Atmos. Chem. Phys., 10, 7997–8009, 2010
www.atmos-chem-phys.net/10/7997/2010/
doi:10.5194/acp-10-7997-2010
© Author(s) 2010. CC Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Optical extinction by upper tropospheric/stratospheric aerosols and
clouds: GOMOS observations for the period 2002–2008
F. Vanhellemont1 , D. Fussen1 , N. Mateshvili1 , C. Tétard1 , C. Bingen1 , E. Dekemper1 , N. Loodts1 , E. Kyrölä2 ,
V. Sofieva2 , J. Tamminen2 , A. Hauchecorne3 , J.-L. Bertaux3 , F. Dalaudier3 , L. Blanot3,4 , O. Fanton d’Andon4 ,
G. Barrot4 , M. Guirlet4 , T. Fehr5 , and L. Saavedra5
1 Belgian
Institute for Space Aeronomy, Brussels, Belgium
Meteorological Institute, Helsinki, Finland
3 Service d’Aéronomie du CNRS, Verrieres-le-Buisson, France
4 ACRI-ST, Sophia-Antipolis, France
5 ESA-ESRIN, Frascati, Italy
2 Finnish
Received: 2 February 2010 – Published in Atmos. Chem. Phys. Discuss.: 26 April 2010
Revised: 18 August 2010 – Accepted: 24 August 2010 – Published: 27 August 2010
Abstract. Although the retrieval of aerosol extinction coefficients from satellite remote measurements is notoriously
difficult (in comparison with gaseous species) due to the lack
of typical spectral signatures, important information can be
obtained. In this paper we present an overview of the current operational nighttime UV/Vis aerosol extinction profile
results for the GOMOS star occultation instrument, spanning the period from August 2002 to May 2008. Some
problems still remain, such as the ones associated with incomplete scintillation correction and the aerosol spectral law
implementation, but good quality extinction values are obtained at a wavelength of 500 nm. Typical phenomena associated with atmospheric particulate matter in the Upper
Troposphere/Lower Stratosphere (UTLS) are easily identified: Polar Stratospheric Clouds, tropical subvisual cirrus
clouds, background stratospheric aerosols, and post-eruption
volcanic aerosols (with their subsequent dispersion around
the globe). For the first time, we show comparisons of GOMOS 500 nm particle extinction profiles with the ones of
other satellite occultation instruments (SAGE II, SAGE III
and POAM III), of which the good agreement lends credibility to the GOMOS data set. Yearly zonal statistics are presented for the entire period considered. Time series furthermore convincingly show an important new finding: the sensitivity of GOMOS to the sulfate input by moderate volcanic
eruptions such as Manam (2005) and Soufrière Hills (2006).
Finally, PSCs are well observed by GOMOS and a first qual-
Correspondence to: F. Vanhellemont
([email protected])
itative analysis of the data agrees well with the theoretical
PSC formation temperature. Therefore, the importance of
the GOMOS aerosol/cloud extinction profile data set is clear:
a long-term data record of PSCs, subvisual cirrus, and background and volcanic aerosols in the UTLS region, consisting
of hundreds of thousands of altitude profiles with near-global
coverage, with the potential to fill the aerosol/cloud extinction data gap left behind after the discontinuation of occultation instruments such as SAGE II, SAGE III and POAM III.
1
Introduction
Upper tropospheric and stratospheric particles (whether liquid or solid) are intensively studied for a number of reasons
that can be roughly summarized as follows: (1) they have
an impact on the Earth radiative balance due to their optical properties, (2) they play a crucial role in heterogeneous
chemistry, and (3) they provide information on the emission
of precursor species from which they originate. Here, the
word ’particles’ should be taken in its most general context:
liquid or solid aerosols of submicron size, cloud droplets or
crystals with sizes of tens or hundreds of microns, etc. From
previous experiences with satellite occultation instruments,
we know that a few particle types are commonly encountered
in the measurements: stratospheric aerosols, tropical subvisual cirrus clouds and Polar Stratospheric Clouds (PSCs).
The stratospheric aerosol layer (or Junge layer, named after its discoverer; see Junge et al., 1961) consists of liquid
droplets composed of a mixture of sulphuric acid and water. The layer manifests itself in a pronounced way after
Published by Copernicus Publications on behalf of the European Geosciences Union.
7998
F. Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
major volcanic eruptions that are powerful enough to inject
teragrams of SO2 into the stratosphere, such as the ones of
Mount Pinatubo (the Philippines, 1991) or El Chichón (Mexico, 1982); see Robock (2000). After an oxidation process,
sulfate aerosols are formed that are subsequently transported
globally, depending on the latitude of the eruption. Eventually they are removed from the stratosphere, although this
process can take years. Nevertheless, the last major eruption (Mount Pinatubo) dates already from 18 years ago, and
the stratospheric aerosol layer returned to its “background”
level around 1997. Since then, no significant trend is observed in data from SAGE II, balloon-borne instruments and
lidars (Deshler et al., 2003, 2006), even when compared with
volcanically quiet years such as 1979. These findings suggest a more or less stable input of sulphuric species in the
stratosphere that maintain the background layer. Crutzen
(1976) suggested carbonyl sulfide (OCS) as the major precursor gas, although this hypothesis has been challenged (see
e.g. Chin and Davis (1995); Leung et al. (2002)). Very recently, increased anthropogenic SO2 emission by coal burning in China has been proposed as a possible origin for an
upward trend in lidar measurements of stratospheric aerosols
(Hofmann et al., 2009). A good overview of stratospheric
aerosol science can be found in (SPARC, 2006).
Polar Stratospheric Clouds have been investigated for over
two decades now. The well-known classification scheme of
PSCs was originally based on observations of backscatter
and depolarization ratios with lidar instruments (Poole and
McCormick, 1988). Type Ia particles are believed to be relatively large crystalline particles consisting of hydrates of
HNO3 such as Nitric Acid Trihydrate (NAT) or Dihydrate
(NAD). The smaller, liquid Type Ib particles consist of a supercooled ternary solution (STS) of HNO3 , H2 SO4 and H2 O.
The crystalline Type II particles are formed of pure water ice.
PSCs have a crucial role in the stratospheric ozone depletion process over the Arctic and Antarctic regions, because
of (1) heterogeneous chemistry on the particle surface and
(2) the denitrification of the atmosphere by the sedimentation of mainly Type Ia particles. Formation of PSCs is driven
by temperature: Type Ia and Ib particles form below about
195 K, while the Type II ice crystals form at the ice frost
point, about 188 to 190 K in the lower stratosphere. These
cold temperatures are provided in the polar regions during
local winter. A more detailed description of PSC formation
can be found in the review paper of Zondlo et al. (2000).
The so-called subvisual cirrus clouds are a fairly recent
discovery for the obvious reason that they are optically thin
and thus invisible from the ground (hence the name): an upper limit for the optical thickness of 0.03 at 694 nm is sometimes mentioned in the literature. Long optical pathlengths
ensure that satellite occultation instruments such as SAGE
II have detected them frequently (Wang et al., 1996). They
are mainly found in the tropics and midlatitudes, around the
tropopause altitude region (16–17 km). Much is still to be
learned about the formation of these clouds, but Jensen et al.
Atmos. Chem. Phys., 10, 7997–8009, 2010
(1996) proposed at least two mechanisms: (1) they originate
from horizontal anvil-shaped outflows of large convective cumulonimbus clouds, and (2) from nucleation inside a humid
layer that experiences slow uplift through the extremely cold
region of the tropopause. Subvisual cirrus form cloud layers
with a very large horizontal extent (hundreds of kilometers)
but are at the same time very thin (smaller than 1 km). In
recent years they received a lot of attention due to their possible role in the radiative balance of the atmosphere, their impact on ozone concentrations through heterogeneous chemistry (Solomon et al., 1997), and their role in the dehydration
of air that enters the tropical lower stratosphere (Jensen et al.,
1996).
The Global Ozone Monitoring by Occultation of Stars
(GOMOS) instrument was primarily intended to deliver accurate altitude profiles of trace gas concentrations. These
retrievals can be performed since each gas is detected by
its specific spectral signature in the measured light intensity. Atmospheric particle populations pose a more challenging problem since the spectral shape is unknown, with the
direct consequence that it is a priori impossible to find out
which kind of particles are in the line of sight of the instrument. This is the reason why GOMOS delivers only one
common product, perhaps somewhat misleadingly dubbed
“aerosol extinction”. It should be understood that this data
product currently embraces all above-mentioned types of
aerosol and cloud particles (or indeed even any unknown extinction phenomenon with a smooth spectral dependence).
However, the distinction will be made in this paper with
the use of additional information, such as time of appearance/disappearance, geolocation and altitude.
First results on GOMOS aerosol/cloud extinction profiles
representing the year 2003 were previously published (Vanhellemont et al., 2005). The data discussed here span a much
longer time period, from 2002 to 2008. Furthermore, a new
data version was used, with as most important feature the use
of a quadratic polynomial of wavelength as aerosol extinction
model, while the previous model as described in (Vanhellemont et al., 2005) was oversimplified and consisted of a fixed
inverse wavelength function.
2
GOMOS: Instrument and obtained data set
The GOMOS instrument has been adequately described elsewhere (Kyrölä et al., 2004; Bertaux et al., 1991, 2000, 2010),
so we only give a brief summary here. GOMOS, onboard
ENVISAT, was launched in a sunsynchroneous orbit on 1
March 2002. GOMOS routine operations started in August
2002, and since then the instrument has been recording data
almost continuously until present. GOMOS observes occultations of stars (chosen from a predefined catalogue) behind
the earth limb, and records the received light intensity in the
UV/Vis/NIR spectral range: 248–690 nm (spectrometers A1
and A2), 755–774 nm (spectrometer B1) and 926–954 nm
www.atmos-chem-phys.net/10/7997/2010/
F. Vanhellemont
et al.: GOMOS
Aerosol/Cloud
observations
Vanhellemont
et al.: GOMOS
Aerosol/CloudExtinction
Extinction observations
4
5
# occultations
1.5
1
0.5
2002
4
15
2004
2006
Year
0.5
2
10
5
0
−2
1
5
x 10
0
2
Star magnitude
4
3
1.5
0
−180
2008
−90
0
90
Longitude [degrees]
2
1
4
x 10
4
1
0.5
0
1
2
3
Star temperature [K]
x 10
0
−90
180
1.5
0
7999
4
x 10
# occultations
0
# occultations
2
# occultations
x 10
# occultations
# occultations
2
3
4
4
x 10
−45
0
45
Latitude [degrees]
90
x 10
3
2
1
0
50
100
150
Solar zenith angle [degrees]
Fig. 1. Occultation statistics for the entire GOMOS data set spanning the period August 2002 - May 2008. Shown are histograms representing
the number
of GOMOS
occultations
as functiondata
of (from
left to right, the
top to
bottom)August
year, longitude,
latitude,
star magnitude,
star histograms
temperature representing
Fig. 1. Occultation
statistics
for the
entire GOMOS
set spanning
period
2002–May
2008.
