AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus GmbHGöttingen, Germany10.5194/amt-8-5223-2015Improved stratospheric aerosol extinction profiles from SCIAMACHY: validation and sample resultsvon SavignyC.csavigny@physik.uni-greifswald.deErnstF.RozanovA.HommelR.EichmannK.-U.RozanovV.BurrowsJ. P.https://orcid.org/0000-0003-1547-8130ThomasonL. W.https://orcid.org/0000-0002-1902-0840Institute of Physics, Ernst Moritz Arndt University of
Greifswald, Felix-Hausdorff-Str. 6, 17489 Greifswald, GermanyInstitute of Environmental Physics/Remote Sensing,
University of Bremen, Otto-Hahn-Allee 1, 28334 Bremen, GermanyLangley Research Center, National Aeronautics and Space
Administration, Hampton, VA 23681, USAC. von Savigny (csavigny@physik.uni-greifswald.de)15December20158125223523523June201510August201526November20151December2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/8/5223/2015/amt-8-5223-2015.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/8/5223/2015/amt-8-5223-2015.pdf
Stratospheric aerosol extinction profiles have been retrieved from
SCIAMACHY/Envisat measurements of limb-scattered solar radiation.
The retrieval is an improved version of an algorithm presented
earlier. The retrieved aerosol extinction profiles are compared to
co-located aerosol profile measurements from the SAGE II solar
occultation instrument at a wavelength of 525 nm. Comparisons were
carried out with two versions of the SAGE II data set (version 6.2
and the new version 7.0). In a global average sense the SCIAMACHY
and the SAGE II version 7.0 extinction profiles agree to within
about 10 % for altitudes above 15 km. Larger relative differences
(up to 40 %) are observed at specific latitudes and altitudes. We
also find differences between the two SAGE II data versions of up
to 40 % for specific latitudes and altitudes, consistent with
earlier reports. Sample results on the latitudinal and temporal
variability of stratospheric aerosol extinction and optical depth
during the SCIAMACHY mission period are presented. The results
confirm earlier reports that a series of volcanic eruptions is
responsible for the increase in stratospheric aerosol optical
depth from 2002 to 2012. Above about an altitude of 28 km, volcanic
eruptions are found to have negligible impact in the period 2002–2012.
Introduction
Stratospheric sulfate aerosols mainly comprise H2SO4 and
H2O, with a H2SO4 weight percentage of 75 % on average
(reviewed by ). The size of the particles depends
on the strength of the release of volcanic precursors (mainly
SO2) in the lower stratosphere and ranges from sub-nanometer
molecular clusters, over a few hundred nanometers in the natural
stratospheric background (volcanically quiescent periods
e.g., ), up to a few micron in the aftermath of
large volcanic injections e.g.,. Sulfuric
acid aerosols are thermodynamically stable in the relatively cold
lower stratosphere below about 35 km, forming a global layer –
the so-called Junge layer . The main sources of
sulfur for stratospheric sulfate aerosols are thought to be
the transport of naturally produced COS (carbonyl sulfide) from the
troposphere into the stratosphere, as well as the injection of SO2
and possibly other sulfur compounds into the stratosphere as a
consequence of volcanic eruptions. COS, which has natural and
anthropogenic sources, is the only sulfur compound with a
sufficiently long lifetime to reach the stratosphere
e.g.,. In the stratosphere COS is oxidized
to form SO2, which is then converted to H2SO4. The relative importance of photolysis of COS
and its oxidation by O atoms or excited O atoms is still not well
established. More detailed information on the current
understanding of stratospheric sulfur chemistry was presented by
.
Stratospheric aerosols are of fundamental importance for the
atmosphere's radiative balance, because they scatter solar
radiation, thereby enhancing the Earth's planetary albedo.
Particles also contribute to the greenhouse effect, but the
warming effect only dominates over the cooling effect for
particles with sizes greater than about 2 micron
. However, the typical particle sizes for large
volcanic eruptions such as the Mount Pinatubo eruption in 1991 are
significantly smaller e.g.,. After the
eruption of Mount Pinatubo, the global mean surface temperature
decreased by about 0.4 K during 1992/1993 in large parts of the
world e.g.,.
During the last 15 years, the stratospheric aerosol load has
exhibited significant variability, including an increase likely
caused by a series of volcanic eruptions
e.g.,. Other
suggested causes for this increase include enhanced sulfur
emissions, e.g., due to coal burning, mainly in China
. This suggestion, however, was recently
challenged by . The increased stratospheric
aerosol load, associated with volcanic eruptions, is also one
possible mechanism contributing to the recent global warming
hiatus, as suggested by and further outlined
in . The CMIP-5 model simulations used for the
last IPCC (Intergovernmental Panel on Climate Change) assessment
report assume constant stratospheric aerosol after 2000; i.e.,
they do not consider the changing radiative forcing associated
with changes in stratospheric aerosols. An accurate knowledge of
the temporal evolution of stratospheric aerosol optical depth and
aerosol size is of fundamental importance to the assessment of the
role of stratospheric aerosols during the current warming hiatus
and also for future climate change.
