dissolved hydrogen peroxide (H
2
O
2
)
or ozone (O
3
). Volcanic sulfuric acid particles are typically composed of 75%
by weight of H
2
SO
4
and 25% by weight of H
2
O (Bekki,
1995
).
There has not been a large stratospheric injection of volcanic SO
2
since
the 1991 Mount Pinatubo eruption, which released about 20 Tg of SO
2
into the
stratosphere. For comparison, the current anthropogenic SO
2
'
nucleation
'
flux is about 116 Tg
per year. The 1982 El Chichón eruption injected
7TgofSO
2
into the strato-
sphere. Between the years 2000 and 2012 there has been a series of small- to
moderate-sized eruptions (VEI 3
~
1.3 Tg of SO
2
during
an individual eruption (Nabro, 2011, in Eritrea). In comparison, individual decade-
long CFB eruptive phases would have released up to 5,000 Tg of SO
2
(Self
et al
.,
2008
). The Roza
-
4) injecting at the most
~
flow, the best-studied CFB eruptive phase, released about
1,200TgofSO
2
per year for a decade or longer into altitudes between 7 and
13 km (Thordarson and Self,
1996
). For context, Laki was one of the largest
historic
flood basalt events and injected about 120 Tg of SO
2
in 8 months
(Thordarson and Self,
2003
, and references therein).
The annual sulfur flux from continuously degassing and sporadically erupting
volcanoes is about 13 Tg, based on measurements at 49 volcanoes
'
only
'
(Andres
and Kasgnoc,
1998
). The uncertainties on this estimate are large (about
50%);
hence, there is also a large uncertainty on the magnitude of the radiative effects
the advent of the satellite era, more numerous measurements of the volcanic sulfur
flux have become available (e.g. Carn
et al
.,
2013
) but the picture is still far
from complete.
13.4 Atmospheric and climatic impacts of volcanic aerosol
Figure 13.1
illustrates how volcanic gases and particles affect atmospheric com-
position, chemistry and Earth
is climate. Tephra is a major constituent of a volcanic
cloud associated with explosive eruptions. Airborne tephra particles rapidly fall out
of the atmosphere within minutes to days, hence their climatic effects are mostly
negligible (Robock,
1981
; Niemeier
et al
.,
2009
). Continent-sized ash deposits
produced by super-eruptions (VEI
'
8), however, may induce short-term climate