Shown are
and solar zenith
angle. Blueas
histograms
to the fraction
occultations
dark limb,
red ones
to the fraction
in bright
the number of GOMOS
occultations
functionrefer
of (from
left toofright,
top to inbottom)
year,
longitude,
latitude,
starlimb.
magnitude, star temperature
and solar zenith angle. Blue histograms refer to the fraction of occultations in dark limb, red ones to the fraction in bright limb.
ter sampling is obtained. The actual star spectrum is (in normal mode) calculated from 7 CCD rows of the spectrometers,
to a field
of view of signals,
0.01 degrees.
(spectrometerequivalent
B2). From
measured
transmittances
It is important
to know
uncertainty
on the
obtained
are calculated that
in turn are
usedthat
tothe
derive
altitude
concenprofiles
tempertration profiles
for Ois3largely
, NO2determined
, NO3 , H2by
Othe
andmagnitude
O2 , andand
temperature of the observed star. But even bright stars (such as Sirature profiles.ius)
Furthermore,
aerosol/cloud
extinction
are weak light sources
in comparison
with theprofiles
Sun; proat different wavelengths
are obtained.
files obtained from
stellar occultations have therefore larger
uncertainties
the ones from
solar occultations.
This disThe integration
timethan
to record
a GOMOS
full spectrum
is however
largely compensated
for byby
thethis
fact
is 0.5 s. Theadvantage
actual vertical
sampling
is determined
that stars are abundant in the sky: about 30 to 40 occultatime, togethertions
with
thebeen
vertical
velocity
theorbit,
tangent
point
have
typically
observedofper
although
this
(between 0.5number
and 3.4
km/s, depending
on the obliquity
thean
decreased
to 20 or 30 occultations
per orbit of
after
instrument
malfunction
in optical
2005. Hundreds
thousands
occultation) and
refraction
of the
path atoflower
alti-of
observed
by GOMOS
the start
tudes (whichoccultations
decreaseshave
thebeen
tangent
point
verticalsince
velocity).
of the mission.
A maximum vertical sampling resolution of 1.7 km can be
The occultation statistics for the period August 2002 - May
expected, but2008
during
very
oblique
occultations
a 200
(a total
of about
600,000
events, the entire
datameter
set that
sampling is obtained.
star ofspectrum
is represented
(in norwas availableThe
to usactual
at the time
writing) are
by histograms
in 7Fig.
1. rows
Of course,
thespectrometers,
lower number of
mal mode) calculated
from
CCD
of the
for theof
years
and 2008 are caused by an
equivalent tooccultations
a field of view
0.012002
degrees.
incomplete sampling of these years, while the smaller numIt is important
know
that the
the obtained
ber into2005
resulted
fromuncertainty
the instrumentonfailure
mentioned
profiles is largely
by the magnitude
temperearlier.determined
Longitudinal sampling
is clearly veryand
homogeneous.
Most occultations
occur
at midlatitudes,
ature of the observed
star. But
even
bright starsalthough
(such plenty
as Sir-of
polar
measurements
are
available.
Most
stars
have
ius) are weak light sources in comparison with the Sun;moderprofiles obtained from stellar occultations have therefore larger
uncertainties than the ones from solar occultations. This disadvantage is however largely compensated for by the fact
that stars are abundant in the sky: about 30 to 40 occultations have been typically observed per orbit, although this
number decreased to 20 or 30 occultations per orbit after an
instrument malfunction in 2005. Hundreds of thousands of
occultations have been observed by GOMOS since the start
of the mission.
www.atmos-chem-phys.net/10/7997/2010/
ate to weak brightness, and are rather cold. Also, about half
of the occultations occur in the Sun-illuminated atmosphere
(‘bright
Solar Zenith
Angle SZA
the other
half 2002 - May
Thelimb’,
occultation
statistics
for <
the100),
period
August
in
‘dark(a
limb’
(SZA
≥ 100).600
More
details
about the
the solar
il- data set that
2008
total
of about
000
events,
entire
lumination condition and its consequences are given below.
was available to us at the time of writing) are represented
by histograms in Fig. 1. Of course, the lower number of
3occultations
Retrieval method
for the years 2002 and 2008 are caused by an
incomplete
sampling
3.1 Spectral behaviour of these years, while the smaller number in 2005 resulted from the instrument failure mentioned
As
alreadyLongitudinal
said above, thesampling
particle optical
extinction
specearlier.
is clearly
very
homogeneous.
trum is a priori unknown. Therefore, the actual optical specMost
occultations
occur
at
midlatitudes,
although
plenty of
trum is treated as a product of the retrieval algorithm. It is
polar
are available.
Mostshould
starsbehave moderfar
frommeasurements
clear how such a particle
optical spectrum
parameterized,
we can be and
guided
the factcold.
that usual
ate to weak but
brightness,
arebyrather
Also, about half
atmospheric
particle sizeoccur
distributions
broad, covering a atmosphere
of the occultations
in theareSun-illuminated
few orders of magnitude, and the resulting optical spectra are
(“bright
limb”,
Zenith
SZA<100),
thus
smooth.
HenceSolar
they are
often Angle
described
with analytic,the other half
in ‘dark
limb’ such
(SZA≥100).
More
about
smooth
functions,
as polynomials.
Fordetails
data version
6.0,the solar illuthe
GOMOScondition
Science team
to implement a quadratic
mination
andchose
its consequences
are given below.
polynomial as a function of wavelength for the simple reason
that it is versatile: large (constant spectrum), medium-sized
(peak at mid-visible wavelengths) or small particle spectra
3 Rayleigh
Retrieval
method
(the
limit:
extinction β depends on wavelength λ
as β ∼ λ−4 ) can within good approximation be captured
by
polynomials.
Retrieval algorithms for other satellite
3.1suchSpectral
behaviour
As already said above, the particle optical extinction spectrum is a priori unknown. Therefore, the actual optical spectrum is treated as a product of the retrieval algorithm. It is
far from clear how such a particle optical spectrum should be
parameterized, but we can be guided by the fact that usual
atmospheric particle size distributions are broad, covering a
few orders of magnitude, and the resulting optical spectra are
thus smooth. Hence they are often described with analytic,
smooth functions, such as polynomials. For data version 6.0,
the GOMOS Science team chose to implement a quadratic
Atmos. Chem. Phys., 10, 7997–8009, 2010
8000
F. Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
polynomial as a function of wavelength for the simple reason
that it is versatile: large (constant spectrum), medium-sized
(peak at mid-visible wavelengths) or small particle spectra
(the Rayleigh limit: extinction β depends on wavelength λ as
β∼λ−4 ) can within good approximation be captured by such
polynomials. Retrieval algorithms for other satellite occultation instruments are often also equipped with this feature.
Furthermore, Vanhellemont et al. (2006) showed with simulated data (using a Mie code) that a quadratic polynomial
is a good choice when one wants to obtain good aerosol retrievals without introducing too many degrees of freedom in
the retrieval problem.
other wavelengths (evaluated with the quadratic polynomial
within the spectrometer A range 248-690 nm) are often very
noisy.
Finally, it should be mentioned that during the spectral inversion, the optical extinction spectra from the neutral air
(Rayleigh scattering) are removed instead of retrieved from
the measurements, using external ECMWF (European Centre for Medium-range Weather Forecasts) air density data,
to avoid interferences with the residual scintillation and the
spectrally similar aerosol contribution. Also, only spectrometer A measurements are currently used for the aerosol retrieval.
3.2
3.3
Spectral/spatial inversion
The GOMOS retrieval algorithm has been described in detail in another paper of this special issue (Kyrölä et al.,
2010). Here it suffices to say that the retrieval consists of two
parts: (1) the spectral inversion, where measured transmittance spectra at each tangent altitude separately are inverted
to slant path integrated column densities (for gases) and optical thickness (for particles), and (2) the spatial or vertical
inversion, where the retrievals of the first step are separately
inverted to local concentration profiles (gases) and optical extinction profiles (particles). This second step is performed in
combination with a Tikhonov altitude smoothing (Twomey,
1985; Rodgers, 2000) in order to get partially rid of the perturbations caused by residual scintillation in measurements
taken during very inclined occultations (Sofieva et al., 2009).
The amount of smoothing is determined by a predetermined
target resolution for the profile. For particles, a profile resolution of 4 km was chosen since unsmoothed profiles showed
strong oscillations; aerosol extinction spectra have the tendency to swallow a large portion of the residual scintillation
perturbations. Nevertheless, fine-scale structures (e.g. thin
clouds) are smeared out due to this feature. The choice of
4 km for target resolution was based on the experience that
it gave agreeable results; the actual magnitude of the residual scintillation perturbation on aerosol profiles is still not
adequately determined, although it is known for the ozone
retrievals (Sofieva et al., 2009).
It is in the spectral inversion model that the slant path particle optical thickness is implemented as a quadratic polynomial:
τ (λ)=σref (r0 + r1 1λ + r2 1λ2 )
with 1λ=λ − λref and λref =500 nm a reference wavelength.
For scaling purposes, a “cross section” σref =6×10−10 cm2
was used. Notice that only the first parameter r0 has a direct
physical meaning: σref r0 equals the particle slant path optical thickness at 500 nm. During the spatial inversion, this
parameter receives Tikhonov-style altitude smoothing, while
the other parameters remain unconstrained. Retrospectively,
this turned out to be a poor methodology: while the obtained
extinction profiles at 500 nm look quite good, the spectra at
Atmos. Chem. Phys., 10, 7997–8009, 2010
The homogeneous layer assumption
Occultation measurements have a limited information content. Hence the assumption of homogeneous atmospheric
layers that is so often found in retrieval codes: the number
of unknowns is drastically reduced, leading to a more stable
inversion. For more or less uniformly mixed gases and particles (such as stratospheric aerosols), the assumption should
hold quite well. However, when local phenomena (such as
clouds) are observed, the assumption is wrong: the phenomenon affects the light ray only locally. This should be
taken into account whenever we analyse retrievals of PSCs
and volcanic plumes, for example: the obtained extinction
coefficients should be seen as slant path equivalent values.
3.4
Bright limb versus dark limb retrievals
When the optical light path traverses the Sun-illuminated
atmosphere, parts of the atmosphere located in the fieldof-view scatter additional light into the instrument, which
means that the simple optical transmission model for the retrieval is not correct anymore. This was anticipated: the additionnal upper and lower CCD bands in GOMOS were implemented to correct for this background illumination (Bertaux
et al., 2010; Kyrölä et al., 2010). However, post-launch processing showed that the correction is not perfect. The situation is particularly bad for the aerosol extinction profiles,
since their smooth optical extinction spectrum strongly resembles the limb signal. The aerosol extinction retrievals
typically show unrealistic features at very high stratospheric
altitudes. At present, the best we can do is exclude the bright
limb profiles from data analysis. Investigations showed that
occultation events with a solar zenith angle of 100◦ or larger
deliver unperturbed, dark limb aerosol retrievals.