Stratospheric sulfate aerosols are also of importance for
stratospheric chemistry because they may facilitate chlorine
activation through heterogeneous reactions , and
they also act as nucleation nuclei for polar stratospheric clouds
that play a crucial role in the formation of the Antarctic ozone
hole, as well as catalytic ozone losses within the Arctic polar
vortex during cold Arctic winters. Stratospheric aerosols are
central to the discussion on geoengineering by solar radiation
management e.g.,. Any potential
testing, either using natural phenomena such as volcanic
eruptions, or man made, for such complex management strategies
requires precise measurements of stratospheric aerosol.
Measurement techniques employed to study stratospheric aerosol
parameters include in situ particle sampling, e.g., by optical
means e.g.,, single or multi-wavelength LIDAR
observations e.g.,, satellite
solar e.g., or
stellar e.g., occultation measurements,
as well as satellite observations of limb-scattered solar
radiation .
In this article we present validation results for an improved
version of the SCIAMACHY limb-scatter stratospheric aerosol
extinction profile algorithm, that was developed at the Institute
of Environmental Physics at the University of Bremen. In an
earlier study presented an algorithm
description, error budget and validation results for a previous
version of the SCIAMACHY stratospheric aerosol retrieval. The new
data set presented here is validated with co-located SAGE-II
stratospheric aerosol profile measurements. Using different
retrievals approaches, and
also presented stratospheric aerosol profile retrievals from
SCIAMACHY limb-scatter observations.
The structure of the paper is as follows. In Sect. we provide a description of the SCIAMACHY
instrument focusing on the aspects most relevant to this study. In Sect. the stratospheric aerosol retrieval
algorithm is briefly described, and the differences with the previous
version of the aerosol data product are highlighted. A brief
description of the SAGE II stratospheric aerosol retrievals is
given in Sect. . Section presents comparisons of the SCIAMACHY
stratospheric aerosol extinction profile retrievals with
co-located SAGE II measurements. Sample results are presented in Sect. , and conclusions are given at the end.
Stratospheric aerosol extinction profile retrievals from SCIAMACHY
limb-scatter measurementsSCIAMACHY on Envisat
SCIAMACHY, the SCanning Imaging Absorption spectroMeter for
Atmospheric CHartography was 1 of 10 scientific instruments on
board ESA's Envisat satellite. Envisat was launched on 28
February 2002 from Kourou (French Guiana) into a sun-synchronous
orbit having an equator-crossing time of 10:00 a.m. in a
descending node. SCIAMACHY is an 8-channel grating spectrograph
covering the spectral range from about 220 to 2380 nm with a
wavelength dependent spectral resolution between 0.2 and 1.5 nm.
SCIAMACHY nominal operations started in August 2002 and were
suddenly interrupted in April 2012 due to a spacecraft failure.
During an orbit SCIAMACHY performed observations in alternate
nadir and limb viewing geometry and solar/lunar occultation
geometry, as well as nighttime limb-emission observations and
provided daily measurements of the solar spectral irradiance. A
more detailed description of the SCIAMACHY instrument can be found
in , or
. The stratospheric aerosol profile retrievals
described in this publication are based on SCIAMACHY limb-scatter
observations in the visible spectral range (SCIAMACHY channels 3
and 4). The retrievals are based on SCIAMACHY Level 1 data version
7.0x, and the data were calibrated with all options except flags 0,
6, and 7 corresponding to memory effect correction, polarization
correction and absolute calibration, due to the remaining issues
with these calibration steps. The aerosol retrieval approach used
in this study is insensitive to the absolute radiometric
calibration.
