WORLD METEOROLOGICAL ORGANIZATION, TD-NO. 1172, 265-268, 2003
IDENTIFICATION OF POLAR STRATOSPHERIC AEROSOLS FROM SATELLITE
EXTINCTION MEASUREMENTS
Rong-Ming Hu
Department of Earth and Atmospheric Sciences,
University of Quebec in Montreal, P.O. Box 8888, Station A,
Montreal, Quebec, H3C 3P8, Canada
(email : hu@sca.uqam.ca)
Measurement (POAM) has detected the occurrence
of PSAs during recent cold winters. The instrument
can measure aerosol extinction in six wavelengths (
0.353, 0.442, 0.603, 0.779, 0.922, 1.018 μm ) for
aerosol (Glaccum et al, 1996, Fromm etal., 1999).
Halogan Occultation Experiment (HALOE)
instrument can measure aerosol extinction in four
channels ( 2.45, 3.40, 3.46, 5.26 μm ) (Russell et al.,
1993). The other satellites are also designed to
obtain the global aerosol optical properties with
high quality. Several things need to be further
studied. Can we distinguish the different types of
PSAs from those measurements ? Is it possible to
derive the microphysical properties from those
measurements with certain accuracy ? Which
channel or how many channels can best represent
the 'fingerprints' of aerosols ? Here we investigate
the possible applicability of future satellite
measurement to obtain the properties of PSAs,
which are necessary to be required for the ozone
loss and climate studies. The spectral signatures in
the aerosol extinction measurements have been
analysed from theoretical calculation. Our main goal
is to assess the viability of identification of aerosol
types.
ABSTRACT
This work investigates the achievability of
using existing satellite aerosol extinction
measurements of Polar Stratospheric Aerosols
(PSAs) to identify, for the first time, different
aerosol types. The extinction 'fingerprints' in
multi-wavelength for different PSA types have been
theoretically calculated considering their particle
shapes of the spherical or non-spherical. The
calculations are carried out at the wavelengths
measured by the Halogen Occultation Experiment
(HALOE) and Polar Ozone and Aerosol
Measurement (POAM). From our analysis, The
detection of PSA signals strongly depends on the
selected wavelengths. There is outstanding spectral
extinction difference between the nitric acid and ice
particles for HALOE, while not obvious for POAM.
HALOE shows to be more sensitive to the large
particles due to the similarity of observing
wavelength to particle size. There is no big spectral
difference in extinction calculation between
sphericity and non-sphericity. It is highly possible to
distinguish aerosol types by exploiting the variation
of the extinction coefficient ratio and variance with
wavelength and particle size.
Key words: Extinction measurements, aerosol
types, particles size.
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2
PARTICLE EXTINCTION SPECTRA
The spectroscopic analysis of PSAs could
provide new insight into their composition (Toon
and Tolbert, 1995). Different kind of particles such
as ice, nitric acid trihydrate (NAT) or nitrate acid
dihydrate (NAD) have their own extinction spectra.
Unfortunately, this kind of observation data is so
scarce. In this study, we use the infrared spectral
properties of NAT measured by Tisdale et al (1999)
and the refractive indices in UV, visible and near
infrared region are set to 1.43. For supercooled
ternary solutions (STS) in UV, visible and near
infrared region, we use Lorentz-Lorenz (L-L)
mixing rule to calculate the refractive indices of
particles (Luo et al, 1997). However, the L-L rule
has been broken as the strong absorption of STS
appears in middle infrared region. Some methods
such as the modified extended effective medium
INTRODUCTION
Over past decades, satellite measurements have
provided a large amount of useful information on
the frequency, extent, types and temperatures of
occurrence of Polar Stratospheric Aerosols (PSAs).
Despite years of effort, identifying specific PSAs
and deriving their properties by using this kind of
data remain a difficult task [Hervig et al., 1997,
Fromm 1999, Hervig et al., 2001, Santee et al.,
2001]. As PSAs usually occur at high altitude in
lower stratosphere, the particles are so small that
they are impossible to measure from the surface of
earth. The satellites have the advantage to measure
the PSAs globally. The Polar Ozone and Aerosol
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approximation (EEMAs) approach were designed
for this kind of strong absorbing heterogeneous
particles, even for larger size of spherical or
nonspherical particles (Chylek and Srivastava,
1983). For small particles (size parameter is much
smaller than 1), the classical L-L mixing rule is still
effective with certain accuracy (smaller than 1
percent). As the size of STS is usually small, we still
use classic L-L mixing rule to calculate the
refractive index extending to infrared region. For
ice, the refractive index from ultraviolet to infrared
is from Warren, S. G. , 1984. The extinction spectra
is calculated ranging from 0.1 μm UV up to 10 μm
infrared wavelength. As our main purpose is to try
to find the 'fingerprints' of different kinds of PSAs,
especially identify the difference between ice and
NAT, the single particle in radius 1.0 μm of ice and
NAT has been selected for calculation.