3.5
Retrieval results
In summary, the GOMOS particle extinction profiles have
acceptable quality around 500 nm, but are oversmoothed.
At other wavelenghts, the profile quality is poor, and we
therefore exclude them from further study. Furthermore,
it is best to exclude the aerosol profiles in bright limb
www.atmos-chem-phys.net/10/7997/2010/
Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
F. Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
40
30
Altitude [km]
Altitude [km]
40
Polar stratospheric cloud
20
10
30
20
10
0
2
4
0
Extinction [km−1] −3
x 10
Background aerosol
40
30
20
10
2
4
Extinction [km−1] −3
x 10
Volcanic aerosol
40
Altitude [km]
Altitude [km]
Tropical subvisual cirrus
30
20
10
0
2
4
−1
−3
Extinction [km ]
x 10
0
2
4
−1
−3
Extinction [km ]
x 10
Fig.
IndividualGOMOS
GOMOS
particle
extinction
profiles
asFig. 2.
2. Individual
particle
extinction
profiles
withwith
associsociated
uncertainty:
Polar Stratospheric
Cloud (2003/7/30,
ated uncertainty:
a Polara Stratospheric
Cloud (30/7/2003,
72,57◦ S,
◦
72,57
S, 2.18
W), a subvisual
tropical subvisual
cirrus
cloud (2002/10/7,
2.18◦◦W),
a tropical
cirrus cloud
(7/10/2002,
0.52◦ S,
◦ ◦
◦
78.69S, E),
background
stratospheric
aerosols (12/9/2003,
43.34◦ S,
0.52
78.69
E), background
stratospheric
aerosols (2003/9/12,
◦ ◦
◦
137.36S, W),
volcanic
stratospheric
aerosols (9/1/2007,
4.52◦ S,
43.34
137.36
W), volcanic
stratospheric
aerosols (2007/1/9,
◦ ◦ E). All◦ plots have the same scale, in order to facilitate com27.15
4.52 S, 27.15 E). All plots have the same scale, in order to faciliparisons
of magnitude.
tate
comparisons
of magnitude.
(SZA<100): from the entire data set of 600 000 occultaintroduce bias in the extinction profiles, as comparisons with
tions, about 301 000 remain to be exploited. Among these,
other instruments show (see below).
some are still perturbed by residual scintillation. These perLike all are
profiles
derived
from occultation
measurements,
turbations
visible
in individual
aerosol extinction
profiles.
GOMOS
aerosol
extinction
profiles
are
increasingly
more
When averaging large numbers of profiles (see e.g. the zonal
uncertain
when
we
descend
to
lower
altitudes,
because
the
plots below), the resulting mean profile becomes smooth.
transmitted
light
intensity
becomes
weaker
due
to
increasThere is reason to assume that the scintillation perturbations
ing
atmospheric
gases, aerosols
clouds.
do not
introduce extinction
bias in the by
extinction
profiles, and
as comparIn
principle,
a
cut-off
altitude
can
be
defined,
below
which
isons with other instruments show (see below).
no Like
aerosol
information
is
present
anymore.
This
altitude
deall profiles derived from occultation measurements,
pends
on the
wavelength
considered,
the magnitude
and
GOMOS
aerosol
extinction
profiles and
are increasingly
more
temperature
of the
and therefore
changes from
one the
ocuncertain when
westar,
descend
to lower altitudes,
because
transmitted
light
intensity
becomes
weaker
due
to
increascultation to the next. At 500 nm, an average limit of 10 km
ing be
atmospheric
gases, below
aerosols
and the
clouds.
can
consideredextinction
as a roughbyestimate
which
proIn
principle,
a
cut-off
altitude
can
be
defined,
below
which
files are not trustworthy anymore.
noOn
aerosol
is present
anymore.
Thisextinction
altitude deFig. information
2, we present
individual
GOMOS
repends
on
the
wavelength
considered,
and
the
magnitude
and
trievals at 500 nm for a number of particle types. It is impostemperature
of the star,
therefore
one ocsible
to distinguish
theseand
types
using changes
only onefrom
wavelength;
cultation
to
the
next.
At
500
nm,
an
average
limit
of
10 km
the presented profiles were selected after analysis of the
encan
be
considered
as
a
rough
estimate
below
which
the
protire GOMOS data set and knowledge about geolocation and
files of
areoccurence
not trustworthy
anymore.
time
of these
particle phenomena (see further
on in this paper).
It is clear that the profiles are oversmoothed. For instance,
www.atmos-chem-phys.net/10/7997/2010/
8001
5
tropical subvisual clouds are known to be horizontally exOn Fig. 2, we present individual GOMOS extinction retended cloud layers that are quite thin, having a thickness
trievals at 500 nm for a number of particle types. It is impossmaller than 1 km (Jensen et al., 1996); the example on the
sible to distinguish these types using only one wavelength;
figure
clearlyprofiles
shows that
layer has
spread
the presented
werethe
selected
afterbeen
analysis
of out
the by
en-the
smoothing
constraint.
tire GOMOS data set and knowledge about geolocation and
should mention
here
that phenomena
the obtained(see
particle
timeWe
of occurence
of these
particle
furtherextinction
retrievals
sometimes
assume
negative
values,
usuon in this paper).
ally
at
altitudes
where
the
measured
signals
are
low
(below
It is clear that the profiles are oversmoothed. For instance,
the above-mentioned
cut-off
altitude),
or horizontally
where the particle
tropical
subvisual clouds
are known
to be
exabundance
is
low
(upper
stratosphere
and
higher).
This is
tended cloud layers that are quite thin, having
a thickness
a logical
consequence
of ettheal.,fact
that the
the example
last retrieval
step
1996);
on the
smaller
than
1 km (Jensen
(spatial
inversion)
is
linear
and
that
the
retrievals
are
not
configure clearly shows that the layer has been spread out by the
strained toconstraint.
be positive. All further results that are discussed
smoothing
inWe
thisshould
paper mention
were obtained
by the
processing
data that
here that
obtainedofparticle
ex-include negative
values;
discarding
these
wouldvalues,
lead to usubiased
tinction
retrievals
sometimes
assume
negative
results.
ally
at altitudes where the measured signals are low (below
the Quantifying
above-mentioned
cut-off extinction
altitude), orretrieval
where the
particle
the aerosol
error
is chalabundance
low (upper
stratosphere
andfound
higher).
This ispalenging. Aisdetailed
description
can be
in another
aper
logical
consequence
of the fact
that
the last retrieval
of this
GOMOS special
issue
(Tamminen
et al., step
2010).
(spatial
inversion)
is linear
and that
the retrievals
are not
conWe repeat
the most
important
ideas
and findings
here.
The
strained
be positive.
All is
further
resultsby
thattwo
arecontributions
discussed
randomtoerror
on a profile
determined
inthat
thiswepaper
were obtained
by the
processing
of datanoise
that which
inmentioned
before: (1)
measurement
clude
negative
discarding
would due
leadtotostar
biased
changes
fromvalues;
one stellar
sourcethese
to another
magresults.
nitude and temperature differences, and (2) the uncorrected
Quantifying
the aerosol
extinctionAtretrieval
is chal-the
residual
scintillation
component.
the timeerror
of writing
lenging.
A
detailed
description
can
be
found
in
another
paGOMOS error estimation for the operational data products
et
al.,
2010).
per
of
this
GOMOS
special
issue
(Tamminen
does not yet take the latter into account, so that retrieval erWe
the most
important ideas
andinfluence
findings of
here.
rorsrepeat
are likely
underestimated.
The
starThe
magrandom
error
on
a
profile
is
determined
by
two
contributions
nitude is clear: brighter stars deliver a better signal-to-noise
that
we mentioned
before:determines
(1) the measurement
which
ratio.
Star temperature
the main noise
spectral
emischanges from one stellar source to another due to star magsion range: hot stars emit in the UV, colder ones in the visnitude and temperature differences, and (2) the uncorrected
ible and near-infrared domain. The influence of star temresidual scintillation component. At the time of writing the
perature on aerosol retrievals nevertheless remains limited;
GOMOS error estimation for the operational data products
it is star magnitude that plays the crucial role (Tamminen
does not yet take the latter into account, so that retrieval eret al., 2010). Sources of systematic error are of course (1)
rors are likely underestimated. The influence of star maga possibly wrong aerosol spectral model, and (2) an impernitude is clear: brighter stars deliver a better signal-to-noise
fect ECMWF
air density
profile, the
bothmain
of which
have
been
ratio.
Star temperature
determines
spectral
emisestimated
by
Tamminen
et
al.
(2010).
Retrieval
errors
are
sion range: hot stars emit in the UV, colder ones in the vis- of
course
by standard
propagation
ible
and calculated
near-infrared
domain. error
The influence
of through
star tem-the
retrievalon
chain.
Aerosol
extinction
error estimates
bright
perature
aerosol
retrievals
nevertheless
remains (for
limited;
of magnitude
30 % are obtained
around
an altitude
10 km, 2-10
itstars)
is star
that plays
the crucial
roleof(Tamminen
from
15 toSources
25 km, and
10-50 % from
km. (1)
et%al.,
2010).
of systematic
error25
aretoof40course
As mentioned
before,spectral
the amount
Tikhonov
altitude
a possibly
wrong aerosol
model,ofand
(2) an impersmoothing
is air
determined
by a predefined
target resolution
fect
ECMWF
density profile,
both of which
have been of
4 km, at by
allTamminen
altitudes, et
regardless
of Retrieval
the star magnitude
estimated
al. (2010).
errors are ofand
temperature.
Presentation
averaging
kernels
is therefore
course
calculated
by standardoferror
propagation
through
the
unnecessary;
profile
resolution
is chosen
in advance.
retrieval
chain. the
Aerosol
extinction
error
estimates
(for bright
stars) of 30% are obtained around an altitude of 10 km, 2–
10% from 15 to 25 km, and 10-50% from 25 to 40 km.