Algorithm description
The algorithm used to retrieve stratospheric aerosol extinction
profiles from SCIAMACHY limb-scatter observations has been
described in detail by and ,
and only the most important features are summarized here. The
retrieval is based on a color-index approach employing normalized
limb-radiance profiles at wavelengths of 470 and 750 nm
following . The inverse problem is solved with
an iterative optimal estimation approach as described by
. The radiative transfer model SCIATRAN
is run online during the retrieval. The
earlier version (version 1.0) described in used
the Henyey–Greenstein phase function parametrization. This was
found to be inadequate for an accurate retrieval of stratospheric
aerosol extinction profiles (see also Sect. ). The
current version 1.1 used here is based on scattering phase
functions calculated using Mie scattering theory
e.g., assuming aerosol size parameters
are representative of stratospheric background conditions, i.e., a
mono-modal log-normal aerosol particle size distribution with a
median radius of 0.11µm and a distribution width of
σ = 1.37 following the in situ balloon observations by
. The value of the Ångstrøm exponent used for
the retrievals is α=lnβ750-lnβ470ln750-ln470≈-1.434, with
β750 and β470 being the aerosol extinction
coefficients at 750 and 470 nm. The retrieval uses atmospheric
background profiles for the date, time and location of each
SCIAMACHY limb measurement from the ECMWF (European Centre for
Medium-Range Weather Forecasts) operational analysis. In addition,
the seasonally dependent surface albedo climatology by
is used for the radiative transfer
calculations. The sensitivity of version 1.1 stratospheric aerosol
extinction profile retrievals from SCIAMACHY limb-scatter
observations to errors in the assumed surface albedo was
determined recently by . An albedo error of
±0.15 is associated with aerosol extinction changes of 8 % at
most for altitudes between 15 and 35 km. The extinction changes
are generally below 4 %. also studied the
impact of an optically thick tropospheric cloud layer (optical
depth τ=20) with 3 km vertical extent (below the field of
view) on the stratospheric aerosols profile retrievals. Except for
the highest southern latitudes – associated with scattering
angles exceeding 130∘ – the impact of clouds was found
to be less than about 10 %. At the highest southern latitudes
retrieval errors of up to 40 % can occur. Future versions of the
SCIAMACHY stratospheric aerosol profile retrieval will include an
effective albedo retrieval based on the limb radiances near the
reference tangent height.
We also point out that assuming a log-normal particle size
distribution with constant median radius and distribution width
prevents the actual variability of the stratospheric aerosol
particle size distribution, associated with for example seasonal
variations, QBO (Quasi-Biennial Oscillation) effects or volcanic eruptions to be taken into
account. A future version of the stratospheric aerosol profile
retrieval from SCIAMACHY limb observations will be based on
extinction profile retrievals at several individual wavelengths.
This approach yields simultaneous retrievals of particle size
information.
SAGE II aerosol profile retrievals
The Stratospheric Aerosol and Gas Experiment (SAGE) II was a solar
occultation instrument on ERBS (Earth Radiation Budget Satellite)
that provided global measurements of stratospheric O3 and
NO2 profiles and stratospheric aerosol extinction profiles at
different wavelengths in the optical spectral range, including 525 nm. SAGE II operated from 1984 to 2005 and its stratospheric
aerosol data set is generally considered to be one of the data
sets with the highest accuracy. For this reason it is often used
for validation studies and is well suited for comparisons with the
SCIAMACHY stratospheric aerosol retrievals presented here.
A new version (7.0) of the SAGE II stratospheric aerosol data set
was recently presented by , yielding
stratospheric aerosol extinction profiles that sometimes differ
significantly from the earlier version 6.2. In this study, we use
both SAGE II versions for the validation of SCIAMACHY aerosol
extinction profile retrievals.
Comparison with SAGE II aerosol extinction profiles
We start by comparing globally averaged co-locations between SAGE
II occultation and SCIAMACHY limb-scatter observations. The
co-location criteria are 500 km spatial distance and 6 h
temporal difference at most between the SAGE II and SCIAMACHY
measurements. As a result of the complexity of the radiative
transfer in limb geometry at large solar zenith angles (SZAs),
SCIAMACHY limb measurements for SZAs exceeding 87∘ were
not considered. All available co-locations between 1 January 2003
and 17 August 2005 were used. The SAGE II aerosol extinction
values at a wavelength of 525 nm were used, and the SCIAMACHY
extinction coefficients were evaluated at this wavelength using
the assumed Ångstrøm exponent.
Figure shows a
comparison between SAGE II and SCIAMACHY aerosol extinction
profiles at 525 nm for all co-locations. The left panel of Fig. depicts the averaged
SCIAMACHY (red solid line), SAGE II (version 6.2 shown as black
and version 7.0 as blue solid line) profiles, together with the
SCIAMACHY a priori profile (green solid line). The dashed colored
lines show the corresponding standard deviations. The right panel
of Fig. shows the
relative difference between the SCIAMACHY and SAGE II profiles
relative to SAGE II, i.e., (SCIAMACHY – SAGE)/SAGE for SAGE II
versions 6.2 (black solid line) and 7.0 (blue solid line). The
dashed lines again correspond to the standard deviations of the
relative differences. The total number of co-locations used for
this comparison is 3589. We find average agreement to within about
10 % between SCIAMACHY and SAGE II version 7.0 aerosol extinction
profiles for all altitudes between 15 and 35 km, which can be
considered very good. Interestingly, the relative differences
between SCIAMACHY and SAGE II version 6.2 are with values of up to
about 30 % significantly larger than between SCIAMACHY and the new
SAGE II version 7.0. This suggests that in a global average sense,
differences between SAGE II version 6.2 and version 7.0 of up to
about 20 % have to be expected. This estimate is in good agreement
with the direct comparisons between the two SAGE II versions
presented by .
