AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-9-115-2016Sensitivity of thermal infrared nadir instruments to the chemical and microphysical
properties of UTLS secondary sulfate aerosolsSellittoP.psellitto@lmd.ens.frLegrasB.https://orcid.org/0000-0002-3756-7794Laboratoire de Météorologie Dynamique (LMD), CNRS-UMR8539,
Institut Pierre Simon Laplace, École Normale Supérieure, École
Polytechnique, Université Pierre et Marie Curie, Paris,
FranceP. Sellitto (psellitto@lmd.ens.fr)18January20169111513228June201510August201519November201518December2015This 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/9/115/2016/amt-9-115-2016.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/9/115/2016/amt-9-115-2016.pdf
Monitoring upper-tropospheric–lower-stratospheric (UTLS) secondary
sulfate aerosols and their chemical and microphysical properties
from satellite nadir observations is crucial to better understand
their formation and evolution processes and then to estimate their
impact on UTLS chemistry, and on regional and global radiative
balance. Here we present a study aimed at the evaluation of the
sensitivity of thermal infrared (TIR) satellite nadir observations
to the chemical composition and the size distribution of idealised
UTLS sulfate aerosol layers. The extinction properties of sulfuric
acid/water droplets, for different sulfuric acid mixing ratios and
temperatures, are systematically analysed. The extinction
coefficients are derived by means of a Mie code, using refractive
indices taken from the GEISA (Gestion et Étude des Informations
Spectroscopiques Atmosphériques: Management and Study of
Spectroscopic Information) spectroscopic database and log-normal
size distributions with different effective radii and number
concentrations. IASI (Infrared Atmospheric Sounding Interferometer)
pseudo-observations are generated using forward radiative transfer
calculations performed with the 4A (Automatized Atmospheric
Absorption Atlas) radiative transfer model, to estimate the impact
of the extinction of idealised aerosol layers, at typical UTLS
conditions, on the brightness temperature spectra observed by this
satellite instrument. We found a marked and typical spectral
signature of these aerosol layers between 700 and
1200 cm-1, due to the absorption bands of the sulfate
and bisulfate ions and the undissociated sulfuric acid, with the
main absorption peaks at 1170 and 905 cm-1. The
dependence of the aerosol spectral signature to the sulfuric acid
mixing ratio, and effective number concentration and radius, as well
as the role of interfering parameters like the ozone, sulfur
dioxide, carbon dioxide and ash absorption, and temperature and
water vapour profile uncertainties, are analysed and critically
discussed. The information content (degrees of freedom and retrieval uncertainties)
of synthetic satellite observations is estimated for different instrumental configurations.
High spectral resolution (IASI-like pseudo-observations) and broadband spectral features
(Moderate Resolution Imaging Spectroradiometer (MODIS) and Spinning Enhanced Visible and
InfraRed Imager (SEVIRI)-like pseudo-observations) approaches are proposed and discussed.
Introduction
Secondary sulfate aerosols are sulfate-containing aqueous
solution droplets, generally of submicron size, produced from
gas-to-particle conversion processes involving sulfur-containing
gaseous precursors . They are one of the predominant
types of aerosols in the upper troposphere/lower stratosphere
(UTLS) and can have an important impact on
the atmospheric radiative transfer and climate see
e.g., cirrus formation and their optical
properties see e.g. and chemistry in the
UTLS see e.g.. Monitoring their chemical
composition, i.e. the mixing ratio of sulfates in the aqueous
solution, and their microphysical properties, i.e. the size
distribution parameters, is fundamental to better understand the
processes of formation and their impacts on UTLS chemistry and
radiative transfer. From a satellite perspective, limb-viewing and
occultation UV/VIS/NIR (ultraviolet, visible, near-infrared) and
TIR (thermal infrared) instruments, e.g. the Stratospheric Aerosol
and Gas Experiment (SAGE) II and III e.g.,
the Improved Stratospheric and Mesospheric Sounder (ISAMS) e.g., the
Atmospheric Trace Molecule Spectroscopy (ATMOS)
e.g., the Optical Spectrograph and Infrared
Imager System (OSIRIS) e.g. and
Atmospheric Chemistry Experiment (ACE)-Fourier Transform Spectrometer (FTS) e.g.
have been used to derive vertical profiles of UTLS sulfate
aerosol absorption, especially during strong volcanic
eruptions. These observations have allowed the
identification of the perturbation of the UTLS aerosol layer
produced by stronger volcanic eruptions. The retrieval of some parameters has
also been attempted, like sulfate mass or density e.g.,
effective radius e.g. and sulfate aerosol
composition e.g.. Some of these works rely on
strong assumptions. For example, and used a fixed sulfate mixing
ratio and temperature of the aerosol layer (and so a fixed refractive index)
and and considered sulfate aerosols as purely absorbing.
In addition, limb and occultation observations are not
well adapted to monitor the processes of formation and evolution
of the aerosol population due to the low horizontal resolution and
the scarce distribution of observations. Routine nadir
measurements are more adapted to monitor sulfate aerosols
properties from the regional to the global scale, to constrain
processes and to estimate the global impact of sulfate aerosol
production and evolution, due to their extended spatial
coverage and higher spatial resolution. Nadir satellite
observations in the solar spectral range (UV/VIS/NIR) are insensitive to the chemical
composition of aerosols, while they can provide partial
information on their size distribution, via the fine-to-coarse
aerosol optical depth ratio . Thermal
infrared (TIR) observations are sensitive to the chemical
composition of the aerosols due to the strong spectral variations
of the imaginary part of the refractive index in this band and,
correspondingly, of the absorption, as a function of the composition
e.g.. Unfortunately, the exploitation of
nadir TIR observations for sulfate aerosol layer monitoring
consists today only of a semi-quantitative detection
, as nadir observations are generally regarded
as weakly sensitive to the relatively small and diluted (in terms
of their number concentration) sulfate aerosol droplets. As
a consequence, despite their importance, satellite observation
of sulfate aerosols in the UTLS at the regional scale, and their
properties, is limited.
In this work, we present an analysis of the sensitivity of the
satellite nadir TIR observations to secondary sulfate aerosols in
the UTLS and their chemical and microphysical properties. The
main target of this paper is to link the optical characterisation
of sulfate solutions, as available from the published
spectroscopic laboratory measurements of e.g. ,
to the empiric observation of aerosol signatures in favourable
natural contexts, e.g. for the production of volcanically enhanced sulfates
in the UTLS. Indeed, a peculiar aerosol signature
(increasing absorption between about 700 and 1300 cm-1)
has been observed in the past in those conditions, e.g. by
for the eruption of the Sarychev volcano,
by for the eruption of the Pinatubo
volcano and by for the eruption of the Kasatochi volcano. These signatures have been attributed to the secondary
sulfate aerosols formed in the volcanic plume, without linking
them to the specific spectroscopic features of the different
sulfur-containing species (e.g. undissociated sulfuric acid molecules, sulfate and bisulfate ions) contained in the droplets and their chemical and
microphysical characterisation. Establishing such a link will
improve our understanding of the radiative properties of these
aerosol layers and is a prerequisite for an optimal retrieval of
their properties from satellite data. To do so, the extinction
properties of sulfuric acid/water droplets, for different
sulfuric acid mixing ratios and temperatures, are systematically
analysed. The extinction coefficients are derived by means of
a Mie code, using refractive indices taken from the GEISA (Gestion
et Étude des Informations Spectroscopiques Atmosphériques:
Management and Study of Spectroscopic Information) spectroscopic
database and log-normal size distributions with different
effective radii and number concentrations. IASI (Infrared
Atmospheric Sounding Interferometer) pseudo-observations are
generated using forward radiative transfer calculations performed
with the 4A (Automatized Atmospheric Absorption Atlas) radiative
transfer model, to estimate the impact of the absorption of
idealised aerosol layers, at typical UTLS conditions, on the
brightness temperature (BT) spectra observed by simulated
satellite instruments.
The paper is organised as follows: in Sect. we
introduce the data and methods used in our study. In
Sect. we discuss the spectral absorption of the
sulfate aerosol layer, and the dependence of the absorption on the chemical and
microphysical properties of the layer. The IASI pseudo-observations obtained
using these aerosol optical characterisations are discussed in
Sect. , and the UTLS sulfate aerosol BT signature is
isolated and analysed with respect to the chemical and
microphysical properties of the layer. The role of interfering
parameters, e.g. the uncertainties of the temperature, and water
vapour and ozone concentration profiles, as well as the absorption
by other volcanic species as SO2, CO2 and ash, are
discussed in Sect. . The information content of pseudo-observations
with different simulated instrumental configurations is discussed in Sect. . Conclusions are drawn in
Sect. .