4 AsComparisons
with other
instruments
mentioned before,
the amount
of Tikhonov altitude
smoothing is determined by a predefined target resolution of
detailed
study for the
GOMOS
aerosol extinc4A
km,
at all validation
altitudes, regardless
of the
star magnitude
and
tion profiles Presentation
has not been of
performed
Nevertheless,
Vantemperature.
averagingyet.
kernels
is therefore
hellemont etthe
al. profile
(2008)resolution
already performed
comparisons
unnecessary;
is chosen in
advance. with
the results derived from the imagers of the Atmospheric
Atmos. Chem. Phys., 10, 7997–8009, 2010
Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
7
F. Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
GOMOS/SAGE II
GOMOS/SAGE III
40
35
35
30
30
30
25
25
25
20
15
20
15
20
15
10
10
10
5
5
5
0
−200
−100
0
100
Relative difference [%]
200
0
−200
−100
0
100
Relative difference [%]
GOMOS/POAM III
40
35
Altitude [km]
Altitude [km]
40
Altitude [km]
8002
200
0
−200
−100
0
100
Relative difference [%]
200
Fig. 3. Comparison of GOMOS aerosol extinction profiles with other satellite measurements: SAGE II at 525 nm (left panel), SAGE
III at 596 nm (middle panel) and POAM III at 603 nm (right panel). Shown are the statistics for the relative differences, calculated as
Fig. 3. Comparison of
GOMOS aerosol extinction profiles with other satellite measurements: SAGE II at 525 nm (left panel), SAGE
100 × 2(pGOMOS − pSAT )/|pGOMOS + pSAT |. The median of the set of relative differences is shown with full lines and dots. Dashed lines
III at 596 nm (middleindicate
panel)
III16thatand
603
(right panel). Shown are the statistics for the relative differences, calculated as
the and
spread,POAM
given by the
84thnm
percentile.
100 × 2(pGOMOS − pSAT )/|pGOMOS + pSAT |. The median of the set of relative differences is shown with full lines and dots. Dashed lines
indicate the spread, given by the 16th and 84th percentile.
40
Comparisons with other
instruments
30
Alt. [km]
4
2002
40
30
2003
40
2004
40
2005
For the i-th coincidence,
the difference between GOMOS
30
30
(GOM) and the other instrument (SAT), relative to the mean
20
of20the two, was evaluated
as follows:
Alt. [km]
20
A detailed validation study20 for the GOMOS aerosol
extinction profiles has not been performed
yet.
Nevertheless,
Van10
10
10
10
hellemont et al. (2008) already performed comparisons with
(pGOM,i − pSAT,i )
−50
0
50
−50
0
50
−50 × 2
0
50
−50
0
50
1
=100
i
the results derived from the imagers of the Atmospheric
|pGOM,i + pSAT,i | x 10−3
2008
2006
2007
Chemistry Experiment ACE
Here,
40
40 (Bernath et al., 2005).
40
1
we present first comparisons with aerosol extinction proAs statistical estimators for the entire
data set, we prefered
30
30
30
0.8
files from the Stratospheric Aerosol and Gas Experiment
to use the median (50th percentile)
since
it is more robust
0.6
20
20
II (SAGE II; see Chu et 20al. (1989)), its follow-up
SAGE
with
respect to outliers than the numerical
mean. The vari0.4
III (Thomason and Taha, 102003) and the Polar
Ozone and
ance
of the data set was calculated
0.2 with the 16th and 84th
10
10
Aerosol Measurement POAM III (Lucke et al., 1999). As al0 estimates are shown in
percentile.
The50 obtained statistical
−50
0
50
−50
0
50
−50
0
ready stated, only dark limb measurements
were considered.
Lat. [deg.]
Lat. [deg.]
Lat. [deg.]
Fig. 3. As
can be seen, the comparisons are quite good at
Furthermore we avoided PSCs by excluding occultations inupper tropospheric/lower stratospheric altitudes. Differences
side the polar vortex:
apart
thevalues
wrong
assumption
of at 500
Fig. 4.
Yearly from
zonal median
of (dark
limb) aerosol extinction
nm for
the entireIIGOMOS
mission. 20%
All features
that 10
are toto
be 25 km, a conclusion
with
SAGE
are within
from
present: theinstruments
stratospheric aerosol
layer,two
PSCsdifin the Antarctic region, tropical subvisual cirrus clouds. Notice the enhanced
homogeneous layers,expected
two are
different
with
that
can2006.
also
bethatdrawn
for
the SAGE
III comparisons. The
stratospheric aerosol layer in 2007, resulting from the Soufrière Hills eruption
in May
Years
are sampled
incompletely:
2002 (start
ferent viewing geometries
will
deliver
two(instrument
different
profiles
of the data set:
August
2002), 2005
failure)
and 2008 (end of available
datadifferences
set).
median
with POAM III are even smaller, within
for the same cloud. This is less of a problem for tropical
10% from 11 to 22 km. Notice however that the variance is
subvisual cirrus since they typically have a very wide hormuch larger than for the SAGE II/III comparisons.
izontal extent and are very thin. A coincidence window of
+/−250 km and +/−6 h was used, allowing us to find a fairly
large comparison data set, summarized in Table 1. The ge5 Results
ographic location of the obtained coincidences is mainly determined by the coverage of the used instruments (SAGE II,
5.1 Yearly zonal statistics
SAGE III and POAM III) since GOMOS has a near-global
coverage with several occultation latitudes per orbit. Notice
A coarse idea about the presence of aerosols and clouds in
that we used a spectral channel of each instrument that is
the upper troposphere/lower stratosphere can be gained by
close to the GOMOS reference wavelength of 500 nm. Furconsidering zonal yearly statistics. Ranging from 90◦ S to
thermore, GOMOS data were interpolated to these wave90◦ N, 72 latitude bins with a width of 2.5 degrees were delengths using the retrieved quadratic polynomial. We should
fined. Aerosol extinction profiles were linearly interpolated
also mention that no effort was made to match the vertical
on a common altitude grid ranging from 1 to 50 km with a
resolution between two instruments with averaging kernels;
spacing of 1 km. Yearly statistics were subsequently calcuthis will certainly be done in a full validation study in the
lated on all data within one bin. Once again: we used perfuture.
centiles as statistical estimators since extinction values are
not necessarily normally distributed and because the median
(50th percentile) is rather insensitive to outliers. We should
Atmos. Chem. Phys., 10, 7997–8009, 2010
www.atmos-chem-phys.net/10/7997/2010/
15
15
15
10
10
10
5
5
5
F. Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
0
−200
−100
0
100
Relative difference [%]
200
0
−200
−100
0
100
Relative difference [%]
0
−200
200
8003
−100
0
100
Relative difference [%]
200
Table 1. GOMOS aerosol extinction retrievals: comparison data set; coincidence window: (500 km, 1/2 day)
Fig. 3. Comparison of GOMOS aerosol extinction profiles with other satellite measurements: SAGE II at 525 nm (left panel), SAGE
Instrument Version Wavelength # of coincidences
Time period considered
III at 596 nm (middle panel) and POAM III at 603 nm (right panel). Shown are the statistics for the relative differences, calculated as
100 × 2(pGOMOS
− pII
SAT )/|pGOMOS + pSAT |. The median of the set of relative differences is shown with full lines and dots. Dashed lines
SAGE
6.20
525 nm
6227
27 August 2002–March 5, 2005
indicate the spread, given by the 16th and 84th percentile.
SAGE III
POAM III
2002
40
Alt. [km]
3.00
4.00
596 nm
603 nm
6258
11 641
2003
40
7 November 2002–3 December 2005
26 August 2002–3 December 2005
2004
40
30
30
30
30
20
20
20
20
10
10
10
10
−50
0
50
2006
40
−50
0
50
2007
40
−50
0
50
−50
0
50
−3
2008
40
2005
40
x 10
Alt. [km]
1
30
30
30
20
20
20
10
10
10
−50
0
50
Lat. [deg.]
−50
0
50
Lat. [deg.]
0.8
0.6
0.4
0.2
−50
0
50
Lat. [deg.]
0
Fig. 4. Yearly zonal median values of (dark limb) aerosol extinction at 500 nm for the entire GOMOS mission. All features that are to be
Fig. 4. Yearly
zonal median values of (dark limb) aerosol extinction at 500 nm for the entire GOMOS mission. All features that are to be
expected are present: the stratospheric aerosol layer, PSCs in the Antarctic region, tropical subvisual cirrus clouds. Notice the enhanced
expectedstratospheric
are present:
the stratospheric
aerosolfrom
layer,
PSCs inHills
theeruption
Antarctic
region,
clouds.2002
Notice
aerosol
layer in 2007, resulting
the Soufrière
in May
2006. tropical
Years thatsubvisual
are sampledcirrus
incompletely:
(start the enhanced
stratospheric
in2002),
2007,2005
resulting
fromfailure)
the Soufrière
eruption
May
of theaerosol
data set:layer
August
(instrument
and 2008Hills
(end of
availablein
data
set).2006. Years that are sampled incompletely: 2002 (start
of the data set: August 2002), 2005 (instrument failure) and 2008 (end of available data set).
also mention that yearly zonal statistics are seasonally biased when we use only dark limb measurements, since Arctic/Antarctic summer is not sampled. The median particle
extinction at 500 nm is shown in Fig. 4 for every GOMOS
mission year. A few phenomena are readily observed: (1)
the umbrella-shaped stratospheric aerosol layer that is highest at the equator, lowest at the poles, (2) higher extinction
values in the Antarctic (and to a lesser extent Arctic) stratosphere that are caused by PSCs, and (3) higher extinction
values in a localized tropical zone at an altitude of about 16–
17 km due to subvisual cirrus clouds. The picture is almost
systematic for every year, but there are some significant differences however. First notice that Antarctic PSCs are less
pronounced for the years 2002 and 2008 because the Antarctic PSC season is not well sampled in those years: the data
set starts end of August 2002 and ends in May 2008. More
important, stratospheric aerosol extinction levels are much
higher in 2007 and remain elevated even in 2008, suggesting
the formation of new aerosols following stratospheric injection of SO2 by a volcanic eruption. This is the case; the
Soufrière Hills eruption (see below) in May 2006 is most
www.atmos-chem-phys.net/10/7997/2010/
likely the source. Furthermore, 2005 also seems to exhibit
elevated aerosol levels, although the picture is noisier due to
the incomplete sampling of the year (instrument failure).
Equally interesting is the yearly zonal variability of the
500 nm aerosol extinction values, calculated as half the difference between the 84th and 16th percentile. The variability is of course determined by the S/N-ratio of the measurements and (more importantly) by natural aerosol/cloud
variability, caused by the appearing, disappearing and atmospheric transport of particles. This is clearly visible on
Fig. 5. Typical “on/off-events” such as clouds (tropical cirrus, PSCs) exhibit large variability, while slowly changing
features (such as the stratospheric aerosol layer) vary little
within one year. Notice once again that the Antarctic PSC
variability during 2002 and 2008 is weak since these years
have not been completely sampled. At lower, tropospheric
altitudes, we see large variability due to a combination of
larger profile uncertainty (measured signals are weaker) and
tropospheric clouds.
Atmos. Chem. Phys., 10, 7997–8009, 2010
8004
Version
Wavelength
# of coincidences
Time period considered
SAGE II
SAGE III
POAM III
6.20
3.00
4.00
525 nm
596 nm
603 nm
6227
6258
11641
August 27, 2002 - March 5, 2005
November 7, 2002 - December 3, 2005
August 26, 2002 - December 3, 2005
2002
40
Alt. [km]
Instrument
F. Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
2003
40
2004
40
30
30
30
30
20
20
20
20
10
10
10
10
−50
0
50
−50
2006
40
0
50
2007
40
−50
0
2005
40
50
−50
0
−3
2008
40
50
x 10
Alt. [km]
1
30
30
30
20
20
20
10
10
10
−50
0
50
Lat. [deg.]