Left panel: comparison of average co-located SAGE II version 6.2
(black solid line) and version 7.0 (blue solid line) and SCIAMACHY
version 1.1 (red solid line) aerosol extinction profiles with standard
deviation (dashed lines). The green line corresponds to the a priori
extinction profile used for the SCIAMACHY retrievals. The number in the top
right corner shows the number of co-locations averaged. Right panel: mean
relative difference between SCIAMACHY and SAGE II (version 6.2 in black and
version 7.0 in blue) aerosol extinction profiles with standard deviation
(dashed).
Similar to Fig. but
separated into Southern Hemisphere co-locations (left panels) and Northern
Hemisphere co-locations (right panels) of SCIAMACHY and SAGE II measurements.
Relative difference (%) of the SCIAMACHY aerosol extinction with respect to co-located SAGE II measurements, both averaged in 8 latitude
bins and given in 5- % steps.
Alt. (km)60–80∘ N 40–60∘ N 20–40∘ N 0–20∘ N 0–20∘ S 20–40∘ S 40–60∘ S 60–80∘ S SAGE II version6.27.06.27.06.27.06.27.06.27.06.27.06.27.06.27.01510101010(20)(15)––––(80)(70)1010552005515-10-5-30-30-20-20-5-505-5025-40-25-150-20-5-25-20-30-20-25-20-15-5-10030-50-30-350-25-5-30-20-40-30-40-25-15202525 Mean-20-10-105-150-30-25-30-25-20-15-51055
Left panels: comparison of the retrieved SCIAMACHY 525 nm aerosol
extinction profiles (red solid line) with SAGE II version 6.2 (black solid
line) and version 7.0 (blue solid line) aerosol extinction for eight latitude
bins. The dashed lines show the corresponding standard deviations. The green
line again shows the SCIAMACHY a priori profile. The numbers in the top right
corner show the number of co-locations averaged (“#”) and the average
scattering angle (“S”) in degrees. Right panels: mean relative difference
between SCIAMACHY and both versions of SAGE II aerosol extinction profiles
(solid lines with the same color code as in the left panels) with standard
deviations (dashed).
Left panels: comparison between SCIAMACHY version 1.0 (HG) and
Version 1.1 (MIE) with SAGE II version 6.2 stratospheric aerosol extinction
profiles at 525 nm for both hemispheres. Right panels: similar to left
panels, but for SAGE II version 7.0.
The good agreement between SCIAMACHY and SAGE II version 7.0
aerosol extinction profiles in the global average is promising,
but may not imply similar agreement in different hemispheres or
for specific latitude bands. In order to identify possible
interhemispheric differences in the agreement with SAGE II
measurements, we show in Fig. comparisons
between SCIAMACHY and SAGE II stratospheric aerosol extinction at
525 nm for both hemispheres. The color/line convention is the
same as in Fig. .
SCIAMACHY aerosol extinction profiles are in very good agreement
with SAGE II version 7.0 in the Southern Hemisphere (1635
individual co-locations) – with relative differences of less
than about 10 % between 16 and 33 km. However, in the Northern
Hemisphere (1954 co-locations) the differences reach about 20 % at
altitudes above 25 km with SCIAMACHY aerosol extinction being
lower than the SAGE values.
Left panel: comparison of average co-located SAGE II (black and blue) and
SCIAMACHY (red) aerosol extinction profiles with standard deviation (dashed
lines) for cloud-free SCIAMACHY limb-scatter observations only. The green line
shows the a priori extinction profile used for the SCIAMACHY retrievals. The
number in the top right corner shows the number of co-locations averaged.
Right panel: mean relative difference between SCIAMACHY and SAGE II aerosol
extinction profiles with standard deviation (dashed).
We further refine the comparisons by comparing co-located
SCIAMACHY and SAGE II measurements for 20∘ latitude bins
in Fig. . Table lists relative differences between
SCIAMACHY and both SAGE II versions for different altitudes and
the different latitude bins. Figure
shows that there is no constant bias between SCIAMACHY and SAGE II
aerosol extinction; the differences vary with latitude and
altitude in a complex manner. At tropical latitudes the SCIAMACHY
aerosol extinctions are generally lower than the SAGE II values,
and the differences between the two SAGE II versions are smaller
than for most other latitude bins. In the Northern Hemisphere the
differences between SCIAMACHY and SAGE II version 7.0 decrease
from low to mid latitudes and increase again at polar latitudes.