Data and methods
To analyse the sensitivity of nadir satellite observations in the
TIR spectral region, we have generated a set of IASI
pseudo-observations with the Automatized Atmospheric Absorption
Atlas OPerational (4A/OP) radiative transfer model, see
e.g.. 4A/OP is a fast and accurate line-by-line
radiative transfer model, developed by the Laboratoire de
Météorologie Dynamique and the NOVELTIS company
(http://www.noveltis.com/) with the support of the CNES
(Centre National d'Études Spatiales), to simulate the radiative
transfer in planetary atmospheres, with a particular focus on the
IR spectral region. In 4A/OP simulations, as well
as simulations from nine other radiative transfer models, were
compared to IR high spectral resolution observations (aircraft observations with
the High-resolution Interferometer Sounder and the Airborne Research Interferometer Evaluation System) and found
capable of reproducing the observations to within the observation
noise. 4A/OP has been used in support of the IASI missions
. Please refer to the website
http://4aop.noveltis.com/ for more details of 4A/OP.
In the present work, IASI observations are simulated by means of
the observation geometry, the instrument spectral response
function (ISRF) and the radiometric noise. Typical nadir
observations have been considered, with zero viewing zenith
angle. We have simulated radiances in the range
700.0–1300.0 cm-1, with a spectral resolution of
0.25 cm-1 before apodization (0.50 cm-1 apodized spectral resolution), a spectral sampling
of 0.25 cm-1 and a Gaussian ISRF. The
radiometric noise is set at a noise equivalent brightness
temperature of 0.25 K. These parameters are implemented
into the standard distribution of 4A/OP, and based on IASI
preparatory studies (see e.g. ).
A baseline 4A/OP run, i.e. a run with no sulfate aerosols, is
first performed to compare with different configurations of the
UTLS sulfate aerosol layer, i.e. with different chemical and
microphysical properties. Then different runs with varying
H2SO4 mixing ratios and aerosol size distributions are
performed. For all these simulations, a typical tropical
atmosphere is considered in terms of temperature, pressure and trace gas
vertical profiles. The altitude of the sulfate
aerosol layer is fixed at about 150 hPa. 4A/OP is capable of
simulating aerosol scattering and absorption by means of the DISORT
(Discrete Ordinate Radiative Transfer) scheme, using the
extinction (absorption plus scattering) coefficient, the single
scattering albedo and the asymmetry parameter as optical
inputs. These optical parameters for the different simulated
sulfate aerosol layers are obtained with IDL (Interactive
Data Language,
http://www.exelisvis.com/ProductsServices/IDL.aspx) Mie
scattering routines of the Earth Observation Data Group of the
Department of Physics of Oxford University. Starting from the
refractive index and the size distribution of the aerosol layer,
the Mie theory allows the calculation of the extinction,
scattering and absorption coefficients, as well as derived
quantities like the single scattering albedo, and the angular
distribution of the radiation fields (by means of the phase
function), which can be represented by the integral asymmetry
parameter . It is worth mentioning that using Legendre moments
would be a better approximation of the phase function, when scattering processes are more important.
In this work we assume that sulfate aerosols are binary
systems of H2SO4/H2O solution droplets, with varying
H2SO4 mixing ratios. Ternary
HNO3/H2SO4/H2O solutions, which are important
for the formation of nitric acid hydrates in polar stratospheric clouds
, are excluded from this study. We have used
temperature-dependent refractive indices for different mixing
ratios of the binary system H2SO4/H2O from the
laboratory study of . These data are available
from the GEISA database . Refractive indices are
available for a wide array of temperatures, from 183 to
293 K, and H2SO4 mixing ratios (in mass, i.e. the
ratio of the H2SO4 mass to the total mass of the
droplets) ranging from 10 to 80 %, so covering
tropospheric and stratospheric conditions. These refractive
indices are measured in the region 500.0–5000.0 cm-1,
where major features of the absorption spectra of sulfate
droplets-radiation can be found, i.e. the OH stretching region
(3200–3500 cm-1), bisulfate ion absorption (1341,
1030, 1050, 885 and 593 cm-1) and sulfate ion
absorption (1104 and 613 cm-1). For higher
concentrations, features of the undissolved H2SO4
molecules (absorption at 1370, 1170, 965, 905 and
564 cm-1) appear as well. Please refer to
for more details on this data set and to e.g.
and for a detailed description of the
spectroscopic absorption vibration modes for the sulfate and
bisulfate ions, and the sulfuric acid molecule.
The size distributions for our simulated layers (i.e. a function
n(r) defined so that n(r)dr is the number of particles per
unit volume, with a radius between r and r+dr) are modelled as
log-normal distributions:
n(r)=N0rlnσr2πe-12ln(r/rm)lnσr2.
In Eq. (), N0 is the total number concentration (in
particles cm-3), rm is the mean radius and
lnσr is the unitless standard deviation of
ln(r/rm). For our simulations, we fixed
σr to 1.86 (a typical value; see
e.g. ) and we varied N0 and rm, to
study the impact of different number concentrations and mean size
on the absorption properties of the sulfate aerosol layers. We
simulate layers with N0=8, 9, 10, 12, 15, 20, 25 and 30
particles cm-3, and rm=0.06, 0.07, 0.08,
0.1, 0.15, 0.2, 0.3 and 0.4 µm. Here we recall that
a typical UTLS sulfate aerosol layer in background conditions,
i.e. without any perturbation from emissions of sulfur compounds
from natural (volcanic explosive eruptions) or anthropic (severe
UTLS pollution) sources, can be represented by a log-normal
distribution with N0 of a few particles cm-3 and
a rm lower than 0.1 µm, while
a volcanically perturbed layer can be represented by a log-normal
distribution with N0 of a few tens particles cm-3
and a rm higher than 0.2 µm. Thus, our database can represent several naturally
occurring conditions. It must be noted here that multi-modal
log-normal distributions have also been observed, especially in
volcanically perturbed situations see e.g.,
but are not simulated in the present study. We plan in the future
to extend our simulations to multi-modal log-normal size
distributions.
From H2SO4 concentration and aerosol size distribution,
other parameters can be derived, which are useful metrics for
remote sensing measurements. It is often convenient to express the
size distribution with compact parameters, like the effective
radius re and the effective number concentration
Ne. The effective radius is directly linked to the
extinction properties of the layer. These two quantities are defined
as follows see e.g.:
re=∫r3n(r)dr∫r2n(r)drNe=(∫r2n(r)dr)3(∫r3n(r)dr)2.
Because σr is fixed in our simulations, it follows that
re=rme2.5ln2σr=2.619rmNe=N0e-3.0ln2σr=0.315N0.
The two quantities are defined in such a way that
Nere2=N0rsurf2,
where rsurf=1N0∫r2n(r)dr
is the mean surface area of the aerosol layer. Please find in
Tables and the total number
concentrations and the mean radii used in this work, with the
corresponding effective radii and number concentrations.
Total number concentrations and effective number concentrations used in the present work.
Number concentrationEffective number concentration(particles cm-3)(particles cm-3)82.5292.83103.15123.78154.72206.83257.87309.45
Mean radii and effective radii used in the present work.
Mean radius (µm)Effective radius (µm)0.060.160.070.180.080.210.100.260.150.390.200.520.300.790.401.05
Then, from these two quantities and the sulfate aerosol density,
the effective mass volume concentration (in g m-3) can
be defined as follows:
Me=43πρ(c)re3Ne.
In Eq. (), ρ(c) is the density of the sulfate
aerosols (a function of the H2SO4 concentration), which
has been considered as fixed in a given aerosol layer. The
effective mass volume concentration can then be used to calculate
the total mass (in g, by multiplying by the volume occupied by the
aerosols) or the effective columnar abundance (in g m-2
by multiplying by the vertical altitude interval occupied by the
aerosols). This latter quantity,
Mcol=Me⋅Δz,
is linked to the total absorption due to the presence of sulfate
aerosols and is considered in the following discussions.
