JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, D11S05, doi:10.1029/2005JD006315, 2006

Intercomparison of ILAS-II version 1.4 aerosol extinction coefficientat 780 nm with SAGE II, SAGE III, and POAM III
N. Saitoh,1,2 S. Hayashida,3 T. Sugita,1 H. Nakajima,1 T. Yokota,1 M. Hayashi,4K. Shiraishi,4 H. Kanzawa,5 M. K. Ejiri,6 H. Irie,7 T. Tanaka,1 Y. Terao,8R. M. Bevilacqua,9 C. E. Randall,10 L. W. Thomason,11 G. Taha,12
H. Kobayashi,13 and Y. Sasano1
Received 2 June 2005; revised 25 January 2006; accepted 15 February 2006; published 7 June 2006.

[1] The Improved Limb Atmospheric Spectrometer (ILAS)-II on board the Advanced

Earth Observing Satellite (ADEOS)-II observed stratospheric aerosol in visible/nearinfrared/infrared spectra over high latitudes in the Northern and Southern hemispheres,intermittently from January to March and continuously from April through October 2003.
This study assesses the data quality of ILAS-II version 1.4 (V1.4) aerosol extinction
coefficient at 780 nm. In the Northern Hemisphere (NH), aerosol extinction coefficient(AEC) from ILAS-II agreed with extinctions from SAGE II and SAGE III within ±10%
and with extinction from POAM III within ±15% at heights below 20 km. From 20 to
26 km, ILAS-II AEC was smaller than extinctions from the other three sensors;differences between ILAS-II and SAGE II ranged from 10% at 20 km to 34% at 26 km in
the NH. Over the Southern Hemisphere (SH), ILAS-II AEC from 20 to 25 km in February
was 12–66% below SAGE II extinction. The difference increased with increasing
altitude. Comparisons between ILAS-II and POAM III from January to May in the SH(‘‘non-PSC season’’) yielded qualitatively similar results. From June to October(‘‘PSC season’’), ILAS-II extinction was also smaller than POAM III extinction above
17 km; however, ILAS-II extinction agreed with POAM III extinction to within ±15%from 12 to 17 km during the PSC season. The comparisons indicate that in bothhemispheres the ILAS-II V1.4 AEC is comparable to extinctions from other measurements
below approximately 20 km and systematically low above approximately 20 km although
the mean difference is as small as 2  105 km1 during the non-PSC season.
Citation: Saitoh, N., et al. (2006), Intercomparison of ILAS-II version 1.4 aerosol extinction coefficient at 780 nm with SAGE II,
SAGE III, and POAM III, J. Geophys. Res., 111, D11S05, doi:10.1029/2005JD006315.

1. Introduction[2] Stratospheric aerosols greatly impact stratosphericchemistry, including destruction of ozone. In polar regions,
Polar Stratospheric Clouds (PSCs), which form undercold conditions in winter and early spring, lead to severe
ozone depletion by providing the surface required forheterogeneous reactions that convert inactive chlorine into
active chlorine [e.g., Solomon, 1999]. Moreover, PSCs
irreversibly remove nitric acid from the gas phase throughsedimentation (denitrification) [Solomon, 1999], and theremoval could facilitate springtime ozone depletion [World
Meteorological Organization, 2002]. Observation of stratospheric aerosols, particularly PSCs, is thus crucial for theunderstanding of ozone destruction processes. Many stratospheric aerosol measurements have been performed withlidar and Optical Particle Counter (OPC) [Stratospheric
Processes and their Role in Climate, 2005, and references
therein]. Space-borne sensors that provide observations overa large area have also monitored stratospheric aerosols. The
Stratospheric Aerosol Measurement (SAM) II is a solar
occultation sensor on board a polar-orbiting satellite. It
continuously observed stratospheric aerosols and PSCs over1

National Institute for Environmental Studies, Tsukuba, Japan.
Now at Center for Climate System Research, University of Tokyo,
Kashiwa, Japan.
3
Faculty of Science, Nara Women’s University, Nara, Japan.
4
Faculty of Science, Fukuoka University, Fukuoka, Japan.
5
Department of Earth and Environmental Science, Graduate School ofEnvironmental Studies, Nagoya University, Nagoya, Japan.
6
Center for Atmospheric and Space Sciences, Utah State University,
Logan, Utah, USA.
2

Copyright 2006 by the American Geophysical Union.
0148-0227/06/2005JD006315$09.00

7
Frontier Research Center for Global Change, Japan Agency for
Marine-Earth Science and Technology, Yokohama, Japan.
8
Division of Engineering and Applied Science and Technology,
Harvard University, Cambridge, Massachusetts, USA.
9
Naval Research Laboratory, Washington, D. C., USA.
10
Laboratory for Atmospheric and Space Physics, University ofColorado, Boulder, Colorado, USA.
11
NASA Langley Research Center, Hampton, Virginia, USA.
12
Science Systems and Applications, Inc., Lanham, Maryland, USA.
13
Central Research Institute of Electric Power Industry, Tokyo, Japan.