In the Southern Hemisphere the differences between SCIAMACHY and
SAGE II version 7.0 become smaller and change their sign. It is
important to point out that some of the observed latitudinal
variation of the difference between SAGE II and SCIAMACHY
stratospheric aerosol extinction profiles is likely related to
differences between the assumed scattering phase function – which
is based on the assumed size parameters – and the actual
scattering phase function in combination with the latitudinal
variation of the scattering angle of the SCIAMACHY limb-scatter
observations. The scattering angle varies from about 30∘
at high northern latitudes to about 150∘ at high southern
latitudes. Errors in the assumed scattering phase function can
easily lead to differences in retrieved aerosol extinction of
several tens of % and may also lead to significant
interhemispheric differences in the aerosol extinction retrieval
errors from SCIAMACHY limb-scatter observations (see also Sect. ).
In terms of the agreement between the two SAGE II versions we find
that the differences generally increase with increasing altitude
and version 7.0 aerosol extinction is almost always smaller than
the version 6.2 values. Furthermore, the differences are
relatively small at low latitudes, increase towards mid-latitudes
– particularly at altitudes above 25 km – and decrease again at
the highest latitudes.
Comparison of SCIAMACHY version 1.0 and 1.1
stratospheric aerosol data sets
As discussed in Sect. the previous version (1.0) of the SCIAMACHY stratospheric aerosol data product was
based on a Henyey–Greenstein scattering phase function with an
asymmetry parameter of g=0.712. In
contrast, version 1.1 is based on a more realistic Mie phase
function, which was implemented to reduce the large
interhemispheric differences between SCIAMACHY and co-located SAGE
II extinction profiles present in SCIAMACHY version 1.0. Figure shows comparisons between SCIAMACHY
and SAGE II aerosol extinction profiles for SCIAMACHY version 1.0
(HG for Henyey–Greenstein) and version 1.1 (MIE) with SAGE II
version 6.2 (left panels) and SAGE II version 7.0 (right panels)
for both hemispheres separately. The comparisons with both SAGE II
data versions show that the interhemispheric difference found in
SCIAMACHY version 1.0 is significantly smaller in SCIAMACHY
version 1.1. For SCIAMACHY version 1.0 the SCIAMACHY stratospheric
aerosol extinctions are significantly high-biased in the Southern
Hemisphere, compared to the Northern Hemisphere and relative to
SAGE II aerosol profiles. The reduction in interhemispheric
asymmetry for SCIAMACHY version 1.1 clearly shows that the Mie
phase function is a much more adequate aerosol scattering phase
functions compared to the Henyey–Greenstein parametrization used
for SCIAMACHY version 1.0.
Aerosol extinction coefficient fields at 525 nm wavelength as a
function of time and latitude at 18 km (top left panel), 22 km (top right
panel), 26 km (bottom left panel) and 30 km altitude (bottom right panel).
The aerosol extinction profiles were daily and zonally averaged. The capital
letters in the top left panel indicate volcanic eruptions (Manam: A, January
2005, 4∘ S; Soufriere Hills: B, May 2006, 16∘ N; Tavurvur:
C, October 2006, 4∘ S; Kasatochi: D, August 2008, 52∘ N;
Sarychev Peak: F, June 2009, 48∘ N; Mount Merapi: G, October 2010,
7∘ S; Nabro: H, June 2011, 13∘ N) and the Australian bush
fires (E) of February 2009 at 38∘ S. The white stripe indicates a
gap in the standard limb measurement coverage. The bottom right panel also
shows the -30 m s-1 contour line of the monthly and zonally averaged
zonal wind at the 10 hPa level taken from ERA Interim.
Top panel: stratospheric aerosol optical depth field at 525 nm
wavelength and integrated from the 380 K isentrope up to 40 km as a function
of time and latitude with daily resolution. The letters indicate the same
events as in Fig. . Bottom panel: stratospheric
aerosol optical depth at 525 and 750 nm wavelength integrated from the
380 K isentrope up to 40 km and corresponding to a surface-area weighted
average between 60∘ S and 60∘ N (left ordinate). The right
ordinate of the bottom panel shows the stratospheric aerosol radiative
forcing determined using the approach discussed by as
described in the text (Note inverted scale). The right ordinate only applies
to the blue line, i.e., 525 nm wavelength.
Stratospheric aerosol optical depth (between 380 K potential
temperature and 40 km altitude) time series at 525 and 750 nm wavelength
for different 20∘ latitude bins. Note that the right ordinate only
applies to the blue line, i.e., 525 nm wavelength.