−50
0
50
Lat. [deg.]
0.8
0.6
0.4
0.2
−50
0
0
50
Lat. [deg.]
Fig. 5. Yearly zonal variability of (dark limb) aerosol extinction at 500 nm for the entire GOMOS mission. Variability is calculated as half
Fig. 5. Yearly
zonal variability of (dark limb) aerosol extinction at 500 nm for the entire GOMOS mission. Variability is calculated as
the difference of the 84th and 16th percentile, and is determined by measurement error and (more importantly) by natural aerosol/cloud
half the difference
of the
84thevents
and 16th
percentile,
and cirrus,
is determined
by measurement
(morewhile
importantly)
by natural
variability.
’On/off’
such as
clouds (tropical
Antarctic PSCs)
of course exhibiterror
large and
variability,
slowly changing
features aerosol/cloud
(backgrounds
aerosol
vary little.
Years that
are sampled
incompletely:
(start
of the large
data set:
august 2002),
2005slowly
(instrument
variability. “On/off”
events
suchlayer)
as clouds
(tropical
cirrus,
Antarctic
PSCs) of 2002
course
exhibit
variability,
while
changing features
andlayer)
2008 (end
of available
data set).
(backgroundsfailure)
aerosol
vary
little. Years
that are sampled incompletely: 2002 (start of the data set: august 2002), 2005 (instrument
failure) and 2008 (endVanhellemont
of available
data
set). Aerosol/Cloud Extinction observations
et al.:
GOMOS
9
Aerosol. Ext. [1/km]
uary 27 and 28 (Kamei et al., 2006). Manam is one of
2006, in a tropical latitude band from 6◦ N to 26◦ N. Three
the most active volcanoes
in the region; the above mendistinct periods can −3
be seen. First, we observe low back°
°
2004 to 2006 − Aerosol ext. 500 nm, alt. = 20 km, lat. = 6 N − 26 N
x 10
tioned eruptions followed ongoing volcanic activity that
ground aerosol 3levels until the end of 2004. Second, from
started already in October 2004. An image taken by the
early 2005 to the beginning of June 2006, large data gaps
infrared Aqua/MODIS (Moderate Resolution Imaging Specare present due2.5to instrument failure. Nevertheless, the data
troradiometer) indicates that the ash clouds of the first erupseem to suggest a declining tail following a volcanic erup2 we take the small set of data points at early
tion on Jan. 27 reached to over 20 km altitude, well into
tion, especially if
the stratosphere. The ash clouds of the second eruption on
2005 into account. And third, in early July 2006 we observe
1.5 by about a factor of 3 in extinction levels.
Jan. 28 ascended to 18 km altitude (Smithsonian Institution,
a sudden increase
2005). Several days later, stratospheric aerosol layers were
The same conclusions are drawn when inspecting the evolu1
detected twice between about 18 to 20 km altitude (layer
tion in latitude and
time on Fig. 7. It is clear that at least two
thickness ranging from 0.2 to 1.4 km) by a shipboard lidar usvolcanic events in the tropics should be considered.
0.5
ing a Nd:YAG laser operated at 1064 nm and 532 nm (Kamei
The elevated values in 2005 are most likely caused by
et al., 2006). The layers were detected in the Western Pacific
stratospheric sulfate
aerosols, formed out of the SO2 cloud
0
around 0 - 2◦ N, 156◦ E (Feb. 3-4 2005) and 7 - 9◦ N, 156◦ E
injected in the stratosphere by the eruptions of the Manam
◦
◦
(Feb. 9 - 10). Inspecting Fig. 7, we see that the amount of
volcano (Papua
New
Guinea,
4.080
S,
145.037
E)
on
Jan−0.5
−1
Feb04
Jun04
Sep04
Dec04
Mar05
Jul05
Date
Oct05
Jan06
May06
Aug06
Nov06
Fig. 6. A 2004-2006 GOMOS aerosol extinction time series at an altitude of 20 km. All data within a latitude band from 6◦ N - 26◦ N are
Fig. 6. A 2004-2006shown.
GOMOS
extinction
an altitude
20 km.
data within
a latitude
band from 6–26◦ N are
Clearly aerosol
visible are the
background time
aerosol series
conditionat
in 2004,
the possibleof
declining
tail ofAll
the post-eruptive
aerosols
resulting from
Manam
(Papua New
Guinea, condition
Jan. 27-28, 2005;
first vertical
and thedeclining
enhanced aerosol
following
the eruption aerosols
of
shown. Clearly visibletheare
the volcano
background
aerosol
in 2004,
the line),
possible
tail levels
of the
post-eruptive
resulting from
Soufrière Hills (Montserrat, May 20, 2006, second vertical line).
the Manam volcano (Papua
New Guinea, Jan. 27-28, 2005; first vertical line), and the enhanced aerosol levels following the eruption of
Soufrière Hills (Montserrat, 20 May 2006, second vertical line).
injected sulfur was large enough to leave a significant aerosol
trace in the GOMOS data during the largest part of 2005.
The source for the elevated aerosol levels in the second
5.2 Volcanic stratospheric
aerosols
half of 2006 and
2007 has been identified as the eruption
of the Soufrière Hills volcano (16.72◦N, 62.18◦W, Montserrat, West Indies), on May 20, 2006. Immediately following
the collapse of the eastern volcano flank, an eruption column
The observed elevated
extinction
levels
in 2005
reof ash
and gases rose
to at least
17 km. and
Prata2007
et al. (2007)
usedinvestigation.
a combination of satellite
instruments
quire a more detailed
On Fig.
6, we(Aqua/AIRS,
present
MSG/SEVIRI, MLS, OMI and CALIPSO/CALIOP) to reaerosol extinction values
altitude
of 20 km
(which
constructat
thean
event
and its immediate
aftermath.
Theylies
found
an estimated
0.1 Tg(S)for
was injected
in the stratosphere
above the subvisualthatcirrus
altitudes)
the period
2004–
in the form of SO2 , after which
the gas cloud
west◦ traveled
2006, in a tropical latitude
6◦20Nkm.
to 26
Three
ward at an band
altitudefrom
of about
CarnN.
et al.
(2007)
on the other hand used OMI data to obtain an estimated
0.22 Tg of SO2 . They also reported CALIPSO/CALIOP lidar 10,
backscatter
measurements
of the associated stratospheric
Atmos. Chem. Phys.,
7997–8009,
2010
aerosol layer as early as June 7, 2006, at an altitude of 20 km.
The layer remained visible in the CALIOP data until July
6. However, as Thomason et al. (2007) remarked, in general the stratospheric aerosol layer remains quite invisible in
CALIOP backscatter measurements, while occulation instru-
path lengths. Nevertheless, very recently Vernier et al. (2009)
showed how CALIPSO/CALIOP backscatter data improved
strongly after the introduction of a new cloud mask and a
new
calibration,
and howcan
an improved
stratospheric
distinct
periods
be seen.
First,aerosol
we observe low backpicture emerged after sufficient data averaging. In any case,
ground
aerosol
levels
until
the
end
of
2004. Second, from
the fact that GOMOS identifies stratospheric sulfate aerosols
early
to the
June
2006, large data gaps
well
after2005
the eruption
datebeginning
is illustrated inof
Fig.
8, which
shows the appearing and poleward transport of the sulfuric
are present due to instrument failure. Nevertheless, the data
acid particles.
seem
a declining
tail following
a volcanic erupFigs. 7to
andsuggest
8 also indicate
that it took roughly
one year
for
the Soufrière
Hills aerosol
the entireset
globe.
tion,
especially
if wecloud
taketo cover
the small
of data points at early
Elevated extinction levels remained present until at least the
2005
into Inaccount.
third,
earlyHills
July 2006 we observe
end
of 2007.
the monthsAnd
following
the in
Soufrière
eruption, a few other eruptions possibly impacted the stratospheric aerosol layer. Hofmann et al. (2009) briefly mentioned the July 14, 2006 Tungurahua eruption (Ecuador,
1,467◦S, 78,442◦W), less www.atmos-chem-phys.net/10/7997/2010/
than a month after the Soufrière
Hills eruption. And Thomason et al. (2007) showed CALIOP
observations of the volcanic plume resulting from the October 7, 2006 Tavurvur eruption (Rabaul, Papua New Guinea,
4.271◦S, 152.203◦E). Both are equatorial volcanoes that possible injected sulfur into the stratosphere, hereby enhanc-
10
Vanhellemont
et al.: GOMOS Aerosol/Cloud Extinction observations
F. Vanhellemont
et al.: GOMOS Aerosol/Cloud Extinction
observations
Lat. [deg.]
x 10
2
45
1.5
0
1
−45
−90
Jan05
0.5
Jan06
Jan07
Jan08
x 10
2
45
1.5
0
1
−45
−90
Jan05
0
−3
Aerosol extinction (500 nm) at 20 km altitude
90
Lat. [deg.]
−3
Aerosol extinction (500 nm) at 18 km altitude
90
8005
0.5
Jan06
Jan07
Jan08
0
Fig. 7. Checkerboard plots showing the median of binned 500 nm aerosol extinction data as function of latitude and time, at an altitude
7. Checkerboard
showing
the panel).
median ofNotice
binned 500
aerosol extinction
data early
as function
of latitude
and the
time,end
at anofaltitude
18 The global
of 18 km Fig.
(upper
panel) andplots
20 km
(lower
thenm
elevated
values after
2005,
and after
May of
2006.
and in
20 the
km (lower
panel). Notice the elevated values after early 2005, and after the end of May 2006. The global spreading
spreadingkm
is (upper
clearlypanel)
visible
latter case.
is clearly visible in the latter case.
a suddenetincrease
by From
aboutGOMOS
a factordata
of 3it in
levels.
al. (2009).
is extinction
at present unclear
or not the are
existing
volcanic
layer was
The samewether
conclusions
drawn
whenaerosol
inspecting
thereplenevoluished with
sulfate
aerosols
fromat
these
eruption in latitude
andnew
time
on Fig.
7. Itoriginating
is clear that
least
two
volcanictions.
events in the tropics should be considered.
The elevated
values
in 2005Clouds
are most likely caused by
5.3 Polar
Stratospheric
stratospheric sulfate aerosols, formed out of the SO2 cloud
detects PSCs quite
well.
Strongly of
enhanced
optiinjected GOMOS
in the stratosphere
by the
eruptions
the Manam
extinction
measured4.080
every◦year
in the Antarctic
◦ E) onPSC
volcano cal
(Papua
NewisGuinea,
S, 145.037
Janseason (roughly from the end of May to October), with valuary 27 ues
andtypically
28 (Kamei
et
al.,
2006).
