JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D24, 8205, doi:10.1029/2001JD000595, 2002

Characteristics of Arctic polar stratospheric clouds in thewinter of 1996/1997 inferred from ILAS measurements
N. Saitoh,1 S. Hayashida,1 Y. Sasano,2 and L. L. Pan3
Received 5 March 2001; revised 12 October 2001; accepted 6 November 2001; published 19 September 2002.

[1] The Improved Limb Atmospheric Spectrometer (ILAS) captured many polar

stratospheric cloud (PSC) events in the Northern Hemisphere during the winter andearly spring of 1997. Simultaneous measurements of nitric acid and aerosols by ILAS
made it possible to infer PSC composition. The aerosol extinction coefficient and nitricacid data were compared with the theoretically predicted values for supercooled ternary
solution (STS), nitric acid dihydrate (NAD), and nitric acid trihydrate (NAT) at
thermodynamic equilibrium to classify PSC types. The observations showed that in1997, both nitric-acid-containing solid and liquid PSCs formed over the Arctic during
winter and early spring, until mid-March. The STS PSCs were observed early in the
PSC season, in mid-January. Most of the PSCs observed late in the PSC season hadfeatures of nitric-acid-containing hydrates. An intensive analysis of the temperature
histories suggested that most of the STS events observed in January had experienced thethermal conditions necessary for the formation of liquid PSCs. The nitric-acidcontaining hydrates observed in March seemed not to have been influenced by any
mountain-induced lee waves. The process of nitric-acid-containing hydrate formation
INDEX TERMS: 0305
based on synoptic scale temperature change is discussed.
Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0340 AtmosphericComposition and Structure: Middle atmosphere—composition and chemistry; 1640 Global Change: Remotesensing; 1610 Global Change: Atmosphere (0315, 0325)
Citation: Saitoh, N., S. Hayashida, Y. Sasano, and L. L. Pan, Characteristics of Arctic polar stratospheric clouds in the winter of
1996/1997 inferred from ILAS measurements, J. Geophys. Res., 107(D24), 8205, doi:10.1029/2001JD000595, 2002.

1. Introduction[2] Polar stratospheric clouds (PSCs) cause large amountsof ozone depletion in the lower stratosphere in both poleregions, by providing a situation that allows heterogeneous
reactions that convert inactive reservoir chlorine into active
chlorine [Solomon, 1999]. PSCs also remove nitric acidfrom the gas phase irreversibly, which prevents deactivation of active chlorine. The phase and chemical composition of PSCs are important factors in accurately estimating
the amount of ozone loss and revealing the mechanism ofozone destruction during polar winter in more detail.
Ravishankara and Hanson [1996] and Borrmann et al.
[1997] suggested that liquid droplets convert inactive
chlorine to an active form more efficiently than do frozen
particles. In addition to their function in heterogeneous
reactions, solid particles play an important role in redistribution of NOy and water vapor via sedimentation. The
temperatures over the Arctic seldom fall below the temperature at which ice particles are formed; therefore, solid
1

Faculty of Science, Nara Women’s University, Nara, Japan.
National Institute for Environmental Studies, Tsukuba, Ibaraki, Japan.
3
Atmospheric Chemistry Division, National Center for AtmosphericResearch, Boulder, Colorado, USA.
2

Copyright 2002 by the American Geophysical Union.
0148-0227/02/2001JD000595$09.00

ILS

particles containing nitric acid (nitric acid hydrates) arethought to be a predominant factor in the large-scale
denitrification observed in the Arctic [e.g., Waibel et al.,
1999; Fahey et al., 2001; Tabazadeh et al., 2001].
[3] Lidar measurements are an effective technique forinferring the composition of PSCs directly. The scattering
ratio and depolarization observed with lidar make it possible
to categorize the PSC types as type 1a (large depolarizationand small scattering ratio), type 1b (small depolarization andlarge scattering ratio), or type 2 (large depolarization andlarge scattering ratio) [Browell et al., 1990]. Carslaw et al.
[1994] showed that the observed type 1b PSCs would besupercooled ternary solutions (STS), based on a thermodynamic model, and measurements of the infrared spectra of
PSCs support the existence of STS [Toon and Tolbert, 1995].
Backscatter sonde measurements show substantial growth ofliquid droplets at several degrees below TNAT, suggesting theexistence of STS [Larsen et al., 1996]. While both thecomposition and formation mechanism of liquid type 1b
PSCs are clear to some extent, those of solid type 1a PSCs
remain unclear [Tolbert, 1996]. Type 1a PSCs could be nitricacid hydrates, such as nitric acid trihydrate (NAT), nitric aciddihydrate (NAD), or nitric acid pentahydrate (NAP) [Hanson and Mauersberger, 1988; Worsnop et al., 1993; Marti
and Mauersberger, 1994].
[4] Tolbert [1994, 1996] reviewed the formation mechanisms of PSCs over a polar winter. The solid particles that