Effect of clouds on the SCIAMACHY/SAGE II comparisons
In order to investigate the effect of clouds on the SCIAMACHY
stratospheric aerosol extinction profile retrievals we employ the
cloud occurrence data base also obtained from SCIAMACHY
limb-scatter observations in the near-IR spectral range using the
cloud flagging algorithm SCODA (SCIAMACHY Cloud Detection
Algorithm) described by . Due to
the relatively large geographical footprint of individual
SCIAMACHY limb measurements of about 250 km across viewing
direction by about 500 km in viewing direction the majority of all
limb-scatter observations are affected by tropospheric clouds.
According to (their Fig. 8b) the annually
averaged tropospheric cloud occurrence rate measured with
SCIAMACHY limb-scatter observations exceeds 90 % at almost all
geo-locations. Using cloud-free SCIAMACHY measurements for the
comparison with SAGE II, only 138 out of the total of 3589
co-locations between the two instruments remain. Figure shows the comparison
between SCIAMACHY and SAGE II aerosol extinction profiles with the
cloud filter applied. Interestingly, the relative differences are
very similar compared to the differences shown in Fig. without the cloud
filter applied. This implies that any inhomogeneity of the
limb-scattered radiance across the field of view of SCIAMACHY is
not the major source of the differences.
Sample results
In this section we present results on the latitudinal and temporal
variability of stratospheric aerosol extinction and stratospheric
aerosol optical depth. In order to avoid contamination by
tropospheric clouds, which can reach altitudes of up to 16–17 km at low latitudes, stratospheric aerosol optical depth is
determined by integrating the extinction profiles from the
Θ=380 K isentrope up to 40 km altitude, following
.
Figure shows the retrieved
stratospheric aerosol extinction at a wavelength of 525 nm and at
18 km (top left panel), 22 km (top right panel), 26 km (bottom
left panel) and 30 km (bottom right panel) altitude for the period
from 1 January 2003 to 31 December 2011. Note that the aerosol
extinction profiles were daily and zonally averaged. The white
letters in the top left panel indicate volcanic eruptions (see
Fig. caption) and the “Black Saturday” pyrocumulus event in
February 2009 (E; e.g., ). During approximately
the first 2.5 years of the SCIAMACHY mission, that were relatively
unaffected by volcanic activity, the aerosol extinction at 18 km
shows a pronounced annual cycle at mid-latitudes in both
hemispheres with a wintertime maximum, and low values in the
tropics and subtropics, where the aerosol layer resides at higher
altitudes. This annual variation is consistent with LIDAR
measurements of the stratospheric aerosol backscatter ratio
observed from Boulder (40∘ N) and Hawaii (19∘ N)
and is linked to
seasonal variations in stratospheric temperature and moisture as
well as the meridional transport associated with the Brewer–Dobson
circulation . The volcanic eruptions
lead to significantly enhanced stratospheric aerosol extinction
values at 18 km altitude. The effects of mid and high latitude
eruptions – e.g., Kasatochi in August 2008
e.g., and Sarychev Peak in June
2009 e.g., – are limited to the
corresponding hemisphere, but can affect tropical latitudes.
The season during which eruptions occur has a significant impact
on the meridional transport of the volcanic aerosol and of
pyrocumulus injections into the upper troposphere. The aerosol
detected after the Australian Black Saturday bushfires (E in Fig. ) in winter 2009 was predominantly
transported equatorward, whereas the aerosol produced by the Mount
Merapi eruption (G in Fig. ) in fall
2010 was mainly transported poleward.
The aerosol extinction fields at 18 and 22 km (top panels of Fig. ) show signatures of polar
stratospheric clouds (PSCs) during hemispheric spring,
particularly pronounced in the Southern Hemisphere at the highest
latitudes during September and October. We have to point out
that the SCIAMACHY stratospheric aerosol profile retrieval assumes
a Mie phase function characteristic for stratospheric background
aerosol, not for PSCs. The retrieved aerosol extinction for
measurements affected by PSCs is therefore only an indicator of
their enhanced extinction, and the extinction values may be
affected by larger uncertainties than the stratospheric background
aerosol retrievals. Whilst type Ib PSC particles are liquid and
thus may be well described by Mie theory, type Ia PSC particles,
which consist of solid nitric acid trihydrate,
HNO3⋅3H2O or NAT and type II PSC particles, which
consist of ice, are not spherical particles. More accurate phase
functions for NAT and ice are required to obtain accurate
extinction profiles.