The extinction coefficient βext, discussed in the
following sections, can be expressed as follows:
βext(c,re,Ne,ν)=∫πr2Qext(m(c),r,ν)n(r)dr=∫πr2Qext(m(c),r,ν)N0rlnσr2πe-12ln(r/rm)lnσr2dr,
where Qext(m(c),r,ν) is the single particle
extinction efficiency factor from Mie theory, m is the complex
refractive index, which is a function of the H2SO4 mixing
ratio (c), and n(r) is the considered log-normal size
distribution. The mean radius and the total number concentration
are linked to the effective radius and number concentration by Eqs. (4) and (5). The dependence of the βext spectra on c
is transferred from Qext, and the dependence on Ne
and re is transferred from n(r). From the Eq. (), it
follows that the extinction coefficient can be separated in two
factors:
βext(c,re,Ne,ν)=NeF(c,re,ν),
where the function F(c,re,ν) does not depend on
the effective number concentration Ne.
Optical characterisation of the sulfate aerosol layers: dependence of the extinction coefficient on chemical and microphysical propertiesSulfate aerosol absorption and scattering
We first analyse the spectral extinction coefficient for different
chemical and microphysical properties of the sulfate aerosols
layer. Previous studies on the retrieval of sulfate aerosols have
neglected the scattering component of the extinction coefficient in
the TIR spectral region; see e.g. and . In
Fig. we show the single scattering albedo (the ratio
of the scattering and the extinction coefficient) of sulfate
layers with two different H2SO4 mixing ratios (64 and
75 %) and two typical size distributions: a background,
volcanically quiescent aerosol layer (N0=9cm-3,
rm=0.07µm, σm=1.86),
and a moderately perturbed volcanic aerosol layer
(N0=30cm-3, rm=0.2µm,
σm=1.86). In volcanically perturbed conditions,
the number concentration N0 is higher due to the enhanced
formation of new particles from sulfur dioxide emissions and the
mean radius rm is bigger due to the enhanced
coagulation of droplets in denser environments see
e.g.. In Fig. , as well as in
the other figures of this section, the spectral discretization
(available wavenumbers) originates from the spectral discretization
of the refractive indices in the database of
. While the absorption dominates the extinction in
the TIR spectral region, the scattering component has increasing
values for bigger particles and can reach values up to about
20 % in volcanic conditions. This result suggests that the
scattering component of the extinction, even if relatively small
with respect to the absorption, cannot be
neglected in volcanically perturbed conditions. Correspondingly, in the following sections we study the
extinction coefficient of the layer. The parameters of
the aerosol layer are the H2SO4 mixing ratio, the
temperature and the size distribution.
Spectral single scattering albedo, in the spectral range
600–1400 cm-1, for sulfate aerosol layers with different
H2SO4 mixing ratios (64 and 75 %, at 213–215 K),
for typical background size distribution (dark and light blue lines) and for
a moderate volcanically perturbed size distribution (dark and light red
lines). See the text for further details.
Dependence of the extinction coefficient on the H2SO4 mixing ratio and the temperature
To study how the extinction of the layer varies with respect to the
H2SO4 mixing ratio and the temperature, we use the
tabulated combinations of mixing ratios and temperatures in GEISA,
as described in Sect. , and we fix the size
distribution. The whole set of temperature/mixing ratio combinations, i.e. not
constrained to UTLS conditions, is analysed in the present section. This has been done to
have a more complete view of this dependency. We consider the two size distributions, background
and volcanic, introduced in Sect. . As said before, the
absorption dominates over the scattering and so the extinction
coefficient discussed in the following is mostly defined by the
sulfate absorption spectral features, except for a small
correction for layers with bigger particles (volcanic conditions, see Fig. and inherent text).
Figure shows the extinction coefficient in
the range 600 to 1400 cm-1 for sulfate aerosol layers
at temperatures of 293 and 213 K, and different
H2SO4 mixing ratios, depending on the availability of
mixing ratio/temperature combinations in the data set. The complete
data set, for all available temperatures in the interval
188–293 K can be found in Figs.S 1 and S2 of the
Supplement. In all cases, the extinction in volcanically perturbed
conditions is at least 50 times larger than in non-volcanic
conditions, as a result of more (N0=20 vs. 8 cm-3)
and bigger (rm=0.3 vs. 0.06 µm)
particles. This suggests a strong sensitivity of the sulfate
aerosol extinction to the size distribution, which is more
thoroughly discussed in Sect. . The H2SO4
mixing ratio is a relatively sensitive parameter as well, as the
variability of the extinction coefficient with respect to this
parameter is over a factor 3 to 5 for each given temperature. The
sulfate absorption region is located in this spectral range and is
clearly visible. A minimum extinction occurs at
650–800 cm-1. Then, from 900 to 1300 cm-1
the absorption bands of the sulfate ion (ν3 SO42-
asymmetric stretch band, centred at about 1104 cm-1),
the bisulfate ion (ν1 SO3- symmetric stretch band,
centred between 1030 and 1050 cm-1) and of the
undissociated sulfuric acid molecule (combination of bend and
stretch of different groups, with absorption peaks centred at 905,
965 and 1170 cm-1) dominate, generating a general
increase with wavelength of the absorption (and then of the
extinction) in this spectral region, with additional absorption
features at smaller spectral resolution. At mixing ratios larger
than 50 % and at all temperatures, a maximum
of the H2SO4 molecule absorption appears at 1170 cm-1
that is quite elevated; for smaller mixing ratios and lower temperatures, the
sulfate ion absorption feature at 1104 cm-1 dominates,
while the impact of the H2SO4 molecular absorption at
1170 cm-1 is less marked. Two further peaks can be found
at about 905 cm-1 (sulfuric acid molecular band) and
1050 cm-1 (bisulfate ion bands). In general, the
H2SO4 absorption bands are more important for concentrated
solutions (>60%), as first noticed and
attributed to different auto-dissociation equilibria occurring at
such higher concentrations. For more diluted solutions, the
dependence of the absorption spectra on temperature is due to the
dependence of the dissociation constant of the sulfate/bisulfate
system on temperature . Except for this effect, the
temperature seems a much less important factor than the mixing
ratio, to determine the absorption properties of the aerosol layer,
especially in real world UTLS conditions (H2SO4
mixing ratios greater than 60 %, temperatures smaller than
220 K).
Spectral extinction coefficients for sulfate aerosol layers with
different H2SO4 mixing ratios, from 0 to 80 % as indicated
by the colour bar, and temperatures of 293 K (upper row) and
213 K (bottom row). Different mixing ratio/temperature combinations
are shown, depending on the availability in the refractive indices data set of
. The extinction coefficients are shown for a typical
background size distribution (left column) and for a moderate
volcanically perturbed size distribution (right column). See the text for
further details.
A general increasing absorption by UTLS secondary sulfate aerosols
in the range 700–1300 cm-1 has been indeed observed in
the past by satellite limb observations, due to the Mount Pinatubo
eruption of 1991 . The two absorption peaks at 905
and 1170 cm-1 have also been observed by MIPAS-B
(Michelson Interferometer for Passive Atmospheric Sounding – Balloon-borne version) limb observations at altitudes between 11
and 16 km for the same volcanic event (see the spectra of
Fig. 3 in , which have shapes and absolute values
comparable to those of Fig. of the present
paper, for volcanic conditions, higher H2SO4 mixing ratios
and temperatures of about 200 K). have based a detection algorithm
for sulfate aerosols from NOAA (National Oceanic and Atmospheric Administration) -10, -11 and -12 observations
on the simulated spectral absorption differences between about 800 and 1200 cm-1. Nevertheless,
, and did not explicitly relate the
observed absorption coefficient spectra to these specific molecular
H2SO4 vibration modes. An increasing absorption has been observed in IASI observations
of the volcanic plume emanated by Kasatochi eruption of 2008 and erroneously attributed to sulfate
ionic absorption bands .
To further demonstrate the very small sensitivity of the sulfate
extinction spectra to temperature, and the much more important
contribution of the H2SO4 mixing ratio, we made a linear
regression of the maximum extinction at 1170 cm-1 and
its ratio to the minimum extinction near 800 cm-1, as
a function of the temperature, at fixed mixing ratios (parameters
of the regressions in Tables S1 and S2 in the Supplement), for
background and volcanically enhanced situations. In general, the
extinction does not vary much with temperature, except for a few
cases, e.g. a 20 % variation of the absolute extinction,
from 183 to 293 K, with a mixing ratio of 57 % in
both background and volcanic conditions. For other mixing ratios,
the variation with temperature is less than 10 %. This
variability is much more limited than that due to mixing ratio
variations, which can be larger than 100 % (for example,
the maximum absorption at 1170 cm-1 varies from about
2.5×10-3 to about 6.0×10-3km-1,
and from about 2.5×10-5 to about 6.0×10-5km-1, in volcanic and background conditions,
respectively, with mixing ratios spanning 30 to 64 %). The
variability of the relative extinction 1170–800 cm-1
with temperature is even smaller.