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SAITOH ET AL.: ARCTIC POLAR STRATOSPHERIC CLOUDS IN THE WINTER OF 1996/1997

are often observed over the Arctic [e.g., Browell et al., 1990]
cannot simply be interpreted as nucleation on ice nuclei,
because the synoptic temperatures seldom fall below the ice
frost point over the Arctic. Recently, many studies haveexamined the effect of mountain-induced lee waves on solid
particle formation [Tsias et al., 1997; Carslaw et al., 1998,
1999; Riviere et al., 2000]. The formation mechanisms of
PSCs are so complex that the thermal conditions the particles
have experienced should also be examined closely, in
addition to the temperature where they are observed. Many
studies of synoptic temperature histories have been carriedout using in situ [Larsen et al., 1996, 1997] and satellite[Santee et al., 1998; Riviere et al., 2000] measurements.
[5] Some studies suggest that satellite measurements
have good potential for investigating the chemical composition and physical state of PSCs. Massie et al. [1997] usedthe data obtained with the Microwave Limb Sounder(MLS), the Cryogenic Limb Array Etalon Spectrometer(CLAES), and the Improved Stratospheric and MesosphericSounder (ISAMS) on board the Upper Atmosphere
Research Satellite (UARS) to study the formation of PSCs
over Scandinavia. Hervig et al. [1997] compared the Halogen Occultation Experiment (HALOE) aerosol data withthe calculated volume of several types of PSCs. Santee et al.
[1998] investigated the composition of PSCs over the
Antarctic using nitric acid data measured with the MLS,
and Santee et al. [2000] combined the MLS nitric acid datawith the aerosol extinction coefficient data obtained fromthe Polar Ozone and Aerosol Measurement (POAM) II.
[ 6 ] The Improved Limb Atmospheric Spectrometer(ILAS) on board the Advanced Earth Observing Satellite(ADEOS) is a solar occultation sensor [Sasano et al., 1999]
with two spectrometers: a 44-channel infrared (IR) spectrometer operating from 6.21 to 11.76 microns, and a 1024channel near-visible spectrometer operating at wavelengths
from 753 to 784 nm [Suzuki et al., 1995; Nakajima et al.,2002]. ILAS was designed to observe the profiles of minor
stratospheric species, such as ozone, nitric acid, and water
vapor, and that of the stratospheric aerosol extinction coefficient at 1-km height intervals [Yokota et al., 2002]. Due tothe Sun-synchronous polar orbit of ADEOS, ILAS observed
14 circumpolar points in each hemisphere every day at highlatitudes (57.1 – 72.7N and 64.3 – 88.2S). ILAS started
regular operation in November 1996 and continued until
June 1997. In inferring PSC composition, simultaneous
measurements of aerosol extinction and nitric acid for thesame air mass are desirable. Hayashida et al. [2000a]reported that the ILAS captured more than 60 PSC profiles
in the Northern Hemisphere during the winter and early
spring of 1997, and showed their temporal and spatial