The impact of the volcanic eruptions decreases with increasing
altitude, as expected. At 30 km (bottom right panel of Fig. ) a volcanic signature is hardly
apparent in the latitudinally resolved time series and the main
variability component at low latitudes has a period characteristic
of the Quasi-Biennial Oscillation (QBO, reviewed by
). The enhanced tropical aerosol extinction
values at 30 km altitude occur during easterly shear conditions of
the QBO . The bottom right
panel of Fig. also shows the -30 m s-1
(i.e., easterlies) contour line of the monthly and zonally averaged
zonal wind at 10 hPa taken from the ERA Interim reanalysis. The
results demonstrate that enhancements in aerosol extinction at 30 km altitude are linked to strong easterlies at the 10 hPa pressure
level. These enhancements in aerosol extinction are caused by an
upward motion, which is a manifestation of the QBO secondary
oscillation induced by meridional temperature gradients caused by
the wave-driven vertical wind shear
.
recently discussed the underlying physical
mechanisms, also based on SCIAMACHY stratospheric aerosol
measurements, in more detail.
The top panel of Fig. shows the
latitude and time variation of stratospheric aerosol optical depth
(between Θ=380 K and 40 km altitude) at a wavelength of
525 nm. The effects of the volcanic eruptions are again clearly
visible and are associated with local enhancements of the aerosol
optical depth of up to a factor of about 5. The bottom panel of Fig. shows the temporal evolution
of stratospheric aerosol optical depth averaged over latitude and
weighted with the surface area of the latitude bins, from
60∘ S to 60∘ N, again for the period 1 January
2003–31 December 2011. The optical depth is shown at
wavelengths of 525 and 750 nm, in order to facilitate
comparisons with previous studies that present optical depth at
different wavelengths. Note that the ratio of the optical depths
at these two wavelengths is constant and determined by the fixed Ångstrøm exponent mentioned in Sect. 2.2. In 2003 and 2004 the globally averaged
aerosol optical depths are 3–4 × 10-3 and around 2 × 10-3 at 525 and 750 nm, respectively. The
temporal evolution exhibits intermittent enhancements associated
with the volcanic eruptions as evident from the comparisons of the
upper and lower panel of Fig. . The
maximum stratospheric aerosol optical depth values of about 8 × 10-3 and 4–5 × 10-3 at 525 and 750 nm, respectively, during the time period covered by SCIAMACHY
measurements occur in summer 2011 after the eruption of Nabro, and
correspond to roughly 2 times the 2003/2004 values. Following
, we can convert stratospheric aerosol optical
depth (OD) to radiative forcing (RF) using the formula RF =-25×OD(λ=525 nm), as in . The right
axis of the bottom panel of Fig.
shows the radiative forcing caused by the stratospheric aerosol.
The radiative forcing varies from about -0.1 W m-2 in
2003/2004 to a peak value of about -0.2 W m-2 immediately
after the Nabro eruption in summer 2011. Please note that the
radiative forcing shown in the bottom panel of Fig. applies to the optical depth at 525 nm (blue line).
Similar to the top panel of Fig.
we show in Fig. time
series of stratospheric aerosol optical depth and radiative
forcing for different latitude bins with 20∘ meridional
extent. For the 20–40∘ and 40–60∘ latitude bins and during volcanically undisturbed
conditions, the winter/spring maximum is clearly visible in both
hemispheres. Particularly the low latitudes exhibit significant
variability indicating that the stratospheric aerosol layer is
highly variable even during periods without major volcanic
eruptions . This implies that the
Brewer–Dobson circulation and the QBO both play an important role
for the exchange of COS and any other sources of stratospheric
aerosol. The overall increase – with high variability – of the
stratospheric aerosol loading during the SCIAMACHY mission period
is in good general agreement with the results presented by
.
We now discuss how the stratospheric aerosol optical depths
retrieved in this study compare to previously published
measurement and model results. The comparison of the stratospheric
aerosol optical depth derived from the SCIAMACHY retrievals with
earlier results is not straightforward, because different
definitions of the stratospheric aerosol optical depth are used in
different studies. use the 185 hPa pressure
level as the lower boundary, integrate from 15
to 40 km altitude, and
integrate from the Θ=380 K isentrope up to 40 km
altitude. Moreover, aerosol optical depth is presented at
different wavelengths (530 nm in , presumably
525 nm in – this is not explicitly mentioned
– and 750 nm in and )
which further complicates direct comparisons. In order to simplify
the comparison of our results with these published results we show
in Fig. (bottom panel) and Fig. stratospheric aerosol
optical depth at two wavelengths (525 and 750 nm).
show the temporal evolution of stratospheric
aerosol optical depth from 1994 to 2010 for the tropics and the
50∘ S–50∘ N latitude range (their Fig. 2). For
2003/2004 they find values of about 5 × 10-3 for both
latitude ranges. These values are slightly larger than the 3–4 × 10-3 found in our study. The differences may be
related to the differences in the lower altitude limit.