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high latitudes for about 15 years from October 1978 to
December 1993 [e.g., McCormick et al., 1982; Poole andPitts, 1994]. Subsequently, several other solar occultation
sensors, including the Polar Ozone and Aerosol Measurement (POAM) II (September 1993 to November 1996)[Randall et al., 1996, 2000; Fromm et al., 1997, 1999],
the Improved Limb Atmospheric Spectrometer (ILAS)(November 1996 to June 1997) [Hayashida et al., 2000;
Saitoh et al., 2002], and POAM III (April 1998 to present)[Randall et al., 2001; Bevilacqua et al., 2002; Strawa et al.,2002] have obtained stratospheric aerosol and PSC data for
latitudes similar to those covered by SAM II. In addition,
the Stratospheric Aerosol and Gas Experiment (SAGE) II, asolar occultation sensor on board an inclined-orbiting satellite that can make more extensive observations from low/
mid latitudes to high latitudes, began regular observations ofstratospheric aerosols in October 1984 [e.g., Thomason,
1991; Hitchman et al., 1994; Thomason et al., 1997] andis still in operation. SAGE III, which is the successor to
SAGE II on board a polar-orbiting satellite, has measured
stratospheric aerosols since February 2002 at high latitudes
in the Northern Hemisphere (NH) [Poole et al., 2003] and at
mid latitudes in the Southern Hemisphere (SH) [Thomason
and Taha, 2003].
[3] ILAS-II [Sasano et al., 2001] is the successor to ILAS
[Sasano et al., 1995, 1999], and was launched on board theAdvanced Earth Observing Satellite (ADEOS)-II (polarorbiting satellite) on 14 December 2002 [Nakajima et al.,2004]. It made about 150 preoperational observations fromJanuary to March, and measured continuously for about
7 months from 2 April through 24 October 2003, at whichtime ADEOS-II lost its function because of solar paddle
failure. ILAS-II is designed to observe profiles of stratospheric minor gases such as O3, HNO3, NO2, N2O, CH4,
H2O, CFC-11, CFC-12, ClONO2, and N2O5 as well as
profiles of extinction by stratospheric aerosols and PSCs athigh latitudes of both hemispheres (53.9 – 71.1N and 63.6 –88.0S). ILAS-II carried three infrared spectrometers (ch.1:
6.21 – 11.76 mm, ch.2: 3.00 – 5.70 mm, ch.3: 12.78 –12.85 mm),one visible spectrometer (ch.4: 753 – 784 nm), and a sun-edge
sensor [Nakajima et al., 2006]. Aerosol extinction coefficient(AEC) is retrieved from eight different spectral window
element data; one of them is at 780 nm in the visible
spectrometer and the others are at 3.0, 3.8, 5.1, 7.1, 8.3,
10.6, and 11.8 mm in the infrared spectrometers. In this study,
we assess the data quality of ILAS-II AEC at 780 nm
processed with the version 1.4 (V1.4) retrieval algorithm.
[4] In this paper, data quality was assessed over the NH
by comparing ILAS-II AEC with SAGE II, SAGE III, and
POAM III. Over the SH, ILAS-II data were examined in
two separate groups. The first group included data fromJanuary through May, when PSCs do not occur (hereafterreferred to as the ‘‘non-PSC season’’). ILAS-II AEC datafrom the non-PSC season were compared to SAGE II and
POAM III extinction data. In addition, an OPC and a Laser
Particle Counter (LPC) measured aerosol number density
profiles in various diameter ranges over Syowa Station on
22 February 2003. These data were converted into extinction at 780 nm and compared to the nearest ILAS-II AEC
data obtained on the same day. The second group included
data from June through October, when PSCs are frequently
observed (‘‘PSC season’’). During the PSC season, it is

D11S05difficult to complete comparisons with other measurements
because of the inhomogeneity of PSCs. Almost all the
ILAS-II and POAM III profiles during the PSC season
included enhanced layers suggesting the existence of PSCs.
However, we made statistical comparisons between ILAS-IIand POAM III for these PSC profiles rather than individualcomparisons.

2. ILAS-II Aerosol Extinction Data andCharacteristics[5] ILAS-II measures solar radiance in the exosphere(direct sunlight) and radiance attenuated as sunlight travels
through the atmosphere as a function of tangent height.
Sunlight incident on the entrance slit of the visible spectrometer is dispersed by the spectrometer grating, and thendetected with a 1024-element metal-oxide-semiconductor(MOS) photodiode array with spectral resolution of
0.06 nm. The entrance slit size corresponds to an instantaneous field of view (IFOV) of 1 km in the vertical and
2 km in the horizontal at a tangent point. Nakajima et al.
[2006] detailed the ILAS-II hardware characteristics and its
performance in orbit.
[6] The visible spectrometer of ILAS-II measures temperature and pressure by obtaining the absorption spectrumby oxygen molecules (O2 A-band). Simultaneously, AEC isretrieved from transmittance averaged for 12 elements at
approximately 780 nm, outside the O2 A-band. The ILAS-II
algorithm to retrieve AEC from the spectral data obtainedwith the visible spectrometer is similar to the algorithm used
for ILAS data [Hayashida et al., 2000; Yokota et al., 2002].
ILAS-II can measure solar luminosity of the whole solar
disk in the exosphere (solar scan data acquisition)[Nakajima et al., 2006]. Thus sunspot and limb darkening
effects on the AEC data can be estimated from the measured
solar luminosity in the ILAS-II retrieval, while theoreticalluminosity was used in the ILAS retrieval. T. Yokota et al.
(Improved Limb Atmospheric Spectrometer-II (ILAS-II)
version 1.4 algorithm for retrieval of gas and aerosol
profiles in the stratosphere, manuscript in preparation,2006) discussed the improvements of the ILAS-II AEC
retrieval over the ILAS retrieval.
[7] In the ILAS AEC data product, ‘‘internal error’’ wasprovided, which comprises random noise in the observed
solar signals, error in the estimate of direct sunlight during
atmospheric transmission (100% level), and error in theestimate of ozone absorption in the Wulf band [Hayashida
et al., 2000; Yokota et al., 2002]. In the ILAS-II V1.4
product, ‘‘repeatability error’’ is defined and provided
instead of the internal error; it is estimated on the basis ofmeasurement repeatability. In addition, error in the estimateof Rayleigh scattering by atmospheric molecules is estimatedfrom uncertainties in the United Kingdom Meteorological
Office (Met Office) temperature data that we assume;
assumed uncertainties in temperature of ±2 K at 10 km
and ±5 K at 70 km resulted in errors of two orders ofmagnitude smaller than extinction values below 20 km[Yokota et al., 2002, Figure 6]. This error is provided as‘‘external error’’ in the ILAS-II product, as in the case of
the ILAS product. The Root-Sum-Square (RSS) of therepeatability and external errors is defined as the ‘‘total
error’’ (T. Yokota et al., manuscript in preparation, 2006).