distributions. The aim of this study was to infer the composition of the PSC particles observed from ILAS aerosol andnitric acid data. The synoptic temperature histories of air
masses containing the observed PSCs are also examined tocompare the results with PSC formation theory.
[7] The latest retrieval (version 5.20) of ILAS data wasused in this study. The version 3.10 and 5.20 ILAS nitric aciddata were validated by Koike et al. [2000] and Irie et al.
[2002], respectively, demonstrating the reliability of the data.
The version 5.20 ILAS water vapor data was validated by
Kanzawa et al. [2002]. The inversion algorithm and validityof version 4.20 of the ILAS aerosol extinction data have been
described by Hayashida et al. [2000a]. Details of the version
5.20 algorithm were described by Yokota et al. [2002]. Here,
we briefly present the differences between version the 5.20
and version 4.20 data. Figure 1 is comparable with the
Figure 3 of Hayashida et al. [2000a], which compared
Stratospheric Aerosol and Gas Experiment (SAGE) II version 6.0 data and ILAS version 5.20 data. For comparison,
we selected pairs of coincident measurements with ILAS and
SAGE II when the distance between the two measurement
locations was within 300 km and the measurement timedifference was less than 120 min. As a result, 38 profiles in
November 1996 and 20 profiles in February 1997 wereselected over the Southern Hemisphere, and 11 profiles in
May 1997 over the Northern Hemisphere. The November
data were separated into two groups, containing profiles
inside (33 profiles) or outside (5 profiles) the polar vortex.
Each panel depicts the mean fractional difference between
the ILAS and SAGE II data (solid line), the root mean squareof the fractional difference (dotted line), and the ILAS
(dashed line) and SAGE II (dashed and dotted line) measurement errors. The mean fractional differences are within theuncertainties of the ILAS and SAGE II measurement errors,
and are in the range of 10– 20% where the extinction values
are larger than 1.0  10 5 km 1, demonstrating the validityof the ILAS version 5.20 aerosol data for scientific use.

2. PSC Identification[8] Our approach to identifying PSCs is similar to thoseof Poole and Pitts [1994] and Fromm et al. [1997]. In thesestudies, threshold levels were determined from the background aerosol levels, and then events with an extinction
exceeding the threshold levels were regarded as PSC events.
In the method that we adopted by Hayashida et al. [2000a],
all the extinction data inside the polar vortex were averaged
for each altitude interval and for each 10-day period where
the collocated temperature was above 200 K, and the mean
was defined as the background aerosol level of each period.

Figure 1. (opposite) Comparison of the extinction coefficient at 780 nm using ILAS version 5.20 and SAGE II version
6.0. For comparison, we selected pairs of coincident ILAS and SAGE II measurements when the distance between the two
measurement locations was within 300 km and the measurement time difference was less than 120 min. The 38 coincident
profiles observed in November 1996 were divided into two groups depending on whether they were inside or outside thepolar vortex. From the left, comparisons of (a) 33 profiles over the Southern Hemisphere in November 1996 inside thevortex, (b) 5 profiles outside the vortex in the same month, (c) 20 profiles over the Southern Hemisphere in February 1997,
and (d) 11 profiles over the Northern Hemisphere in May. Each panel depicts the mean fractional difference between the
ILAS and SAGE II data (solid line), the root-mean-square of the fractional difference (dotted line), the ILAS measurement
error (dashed line), and the SAGE II measurement error (dashed and dotted line). The SAGE II extinction coefficients at
525 nm and 1020 nm were converted to 780 nm by linearly interpolating the logarithms of the extinction coefficients.

SAITOH ET AL.: ARCTIC POLAR STRATOSPHERIC CLOUDS IN THE WINTER OF 1996/1997

(a)

(b)

Nov 1996 (inside vortex)

Nov 1996 (outside vortex)

Altitude (km)20

Altitude (km)20

(ILAS-SAGEII)/SAGEIIStandard deviation
Relative error(ILAS)
Relative error(SAGEII)(ILAS-SAGEII)/SAGEIIStandard deviation
Relative error(ILAS)
Relative error(SAGEII)

Observation Mode: Sunset
No. of data: 33

Observation Mode: Sunset
No. of data: 5

0

0
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-20
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20
40
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-40

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Mean relative residual (%)

(c)

(d)

Feb 1997

May 1997

Altitude (km)20

Altitude (km)20

(ILAS-SAGEII)/SAGEIIStandard deviation
Relative error(ILAS)
Relative error(SAGEII)(ILAS-SAGEII)/SAGEIIStandard deviation
Relative error(ILAS)
Relative error(SAGEII)

Observation Mode: Sunset
No. of data: 20

Observation Mode: Sunrise
No. of data: 11

0

0
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-20
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-40

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Mean relative residual (%)( + : Version V05.20 >SAGEII)

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SAITOH ET AL.: ARCTIC POLAR STRATOSPHERIC CLOUDS IN THE WINTER OF 1996/1997