present measurement and model results of
stratospheric aerosol optical depth from 2002 to 2011 for the
20∘ S–20∘ N and the 45–70∘ N latitude ranges (their Fig. 11). For the tropical
latitude range optical depths of around 4 × 10-3 at
530 nm are found in 2003/2004 in both the EMAC model results and
the SAGE/CALIPSO measurements. These values are in good agreement
with the results presented here. The quantitative enhancements in
aerosol optical depth due to the volcanic eruptions are slightly
different in compared to our results, e.g., for
the case of the Soufriere Hills and Tavurvur eruptions in 2006,
but the overall agreement can be considered good. We can also
compare the aerosol optical depth time series for the
45–70∘ N latitude range in
with the 40–60∘ N results
shown in the bottom right panel in Fig. . In 2003/2004
find values of around 4 × 10-3 (at
530 nm), which compares well with the 3–5 × 10-3 at
525 nm in our results. Good agreement is also found in terms of the optical depth
enhancements associated with the eruptions of Kasatochi, Sarychev
Peak, and Nabro.
The Sarychev results presented by allow a
comparison of our results with the OSIRIS aerosol optical depth
during 2009. During the 2 months prior to the Sarychev eruption
report aerosol optical depths of about 3 × 10-3 at 750 nm and latitudes between 50–60∘ N (their Fig. 5, middle panel). The optical depth
for this latitude range increases to about 1 × 10-2 in
August and September. Our results also show optical depths of
about 3 × 10-3 at 750 nm and latitudes between
40–60∘ N (bottom right panel of Fig. ) before the eruption and
values of about 1 × 10-2 in August and September; i.e.,
the results presented by are in very good
agreement with our results. Similar agreement is found between the
Kasatochi results presented by and our
results.
In summary, the stratospheric aerosol optical depths presented
here are in good quantitative agreement with previously published
results in terms of both the relatively quiet conditions in
2003/2004 and the volcanic enhancements that occurred since 2006.
Conclusions
Stratospheric aerosol extinction profiles in the visible spectral
range were retrieved from SCIAMACHY limb-scatter observations
using an improved retrieval method. A near-global stratospheric
aerosol extinction profile data set covering the altitude range
from the tropopause up to about 40 km has been retrieved for the
period from August 2002 to April 2012. The retrieved aerosol
extinction profiles were validated by comparison with co-located
solar occultation measurements with SAGE II between January 2003
and the end of the SAGE II mission. A global mean comparison with
the latest SAGE II data set (version V7.0) shows agreement within
10 % for altitudes above 15 km. An earlier version of the data
set was using a Henyey–Greenstein scattering phase function and
showed a strong interhemispheric difference of the comparisons
with SAGE II. Comparisons made for different latitude ranges in
opposite hemispheres showed that the retrieval improved
significantly with the implementation of a Mie scattering phase
function based on realistic aerosol particle sizes. We also find
relative differences between aerosol extinction coefficients in
SAGE II version 6.2 and 7.0 data of up to 40 % for certain
altitudes and latitudes. The SCIAMACHY stratospheric aerosol
extinction profile data set covers a period characterized by small
and medium-sized volcanic eruptions and provides important
measurements for the determination of the climate effects of
stratospheric aerosol evolution from 2002 to 2012. Over the
period of SCIAMACHY measurements, the changes in aerosol
extinction have been shown to be strongly dependent on (a) the
volcanic activity up to about 28 km and (b) dynamical processes,
such as the Brewer–Dobson circulation and the quasi biennial
oscillation. We find good quantitative agreement of the
stratospheric aerosol optical depth derived in this study with
previously published results. The radiative forcing of the
stratospheric aerosol over the period 2003–2011 has also been
assessed. In summary, the SCIAMACHY observations are providing a
unique and milestone global record of the stratospheric aerosol
between the tropopause and about 40 km altitude for the period
between 2002 and 2012.
Acknowledgements
This work was in part supported by the German Ministry of
Education and Research (BMBF) within the project ROMIC-ROSA
(grants 01LG1212A and 01LG1212B). This study was made possible by
some funding from the ESA SPIN project and the DLR (German
Aerospace) funding for SCIAMACHY. Similarly, this study would not
have been possible without the base funding of the Institute of
Remote Sensing of the University of Bremen and the Institute of
Physics of Ernst Moritz Arndt University of Greifswald. SCIAMACHY
is jointly funded by Germany, the Netherlands and Belgium. We are
also indebted to ESA for providing the SCIAMACHY Level 1 data used
in this study.
Edited by: E. Kyrölä
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