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Figure 1. Time series of latitudinal coverage of ILAS-II(black dots), POAM III (light gray circles), SAGE II (thickgray line), and SAGE III (solar occultation mode only; thingray line) during ILAS-II operations from January to
October in 2003. The star denotes the position of Syowa
Station (69.0S, 39.6E), where the OPC/LPC measurement
was conducted on 22 February.
[8] Measurement repeatability is defined as the ‘‘closeness of the agreement between the results of successive
measurements of the same measurand’’ [International
Organization for Standardization, 1993]. Measurement
repeatability was calculated empirically in the ILAS-II
V1.4 retrieval as follows. The mean (referred to as ‘‘x’’)
and 1s standard deviation (‘‘s’’) of ILAS-II AEC data were
calculated for every successive 100 occultation events (7 –8 days) at each altitude level in each hemisphere. The
smallest value of relative standard deviation defined as e =s/x was selected for each altitude level and defined as themeasurement repeatability of the altitude level. In this way,the e value calculated for the period when variability in
AEC was smallest during ILAS-II operations is selected as
the measurement repeatability. Here, the measurement repeatability is regarded as the precision of the ILAS-II AEC
measurements. It includes the contribution from naturalvariability to some extent and therefore would give an
upper limit of the true measurement precision. The precision
was 5 – 15% at 12– 26 km in the NH and 6 – 20% at 12–
23 km in the SH (T. Yokota et al., manuscript in preparation,2006); the precision was smaller than 10% between 15 and
25 km in the NH and between 15 and 21 km in the SH.

D11S05[9] At almost all altitudes below 25 km, the magnitude of
the measurement repeatability over the SH was larger thanthe magnitude over the NH (e.g., five times larger at 25 km).
ILAS-II made continuous observations for less than an
entire year, only for seven months from April to October.
The values of the measurement repeatability over the SH
were estimated from data obtained in April or in Octoberwhen the atmosphere is sometimes perturbed because of theformation or break up of the polar vortex at ILAS-II
measurement latitudes. In contrast, the values of the measurement repeatability over the NH were estimated fromdata acquired primarily in the summer (July to October), a
more quiescent time of year. The values of the measurement
repeatability over the SH reflect larger variability in atmospheric aerosol compared to the NH because of the seasons
sampled.
[10] This study focused on AEC data below 30 km in the
NH and below 25 km in the SH to avoid the effect ofsunspots. At least about one sixth of all the ILAS-II AEC
data at 780 nm are affected by sunspots judging from theobserved solar luminosity data. Sunspots effect on the
AEC data can be estimated from the solar luminosity in
the ILAS-II retrievals (T. Yokota et al., manuscript in
preparation, 2006), but the effect cannot always be correctedin the current version because of a hardware problem on
ILAS-II [Nakajima et al., 2006]. In the current version,
most of the AEC data still include sunspots effect above30 km in the NH and above 25 km in the SH. Sunspots
could affect ILAS-II AEC data at lower altitude levels in the
SH compared to the NH, probably because ILAS-II AEC
values in the SH were generally smaller than those in the
NH at the same altitude levels. Those disturbed data are notrecommended for scientific use.