These analyses of the dependence of the extinction coefficient on
the temperature and H2SO4 mixing ratio point out that the
temperature of the aerosol layer (and therefore its altitude) cannot be
retrieved from aerosol extinction measurements, especially for
mixing ratios greater than 60 %. From a different
perspective, this is beneficial for the retrieval of sulfate
aerosol chemical and microphysical parameters because this
property limits the number of sensitive factors affecting the
extinction spectra variability. The extinction, and more in
particular the absorption signature, is very sensitive to the
H2SO4 mixing ratio of the binary solution
H2SO4/H2O, both in background and volcanically enhanced
situations, paving the way to a retrieval.
Dependence of the extinction coefficient on the size distribution
Figure suggests that the sulfate
extinction signature is sensitive to the size distribution of the
aerosol layer. In this section, we systematically investigate this
dependence by estimating the individual contributions of the
number concentration and effective radius. To conduct targeted
analyses on the UTLS sulfate aerosols, we have limited the
H2SO4 mixing ratio to four values, 60, 64, 70 and
75 %, and the temperature to two close values, 213 and
215 K, among those available in the database. This is
intended to simulate typical tropical UTLS temperatures
see e.g. and typical mixing ratios within
dehydrated atmospheric regions . In this section, we let the size distribution parameters vary independently.
Spectral extinction coefficient for a sulfate aerosol layer, as
a function of (a, b) the effective number concentration
Ne (different number concentrations in different colours; see
colour-coded legend) (c, d) the effective radius re
(different radii in different colours, see colour-coded legend). Sulfate
aerosol layers with 60 and 75 % (values indicated in the plots)
H2SO4 mixing ratios are considered at 213–215 K. Please
note the logarithmic ordinate in (c, d).
Figure a and b show the extinction coefficient in
the spectral range 600–1400 cm-1 for these UTLS
conditions, as a function of the effective number concentration
Ne, illustrating the proportionality shown in
Eq. (). The range of variation of Ne (see
Table ) covers background to severe volcanic
conditions (i.e. Pinatubo-like). The effective radius re (corresponding
rm in parentheses) is fixed at 0.79
(0.30) µm. The H2SO4 mixing ratio is fixed at
60 % (a) and 75 % (b). Figure c
and d show the extinction coefficient in the spectral range
600–1400 cm-1 as a function of the effective radius
re. Note that in this figure the ordinate axis is
logarithmic, to better show the spectral behaviour of the
different curves. The effective number concentration
Ne (corresponding N0 in parentheses) is now
fixed at 7.87 (20.00) cm-3. The H2SO4 mixing
ratio is fixed at 60 % (c) and 75 % (d). The
spectral patterns of the sulfate/bisulfate ions and the
sulfuric acid molecule are clearly visible for all effective
number concentrations and radii, with the same three marked peaks that
emerged in Fig. . Figure c
and d show that F(c,re,ν) depends strongly on
re, growing monotonously with re. The
growth is not, however, uniform in ν, as spectral extinction
gets flatter with bigger sulfate particles. In general, the
extinction is very small for smaller particles. Comparing
Fig. a, b and a, c, it appears that
the most important condition leading to significant extinction is
a large effective radius of the layer. At a fixed H2SO4
mixing ratio, the variability of the extinction, e.g. at the
maximum absorption at 1170 cm-1, due to the effective
radius can be up to 2 orders of magnitude stronger than that due to
the effective number concentration.
Discussion on sulfate aerosol extinctionBroadband spectral features
The spectral extinction of sulfate aerosol layers is, to
different extents, sensitive to the three chemical and
microphysical parameters under investigation, namely the
H2SO4 mixing ratio, the effective radius and the
effective number concentration.
In Sects. and we have analysed the
dependence of the spectral extinction curves as a whole, in
function of the chemical and microphysical properties:
H2SO4 mixing ratio, the effective radius and the
effective number concentration. We now fix the wavenumber. At
a selected wavenumber ν‾ the extinction coefficient
AE (absolute extinction) is (see Eq. )
AE=βext(c,re,Ne,ν‾)=NeF(c,re,ν‾).
We introduce the ratio RE (relative extinction) between the
extinction coefficient at two different wavenumbers ν1 and
ν2:
RE=βext(c,re,Ne,ν1)βext(c,re,Ne,ν2)=NeF(c,re,ν1)NeF(c,re,ν2)=G(c,re,ν1,ν2).
The two functions F(c,re,ν‾) and
G(c,re,ν1,ν2), as well as AE and RE,
depend on the choice of the wavenumbers ν‾, ν1
and ν2. The absolute and relative extinction at selected
wavenumbers is important for remote sensing applications, in
particular for radiometers that have broadband channels at
a number of wavenumbers. Here we select specific wavenumbers where
informative features of the spectral extinction of sulfate aerosol
layers are found, as discussed in Sects. and . Correspondingly, we introduce three spectral
quantities: (1) the maximum AE at 1170 cm-1 (hereafter
referred to as ME, maximum extinction), (2) its relative extinction
with respect to the minimum extinction at 800 cm-1 (the
extinction ratio between 1170 and 800 cm-1, hereafter
referred to as RE1) and (3) the relative extinction of the secondary
peak at 905 cm-1 with respect to the minimum extinction
at 800 cm-1 (the extinction ratio between 905 and
800 cm-1, hereafter referred to as RE2). We recall that
the extinction peaks at 1170 and 905 cm-1 are mostly due
to molecular H2SO4 absorption bands. Another important
spectral feature in the sulfate signature is the extinction peak
due to the ionic absorption at 1050 cm-1, but we exclude
the latter because the information in this band is disturbed by
the ozone absorption band region at 9.6 µm (more details
in Sect. ).
RE1 (a) and RE2 (b) as a function of the effective
radius and H2SO4 mixing ratio. Please note the different scaling of
the colour bars. Arc cosine of the scalar product ∇RE1⋅∇RE2(c). The gradients are
normalised.
The variability of the spectral parameter ME as a function of the
H2SO4 mixing ratio and effective radius and number
concentration has been discussed in Sect. . As it
can be seen from Fig. , ME varies between near-zero
values (for small Ne, re and c) and
40×10-3km-1 (large values of the three
aerosol parameters). ME is very small for small effective radii,
disregarding the value of the effective number concentration and
H2SO4 mixing ratio. Higher values (>10×10-3)
are only found for effective radii bigger than about 0.5 µm. This is an indication of the small sensitivity of the
spectral extinction to sulfate aerosols in background conditions,
and the dominating role of the particle size in ME, as well as in the
depth and the whole curve of the sulfate spectral extinction
signature. Figure a and b show the variability of the
spectral parameters RE1 and RE2 as a function of effective radius
and H2SO4 mixing ratio, as RE1 and RE2 are independent
of the effective number concentration (see Eq. ). The
behaviour of RE1 and RE2 is similar, with increasing values for
higher H2SO4 mixing ratios and smaller particles.
As both RE1 and RE2 depend on c and re but not on
Ne, it is useful to determine whether they differ in
this dependency, having in mind the possibility of retrieving c and
re from spectrometric measurements. Figure c shows the angle α=cos-1∇RE1(c,re)⋅∇RE2(c,re) of the
gradients of RE1 and RE2 with respect to c and re,
normalised to vary over the same range. If the angle is near-zero,
the gradients are parallel vectors and then the variations of RE1
and RE2 carry a very dependent information content. Conversely, for
angles departing from zero, the information content about the
aerosol parameters is more and more independent. It is visible
that the angle remains small for small re and when
both re and c are large, and that there is an
optimal sensitivity range which more or less extends about
a diagonal from the upper left of the diagram, with angles between
15 and 30∘.