The mean plus five standard deviations was adopted as thethreshold value after close examination; events whose
extinction value exceeded the threshold value for the same
period, and whose relative error did not exceed 100%, wereidentified as PSCs (‘‘5sigma-selected’’ events) (about theestimation of total error, see the Appendix of Hayashida
et al. [2000a]). In the analysis of Hayashida et al. [2000a],
insufficient data on background aerosols (T > 200 K),
especially during the cold period in mid-January, reduced
the statistical reliability of the threshold values for theperiod [see Hayashida et al., 2000a, Figure 5].
[9] Our improved approach considered the data obtainedfor 5 days before and after each 10-day period in addition to
the data during the period. This approach produced more
realistic thresholds. Nevertheless, appropriate threshold values for mid-January upper than 26 km could not be
determined. However, this did not affect our analysis significantly, because PSCs were rarely observed at such highaltitudes. We identified about 250 events using thisimproved approach; it did not alter the trend of PSC
distribution shown by Hayashida et al. [2000a]. AlthoughHayashida et al. [2000a] had adopted the NAT formation
temperature, TNAT, as one possible reference to validate
PSC identification, we also investigated all PSC events
exceeding the threshold, including those at collocatedtemperatures higher than TNAT.

3. Comparison of the ILAS Data With
Theoretical Predictions[10] Comparison of the observed aerosol mass or nitricacid data with the values predicted from thermodynamic
theory for various PSC types allows us to infer the chemicalcomposition of observed particles [e.g., Massie et al., 1997;
Santee et al., 1998, 2000]. As the amounts of nitric acid andwater vapor in the atmosphere are closely concerned with
PSC composition, it is important to estimate those amounts
properly in inferring PSC composition.
[11] The ILAS observed the vertical profiles of nitric acidand water vapor, and the aerosol extinction coefficientsimultaneously. Using these observed data to calculate thetheoretically predicted values for various PSC types, we caninfer PSC composition more precisely than using climatological values, as done in earlier works. In the following, we
describe how we determine the total amounts of nitric acid,water vapor, and sulfuric acid properly, based on the ILAS
collocated gas and aerosol data. The total amount is theequivalent mixing ratio of gas and condensed phase combined. In this study, the ILAS data were compared withtheoretical values of particle volume and gaseous nitric acid,assuming the existence of STS, NAD, and NAT.
3.1. Total Amounts of Nitric Acid and Water Vapor
in the Background Stratosphere[12] The total amounts of nitric acid and water vapor in
the atmosphere required in theoretical calculations were
estimated from ILAS nitric acid and water vapor data. All
nitric acid and water vapor data inside the polar vortex wereaveraged for each altitude level and for each 10-day period
in the region where the collocated temperature exceeded
200 K, and the average values were taken as the total
background amounts of nitric acid and water vapor, respec-tively, for each period. Data with a relative error exceeding
100% were excluded from the calculation. For example, theamounts of nitric acid and water vapor at 20 km near 65Nin mid-January were estimated from the observations as
11.4 ppbv and 5.2 ppmv.
3.2. Total Amount of Sulfate in the Background
Stratosphere[13] The weight percentage of sulfate in stratosphericparticles depends only on temperature and humidity [Steeleand Hamill, 1981]. The total amount of sulfate in theatmosphere can be derived from the ILAS extinction profiles. The analytic expression of Carslaw et al. [1995] gives
the particle volume of H2SO4/H2O binary aerosol in warmer
conditions.
[14] We calculated the particle volumes in the background condition for various quantities of sulfate, applying
the values of water vapor estimated from the ILAS data. The
particle volume was converted into extinction coefficients at
780 nm for comparison with ILAS extinction data. The
conversion factors from volume to extinction were derived
by applying Mie scattering theory with the refractive indicesof H2SO4/H2O binary solution (n = 1.449, T = 223 K,
0.781 mm) [Russell et al., 1996]. Two different size distributions for the background conditions were assumed in thecalculations. One was that measured with the optical particle counter (OPC) at midlatitudes [Hofmann and Rosen,1983], and the other was that from the OPC data obtained
over Esrange/Kiruna (67.88N, 21.06E) in February 1997
during the ILAS validation campaign [Deshler et al., 2000],
when no PSCs were detected. Comparison showed that theconversion factors for the different size distributions were
not significantly different. The conversion factors were also
determined based on the Ångström parameter derived fromSAGE II aerosol extinction coefficient data [Hayashida
et al., 2000b]. They were consistent with the above twoconversion factors quantitatively, which confirms the validity of the conversion factors used here. For example, theestimated amount of sulfate was 0.25 ppbv at 20 km near
65N in mid-January. Details of the scheme to estimate total
sulfate were described by Hayashida et al. [2000b].
3.3. Calculation of Nitric Acid and Particle Volume
Based on Thermodynamic Theory[15] The theoretical particle volume for the formation of
STS and the amount of remaining gaseous nitric acid in theatmosphere were calculated by applying Carslaw et al.’s[1995] analytic expression. The total amounts of nitric acid,water vapor, and sulfate were derived as described above.
The vapor pressures of nitric acid in equilibrium over NAD
and NAT were calculated, based on Worsnop et al. [1993]
and Hanson and Mauersberger [1988], respectively. The
expected particle volumes for NAD and NAT were calculated using a mass density of 1.62 g/cm3 for NAT [Hofmann
and Deshler, 1991].
[16] The theoretical particle volumes have to be converted into 780 nm extinction coefficients for comparison
with the ILAS aerosol extinction data. The conversion
factors for PSC cases were derived using the temperaturedependent refractive indices of STS [Luo et al., 1996],
NAD, and NAT [Toon et al., 1994]. The bimodal size
distribution fitted to OPC data measured over Andoya