3. Data for Comparison[11] Figure 1 shows latitudinal coverage of ILAS-II(black dots), POAM III (light gray circles), SAGE II (thickgray line), and SAGE III (solar occultation measurements
only; thin gray line) during ILAS-II operations from January to October 2003. POAM III observations occurred atsimilar latitudes and times as ILAS-II in both hemispheres.
Coincident pairs of ILAS-II and POAM III observations
were chosen when the distance between the two measurement locations was less than 150 km and the time difference
was less than 1 hour in the NH and the non-PSC season in
the SH. A more stringent criterion for coincident pairs wasapplied during the PSC season in the SH because of thespatial inhomogeneity of PSCs. Here, a maximum distanceof 50 km and a maximum time difference of 1 hour wereapplied for the selection. Comparisons in the NH only usedaerosol data after mid-April. The polar vortex, as defined by
the definition of Nash et al. [1996], persisted at approximately 20 km at ILAS-II measurement latitudes until midApril. At the boundary of the polar vortex, large gradients instratospheric aerosol concentration exist [Thomason andPoole, 1993].
[12] SAGE II and SAGE III usually made observations atgreater distances from ILAS-II measurements than POAM
III did owing to the difference in satellites’ orbit. Thus the
distance constraint between the two measurement locations
was set to be 300 km, although the same 1-hour maximum

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time difference was used. These criteria yielded sufficientnumbers of coincident pairs in the NH. For the SH,
however, the criteria yielded only a few coincident pairs.
Thus looser criteria were applied. Coincident pairs in the SH
were required to be within 500 km of each other, with a timedifference of at most 12 hours. These looser criteria allowed
multiple SAGE II observations to match a single ILAS-II
data point. In that case, all observations were accepted as
individual pairs.
[13] SAGE II does not observe aerosol extinction at
780 nm. SAGE II extinction data at 525 nm and 1019 nm
were interpolated to extinction at 780 nm by assuming thatthe logarithm of aerosol extinction is roughly proportional
to the logarithm of wavelength in the lower stratosphere, as
in the works by Burton et al. [1999] and Hayashida et al.
[2000]. The reported errors of SAGE II extinction data werealso interpolated. This study uses SAGE II version 6.2(V6.2) aerosol extinction data. Although the V6.2 extinction
data have not yet been validated, we can confirm thereliability of the data set by comparing to the previous
versions of 5.93 [e.g., Osborn et al., 1989; Ackerman et al.,
1989; Russell and McCormick, 1989] and 6.0 [Hervig and
Deshler, 2002] extinction data that have no clear bias at thetwo relevant wavelengths. The V6.2 extinction data agreed
to the V5.93 data within the RSS of both reported errors.
The difference was within 10% from 18 to 30 km,
although the V6.2 extinction data were on average smaller
than the V5.93 data below 18 km. Better agreement wasseen in comparisons between the V6.2 and the V6.0 below
18 km, although the V6.2 extinction data were on averagelarger than the V6.0 data above 25 km. We can conclude
that SAGE II V6.2 extinction data interpolated to 780 nm
should have no systematic bias.
[14] SAGE III aerosol extinction data and errors at
756 nm and 869 nm were interpolated to extinction at
780 nm using a method similar to that applied to SAGE II
extinction data. SAGE III version 3.0 (V3.0) aerosol
extinction data are used in this study. These data, nearly
identical to the V2.0 data [Taha et al., 2004], have apositive bias at all altitudes at 756 nm and a negative bias
above 24 km at 869 nm; extinctions at these two wavelengths do not follow the power law that is determinedfrom extinctions at the other SAGE III wavelengths[Thomason and Taha, 2003]. The positive bias in extinction at 756 nm and no bias at 869 nm below 24 km should
produce a slightly positive bias in extinction interpolated
to 780 nm. Here, bias in SAGE III extinction datainterpolated to 780 nm was estimated by comparing theinterpolated values to SAGE II extinction data interpolated
to 780 nm. April and September SAGE II and SAGE III
measurements yielded 32 coincident pairs in the NH whena distance maximum of 300 km and a time difference
maximum of 1 hour were applied as criteria for the datafrom April to October 2003. The relative difference,
DSAGE3-SAGE2 (%), is as follows:
DSAGE3-SAGE2 ð%Þ 

100  ðSAGE III  SAGE IIÞ:
SAGE II

DSAGE3-SAGE2 was calculated for all 32 coincident pairs.
Figure 2a shows close agreement between SAGE III and
SAGE II extinction data above 12 km during ILAS-II

D11S05operations, although SAGE III extinction was slightly larger
at almost all altitude levels, as expected.
[15] POAM III measures aerosol extinction at 780 nm, so
a direct comparison can be made with ILAS-II AEC data.
POAM III aerosol extinction data (version 4.0, abbreviatedas V4.0) at 780 nm used in this study have not beenvalidated through intercomparisons with other measurements. Here, POAM III extinction data at 780 nm werealso compared with SAGE II extinction data interpolated to
780 nm. Comparison criteria in the NH were a distancemaximum of 300 km and a time difference maximum of
1 hour, yielding 21 total coincident pairs in April, July, andSeptember 2003. Comparison criteria in the SH were a
distance maximum of 500 km and a time difference maximum of 12 hours, which yielded 38 coincident pairs in
February. The relative difference between SAGE II and
POAM III, DPOAM3-SAGE2 (%), was calculated for all
coincident pairs as follows:
DPOAM3-SAGE2 ð%Þ 

