,,Volcanic sulphate aerosols are formed as a result of oxidation
of the sulphur gases emitted by explosive volcanic eruptions into
the stratosphere. The process of gas-to-particle conversion has
an e-folding time of roughly 35 days (Bluth et al., 1992; Read
et al., 1993).
The e-folding time (by mass) for sedimentation of
sulphate aerosols is typically about 12 to 14 months (Lambert
et al., 1993; Baran and Foot, 1994; Barnes and Hoffman, 1997;
Bluth et al., 1997). Also emitted directly during an eruption
are volcanic ash particulates (siliceous material). These are
particles usually larger than 2 μm that sediment out of the
stratosphere fairly rapidly due to gravity (within three months
or so), but could also play a role in the radiative perturbations
in the immediate aftermath of an eruption. Stratospheric aerosol
data incorporated for climate change simulations tends to be
mostly that of the sulphates (Sato et al., 1993; Stenchikov et
al., 1998; Ramachandran et al., 2000; Hansen et al., 2002; Tett
et al., 2002; Ammann et al., 2003). As noted in the Second
Assessment Report (SAR) and the TAR, explosive volcanic
events are episodic, but the stratospheric aerosols resulting
from them yield substantial transitory perturbations to the
radiative energy balance of the planet, with both shortwave and
longwave effects sensitive to the microphysical characteristics
of the aerosols (e.g., size distribution).''
(..)
,,The radiative effects due to volcanic aerosols from major
eruptions are manifest in the global mean anomaly of reflected
solar radiation; this variable affords a good estimate of radiative
effects that can actually be tested against observations. However,
unlike RF, this variable contains effects due to feedbacks (e.g.,
changes in cloud distributions) so that it is actually more
a signature of the climate response. In the case of the Mt.
Pinatubo eruption, with a peak global visible optical depth of
about 0.15, simulations yield a large negative perturbation as
noted above of about –3 W m–2 (Ramachandran et al., 2000;
Hansen et al., 2002) (see also Section 9.2). This modelled
estimate of reflected solar radiation compares reasonably
with ERBS observations (Minnis et al., 1993). However, the
ERBS observations were for a relatively short duration, and the
model-observation comparisons are likely affected by differing
cloud effects in simulations and measurements. It is interesting
to note (Stenchikov et al., 2006) that, in the Mt. Pinatubo case,
the Goddard Institute for Space Studies (GISS) models that use
the Sato et al. (1993) data yield an even greater solar reflection
than the National Center for Atmospheric Research (NCAR)
model that uses the larger (Ammann et al., 2003) optical depth
estimate.''
Zie de link, pagina 193 en verder (over explosieve vulkaanuitbarstingen).
Gr. Ben
Quote selectie