Manam
is
one
of
3 or 4 times larger than in normal background
the mostconditions.
active volcanoes
in
the
region;
the
above
menThese kind of criteria (latitude, time of year, extioned eruptions
followed
ongoing
volcanic
activity
that
tinction values
above a certain
treshold)
allow us
to identify
the presence
of a PSC. 2004.
It is however
that by
is the
started already
in October
An temperature
image taken
the
driver for the
formation Resolution
of PSCs, as can
be seen
on
infrared main
Aqua/MODIS
(Moderate
Imaging
Specthe top panel of Fig. 9. Shown are all GOMOS aerosol extroradiometer)
indicates that the ash clouds of the first eruptinction measurements at 500 nm taken in 2004 at an alti◦
tion on 27
January
to over
20from
km 90
altitude,
into
tude of 20 kmreached
for the latitude
band
S to 55◦well
S. Temthe stratosphere.
The
ash
cloudsfrom
of the
second product
eruption
perature data
were
obtained
the GOMOS
fileson
consist oftoECMWF
The selected
data
Jan. 28and
ascended
18 kmanalysis
altitudeprofiles.
(Smithsonian
Instituset represents
of measurements
insideaerosol
and outside
tion, 2005).
Severala mixture
days later,
stratospheric
laythe Antarctic vortex (PSCs and stratospheric aerosols), but
ers wereplotted
detected
twice
between
about
18
to
20
km
altitude
against temperature the differentiation is very clear:
(layer thickness
to195
1.4 K,
km)
a shipboard
clouds areranging
formed from
below 0.2
about
theby
known
formalidar using
Nd:YAG laser
operated
1064Itnm
and 532less
nm
tionatemperature
of NAT
and STSatPSCs.
is however
clear
the data
not Type
II PSCs
at even
al.,from
2006).
Thewhether
layers or
were
detected
in form
the Western
(Kamei et
◦ E (3–4
lower temperatures.
Ideally,
differentiation
of 2005)
differentand
types7–
Pacific around
0–2◦ N, 156
February
◦
◦
9 N, 156 E (9–10 February). Inspecting Fig. 7, we see that
the amount of injected sulfur was large enough to leave a significant aerosol trace in the GOMOS data during the largest
part of 2005.
The source for the elevated aerosol levels in the second
half of 2006 and 2007 has been identified as the eruption
of the Soufrière Hills volcano (16.72◦ N, 62.18◦ W, Montserrat, West Indies), on 20 May 2006. Immediately following
the collapse of the eastern volcano flank, an eruption column
www.atmos-chem-phys.net/10/7997/2010/
Prata et al. (2007)
of ash
gases rose
at least
17 km.
of PSC
withand
UV/Vis/NIR
datato
should
be done
by inspecting
particle
of theinstruments
associated opti-(Aqua/AIRS,
usedsize
a distributions,
combinationor the
of shape
satellite
calMSG/SEVIRI,
spectrum. MethodsMLS,
have already
in the past
OMI been
and devised
CALIPSO/CALIOP)
to refor the SAGE and POAM instruments to make a distinction
construct the event and its immediate aftermath. They found
between Type Ia and Ib particles, based on the assumption
estimated
0.1than
Tg(S)
was injected
in the stratosphere
thatthat
the an
latter
are smaller
the former,
with a different
in
the
form
of
SO
,
after
which
the
gas
cloud
traveled westextinction spectrum as a2 consequence (Strawa et al., 2002;
Poole
et al.,
The second
panel of20Fig.
ward
at 2003).
an altitude
of about
km.9 shows
Carntheet al. (2007)
ratio
600 other
nm to 400
nm used
GOMOS
optical
extinction.
The-an estimated
onofthe
hand
OMI
data
to obtain
oretically, very small particles (β(λ) ∼ λ−4 ) should lead to a
0.22 Tg of SO2 . They also reported CALIPSO/CALIOP livalue of 0.2, while large particles (β(λ) = constant) should
dara value
backscatter
measurements
of the
associated
have
of 1. The
data are extremely
noisy
due to thestratospheric
aerosol
layer
as
early
as
7
June
2006,
at iman altitude of
already mentioned problem with the aerosol spectral law
plementation.
the data
cloud in
is more
or less data until
20 km. TheNevertheless,
layer remained
visible
the CALIOP
situated
between
the theoretical
limits,etand
particles
6 July.
However,
as Thomason
al.larger
(2007)
remarked, in genare observed below 195 K, as expected. The data show also
eral the stratospheric aerosol layer remains quite invisible in
that it is currently impossible to differentiate between differentCALIOP
PSC types.backscatter measurements, while occulation instru-
ments such as SAGE II have no problem with the detection.
No doubt this is caused by the preferential forward scattering
The
presented
PSC results
do notinadd
significantly with
new inof light
by small
particles,
combination
longer optical
formation to the current knowledge on PSC formation; for
Vernier
et al. (2009)
path
lengths.
Nevertheless,
very
recently
this, much more detailed analysis is needed (taking into acshowed
how
CALIPSO/CALIOP
backscatter
data
count all thermodynamic parameters and air parcel dynam- improved
strongly
the measurements
introduction clearly
of a new
cloud
ics).
But theafter
GOMOS
contain
ele- mask and a
ments
particle
new(temperature
calibration,dependence
and how of
anextinction
improvedandstratospheric
aerosol
sizes)
that are
crucial inafter
such sufficient
a detailed study.
picture
emerged
data averaging. In any case,
the fact that GOMOS identifies stratospheric sulfate aerosols
well after the eruption date is illustrated in Fig. 8, which
shows the appearing and poleward transport of the sulfuric
acid particles.
Figures 7 and 8 also indicate that it took roughly one
year for the Soufrière Hills aerosol cloud to cover the entire
globe. Elevated extinction levels remained present until at
least the end of 2007. In the months following the Soufrière
Hills eruption, a few other eruptions possibly impacted the
Atmos. Chem. Phys., 10, 7997–8009, 2010
Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
11
8006
F. Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
May 2006
Alt. [km]
30
June 2006
30
July 2006
30
30
20
20
20
20
10
10
10
10
−3
x 10
2
Aug. 2006
1.8
1.6
−50
Alt. [km]
50
Sep. 2006
30
−50
0
50
Oct. 2006
30
20
10
−50
0
50
Jan. 2007
10
−50
0
50
Nov. 2006
30
20
30
Alt. [km]
0
0
50
Feb. 2007
30
10
0
50
1.2
20
−50
30
0
50
Lat.
Mar. 2007
10
1
0.8
−50
30
0
50
Lat.
Apr. 2007
20
20
20
0.2
15
10
−50
0
Lat.
50
10
−50
0
Lat.
50
10
−50
0
Lat.
50
10
0.6
0.4
25
20
1.4
Dec. 2006
30
20
−50
−50
−50
0
Lat.
50
0
8. Evolution
volcanic
stratospheric
aerosolsresulting
resultingfrom
fromthe
theSoufrière
SoufrièreHills
Hillseruption
eruptionon
onMay
20 May
Fig.Fig.
8. Evolution
of of
thethe
volcanic
stratospheric
aerosols
20, 2006.
2006. Shown
Shown are
are monthly
monthly
zonal median aerosol extinction coefficients at 500 nm. Starting from the eruption site (Montserrat, 16.72◦◦ N, 62.18◦◦ W), the plume disperses
zonal median aerosol extinction coefficients at 500 nm. Starting from the eruption site (Montserrat, 16.72 N, 62.18 W), the plume disperses
poleward and encircles the entire earth within a year.
poleward and encircles the entire earth within a year.
stratospheric aerosol layer. Hofmann et al. (2009) briefly
6 Conclusions
mentioned the 14 July 2006 Tungurahua eruption (Ecuador,
1,467◦ S, 78,442◦ W), less than a month after the Soufrière
The current GOMOS operational aerosol/cloud product conHills eruption. And Thomason et al. (2007) showed CALIOP
sists of optical extinction profiles at 500 nm and additional
observations of the volcanic plume resulting from the Octospectral
to evaluate
extinction
at other
waveber 7,coefficients
2006 Tavurvur
eruptionthe
(Rabaul,
Papua
New Guinea,
lengths
as
well.
The
quality
of
the
product
is
not
optimal
◦
◦
4.271 S, 152.203 E). Both are equatorial volcanoesyet,
that
duepossible
to a problematic
spectral
combined
injected sulfur
intolaw
the implementation,
stratosphere, hereby
enhancwith
altitude
smoothing
that is tooHills
strong.
Furthermore,
inganthe
already
existing Soufrière
perturbation.
The
profiles
derived
from
bright
limb
measurements
currently
Tavurvur case was recently demonstrated inare
a convincing
notway
usable.
Nevertheless,
we restrict ourselvesdata
to the
of
using
the improvedifCALIPSO/CALIOP
by use
Vernier
500etnm
extinction
profiles
retrieved
from
dark
limb
measureal. (2009). From GOMOS data it is at present unclear
ments,
then
can be
considered
as layer
good,was
although
wether
or the
not quality
the existing
volcanic
aerosol
replenfineished
structure
(thin
cirrus
clouds,
cloud
inhomogeneities
with new sulfate aerosols originating from these etc.)
eruphastions.
been smoothed. For the first time, a comparison with
SAGE II, SAGE III and POAM III was presented, showing
good
within 20 % Clouds
in the upper troposphere/lower
5.3agreement
Polar Stratospheric
stratosphere, from 10 km to about 25 km.
All
the atmospheric
typesStrongly
that areenhanced
expectedoptito
GOMOS
detects PSCsparticle
quite well.
be cal
observed
by aissatellite
occultation
instrument
have been
extinction
measured
every year
in the Antarctic
PSC
detected
GOMOS:
sulfate
seasonby
(roughly
fromthe
thebackground
end of May stratospheric
to October), with
valaerosol
layer (or
layer)
typical background
umbrellaues typically
3 orJunge
4 times
largerwith
thanits
in normal
shaped
form, sulfate
aerosols
from volcanic
conditions.
These kind
of criteria
(latitude, origin,
time of tropical
year, exsubvisual
below
the tropopause
at identify
an altinctioncirrus
valuesclouds
above ajust
certain
treshold)
allow us to
titude of about 16-17 km, and PSCs in the polar regions.
Atmos. Chem. Phys., 10, 7997–8009, 2010
the presence of a PSC. It is however temperature that is the
The cloud-type events (PSCs and cirrus) have a strong yearly
main driver for the formation of PSCs, as can be seen on
variability, while the Junge layer remains remarkably conthe top panel of Fig. 9. Shown are all GOMOS aerosol exstant within one year.
tinction measurements at 500 nm taken in 2004 at an altiTheoflast
SO2band
injection
into
stratosphere
◦ Sthe
◦ S. Temtude
20 major
km forvolcanic
the latitude
from 90
to 55
dates
already
from
almost
18
years
ago
(Mount
Pinatubo,
perature data were obtained from the GOMOS product
files
1991),
with
the
consequence
that
current
aerosol
levels
are
and consist of ECMWF analysis profiles. The selected data
extremely
low.