100  ðPOAM III  SAGE IIÞ:
SAGE IIAs shown in Figure 2b, POAM III extinction agreed with
SAGE II extinction in the NH to within 1s standard
deviation, although POAM III extinction was 14– 22%
larger on average than SAGE II extinction from 22 to 24 km.
In contrast, from 13 to 23 km in the SH, POAM III
extinction was more than 1s standard deviation than SAGE
II extinction, as shown in Figure 2c; POAM III extinction
data clearly showed a 11 – 30% systematic positive bias
relative to SAGE II extinction data. These results are
consistent with the results of comparisons of coincident
measurements at 450 nm and 1020 nm between POAM III
and SAGE II (C. E. Randall et al., Validation of POAM III
version 4.0 aerosols, manuscript in preparation, 2006).
[16] Measurements of balloon-borne OPC and LPC weretaken at Syowa Station (69.0S, 39.6E, star in Figure 1) by
the 43rd and 44th Japanese Antarctic Research Expedition(JARE) on 22 February in 2003 during the ILAS-II preoperational period. Aerosol extinction coefficients estimatedfrom the size distribution measurements were compared to
the nearest ILAS-II AEC data. Syowa Station is about
599 km from the nearest ILAS-II measurement location(74.0S, 36.6E) on the same day.
[17] Comparisons to data from in situ measurements like
OPC and LPC are meaningful even for just one comparison,because the measurement principle differs from that ofsatellite sensors. A particle counter counts the number ofparticles and measures their sizes by detecting light scatteredby particles that are exposed to incident light. The OPC and
LPC use a halogen lamp and a He-Ne laser, respectively, as alight source. The OPC (LPC) measures particles with radiifrom 0.15 to 3.5 mm (from 0.056 to 0.25 mm). By assuming
refractive indices for sulfate aerosols, a bimodal lognormal
distribution function was fit to the measured cumulative
number concentrations for each particle size range. Extinction at 780 nm was then derived from the particle size
distribution by performing Mie scattering calculations. Here,the refractive indices (1.439 – 1.452) were estimated following Steele and Hamill [1981]. Simultaneously observed
temperature and pressure data were used, and a mixing ratioof water vapor of 4 ppmv and a sulfate content of an aerosol

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D11S05

D11S05Figure 2. (a) Profiles of the mean difference (black line) between SAGE II (V6.2) and SAGE III (V3.0)
aerosol extinction data both interpolated to 780 nm (DSAGE3-SAGE2) and the 1s standard deviation (graylines) in the NH. Here, the logarithms of SAGE II extinction at 525 nm and 1019 nm and SAGE III
extinction at 756 nm and 869 nm are linearly interpolated with the logarithms of the wavelengths,
yielding the extinctions at 780 nm. The reported errors of SAGE II and SAGE III extinctions were alsointerpolated. (b) Profiles of the mean difference between SAGE II and POAM III (V4.0) extinction data
at 780 nm (DPOAM3-SAGE2) and the 1s standard deviation in the NH. (c) Profiles of the mean difference
between SAGE II and POAM III extinction data at 780 nm (DPOAM3-SAGE2) and the 1s standard
deviation in the SH. Criteria for coincident pairs are a distance from a SAGE II measurement location ofno more than 300 km and a measurement time difference of no more than 1 hour for the NH, and a
distance of no more than 500 km and a time difference of no more than 12 hours for the SH.
droplet of 1.0  1015 g were assumed. Error in the OPC/
LPC data was defined as the RSS of the systematic measurement error and statistical random error of the counting. The
systematic measurement error, which includes uncertainty inthe flow rate, was around ±5%
pﬃﬃﬃﬃ for all altitude levels.
Uncertainty in a count C is ± C [e.g.,pﬃﬃﬃﬃ Willeke and Liu,
1976]. The relative counting error (1/ C ) in this study wascalculated for each particle size range for each level. The RSSof the calculated counting errors was then defined as thestatistical counting error and ranged from 0.4 to 44%.

4. Results
4.1. Northern Hemisphere (NH)[18] Figure 3 (top) compares ILAS-II extinction to
SAGE II extinction in the NH. Here, the relative percent
difference, D (%), is defined as follows:D

100  ðILAS-II  other sensorsÞ
:12  ðILAS-II þ other sensorsÞ

The RSS of the total error of individual ILAS-II AEC data
and the interpolated error of the coincident SAGE II
extinction data (defined as ‘‘combined error’’) was divided
by the mean of both extinction data, the denominator of theexpression for D, to define a relative error. At altitude levels
between 12 and 19 km, ILAS-II extinction was withinaround ±10% of SAGE II extinction; the difference waswithin the range of the combined error. In contrast, D (meanof D) values of ILAS-II and SAGE II from 20 to 26 km
ranged from 10 to 34%, which were more than 1s
standard deviation (6 – 29%). Furthermore, the values
exceeded the range of the combined error (7 – 18%).
ILAS-II extinction was smaller than SAGE II extinction
when both extinctions were less than 104 km1. The
mean absolute differences there were as small as 0.7 –1.8 
105 km1. One of the possible reasons for the negative
bias seen in ILAS-II AEC data is uncertainty in the tangent
height registration of the V1.4 retrieval, as described below.
[19] ILAS-II and SAGE III extinction data are comparedin Figure 4. As above, D was calculated for each of the
ILAS-II and SAGE III coincident pairs. Comparisons between ILAS-II and SAGE III are similar to comparisons
between ILAS-II and SAGE II shown in Figure 3 (top).
Below 20 km, ILAS-II and SAGE III extinction data werealso within around ±10% of each other, and that difference
was within the range of the combined error. From 20 to 26 km,
ILAS-II extinction values were smaller than SAGE III
extinction values; D and 1s standard deviation ranged from
12 ± 8 to 45 ± 26%, exceeding the combined errors (7 –21%). However, the difference between ILAS-II and SAGE III
occurred when both extinctions were below 104 km1,
and the magnitude of the mean absolute extinction differences was as small as 1.1– 2.2  105 km1.