To conclude this section, we suggest that these three
broadband spectral parameters, ME, RE1 and RE2, could be used to detect and
extract some quantitative information on UTLS sulfate aerosols, in
the case of volcanically enhanced conditions. ME is sensitive to
the three aerosol parameters, while RE1 and RE2 are only sensitive
to the H2SO4 mixing ratio and the effective radius. In
principle, the information content carried by ME, RE1 and RE2 on
the H2SO4 mixing ratio, effective radius and number
concentration is partially independent for most conditions and then
broadband retrieval schemes could be developed using these three
spectral parameters. One option could be to infer the
H2SO4 mixing ratio and the effective radius from RE1 and
RE2, and then use this information to derive the number
concentration with ME1. In practice, due to the partial correlation
of the information content of RE1 and RE2 to the H2SO4
mixing ratio and the effective radius, and the scarce sensitivity
of ME to the effective number concentration for background
non-volcanic conditions, the sulfate aerosol parameters might be
hardly retrievable in those conditions. However, in
volcanically enhanced conditions, there is an enhanced variability
of ME with respect to the three aerosol parameters but the
variability of RE1 and RE2 with respect to the H2SO4
mixing ratio is relatively small, and the correlation of their
information content is high for bigger values of the H2SO4
mixing ratio. All these considerations suggest that the three
aerosol parameters are only retrievable as independent quantities
for limited conditions when using broadband sulfate extinction
spectral features, and that constraints should be given to at least one
parameter (e.g. the effective number concentration). The approach
of using absolute and relative extinction features is applicable to
both high-resolution and broadband TIR satellite sensors as MODIS
(Moderate Resolution Imaging Spectroradiometer) or
SEVIRI (Spinning Enhanced Visible and InfraRed Imager), even if
these latter instruments may not have optimised channels for
sulfate aerosols observations. The information content of pseudo-observations
based on broadband features (with different nadir instruments), and on high
spectral resolution are discussed in Sect. . It can be noticed
that the use of satellite observations at similar spectral intervals, i.e. broadband
observations at about 800, 900 and 1200 cm-1, has been proposed in the past to observe
secondary sulfate aerosols after strong volcanic eruptions e.g. .
Size distribution sensitivity at fixed mass
In the previous sections, the variability of the sulfate aerosol extinction
coefficient has been studied as a function of the three parameters
Ne, re and c, by varying one parameter with the
other two kept fixed. This has been done, having in mind the possibility of
retrieving the three parameters from observations of the sulfate aerosol
extinction. In any case, it must be considered that in real-world conditions,
the variability of these parameters is constrained by the available sulfate
mass. Then, it is instructive to study the variability of the spectral
extinction coefficient as a function of these parameters, by keeping the mass
fixed.
Figure shows the extinction coefficient at
1170 cm-1 (the maximum extinction or ME; see Sect. ) as
a function of the effective radius re and effective number
concentration Ne, for fixed sulfate aerosol effective mass
volume concentrations Me, as defined in Eq. ().
The ME variabilities are shown for two cases: background and
volcanically enhanced conditions. Typical size distributions for these two
cases, as defined in Sect. , are used to fix the background and
volcanically enhanced mass. The value of c has been kept fixed at 60
(background) and 75 % (volcanically enhanced conditions). The fixed
Me values are then 0.062 µg m-3 (background)
and 31.002 µg m-3 (volcanically enhanced conditions).
Extinction coefficient at 1170 cm-1 (maximum extinction, ME)
as function of the effective radius re (green line) and
effective number concentration Ne (red line), for fixed
sulfate aerosol effective mass volume concentrations Me
(0.062 µg m-3: background conditions – left panel; 31.002 µg m-3:
volcanically enhanced conditions – right panel).
The sulfate extinction is 2 orders of magnitude stronger for a
volcanically typical mass with respect to background. By keeping
Me fixed, the sulfate extinction varies by more than 20 %
from smaller to bigger re, in both conditions. The dependence
on Ne, on the contrary, is very limited. These further
simulations allow most of the variability to be attributed to the size
distribution parameters, as described in Sect. , to the
increment of sulfate mass when increasing Ne or/and
re. In particular, the dominant role of re in
determining significant extinction signatures is mostly due to an increase of
the sulfate mass in the presence of bigger particles. Nevertheless, even at
fixed Me, there is a significant additional variability of the
extinction with varying re. This evidence confirms that
re is the dominant factor determining the sensitivity of the
sulfate aerosol extinction spectra.
Spectral sensitivity of IASI pseudo-observations to chemical and microphysical properties
In order to test the sensitivity of satellite nadir observations in
the TIR to sulfate aerosols and the perturbation brought by other
atmospheric species under real conditions, the spectral extinction
coefficients of the sulfate aerosol layers obtained in
Sect. are used here as inputs of the forward
radiative transfer modelling described in Sect. . As
done for Sects. and , we restrict
our attention to H2SO4 mixing ratios of 60, 64, 70 and
75 % and temperature of the droplets of
213–215 K, within the set of tabulated values. As
demonstrated in Sect. , the extinction properties of
the H2SO4/H2O droplets layers are very weakly dependent
on the temperature. The chosen parameters are intended to mimic
a realistic tropical UTLS situation, given the constraints of our
data set. The altitude of the sulfate aerosol layer has been fixed
at about 150 hPa. A typical tropical atmosphere
(temperature, humidity and gas concentration profiles) is selected
from those available in 4A/OP. Then we conduct 256 simulations by
varying the four H2SO4 mixing ratio/temperature
combinations and all the available effective number concentrations
and effective radii (see Tables and ). A further simulation is made with the same
atmosphere but without a sulfate aerosol layer, to define
a baseline and to isolate the sulfate aerosol impact on the output
spectra. Finally, 256+1 IASI spectral BT pseudo-observations are
obtained. We then define a sulfate aerosol brightness temperature
signature (hereafter simply referred to as BT signature)
as the spectral differences between each of the 256 brightness
temperature spectra and the one obtained with the baseline
run. Figure shows examples of BT signatures for
different combinations of the effective number concentration, the
effective radius and the H2SO4 mixing ratio. In the
spectral region 700–1300 cm-1 a few main interfering
features emerge, like the ubiquitous weak absorption bands of
H2O and the strong rotational-vibrational ozone band
centred at 9.6 µm (1041.67 cm-1) and
affecting the spectral region of approximately
980–1080 cm-1 (see also Fig. ). An initial
finding is that the signal of the bisulfate ion bands at 1030
and 1050 µm is affected by the
interference with the ozone band. This justifies discarding these
absorption features in the discussion of Sect. . The
sulfate ion band at 1104 cm-1 and the weaker
bisulfate band at 965 cm-1 are partially also affected
by the peripheral region of the ozone band. Therefore, it is
very hard to use the ionic bands to infer useful information on
sulfate aerosols. This is mostly inconvenient for small mixing
ratios, where ionic absorption dominates over molecular absorption
(see Fig. ). On the contrary, the molecular
H2SO4 absorption band at 1170 cm-1, as well as
the weaker H2SO4 band at 905 cm-1 and the
background region at 800 cm-1 are clearly visible, while
still partly affected by the weak, fine-scale, ubiquitous water
absorption lines.
The stronger band at 1170 cm-1 is the most important
feature in the sulfate aerosol extinction spectra and is
responsible of the typical increasing extinction or
decreasing BT signature shape between 700 and
1200 cm-1, which has been empirically observed in the
past see
e.g.. In
Sect. it was found that the extinction signature of
the sulfate aerosols, and the three partially independent spectral
parameters, ME, RE1 and RE2, are sensitive to the H2SO4
mixing ratio and to the size distribution, and this sensitivity is
dominated by the effective radius. Consistently, from
Fig. it can be seen that the depth of the BT
signature (and then ME) is strongly dependent on the effective
radius and much less sensitive to the effective number
concentration and the H2SO4 mixing ratio. However, for
fixed effective radius and number concentration, there is
a variation of the spectral slope between 700 and
1200 cm-1 with the H2SO4 mixing ratio. This is
again consistent with the results of Sect. . In
general, the BT signatures at 1170 cm-1 are less than
0.7 K for small particles, with effective radii smaller
than 0.52 µm for all number concentrations and
H2SO4 mixing ratios. The BT signatures at
1170 cm-1 can reach values of 3.0 to 5.0 K for
bigger particles, with higher values for larger number
concentration and H2SO4 mixing ratio. Values of
re of 0.6–0.7 µm and higher are typical
of coagulation effects in more or less severe volcanically enhanced
conditions. Thus, it seems very arduous to observe the background
sulfate aerosol layers, while BT signatures of volcanic sulfates
can be relatively strong.
BT signature of sulfate aerosol layers, for effective number
concentrations of 2.52 (left panel) and 6.30 particles cm-3
(right panel), for effective radii of 0.16, 0.52 and 1.05 µm (see
text in the figures) and H2SO4 mixing ratios of 60, 64, 70 and
75 % (in different colours).
