Geoscience Reference
In-Depth Information
1 Introduction
Stratospheric aerosols (SAs) are spherical droplets comprised of about 75 % sul-
furic acid (H
2
SO
4
) and about 25 % water (H
2
O) (e.g., Rosen
1971
). Precursors of
SAs are sulfur dioxide (SO
2
) and carbonyl sul
de (COS). One key source are
volcanic eruptions, which release SO
2
and may cause a pyrocumulus cloud trans-
porting the SO
2
from the troposphere into the stratosphere. A weaker but permanent
source is the tropical injection of tropospheric air containing SO
2
and COS, and to a
lesser extent sulfate aerosols, which have been formed in the troposphere. The
actual composition and the constituents of SAs depend on several parameters such
as, e.g., temperature and altitude. Stratospheric aerosols can also partly comprise
meteoric dust, especially in the middle and upper stratosphere (Thomason and Peter
2006
).
Stratospheric aerosols play an important role in the Earth
'
s radiative balance and
climate, as they increase the Earth
s planetary albedo by scattering solar radiation
(e.g., Valero and Pilewskie
1992
; Deshler
2008
). Especially after strong volcanic
eruptions, the effect of stratospheric aerosols on the Earth
'
is radiation budget is
stratospheric warming and tropospheric cooling (e.g., Lacis et al.
1992
; Deshler
2008
). Furthermore, SAs in
'
uence stratospheric chemistry. They are precursors for
polar stratospheric clouds and thus support the destruction of ozone inside the polar
vortex (e.g., Hofmann and Solomon
1989
). They may even lead to a halogen-driven
ozone destruction outside polar vortices (Erle et al.
1998
). Based on these prop-
erties, the Global Climate Observing System (GCOS) de
fl
ned stratospheric aerosols
as an Essential Climate Variable (ECV). Aerosol size plays an important role for the
climate response of stratospheric sulfate aerosols (e.g., Timmreck et al.
2010
),
however, the exact aerosol size distribution is often not known. In addition, the
indirect radiation impact, like the aerosol cirrus cloud formation potential, is poorly
quanti
ed (e.g., Thomason and Peter
2006
; Deshler
2008
).
A distinction is drawn between volcanic and non-volcanic aerosols, as they differ
in size and concentration. The radii of non-volcanic SAs, also referred to as
background SAs, are 0.1
0.5
µ
m and the corresponding concentrations are 0.5
-
0.005 cm
−
3
. After large volcanic eruptions, the concentrations increase by a factor
of 10
-
4 (Deshler
2008
). The concentration
maximum of background SAs in the low and mid-latitudes is at about 20 km
altitude (e.g., Junge et al.
1961
). The background SA concentration typically varies
with time and space (Solomon et al.
2011
), nevertheless, there appears to be no
long-term trend between 1970 and 2005 (Deshler
2008
). Variability of the SA load
in the lower stratosphere (from the tropopause to about 24 km altitude in the low
latitudes) is mainly driven by volcanic eruptions (Vernier et al.
2011
) and to a lesser
extent by atmospheric dynamics. However, as shown in this work, the primary
reason for the variability of the SA load in the middle stratosphere is atmospheric
dynamics.
As stratospheric aerosols have typical lifetimes of several years (Hamill et al.
1997
), they are well suited tracers for dynamical processes in the stratosphere with
1,000 and the radii by a factor of 2
-
-
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