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D11S05Figure 4. As in Figure 3 but for 87 coincident ILAS-II and
SAGE III (interpolated to 780 nm) pairs in the NH.

Figure 3. Comparison between ILAS-II and SAGE II
extinctions (interpolated to 780 nm) in the NH. (top) 34
coincident pairs in the NH. (bottom) 8 coincident pairs in
February in the SH. (left) Profiles of the mean of ILAS-II(black line) and SAGE II (gray line) extinctions used in thecomparison. Dashed lines are the 1s standard deviation
profiles. Triangles show the difference between ILAS-II and
SAGE II mean extinction values. Black (gray) triangles
correspond to negative (positive) values of ILAS-II
extinction minus SAGE II extinction. (right) Mean ofrelative percent difference (D) computed for all coincident
ILAS-II and SAGE II extinction data (thick black line) andthe 1s standard deviation (thin black lines). See text for thedefinition of D. Gray dashed lines indicate the mean of therelative error defined as the RSS of the total error of ILAS-II
and the interpolated error of SAGE II divided by the meanof ILAS-II and SAGE II extinctions (equal to thedenominator of the expression for D).
[20] Figure 5 (top) compares ILAS-II and POAM III
extinction data in the NH. From 11 to 19 km, ILAS-IIand POAM III difference was within around ±15% andalmost within the range of the combined error. From 20 to
24 km, where both extinctions were less than 104 km1,
ILAS-II extinction values were small compared to POAM
III extinction values. D and 1s standard deviation ranged
from 17 ± 12 to 37 ± 25%, exceeding the combined

Figure 5. As in Figure 3, but for comparison between
ILAS-II and POAM III. (top) 247 coincident pairs in theNH. (bottom) 198 coincident pairs in the non-PSC season in
the SH.

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Figure 6. Comparison between ILAS-II (74.0S, 36.6E)
and OPC/LPC (69.0S, 39.6E) on 22 February. The
distance between the two measurement locations is599 km. (left) ILAS-II extinction profile (thick black line)
and the total error profile (thin black line). Thick gray andthin gray lines indicate the extinction profile calculated fromthe OPC/LPC data and the error profile, respectively. See
text for the detail of the estimate of error in the OPC/LPC
extinction data. (right) Relative percent difference (D)between ILAS-II and OPC/LPC extinction data and therelative error defined as the RSS of the both errors divided
by the mean indicated by black and gray dashed lines,
respectively.
errors (8– 16%). At these altitudes, the magnitudes of the D
and the mean absolute extinction difference (1.9 – 3.5 
105 km1) between ILAS-II and POAM III were slightly
larger than those between ILAS-II and SAGE II and thosebetween ILAS-II and SAGE III at the same altitudes.
4.2. Southern Hemisphere (SH)4.2.1. Non-PSC Season[21] Figure 3 (bottom) compares ILAS-II and SAGE II
extinction data in February. ILAS-II extinction was larger
than SAGE II extinction from 10 to 16 km. Four of the eight
coincident ILAS-II AEC data were truncated at higher
altitudes than usual, around 12– 13 km, and the extinction
values were unusually large, resulting in poor agreementwith SAGE II extinction. However, the other four cases
showed good agreement at these altitudes; they agreed towithin around ±5% from 12 to 16 km. These two features in
the comparison yielded large standard deviations from 10 to
16 km, as shown in Figure 3 (bottom). From 17 to 19 km,
ILAS-II and SAGE II were within ±10% of each other.
From 20 to 25 km, ILAS-II extinction was smaller than
SAGE II extinction. D and 1s standard deviation ranged
from 12 ± 12 to 66 ± 23%, exceeding the combined
errors (9 –24%). These results are similar to those in the
NH. ILAS-II extinction was smaller than SAGE II extinction
when both extinctions were below 104 km1. The mean
absolute extinction differences of 1.4 – 2.2  105 km1 from
20 to 25 km were slightly larger than the absolute differencesin the NH (0.7 – 1.8  105 km1).

D11S05[22] Figure 5 (bottom) shows comparisons between
ILAS-II and POAM III extinctions in the non-PSC season(January –May). They agreed to within ±5% from 11 to
14 km, but ILAS-II extinction was smaller than POAM III
extinction above 15 km. D and 1s standard deviation
between 15 and 24 km ranged from 13 ± 9 to 94 ±
51%. These values exceeded the combined errors (8 – 45%).
The mean absolute extinction differences of 2.5– 5.5 
105 km1 were larger than the absolute differences between ILAS-II and SAGE II in the non-PSC season.
[23] Figure 6 shows a comparison between ILAS-II andOPC/LPC data. The differences between the two profiles
were comparable to the error bars at most altitude levels.
The error in the OPC/LPC data was estimated to be 5 –44%;
however, this is probably an underestimate, because thesystematic error of the measurement and the statistical errorof the counting were used as an alternative to the error of the
OPC/LPC extinction data. A more proper error estimate
would use a Monte Carlo simulation to infer the impact ofthe measurement uncertainties on the lognormal parameters
and the derived parameters such as aerosol extinction, as
done by Deshler et al. [1993]. The ILAS-II and the OPC/
LPC data do agree to within their error bars if the insufficient error estimate is considered. The difference was within±15% from 13 to 18 km, although ILAS-II extinction
was smaller than the OPC/LPC extinction at all levels
above 15 km, which is similar to the results in thecomparisons with SAGE II and POAM III.
4.2.2. PSC Season[24] Figure 7 shows comparisons between ILAS-II and
POAM III extinction data during the PSC season (June –October). Figure 7 (left) shows that extinction data fromboth instruments in the PSC season were 5 to 10 times
larger than those during the non-PSC season. In addition,
both 1s standard deviations during the PSC season werealso larger than those in the non-PSC season, because theheight at which PSCs occurred from June to October varied
depending on temperature profiles coupled with the movement of cold region. As shown in Figure 7 (right), ILAS-II