Spectral BT signatures for a sulfate aerosol layer at about
150 hPa altitude, as a function of the effective number concentration
Ne for a fixed effective radius
re=0.79µm(a, b), and as a function of the
effective radius re for a fixed effective number concentration
Ne=7.87 particles cm-3(c, d). Sulfate
aerosol layers are considered with 60 %(a, c) and 75 %(b, d)H2SO4 mixing ratios at 213–215 K.
Individual (grey) and average TIGR tropical vertical profiles with
standard deviations (black), for temperature (a) and water vapour
mixing ratio (b). The standard deviation of the Gaussian noise used
to generate the perturbed profiles is in red in both figures; spectral BT
sensitivity for temperature profile (mean differences in black, 1 standard
deviation interval in grey) and water vapour profile variability (mean
differences in red, 1 standard deviation interval in orange) (c).
See text for further details on how this sensitivity is evaluated.
The variability of the spectral BT signature with effective number
concentration, effective radius and H2SO4 mixing ratio is
more systematically displayed in Fig. . This figure
shows the BT signature in the spectral range
700–1300 cm-1, as a function of the effective number
concentration Ne, for a fixed effective radius
re=0.79µm, and as a function of the
effective radius re, for a fixed effective number
concentration Ne=7.87 particles cm-3. The
same patterns evidenced in Fig. , the discussion
above and the results of Sect. are systematically
observed in these plots. A marked maximum of the BT signature at
1170 cm-1, as well as a secondary maximum at
905 cm-1, both due to the molecular H2SO4
absorption, are apparent. The dramatic difference for
Fig. c and d from smaller to bigger effective radii,
is a confirmation of the relative importance of the effective
radius in determining the signature of the UTLS sulfate aerosols
in TIR spectra, and the very low sensitivity at background
conditions.
The analyses of the present section, in conjunction with the
discussion of Sect. , suggest that information on
chemical and microphysical properties of sulfate aerosols can be
extracted using broadband spectral features, e.g. the maximum
absorption near 1170 cm-1, and the spectral ratios
between 1170 and 800 cm-1 and between 905 and
800 cm-1. Nevertheless, the presence of relatively well-separated ionic and molecular absorption signatures justify the use
of a spectral fitting approach to extract the full information
content using finer spectral resolution. These fine-scale structures
are typical of liquid aerosols. Thus, we wish to stress how high-spectral-resolution observations, like those made by IASI, can
bring a significant added value to the characterisation of such
aerosols.
Interfering parametersInterference with temperature and humidity profiles variability
We consider here how uncertainties in the temperature and humidity
profile may mask the sensitivity to sulfate aerosols. The TIGR
(Thermodynamic Initial Guess Retrieval) database is a library of
2311 representative atmospheric situations (tropical, mid-latitude
and polar) selected by statistical methods from an ensemble of
80 000 radiosonde measurements . We have
calculated the average temperature and water vapour mixing ratio
profile from TIGR. Only the tropical atmosphere situations (872
profiles) have been considered. Figure a and b
show the individual tropical temperature and water vapour
profiles, respectively, and the average profiles with standard
deviations. We then generate two data sets of 128 profiles each, by
perturbing the average temperature (one set of 128 perturbed
profiles) and water vapour profile (one set of 128 perturbed
profiles) with a height-dependent Gaussian noise. For each level,
we added a random value from a Gaussian distribution of
zero mean value and variable standard deviation, depending on the
altitude level, to simulate the actual temperature and water vapour
retrieval uncertainties of IASI, as estimated e.g. by
. In this latter work, temperature and water
vapour profile uncertainties are estimated by comparing a data set
of IASI temperature and water vapour profile retrievals with
co-located radiosonde profiles. The standard deviations of the
Gaussian perturbation are assumed between 0.5 and 2.0 K
(temperature) and between 3 and 10 % (water vapour mixing
ratio), as shown in Fig. a and b,
respectively. We then performed two sets of 128 4A/OP forward
radiative transfer runs with (1) the average temperature profile
and the 128 perturbed water vapour profiles (H2O-perturbed
runs), (2) the average water vapour profile and the 128 perturbed
temperature profile (T-perturbed runs), and a further run with
average temperature and water vapour profiles (baseline run for
this analysis). For these runs, the sulfate aerosol layer size
distribution and chemical composition have been taken fixed at
moderate volcanic conditions (log-normal distribution with
N0=20cm-3, rm=0.2µm,
σm=1.86µm, 70 %H2SO4 mixing ratio) at about 150 hPa altitude. Two
sets of spectral differences are finally calculated: a first set of
differences between the T-perturbed and the baseline runs, and
a second set of differences between the H2O-perturbed and
the baseline runs. Figure c shows the mean values
of T-perturbed minus baseline, and H2O-perturbed minus
baseline differences and spectral variability (represented by its
1 standard deviation interval) of these differences.
The temperature profile uncertainties dominate the spectral brightness
temperature variability, with a sensitivity up to ±1.4 K,
peaking in the sulfate-sensitive spectral region between 1100 and
1200 cm-1, while the impact of water vapour concentration profile
uncertainties is generally smaller than ±0.2 K. The broadband
magnitude of these fluctuations indicates a further limitation of the
observations of sulfate aerosol chemical and microphysical properties in
non-volcanically perturbed conditions with IASI-like instruments, at least
for BT sulfate signatures smaller than about 2.0 K (see
Fig. ). At a finer spectral resolution, it is clear that the
regions characterised by water vapour absorption lines must be avoided by
a careful selection of the spectral micro-windows of the retrieval algorithm.
The previous analysis was aimed at the evaluation of the impact of the
temperature and humidity profile uncertainties when these quantities
are retrieved with the same TIR observations used to infer the
sulfate aerosol properties. This discussion is then of interest when
considering near-real time multi-parameter retrievals. In offline
retrieval schemes, temperature and humidity profiles can be taken from
model reanalyses. It must be noted that these profiles have
significantly smaller uncertainties than profiles obtained with TIR
observations, then in these cases their impact on the retrieval of
sulfate aerosols is more limited.
Other interfering species: SO2, CO2 and ash
In the TIR spectral region where sulfate aerosols have their
absorption features (700–1300 cm-1), other gaseous
species have absorption bands, which can interfere with the
sulfate BT signature. Figure shows the
absorption cross sections of water vapour, carbon dioxide, ozone
and sulfur dioxide in this spectral range. As discussed in
Sects. and , the water vapour lines
are ubiquitous and must be taken into account when selecting the
spectral micro-windows of a retrieval algorithm; at the same time,
the ozone band perturbs the region between about 980 and
1080 cm-1, thus preventing from using the sulfate ions
bands around 1050 cm-1. The sulfur dioxide absorption
bands lie in the range 1080–1230 cm-1 and then can
partially affect the maximum sulfate absorption region around
1170 cm-1. The use of high-resolution TIR
spectrometers, like IASI, is then recommendable to select targeted
spectral micro-windows to minimise the effect of the sulfur
dioxide interference, e.g. around 1150 cm-1. The
carbon dioxide bands are stronger for wavenumbers less then about
750 cm-1, and then of lower concern for the present
study. It is interesting to note that enhanced concentrations of
sulfur dioxide, carbon dioxide and water vapour are co-existent
with sulfate aerosols in volcanic plumes, where the sulfate
BT signature is stronger.
Absorption cross sections of water vapour (multiplied by a factor of 30
to enhance visualisation; light blue), carbon dioxide (grey), ozone (orange)
and sulfur dioxide (violet) in the spectral range 700–1300 cm-1.
A typical sulfate aerosol BT signature is overplotted (black). The spectral
ranges of the channels used for IASI BB, MODIS BB and SEVIRI BB instrumental
configurations (broadband features) are also reported (sky blue, green and
red lines and points).