So
low
that
the
effect
of
‘moderate’
volset represents a mixture of measurements inside and outside
canic
eruptionsvortex
becomes
in the GOMOS
aerosol
the Antarctic
(PSCsvisible
and stratospheric
aerosols),
but
record.
One
might
wonder
if
the
so-called
background
is not
plotted against temperature the differentiation is very clear:
only
maintained
bybelow
the typically
mentioned
sources
(OCS,
clouds
are formed
about 195
K, the known
formation
recently
even
anthropogenic
sulfate
from
coal
burning
in
temperature of NAT and STS PSCs. It is however less clear
China),
but
as
well
by
these
moderate
volcanic
eruptions,
that
from the data whether or not Type II PSCs form at even
are
much
weaker thanIdeally,
the catastrophic
events
(Mt. Pinatubo,
lower
temperatures.
differentiation
of different
types
Mt.
St.
Helens,
El
Chichón),
but
much
more
frequent.
Here,
of PSC with UV/Vis/NIR data should be done by inspecting
for
the
first
time,
the
aerosol
enhancements
resulting
from
particle size distributions, or the shape of the associated optithe
Manamhave
(Papua
Newbeen
Guinea)
andinSoufrière
caleruptions
spectrum.of
Methods
already
devised
the past
for the
SAGE and West
POAM
instruments
to make
a distinction
Hills
(Montserrat,
Indies)
have been
identified
in the
between data.
Type Ia
Ib particles,
based onwas
the assumption
GOMOS
In and
the latter
case, GOMOS
clearly able
thethe
latter
aredispersion
smaller than
theaerosols.
former, with
different
tothat
track
global
of the
In thea aftermath
et al.,in2002;
spectrum
as a consequence
(Strawa
ofextinction
the eruption,
two other
possible intrusions
should
prinPoolebeetvisible:
al., 2003).
The second
panel and
of Fig.
9 shows
the
ciple
Tungurahua
(Ecuador)
Tavurvur
(Papua
ratio Guinea).
of 600 nmWe
to 400
nmtoGOMOS
extinction.
TheNew
hope
identifyoptical
them in
future aerosol
oretically,
very small particles (β(λ)∼λ−4 ) should lead to a
profile
improvements.
The dynamics of PSCs have been studied previously with
www.atmos-chem-phys.net/10/7997/2010/
F. Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
x 10
2
1.5
1
www.atmos-chem-phys.net/10/7997/2010/
Acknow
cial aid
the Eur
(Contra
GOMO
0.5
0
−0.5
−1
180
Conclusions
The current GOMOS operational aerosol/cloud product consists of optical extinction profiles at 500 nm and additional
spectral coefficients to evaluate the extinction at other wavelengths as well. The quality of the product is not optimal yet,
due to a problematic spectral law implementation, combined
with an altitude smoothing that is too strong. Furthermore,
profiles derived from bright limb measurements are currently
not usable. Nevertheless, if we restrict ourselves to the use of
500 nm extinction profiles retrieved from dark limb measurements, then the quality can be considered as good, although
fine structure (thin cirrus clouds, cloud inhomogeneities etc.)
has been smoothed. For the first time, a comparison with
SAGE II, SAGE III and POAM III was presented, showing
good agreement within 20% in the upper troposphere/lower
stratosphere, from 10 km to about 25 km.
All the atmospheric particle types that are expected to
be observed by a satellite occultation instrument have been
detected by GOMOS: the background stratospheric sulfate
aerosol layer (or Junge layer) with its typical umbrellashaped form, sulfate aerosols from volcanic origin, tropical
subvisual cirrus clouds just below the tropopause at an altitude of about 16–17 km, and PSCs in the polar regions.
The cloud-type events (PSCs and cirrus) have a strong yearly
variability, while the Junge layer remains remarkably constant within one year.
The last major volcanic SO2 injection into the stratosphere
dates already from almost 18 years ago (Mount Pinatubo,
1991), with the consequence that current aerosol levels are
extremely low. So low that the effect of “moderate” volcanic eruptions becomes visible in the GOMOS aerosol
record. One might wonder if the so-called background is not
only maintained by the typically mentioned sources (OCS,
recently even anthropogenic sulfate from coal burning in
China), but as well by these moderate volcanic eruptions, that
are much weaker than the catastrophic events (Mt. Pinatubo,
Mt. St. Helens, El Chichón), but much more frequent. Here,
2004, alt. = 20 km, lat. = 90S − 55S
2.5
190
200
210
Temperature [K]
220
230
Refere
2004, alt. = 20 km, lat. = 90S − 55S
1
Aerosol Ext. ratio [no units]
6
−3
3
Aerosol Ext. [1/km]
value of 0.2, while large particles (β(λ)=constant) should
have a value of 1. The data are extremely noisy due to the
already mentioned problem with the aerosol spectral law implementation. Nevertheless, the data cloud is more or less
situated between the theoretical limits, and larger particles
are observed below 195 K, as expected. The data show also
that it is currently impossible to differentiate between different PSC types.
The presented PSC results do not add significantly new information to the current knowledge on PSC formation; for
this, much more detailed analysis is needed (taking into account all thermodynamic parameters and air parcel dynamics). But the GOMOS measurements clearly contain elements (temperature dependence of extinction and particle
sizes) that are crucial in such a detailed study.
8007
0.8
0.6
0.4
0.2
0
180
GOMO
PSC te
that pa
This is
crophy
As o
projec
improv
aeroso
the clo
be dev
bution
cle phe
190
200
210
Temperature [K]
220
230
Fig.
measurements of
of Polar
Polar Stratospheric
Stratospheric Clouds.
Clouds. Top
Top
Fig. 9.
9. GOMOS
GOMOS measurements
panel:
at 500
500 nm
nm versus
versus temperature
temperature at
at an
an altitude
altitude
panel: optical
optical extinction
extinction at
of
km, for
for all
all dark
dark limb
limb occultations
occultations in
in 2004
2004 at
at latitudes
latitudes below
below
of 20
20 km,
55◦◦ S.
S. Bottom
Bottom panel:
panel: extinction
extinction ratio
ratio (600/400
(600/400 nm),
55
nm), derived
derived from
from
the same
same measurements.
measurements.
the
for the first time, the aerosol enhancements resulting from
the eruptions of Manam (Papua New Guinea) and Soufrière
Hills (Montserrat, West Indies) have been identified in the
GOMOS data. In the latter case, GOMOS was clearly able
to track the global dispersion of the aerosols. In the aftermath
of the eruption, two other possible intrusions should in principle be visible: Tungurahua (Ecuador) and Tavurvur (Papua
New Guinea). We hope to identify them in future aerosol
profile improvements.
The dynamics of PSCs have been studied previously with
GOMOS data (Vanhellemont et al., 2005). The dependence
of PSC formation on temperature is a complex study topic.
In this paper, we only showed a first qualitative analysis with
GOMOS data, the results of which agree with the theoretical
Atmos. Chem. Phys., 10, 7997–8009, 2010
Bernath
ler,
P.-F.
Dufo
Jenn
Cone
wint
Roch
R., S
D., W
der,
Miss
doi:1
Bertaux
for a
obse
Bertaux
R., K
trend
in Sp
Bertaux
F., S
O., B
Saav
culta
ENV
Carn, S
Krue
serva
sphe
Chin, M
a so
Geop
8008
F. Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
PSC temperature dependence. The data show also roughly
that particle size information is present in the GOMOS data.
This is an important conclusion for future studies on the microphysics of PSCs.
As of February 2009, our team has been involved in a new
project (AERGOM, an ESA financed project) to develop an
improved algorithm that should deliver significantly better
aerosol extinction profiles (at all GOMOS wavelengths) in
the close future. A cloud-type identification method will also
be devised. This will enable us to derive particle size distributions, and to study the microphysics of the observed particle phenomena.
Acknowledgements. This work came into existence with the financial aid of the Belgian Federal Science Policy Office (BELSPO)
and the European Space Agency (ESA) through the AERGOM
project (Contract No. 22022/08/I-OL). ENVISAT is an ESA
mission and GOMOS is an EFI (ESA Funded Instrument).
Edited by: D. Murtagh
References
Bernath, P. F., McElroy, C. T., Abrams, M. C., Boone, C. D., Butler, M., Camy-Peyret, C., Carleer, M., Clerbaux, C., Coheur,
P.-F., Colin, R., DeCola, P., DeMazière, M., Drummond, J. R.,
Dufour, D., Evans, W. F. J., Fast, H., Fussen, D., Gilbert, K.,
Jennings, D. E., Llewellyn, E. J., Lowe, R. P., Mahieu, E., McConell, J. C., McHugh, M., McLeod, S. D., Michaud, R., Midwinter, C., Nassar, R., Nichitiu, F., Nowlan, C., Rinsland, C. P.,
Rochon, Y. J., Rowlands, N., Semeniuk, K., Simon, P., Skelton, R., Sloan, J. J., Soucy, M. A., Strong, K., Tremblay, P.,
Turnbull, D., Walker, K. A., Walkty, I., Wardle, D. A., Wehrle,
V., Zander, R., and Zou, J.: Atmospheric Chemistry Experiment
(ACE): Mission overview, Geophys. Res. Lett., 32, L15S01, doi:
10.1029/2005GL022386, 2005.
Bertaux, J., Kyrölä, E., and Wehr, T.: Stellar occultation technique
for atmospheric ozone monitoring: GOMOS on Envisat, Earth
Obs. Quart., 17–20, 2000.
Bertaux, J. L., Megie, G., Widemann, T., Chassefière, E., Pellinen, R., Kyrölä, E., Korpela, S., and Simon, P.: Monitoring of
ozone trend by stellar occultations: The GOMOS instrument,
Adv. Space Res., 11, 3237–3242, 1991.
Bertaux, J.-L., Kyrölä, E., Fussen, D., Hauchecorne, A., Dalaudier,
F., Sofieva, V., Tamminen, J., Vanhellemont, F., Fanton d’Andon,
O., Barrot, G., Mangin, A., Blanot, L., Lebrun, J. C., Fehr, T.,
Saavedra, L., and Fraisse, R.: Global Ozone Monitoring by Occultation of Stars: Global ozone monitoring by occultation of
stars: an overview of GOMOS measurements on ENVISAT, Atmos. Chem. Phys. Discuss., 10, 9917–10076, doi:10.5194/acpd10-9917-2010, 2010.
Carn, S. A., Krotkov, N. A., Yang, K., Hoff, R. M., Prata, A. J.,
Krueger, A. J., Loughlin, S. C., and Levelt, P. F.: Extended observations of volcanic SO2 and sulfate aerosol in the stratosphere,
Atmos. Chem. Phys. Discuss., 7, 2857–2871, doi:10.5194/acpd7-2857-2007, 2007.