Figure 7. As in Figure 3 but for 164 coincident ILAS-II and
POAM III pairs in the PSC season in the SH. Note that thescale of extinction is one order higher than in Figures 3 – 6.

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Figure 8. PSC profiles of the ILAS-II (73.8S, 88.2E)
and the POAM III (73.7S, 99.3E) on 10 August 2003
indicated by thick black and gray lines, respectively. Thinlines show their errors. The distance between the two
measurement locations and the time difference are 42 km
and 0.1 hour.
extinction was smaller than POAM III extinction from 18 to
24 km; D ranged from 16 to 112%, exceeding the rangeof the combined error (10 – 59%). This characteristic issimilar to the results for the non-PSC season in bothhemispheres. However, ILAS-II and POAM III extinction
data were within ±15% of each other from 12 to 17 km in
the PSC season even when PSCs were present.
[25] Figure 8 shows ILAS-II (black) and POAM III (gray)extinction profiles on 10 August 2003. The distance between the two measurement locations and the time difference were no more than 42 km and 0.1 hour. Both profiles
included two enhanced layers at approximately 19 km and
23 km, indicating the existence of PSCs. In addition to thiscase, ILAS-II and POAM III often obtained extinction
profiles with vertically layered structures similar to eachother when they made observations in very close vicinity.

5. Discussion and Summary[26] We assessed the data quality of ILAS-II AEC at
780 nm processed with the V1.4 retrieval algorithm. The
focus of this study was AEC data below 30 km in the NH
and below 25 km in the SH because ILAS-II AEC data
above those altitude levels would be affected by sunspots inthe current version. Care should therefore be taken whenusing the V1.4 AEC data above these altitudes. The
precision of ILAS-II AEC data as estimated from measurement repeatability was 5– 15% at 12– 26 km in the NH and
6 – 20% at 12 –23 km in the SH.
[27] In the NH, ILAS-II AEC was ±10% to ±15% of
SAGE II, SAGE III, and POAM III below 20 km. Above
20 km, ILAS-II extinction was systematically smaller thanextinctions from the other three sensors. The slightly larger
differences were seen between ILAS-II and SAGE III/
POAM III extinction data than between ILAS-II and SAGE
II extinction data, because SAGE III and POAM III extinction values were slightly larger on average than SAGE II
extinction values (Figures 2a and 2b). It is also consistent

D11S05

with the generally larger variability in POAM III extinction
data than in SAGE II extinction data at 1020 nm [Randall etal., 2001, Figure 6].
[28] ILAS-II and SAGE II comparisons for February in
the SH suggest that ILAS-II AEC has a negative bias
ranging from 12 to 66% as altitude increases from 20
to 25 km, although their agreement was closer from 17 to
19 km. The magnitude of D between ILAS-II and SAGE II
over the SH was slightly larger than that over the NH at thesame altitude levels. It is still unclear why differences werelarger in the SH than in the NH above 20 km. ILAS-II
extinction was systematically smaller than POAM III extinction at all levels above 15 km during the non-PSC
season (January – May). The difference was larger than the
difference between ILAS-II and SAGE II. However, incontrast to the NH, POAM III extinction data have a distinct
positive bias relative to SAGE II extinction data in the SH(Figure 2c). The bias can explain why ILAS-II AEC data
have a larger negative bias against POAM III data during
the non-PSC season in the SH. During the PSC season(June – October), ILAS-II extinction agreed with POAM III
extinction from 12 to 17 km despite the high frequency of
PSC, although ILAS-II was also smaller than POAM III
above 18 km.
[29] Uncertainty in tangent height registration is one
possible cause for extinction differences between ILAS-II
and three other satellite sensors above approximately 20 km.
Tangent height registration error can induce error in retrieved extinction data. Uncertainties in tangent heightregistration were less than 100 m for the SAGE II V6.2
retrieval, around 100 m for the SAGE III V3.0 retrieval[Wang et al., 2002; J. Zawodny, personal communication,2005], and 250 m for the POAM III V4.0 retrieval [Lumpe
et al., 2002; J. Lumpe, personal communication, 2005]. In
the ILAS-II V1.4 retrieval, the transmittance spectrum
method (TS-M) is applied to determine tangent heights
below 30 km. The TS-M utilizes the fact that the absorption
spectra due to oxygen molecules (O2 A-band) are expressed
as a function of temperature and pressure profiles [Nakajima
et al., 2002]. Uncertainty in the TS-M mainly arises fromerror in estimating the baseline component, consisting ofaerosol scattering, Rayleigh scattering by atmospheric molecules, and absorption by ozone in the Wulf band (referred
to as the ‘‘baseline fitting error’’), and error in selecting therange of the O2 A-band for calculating the transmittancespectra (‘‘range selection error’’). Numerical simulations
that were similar to those applied by Nakajima et al.
[2002] showed that errors in the baseline fitting and range
selection would systematically shift the registered tangent
heights, producing systematic errors ranging from 100 to+100 m and from 150 to +150 m, respectively. Including
uncertainties in ILAS-II geometric position and instrument
functions, the uncertainty in the tangent height registration
below 30 km for the ILAS-II V1.4 retrieval is systematic
error ranging from 180 to 180 m and random error of±30 m (T. Tanaka et al., New tangent height registration
method with version 1.4 data retrieval algorithm for thesolar occultation sensor ILAS-II, manuscript in preparation,2006).
[30] A sensitivity study using ILAS-II data suggested thatheight assignments 100 m higher than in the current
retrieval caused a 5% increase in AEC below 20 km