SAITOH ET AL.: ARCTIC POLAR STRATOSPHERIC CLOUDS IN THE WINTER OF 1996/1997(69.3N, 16.0E) in January 1997 [Deshler et al., 2000] wasadopted in this study. Comparison of the conversion factors
for several different particle size distributions for PSCs [Dyeet al., 1992] suggested that the size distributions of PSCs
are more uncertain than those of background aerosols. Thisshould be considered when inferring PSC composition.
3.4. Comparison of the ILAS Data With Theoretical
Predictions
3.4.1. Mid-January[17] Figure 2 shows scatterplots of UKMO temperature
versus the ILAS extinction coefficient (left column), and
UKMO temperature versus the ILAS nitric acid value (rightcolumn), at 20– 23 km in mid-January. Each panel represents a 1-km altitude level. Solid circles represent the data
identified as PSCs. Error bars for the ILAS data are shownin the figure, although not all are distinguishable. The
theoretical curves for STS (red lines), NAD (green line),and NAT (blue line) are shown in the figure. As for NAD
and NAT, their formation curves are derived with the massto-extinction conversion factor for PSC size distributionfunction. The PSC conversion factor for growth curve of
ternary solution should be applied only at cold temperatures
where STS droplets are expected to form, while the background conversion factor should be used at warm temperatures. In Figure 2, STS formation curves are indicated withthe thick solid red lines with the proper conversion factors
depending on temperatures, though the thin dotted red lines
are also shown for reference. The low extinction values atwarmer temperatures are consistent with the theoreticalcurve expected for background (binary) aerosols. In the
figure, the red circles indicate events whose extinction andnitric acid values are closest to theoretical STS values
among three types. Figures 2a and 2b show that theenhancement of the volume corresponds to the decrease in
the ambient nitric acid, suggesting the uptake of nitric acidinto particles as STS forms.
[18] In Figure 2, the data numbered 1 to 4 are from thesame profile, observed on 19 January (65.8N, 21.6E,
Figure 3a), and those numbered 5 to 8 are from 20 January(65.9N, 28.9E, Figure 3b). Figure 3 shows the verticalprofiles of the extinction coefficient (E), nitric acid (N),and the collocated UKMO temperature (T) on these twodays. The 10-day averaged background profile of nitricacid for mid-January is also shown in the figure (dashedline). The black and gray circles represent data identified as
PSCs. The black circles indicate the data that correspond to
the STS formation curve. Figure 3 shows that STS-like
PSCs were observed over a wide vertical range on thesedays. Comparison of the ILAS nitric acid profile with thebackground profile illustrates that nitric acid decreased
significantly in the region where STS PSCs were observed.
[19] Besides the characteristic agreement with thermodynamic model prediction shown above, two other independent analyses support the presence of STS in these observed
PSC events. One is given by the spectral analyses of ILAS
extinction data. The aerosol extinction coefficients derived
from the ILAS visible and four infrared window channels(at 7.12, 8.27, 10.6, and 11.8 mm) show that the spectralfeatures for these events are STS-like (S. Oshchepkov,
private communication, 2001). The other is from coincident
ground based lidar measurements. Both of GKSS Raman-