Another important volcanic effluent that has a spectral signature
in the range 700–1300 cm-1 is the ash. The ash BT
signature has a typical V shape in this range, as shown e.g. by ,
and , which can be modulated
by its mineralogical composition and optical
depth . This ash BT signature is characterised by
a stronger absorption at about 950–1100 cm-1 than at
longer (1200 cm-1) and shorter wavenumbers
(850 cm-1), as shown in Fig. 1b of
. In Fig. the impact of different
UTLS sulfate aerosol layers on the top of the atmosphere (TOA)
radiance spectra, as for our simulations, is shown. The spectra
are expressed in radiance units as a function of the wavelength to
more readily compare the impact of the sulfate absorption with the
impact of ash, as reported in Fig. 1 of . The
reduction of the TOA radiance due to ash is typically 10 to
25 % at its maximum absorption spectral range (about
10–11 µm, 1000–910 cm-1), depending on the
ash optical depth. Sulfates can impact for a few percent, for
small and diluted droplets (green lines in Fig. ), to
up to about 25 % for volcanically enhanced conditions
(red lines in Fig. ) at its maximum extinction
spectral range (about 8.5–9.0 µm,
1175–1110 cm-1). As pointed out by
, the different shapes of the ash and sulfate
absorption signatures facilitate the discrimination of the two
different aerosol particles. In addition, in extreme volcanic
conditions, the magnitude of the sulfate signature is comparable
to that of ash. The comparison of Figs. and 1 of
permits as well the comparison of the impact of
sulfate aerosols and that of sulfur dioxide absorptions on the
TOA radiances. The latter is generally a few percent, with
a maximum at about 9 µm (1110 cm-1), and so it is
comparable to the sulfate aerosol impact for background to
moderate volcanic conditions.
Simulated radiances for baseline no UTLS aerosol conditions (black)
and different UTLS aerosols with different H2SO4 mixing ratios,
effective radii and effective number concentrations (light and dark green and
red; please see legend for details). The abscissa is expressed in both
wavelength (to more readily compare with Fig. 1 of ) and
wavenumber.
Information content analysis: high spectral resolution and broadband spectral features approaches
The information on the three sulfate aerosol parameters [Ne, re, c] contained in the pseudo-observations with different instrumental
configurations is investigated in the present section. In particular, we
define one high-spectral-resolution configuration (IASI HR; the full IASI BT
spectra pseudo-observations are described in Sect. ) and three
configurations based on the broadband spectral features introduced in
Sect. . We have derived related quantities of the three
broadband features (ME, RE1 and RE2) to simulate IASI, MODIS and SEVIRI
observations. For IASI (IASI BB) we have produced three inputs by averaging the BT pseudo-observations in
the following bands: (1) 829–831 cm-1 (minimum sulfates
extinction), (2) 903 and 913 cm-1 (secondary extinction peak
around 905 cm-1) and (3) 1150 and 1154 cm-1 (maximum
sulfates extinction). We have empirically tried to avoid water vapour
absorption lines in these bands by a careful wavenumber selection. For MODIS (MODIS BB) we have convolved the
high-resolution IASI BT pseudo-observations using the relative spectral
response functions of channels 32 (central wavelength 12.02 µm,
central wavenumber 831.9 cm-1), 31 (central wavelength 11.03 µm, central wavenumber 906.6 cm-1) and 29 (central
wavelength 8.55 µm, central wavenumber 1169.6 cm-1).
For SEVIRI (SEVIRI BB) we have convolved the high-resolution IASI BT pseudo-observations using the relative spectral response functions of channels 10 (central wavelength 12.0 µm,
central wavenumber 833.3 cm-1), 9 (central wavelength 10.8 µm, central
wavenumber 925.5 cm-1) and 7 (central wavelength 8.7 µm, central wavenumber 1149.4 cm-1). The relative spectral
response functions for MODIS and SEVIRI are available on the following
websites, respectively: http://modis.gsfc.nasa.gov/about/specifications.php
and http://oiswww.eumetsat.org/WEBOPS/msg_interpretation/msg_channels.php.
The selected wavenumbers of IASI BB and the spectral channels used for
MODIS BB and SEVIRI BB configurations are shown in Fig. .
The full characterisation of the information content of the observations is
provided by the matrices AK (averaging kernel matrix) and
Sx (total error covariance matrix).
AK represents the sensitivity of the retrieved state
x^ to the true state x, and can be calculated as
follows :
AK=∂x^/∂x=(KTSϵ-1K+Sa-1)-1KTSϵ-1.
In Eq. (), K=∂F(x)/∂x
is the Jacobian matrix, and Sϵ and
Sa are the measurement error covariance and the a priori
covariance matrices. The trace of the AK matrix represents the
degrees of freedom (DOF) of the measurement . In our case this
value varies between 0 and 3, with this latter value meaning that the three
aerosol parameters [Ne, re, c] can be retrieved as
perfectly independent quantities.
Sx represents the total uncertainty on the retrieved aerosol
parameters. If the forward model error and the error associated to
non-retrieved parameters (see discussion for temperature and humidity
profiles in Sect. ) is excluded, the total error covariance
matrix can be written as
Sx=Ssm+Sm=(AK-I)Sa(AK-I)T+(KTSϵ-1K+Sa-1)Sϵ(KTSϵ-1K+Sa-1)T.
In Eq. (), Ssm and Sm
represent the contribution due to the correlation of the retrieved parameters
(smoothing error component) and to the spectral measurement noise
(measurement error component). The square roots of the diagonal elements of
Sx represent the total error of the retrieved aerosol
parameters.
The measurement error covariance matrices Sϵ for
the four instrumental configurations are assumed diagonal, with the ith
diagonal element derived from the standard deviation of the BT measurements
in the following way:
Sϵ,i=σϵ,i2. The values
of σϵ,i are assumed as the noise equivalent
brightness temperature (NEBT) for each instrumental configuration. For IASI
HR, the NEBT has been assumed 0.2 K at all wavenumber. For IASI BB, the
standard deviation for each of the three broadband features is calculated as
σϵ,i=0.2K/(ni), where ni is the
number of individual wavenumbers averaged to obtain the ith broadband
feature. For MODIS BB, a NEBT of 0.05 K is assumed for channels 32, 31 and
29 . For SEVIRI BB, NEBTs of 0.93, 0.94 and 0.80 K are assumed
for channels 10, 9 and 7
(www.eumetsat.int/website/wcm/idc/idcplg?IdcService=GET_FILE&dDocName=PDF_TYP_RADIOMET_ACC_MSG-1-2&RevisionSelectionMethod=LatestReleased&Rendition=Web).
The a priori covariance matrix is the same for the four instrumental
configurations. It is assumed diagonal, with the ith diagonal element taken
as 100 % of the three aerosol parameters values. This means that the a
priori knowledge of the parameters [Ne, re, c] is
supposed to be within 100 % of the true value.
The DOF and the total error, calculated for the four instrumental
configurations and for a typical background and volcanically enhanced
conditions (as defined in Sect. ), are listed in
Tables and . One important result is that high
spectral resolution is particularly critical in background conditions. In
these conditions, broadband features provide strongly dependent information
on [Ne, re, c], with all instruments (DOF for IASI,
MODIS and SEVIRI BB are 0.24, 0.22 and 0.01, and total error higher than
100 %, except for c retrievals with IASI BB). The DOF for IASI HR for
aerosols at background conditions is 1.34, so more than one independent piece
of information is retrievable. Nevertheless, even with IASI HR, background
Ne has a total error higher than 100 %, while retrieval
uncertainties on re and c are around 35 %. For
volcanically enhanced conditions, the added value of the high spectral
resolution, with respect to broadband features, is smaller than at
background conditions. Broadband features are reasonably well adapted to
characterise chemical and microphysical properties of sulfate aerosols in
volcanic conditions. In the best case (IASI HR, volcanic conditions) we have
found a DOF of 2.7 and retrieval uncertainties of about 30, 15 and 10 %
for Ne, re and c. This indicates three quasi-independent
pieces of information with relatively small retrieval uncertainties.
Degrees of freedom (DOF) for the retrieval of the three aerosol parameters:
Ne, re and H2SO4 mixing ratio, with
different instrumental configurations (IASI HR refers to the IASI high spectral
resolution; IASI BB refers to IASI broadband features; MODIS BB refers to MODIS broadband
features; SEVIRI BB refers to SEVIRI broadband features. See the text for further
details). DOF are calculated for typical background (Bg) and volcanic (Volc)
conditions.
Total error (%) for the retrieval of the three aerosol parameters:
Ne, re and H2SO4 mixing ratio, with
different instrumental configurations (IASI HR refers to the IASI high spectral
resolution; IASI BB refers to the IASI broadband features; MODIS BB refers to the MODIS broadband
features; SEVIRI BB refers to the SEVIRI broadband features. See the text for further
details). The error is calculated for typical background (Bg) and volcanic
(Volc) conditions.