Atmos. Chem. Phys., 10, 7997–8009, 2010
Chin, M. and Davis, D. D.: A reanalysis of carbonyl sulfide as a
source of stratospheric background sulfur aerosol, J. Geophys.
Res., 100, 8993–9005, 1995.
Chu, W., McCormick, M., Lenoble, J., Brogniez, C., and Pruvost,
P.: SAGE II inversion algorithm, J. Geophys. Res., 94, 8339–
8351, 1989.
Crutzen, P.: The possible importance of OCS for the sulfate layer
of the stratosphere, Geophys. Res. Lett., 3, 73–76, 1976.
Deshler, T., Hervig, M. E., Hofmann, D. J., Rosen, J. M., and
Liley, J. B.: Thirty years of in situ stratospheric aerosol size distribution measurements from Laramie, Wyoming (41N), using
balloon-borne instruments, J. Geophys. Res., 108, doi:10.1029/
2002JD002514, 2003.
Deshler, T., Anderson-Sprecher, R., Jäger, H., Barnes, J., Hofmann,
D. J., Clemesha, B., Simonich, D., Osborn, M., Grainger, R. G.,
and Godin-Beeckmann, S.: Trends in the nonvolcanic component of stratospheric aerosol over the period 1971-2004, J. Geophys. Res., 111, D01201, doi:10.1029/2005JD006089, 2006.
Hofmann, D., Barnes, J., O’Neill, M., Trudeau, M., and Neely, R.:
Increase in background stratospheric aerosol observed with lidar at Mauna Loa Observatory and Boulder, Colorado, Geophys.
Res. Lett., 36, L15808, doi:10.1029/2009GL039008, 2009.
Jensen, E. J., Toon, O. B., Selkirk, H. B., Spinhirne, J. D., and
Schoeberl, M. R.: On the formation and persistence of subvisible
cirrus clouds near the tropical tropopause, J. Geophys. Res., 101,
21361–21375, 1996.
Junge, C., Chagnon, C., and Manson, J.: Stratospheric aerosols, J.
Meteorol., 18, 80–108, 1961.
Kamei, A., Sugimoto, N., Matsui, I., Shimizu, A., and Shibata, T.:
Volcanic aerosol layer observed by shipboard lidar over the tropical western Pacific, Sci. Online Lett. Atmos., 2, 001–004, doi:
10.2151/sola.2006-001, 2006.
Kyrölä, E., Tamminen, J., Leppelmeier, G., Sofieva, V., Hassinen, S., Bertaux, J., Hauchecorne, A., Dalaudier, F., Cot, C.,
Korablev, O., Fanton d’Andon, O., Barrot, G., Mangin, A.,
Theodore, B., Guirlet, M., Etanchaud, F., Snoeij, P., Koopman,
R., Saavedra, L., Fraisse, R., Fussen, D., and Vanhellemont, F.:
GOMOS on Envisat – an overview, Adv. Space Res., 33, 1020–
1028, 2004.
Kyrölä, E., Tamminen, J., Sofieva, V., Bertaux, J.-L., Hauchecorne,
A., Dalaudier, F., Fussen, D., Vanhellemont, F., Fanton d’Andon,
O., Barrot, G., Guirlet, M., Mangin, A., Blanot, L., Fehr, T.,
Saavedra de Miguel, L., and Fraisse, R.: Retrieval of atmospheric
parameters from GOMOS data, Atmos. Chem. Phys. Discuss.,
10, 10145–10217, doi:10.5194/acpd-10-10145-2010, 2010.
Leung, F.-Y. T., Colussi, A. J., Hoffmann, M. R., and Toon,
G. C.: Isotopic fractionation of carbonyl sulfide in the atmosphere: Implications for the source of background stratospheric sulfate aerosol, Geophys. Res. Lett., 29(10), 1474,
doi:10.1029/2001GL013955, 2002.
Lucke, R. L., Korwan, D. R., Bevilacqua, R., Hornstein, J. S., Shettle, E. P., Chen, D. T., Daehler, M., Lumpe, J. D., Fromm, M. D.,
Debrestian, D., Neff, B., Squire, M., König-Langlo, G., and
Davies, J.: The Polar Ozone and Aerosol Measurement (POAM)
III instrument and early validation results, J. Geophys. Res., 104,
18785–18799, 1999.
Poole, L. R. and McCormick, M. P.: Airborne lidar observations
of Arctic polar stratospheric clouds: Indications of two distinct
growth stages, Geophys. Res. Lett., 15, 21–23, 1988.
www.atmos-chem-phys.net/10/7997/2010/
F. Vanhellemont et al.: GOMOS Aerosol/Cloud Extinction observations
Poole, L. R., Trepte, C. R., Harvey, V. L., Toon, G. C., and
Van Valkenburg, R. L.: SAGE III observations of Arctic polar stratospheric clouds – December 2002, Geophys. Res. Lett.,
30(23), 2216, doi:10.1029/2003GL018496, 2003.
Prata, A. J., Carn, S. A., Stohl, A., and Kerkmann, J.: Long
range transport and fate of a stratospheric volcanic cloud from
Soufrière Hills volcano, Montserrat, Atmos. Chem. Phys., 7,
5093–5103, doi:10.5194/acp-7-5093-2007, 2007.
Robock, A.: Volcanic eruptions and climate, Rev. Geophys., 38,
191–219, 2000.
Rodgers, C.: Inverse Methods for Atmospheric Sounding – Theory
and Practice, World Scientific, first edn., 108–110, 2000.
Smithsonian Institution: Bulletin of the Global Volcanism Network, 30(2): February 2005, Manam, http://www.volcano.si.edu/
reports/bulletin, 2005.
Sofieva, V. F., Kan, V., Dalaudier, F., Kyrola, E., Tamminen, J.,
Bertaux, J.-L., Hauchecorne, A., Fussen, D., and Vanhellemont,
F.: Influence of scintillation on quality of ozone monitoring by
GOMOS, Atmos. Chem. Phys., 9, 9197–9207, doi:10.5194/acp9-9197-2009, 2009.
Solomon, S., Borrmann, S., Garcia, R. R., Portmann, R., Thomason,
L., Poole, L. R., Winker, D., and McCormick, M. P.: Heterogeneous chlorine chemistry in the tropopause region, J. Geophys.
Res., 102, 21411–21429, 1997.
SPARC: Report no. 4: Assessment of Stratospheric Aerosol Properties (ASAP) – edited by: Thomason, L. and Peter, Th., Report
WCRP-124/WMO/TD-No. 1295, WMO/ICSU/IOC World Climate Research Programme, 2006.
Strawa, A. W., Drdla, K., Fromm, M., Pueschel, R. F., Hoppel,
K. W., Browell, E. V., Hamill, P., and Dempsey, D. P.: Discriminating Types Ia and Ib polar stratospheric clouds in POAM satellite data, J. Geophys. Res., 107, doi:10.1029/2001JD000458,
2002.
Tamminen, J., Kyrölä, E., Sofieva, V. F., Laine, M., Bertaux,
J.-L., Hauchecorne, A., Dalaudier, F., Fussen, D., Vanhellemont, F., Fanton d’Andon, O., Barrot, G., Mangin, A., Guirlet,
M., Blanot, L., Fehr, T., Saavedra de Miguel, L., and Fraisse,
R.: GOMOS data characterization and error estimation, Atmos. Chem. Phys. Discuss., 10, 6755–6796, doi:10.5194/acpd10-6755-2010, 2010.
Thomason, L., Pitts, M. C., and Winker, D. M.: CALIPSO observations of stratospheric aerosols: a preliminary assessment, Atmos. Chem. Phys., 7, 5283–5290, doi:10.5194/acp-7-5283-2007,
2007.
www.atmos-chem-phys.net/10/7997/2010/
8009
Thomason, L. W. and Taha, G.: SAGE III aerosol extinction measurements: Initial results, Geophys. Res. Lett., 30(12), 1631, doi:
10.1029/2003GL017317, 2003.
Twomey, S.: Introduction to the Mathematics of Inversion in Remote Sensing and Indirect Measurements, Elsevier Scientific
Publishing Company, New York, USA, 122–127, 1985.
Vanhellemont, F., Fussen, D., Bingen, C., Kyrölä, E., Tamminen, J.,
Sofieva, V., Hassinen, S., Verronen, P., Seppala, A., Bertaux, J.L., Hauchecorne, A., Dalaudier, F., Fanton d’Andon, O., Barrot,
G., Mangin, A., Theodore, B., Guirlet, M., Renard, J.-B., Fraisse,
R., Snoeij, P., Koopman, R., and Saavedra, L.: A 2003 stratospheric aerosol extinction and PSC climatology from GOMOS
measurements on Envisat, Atmos. Chem. Phys., 5, 2413–2417,
doi:10.5194/acp-5-2413-2005, 2005.
Vanhellemont, F., Fussen, D., Dodion, J., Bingen, C., and
Mateshvili, N.: Choosing a suitable analytical model for aerosol
extinction spectra in the retrieval of UV/visible satellite occultation measurements, J. Geophys. Res., 111, D23203, doi:
10.1029/2005JD006941, 2006.
Vanhellemont, F., Tétard, C., Bourassa, A., Fromm, M., Dodion, J.,
Fussen, D., Brogniez, C., Degenstein, D., Gilbert, K. L., Turnbull, D. N., Bernath, P., Boone, C., and Walker, K. A.: Aerosol
extinction profiles at 525 nm and 1020 nm derived from ACE
imager data: comparisons with GOMOS, SAGE II, SAGE III,
POAM III, and OSIRIS, Atmos. Chem. Phys., 8, 2027–2037,
doi:10.5194/acp-8-2027-2008, 2008.
Vernier, J. P., Pommereau, J. P., Garnier, A., Pelon, J., Larsen,
N., Nielsen, J., Christensen, T., Cairo, F., Thomason, L. W.,
Leblanc, T., and McDermid, I. S.: Tropical stratospheric aerosol
layer from CALIPSO lidar observations, J. Geophys. Res., 114,
D00H10, doi:10.1029/2009JD011946, 2009.
Wang, P., Minnis, P., McCormick, M., Kent, G., and Skeens, K.: A
6-year climatology of cloud occurrence frequency from Stratospheric Aerosol and Gas Experiment II observations (1985–
1990), J. Geophys. Res., 101, 29407–29429, 1996.
Zondlo, M. A., Hudson, P. K., Prenni, A. J., and Tolbert, M. A.:
Chemistry and microphysics of Polar Stratospheric Clouds and
Cirrus Clouds, Ann. Rev. Phys. Chem., 51, 473–499, doi:10.
1146/annurev.physchem.51.1.473, 2000.
Atmos. Chem. Phys., 10, 7997–8009, 2010