3
APPLICATIONS OF SATELLITE
MEASUREMENTS
As polar nitric acid particles play key role in
ozone loss, it has been received considerable
attention in recent decades. Although lidar
measurements have provided vassive useful
information for these kinds of aerosols. It is still not
enough to be used for globally modelling and
monitoring future ozone trends and climate change.
Satellite measurements can view large geographical
area and even the whole earth. It can meet the
requests for the global data. There are some works
which have shown such potential application to
identify the PSAs by satellite measurements (Hervig
1997, Santee 2001), but it is still far away to identify
the nitric acid particles in all different kinds of
aerosols. In this study, we try to analyse the
potential possibility to distinguish the different
kinds of aerosols for current satellites and get more
knowledge for future measurements designation.
The results show us the refractive index is
crucial for distinguishing the ice and NAT from
extinction in certain wavelength. If we were not
using the spectral properties measured in the
laboratory instead of making some assumptions, we
had missed much information, especially in infrared
wave band.
As mentioned before, the non-spherical effect is
not so strong for extinction, so we use spherical
particles for analysis. To understand particle size
information included in the spectral signature, the
extinction efficiency ratio was used to analyze the
spectral dependence at the HALOE and POAM
wavelengths (Plate 2). The results present that ice
and NAT particles have distinctive spectral 'finger
prints' at the HALOE channels, especially, when the
particle size is around 1 μm. The spectral signature
is not so strong at the POAM channels for ice, NAT
and STS with particle size around 1 μm. For
HALOE channels, the spectral signature exists even
for radii greater than 2 μm, but not for POAM.
We use above refractive index to calculate the
extinction spectra for ice, enhanced NAT, large
NAT, STS and sulfate particles by T-matrix
algorithm (Plate 1). The extinction also depends
upon the particle size distribution. We selected the
microphysical properties of different particles
(Table 1) from our recent retrievals of lidar
measurements (Luo et al., 2001, Hu et al., 2002).
From our results, we find that the strong spectral
'fingerprints' appear in infrared region. There is very
weak spectral signature for the large NAT particles.
It can not be neglected that strong absorption in
infrared region for NAT, STS and sulfate particles.
So the retrieval of concentration of trace gases by
the satellite data in infrared channels may be
contaminated by those particles.
HALOE measured aerosol extinction in several
infrared channels while POAM measured in UV,
visible and near infrared channels. We use these
extinctions to calculate the variance for ice, NAT
and STS in different wavelengths (Plate 3). The
extinctions simulated in different particle size show
strong difference between ice and NAT. For small
size of ice particles, the extinctions increase
monotonically with the variance, but break and
decrease at large radius of ice particles. The NAT
extinctions break also as the particle size increased
to certain value, but remain monotonically decrease
with the larger particle radius. The variance of large
ice particles is increased with the extinction in
contrary to the large NAT particles. Therefore, it is
encouraging for us to discriminate these two kinds
of PSA from extinction pattern. We also analysed
the extinctions for POAM measuring channels.
Unfortunately, there is no such obvious 'fingerprints'
for the ice particles. The NAT extinctions still break
with the increase of particle size. There is no
obvious 'fingerprints' for POAM to distinguish the
ice and NAT particles. It is interesting to find that
the colour ratio of extinctions for POAM have
From our calculation, if there are ice or NAT
particles with same size, it is almost impossible to
identify ice or NAT only by extinction efficiency
from UV 0.1 μm to near infrared wavelength 1.0
μm. However, there is obvious spectral signature in
other infrared bands such as 2.4 μm, 5.2 μm, 7.2
μm. The strongest 'fingerprints' for discriminating
ice and NAT exist in 7.2 μm. From this point of
view, it is necessary to select specific channels in
order to identify the different kinds of PSA pattern
from satellite extinction measurements. In our
calculation, we also consider the influence of the
non-sphericity on the extinction value. There is no
obvious extinction efficiency difference between
spherical and non-spherical particles.
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strong spectral signatures. The particle shape has
effect on the colour ratio of extinction as we notice
that those values are very different between NAT
and ice particles when extinction is not large. For
the large NAT particles are efficient in
denitrification (Fahey et al, 2001), it is essential to
identify those particles (radius 3-10 μm) from small
STS particles. From the extinction signature, it is
possible to distinguish this kind of particles from the
small STS. Liquid aerosols (radius 0.2 μm around)
and thin nitric acid hydrates (radius 3-10 μm) have
completely different wavelength dependence of
extinction, despite the fact that the measured
extinction could be indistinguishable at any one
wavelength. These kinds of spectral features are
stronger in HALOE channels than in POAM
channels.
Chylek, P., and Srivastava, V., Dielectric constant
of a composite inhomogeneous medium. Phys.
Rev., B27, 5098- 5106, 1983.