Our error estimations for re and c can be compared with those made
by . In that work, the retrieval uncertainty is evaluated using
ACE-FTS pseudo-observations, covering the spectral range from 800 to
4750 cm-1. Different spectral configurations (different sub-bands of
the overall spectral region of their pseudo-observations) are discussed,
including a configuration similar to out IASI HR (an interval of about
800–1200 cm-1, called f1 in ). For that configuration,
the total error for re and c is between > 100 and
40 % (depending on the value of re), and about 25 and 20 %
(depending on the value of c), at conditions roughly similar to what we
call background conditions. Our estimated uncertainties (37.1 and 35.9 %,
for re and c), even if comparable, are somewhat lower for
re and higher for c. For extreme re and c values,
found corresponding uncertainties reaching values as low as 10
and 15 % (our estimations for volcanically enhanced conditions are 17.6
and 10.8 %). One reason for these relatively small discrepancies might be
the different instrumental characterisation of ACE-FTS and IASI, including
the different observation geometry. In any case, our results are quite
consistent with those obtained by It is worth noticing that
found that adding other spectral channels, in particular at
shorter wavelengths, significantly decrease the uncertainties, leading to
uncertainties < 2 and 3 % (re and c) when using the
whole spectral range.
Conclusions
In this paper we have presented sensitivity
analyses of the
optical properties and BT signatures of secondary sulfate
aerosols in the UTLS, based on TIR IASI pseudo-observations
obtained with the 4A/OP radiative transfer model. We have modelled
the sulfate aerosols as layers of H2O/H2SO4 binary
solution droplets with varying temperatures, mixing ratios and
size distributions. The optical properties of these layers are
obtained with a Mie code, taking as inputs the complex refractive
indices of H2O/H2SO4 binary solution with different
temperatures and H2SO4 mixing ratios from laboratory
measurements and archived in the GEISA database, and size
distribution with different effective radii and effective number
concentrations.
For these layers, we have determined characteristic BT spectral
signatures in the spectral range 700–1200 cm-1, due to
sulfate and bisulfate ions, and molecular undissociated
H2SO4 absorption bands in the TIR. In this spectral
region, a general decreasing BT signature (increasingly negative
difference with respect to a no-aerosols baseline due to the
increasing extinction coefficient) is observed. This signature is
dominated by the stronger molecular H2SO4 absorption band
at 1170 cm-1, which is particularly pronounced for
realistic tropical UTLS conditions (low temperatures and
H2SO4 mixing ratios greater than 60 %). The BT
signature can reach values as high as -5.0 K at
1170 cm-1, for big particles, and a high number
concentration and H2SO4 mixing ratio. These conditions
are typical of a volcanically perturbed UTLS. Another important
feature that can modulate the shape of the BT sulfate aerosol
signature is the H2SO4 absorption band at
905 cm-1. The extinction of the aerosol layer is found
very weakly sensitive to the temperature of the droplets. The
spectral extinction and then the BT signature is, at different
extents, sensitive to the H2SO4 mixing ratio, the
effective radius and number concentration. In general, the
effective radius qualifies as the dominating factor determining
the sensitivity of the sulfate aerosol extinction spectra and BT
signatures. The main reason for the high sensitivity to the effective radius is
the related change in the effective mass of sulfates, even if a residual sensitivity to
the effective radius is found for simulations at fixed mass. Sulfate aerosol layers in background conditions
(small H2SO4 mixing ratios, and number concentrations and
effective radii) seem not to be fully characterisable with IASI-like TIR instruments
because of (1) small BT signatures, mainly due to the high
sensitivity to the effective radius/mass, and (2) the interference of
the 9.6 µm ozone absorption band, which is superposed
on the main sulfate/bisulfate ion absorption bands. Nevertheless, a residual information
content is found in these pseudo-observations (about 1.3 DOF, and > 100, 37.1
and 35.9 % uncertainties for the retrieval of [Ne, re, c])
in these conditions, meaning that a constrained retrieval of background sulfate
aerosol properties is still feasible. BT spectral
variability induced by the uncertainties on the vertical
temperature profile and surface temperature is a further limiting
factor for the observation of chemical and microphysical
parameters of background sulfate aerosol layers in the
UTLS. Other interfering species are identified, like sulfur
dioxide, carbon dioxide, water vapour and ash. While carbon
dioxide absorption bands are of limited concern in this context
(stronger impact for wavenumbers lower than about
750 cm-1), attention must be paid to the sulfur
dioxide absorption signature, which lies in the region of maximum
sulfate BT signature, and the ubiquitous water vapour lines. The
ash has a different BT signature than sulfates and its impact on
the TOA radiances has a comparable magnitude to the
volcanically enhanced sulfate aerosol layers. At the operational level, these interfering
parameters should be retrieved simultaneously with sulfate aerosols or
constrained with independent data.
From a remote sensing perspective, these analyses show that
broadband features can be identified, like the maximum extinction
at 1170 cm-1 (ME), its ratio with respect to the
minimum extinction at 800 cm-1 (RE1) and the ratio of
the secondary peak at 905 cm-1 with respect to the
minimum extinction at 800 cm-1 (RE2). ME is sensitive
to the H2SO4 mixing ratio, effective number concentration
and radius, while RE1 and RE2 are only sensitive to the
H2SO4 mixing ratio and the effective radius. While the
information content carried by ME, RE1 and RE2 on the three
aerosol parameters is partially independent for most conditions;
(1) in background conditions, ME is scarcely sensitive to the
effective number concentration, and (2) in volcanically enhanced
conditions, the variability of RE1 and RE2 with respect to the
H2SO4 mixing ratio is very scarce, and the correlation of
their information content is higher for higher H2SO4
mixing ratios. In addition, with this broadband approach, the three
aerosol parameters are hardly retrievable as independent
quantities and constraints should be given to at least one
parameter. The analysis of the information content of broadband features approaches
based on IASI (with a limited number of the full spectral resolution wavenumbers), MODIS
and SEVIRI has shown that they are reasonably well adapted to characterise chemical and
microphysical properties of sulfate aerosols in volcanic conditions (2.41, 2.11 and 1.48 DOF
for IASI BB, MODIS BB and SEVIRI BB, with uncertainties of 28 (IASI BB) to 53 % (SEVIRI BB) for
the effective radius, 11 (IASI BB) to 32 % (SEVIRI BB) for the mixing ratio and > 80 % for
the effective number concentration). Even if these broadband spectral features are
appealing to quantitatively characterise sulfate aerosols, an
important factor for a better characterisation is a fine spectral
resolution. The benefits are twofold (1) in a broadband
perspective, in order to accurately select spectral micro-windows
representing ME, RE1 and RE2 or other equivalent spectral
parameters (dedicated channels not always available in moderate
resolution instruments, like MODIS or SEVIRI), and free from water
vapour, sulfur dioxide and ozone interference; (2) in a high-spectral-resolution spectral fitting perspective, to resolve the
concurrent spectral absorption features of sulfates present in the
aerosol droplets, and to precisely estimate the interfering
parameters, as the temperature and humidity profile. The analysis of the information content of
high-spectral-resolution (IASI HR) and broadband feature approaches has shown that the high
resolution is a decisive factor for the characterisation of background sulfate aerosols
(which are not observable with broadband features approaches). In volcanically enhanced
conditions, IASI [Ne, re, c] pseudo-observations, using the full spectral
resolution, have 2.7 DOF, and 30.4, 17.6 and 10.8 % retrieval uncertainties. A further significant
added value is expected from the forthcoming new generation
of satellite TIR instruments, including IASI-NG (New Generation)
that will be launched in the 2020 time frame as part of the EPS-SG
(EUMETSAT Polar System – Second Generation, formerly post-EPS)
mission .
The Supplement related to this article is available online at doi:10.5194/amt-9-115-2016-supplement.
Acknowledgements
The optical parameters of sulfate aerosol layers used in this work
are obtained with the IDL Mie scattering routines developed by the
Earth Observation Data Group of the Department of Physics of Oxford
University, and available via the following website:
http://eodg.atm.ox.ac.uk/MIE/. NOVELTIS is gratefully
acknowledged for the support with the 4A/OP model. Alain Chédin,
Virginie Capelle and Cyril Crevoisier are gratefully acknowledged
for their help with 4A/OP, TIGR and the GEISA database. Hervé Herbin
is gratefully acknowledge for the stimulating discussion during the
last few years. The authors would like to thank the three anonymous referees for their
constructive criticism; their contribution has been substantial towards the production
of the final version of the present manuscript. This project has been
partially supported by the EU
7th Framework Program under grant 603557 (StratoClim).
Edited by: F. Prata
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