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References

Figure 9. Mean profiles of relative percent difference (D)between ILAS-II and SAGE II, SAGE III, or POAM III.
They are identical with the values shown in the right plotsof Figures 3, 4, 5, and 7. (a) NH. (b) SH.
and a 10– 25% increase at approximately 25 km; reversely,height assignments 100 m lower than in the current retrieval
caused 5% and 10– 25% decreases in AEC below 20 km
and at approximately 25 km, respectively. If a heightassignment becomes 100 m higher than the current retrieval,
the negative bias seen in ILAS-II AEC data above approximately 20 km in both hemispheres disappears for the mostpart. The higher altitude assignment also increases AEC
below approximately 20 km where ILAS-II extinction wasslightly larger than extinctions from the other sensors
although the difference was within retrieval errors. However, it does not produce any noticeable bias at lower
altitude levels because the percentage of the impact of
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AEC decreased with decreasing altitudes.
[31] Figure 9 summarizes the comparisons of aerosol
extinction data between ILAS-II and other satellite sensors
in both hemispheres. Over the NH, the magnitudes ofrelative differences in extinctions between ILAS-II andthree other sensors were similar, and the difference characteristics in the vertical were also similar. Over the SH, themagnitudes of relative differences between ILAS-II extinction and SAGE II and POAM III extinctions were different.
Above approximately 20 km in both hemispheres, ILAS-II
AEC is systematically lower, but the mean absolute difference is as small as 2  105 km1 during the non-PSC
season.
[32] Acknowledgments. We thank all members of the ILAS-II
Science Team and Validation Experiment Team and their associates. We
are also grateful to the United Kingdom Meteorological Office (Met Office)for supplying the Met Office assimilation data. OPC/LPC measurements at
Syowa Station for ILAS-II validation were carried out by the 43rd and 44th
Japanese Antarctic Research Expedition (JARE) in cooperation with the
National Institute of Polar Research (NIPR) and the ILAS-II project of the
National Institute for Environmental Studies (NIES) with help of Japan
Meteorological Agency (JMA). The ILAS-II data retrieval processing was
carried out at the ILAS-II Data Handling Facility (DHF) at NIES. The
ILAS-II project has been funded by the Ministry of the Environment ofJapan (MOE). A part of this research work was supported with the Global
Environment Research Fund provided by MOE.

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

R. M. Bevilacqua, Naval Research Laboratory, Code 413BE, Remote
Sensing Division, 4555 Overlook Avenue, Washington, DC 20375, USA.
M. K. Ejiri, Center for Atmospheric and Space Sciences, Utah State
University, Logan, UT 84322, USA.
M. Hayashi and K. Shiraishi, Faculty of Science, Fukuoka University,
Fukuoka 814-0180, Japan.
S. Hayashida, Faculty of Science, Nara Women’s University, Nara 6308506, Japan.
H. Irie, Frontier Research Center for Global Change, Japan Agency for
Marine-Earth Science and Technology, Yokohama 236-0001, Japan.
H. Kanzawa, Department of Earth and Environmental Science, Graduate
School of Environmental Studies, Nagoya University, Nagoya 464-8601,
Japan.
H. Kobayashi, Central Research Institute of Electric Power Industry,
2-11-1 Iwadokita, Komae-shi, Tokyo 201-8511, Japan.
H. Nakajima, Y. Sasano, T. Sugita, T. Tanaka, and T. Yokota, National
Institute for Environmental Studies, Tsukuba 305-0053, Japan.
C. E. Randall, Laboratory for Atmospheric and Space Physics, Universityof Colorado, Boulder, CO 80303, USA.
N. Saitoh, Center for Climate System Research, University of Tokyo,
General Research Building, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 2778568, Japan. (snaoko@ccsr.u-tokyo.ac.jp)
G. Taha, Science Systems and Applications, Inc., 10210 Greenbelt Road,
Suite 400, Lanham, MD 20706, USA.
Y. Terao, Division of Engineering and Applied Science and Technology,
Harvard University, Cambridge, MA 02138, USA.
L. W. Thomason, NASA Langley Research Center, 23a Langley
Boulevard, MS 475, Hampton, VA 23681, USA.

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