4
Hervig, M. E., K. S. Carslaw, T. Peter, T. Deshler,
L. L. Gordley, G. Redaeli, U. Biermann, and
J.M. Russell III, Polar stratospheric clouds due to
vapor enhancement: HALOE observations of the
Antarctic vortex in 1993. J. Geophys. Res., 102,
28185-28193, 1997.
Fahey, D.W., et al., The detection of large
HNO3-containing particles in the winter Arctic
stratosphere. Science, 291, 1026-1031, 2001
Fromm, M. D., R. M. Bevilacqua, J. Hornstein, E.
Shettle, K. Hopple, and J. D. Lumpe, An analysis
of Polar Ozone and Aerosol Measurement
(POAM) II Arctic polar stratospheric cloud
observations, 1993-1996. J. Geophys. Res., 102,
24341-24357, 1999.
Glaccum, W., et al., The Polar Ozone and Aerosol
Measurement instrument, J. Geophys. Res., 101,
14479-14487, 1996.
SUMMARY
Exploratory and monitoring observations of
PSAs are essential for analysing future ozone loss
and climate change over polar regions. It will be
extremely useful to extract the PSA properties from
satellite measurements, but so far it is still limited.
In our study, we have calculated the extinctions for
different PSAs. In order to consider the
non-sphericity of particles, T-matrix algorithm has
been used for calculation. The multi-channel
satellites extinctions can afford more useful
information for retrieval of the microphysical
properties of PSAs. Our analysis shows strong
spectral signature for different kinds of PSAs. The
infrared channels present more promising to identify
the ice, NAT and STS particles, for the absorption
spectra plays more important role in total extinction
value. The particle shape is one of uncertain
parameter for our deriving the particle size from
satellite data, but it has not shown strong influence
on the extinction calculation. By applying the
present and upcoming satellite measurements,
HALOE satellite measuring channels present strong
'fingerprints' for different kinds of PSAs and POAM
shows possible to distinguish the nitric acid particles
from extinction spectral signals.
Hervig, M. E., K. L. Pagan, P. G. Foschi, Analysis
of polar
stratospheric cloud
measurements
from AVHRR. J. Geophys. Res., 106,
10363-10374, 2001.
Hu, R.-M., K. S. Carslaw, C.A. Hostetler, L. A.
Poole, Luo, B.P., T. Peter, S.A. Fueglistaler, T. J.
McGee and J.F. Burris,
The microphysical
properties of wave PSCs retrieved from lidar
measurements during SOLVE-THESEO 2000, J.
Geophys. Res., 107, doi:10.1029/2001JD001125,
2002.
Luo, B. P., U. K. Krieger and T. Peter, Densities and
refractive indices of H2SO4/H2NO3/H2O
solutions to stratospheric temperatures, Geophys.
Res. Lett., 23, 3707-3710, 1996.
Luo, B.P., T. Peter, S.A. Fueglistaler, H. Wernli,
R.-M. Hu, K.S. Carslaw, C.A. Hostetler, L. A.
Poole, T. J. McGee and J.F. Burris, Large
stratospheric particles observed by lidar during the
SOLVE/THESEO 2000 mission, \JGR, submitted,
2001.
ACKNOWLEDGEMENTS
We thank Mishchenko M.I. for enabling us to
use his T-Matrix code. This work was funded by
grant from the U.K. Natural Environment Research
Council as part of the Upper Troposphere--Lower
Stratosphere Thematic Programme.
Russell, J. M., III, L. L. Gordley, J. H. Park, S. R.
Drayson, W. D. Hesketh, R. J. Cicerone, A. F.
Tuck, J.E. Frederick, J. E. Harries, and P. J.
Crutzen, The Halogen Occultation Experiment, J.
Geophys. Res., 98, 10777-10797, 1993.
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Plate 2. Extiction ratio versus particle radius. (a) HALOE
channels. Solid: β (2.45) / β (5.26), dashed: β (5.26)/ β(3.46).
(b) POAM channels. Solid: β (0.353)/ β(1.018), dashed:
β(1.018)/ β(0.603). STS(thin), NAT(very thick) and
ice(thick).
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Plat 1. (a) Extinction spectra for ice, NAT-enh, STS,
NAT-rock and H2SO4 particles. (b) Single scattering
albedo values corresponding to above spectra. (c)
Asymmetry parameter values corresponding to above
spectra. Background aerosol (dashed), liquid PSA (long
dashed), NAT-enh (dotted), NAT-rock (dash dot), ice (solid).
Plate 3. Extinction spectral signatures for different PSAs. (a)
HALOE channels. Solid: 2.45 μm, dashed: 3.46 μm, Dash
dot dot: 5.26 μm. (b) POAM channels. Solid: 0.353 μm,
dashed: 0.603 μm, dash dot dot: 1.018 μm. STS(thin),
NAT(very thick) and ice(thick).
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