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lidar and a Mie-lidar of Bonn University observed type 1bevents on 19 and 20 January continuously at 19.0– 24.5 km
over Esrange/Kiruna (67.88N, 21.06E) [Mehrtens andReichardt, 1997; Dörnbrack et al., 2001]. These Lidar
observations were made in the vicinity of ILAS observations shown in Figure 3 (about 270 km and 450 km awayfrom the profiles in Figures 3a and 3b, respectively),providing good validation to our analyses that ILAS
observed STS.
[20] Figure 2 also shows some discrepancies between themeasurements and the theoretical prediction for STS.
According to the thermodynamic model, the extinction data
for the events from 1 to 8 should be much larger and nitricacid mixing ratio should be much lower than that observed.
Several factors could have contributed to these discrepancies. The first possible factor is nonequilibrium particle
formations under the influence of the mountain waves. Data
numbered from 1 to 8 in the figures were obtained downwind from Scandinavia, and it is possible that the air massobserved has been influenced by lee waves. If that is indeed
what happened, these STS droplets may not have attained
equilibrium with the gas due to mesoscale cooling events, as
discussed by Voigt et al. [2000], who concluded that nonequilibrium compositions are a dominant feature of STSnear Tice under the activity of gravity waves.
[21] The second possibility is the suppression of STS
growth due to the coexistence of ice particles. Pan et al.
[2002] analyzed ILAS water vapor data and found significant gas phase water vapor reduction and dehydration in
mid-January associated with ice particle formation. The
events that show large water vapor reduction include the
PSC events shown in Figure 3. ILAS retrieval error analyses[Yokota et al., 2002] indicate that, without the presence of
NAT particles, the retrieved water vapor data are reliable
even in the presence of ice PSCs. The spectral analyses ofextinction data support the absence of NAT in the midJanuary time period. These lead to the possibility that the
STS growth was suppressed in dehydrated air. In fact, oursensitivity analyses show that if the water vapor used in thethermodynamic model calculation is 1 ppmv lower, theSTS theoretical lines shift leftward and the agreement
between the observations and the STS theoretical lines forthese events is improved.
[22] The third factor that could have contributed to thediscrepancies is the inhomogeneity in the volume sampled
by the satellite sensor. An occultation sensor like the ILAS
has a relatively wide horizontal resolution along its line ofsight, which is about 200 km at an altitude of 20 km. Its
large sampling volume could make the characteristics of theobserved data ambiguous, especially when a mixture ofseveral compositions were observed. When spatial scale of acloud is less than the sampling volume, we see only lessvolume enhancement and less decrease in nitric acid amountthan expected from theory. Therefore, a mixture of STS and
ice would be a reasonable interpretation to explain thesynchronous decrease in nitric acid and water vapor.
[23] The last factor, but not the least, is the uncertainty insynoptic temperature data, such as UKMO data, which
greatly complicates the comparison between the theoreticalprediction and the observations. The magnitude of the bias
and random error in synoptic temperature database varies
with altitude, season, and geographical region, making it

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SAITOH ET AL.: ARCTIC POLAR STRATOSPHERIC CLOUDS IN THE WINTER OF 1996/1997

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Figure 3. The vertical profiles of the extinction coefficient (E), nitric acid (N), and collocated UKMO
temperature (T) observed on 19 January (65.8N, 21.6E) (a) and 20 (65.9N, 28.9E) (b). The 10-day
averaged background profile of nitric acid for mid-January is also shown in the figure (dashed line). The
black and gray circles represent data identified as PSCs. The black circles indicate the data that
correspond to the STS formation curve.
Figure 2. (opposite) Scatterplots of UKMO temperature versus the ILAS extinction coefficient (a1 – a4) and UKMO
temperature versus the ILAS nitric acid (b1 – b4) at 20 –23 km in mid-January. Each panel represents a 1-km altitude level:(a1) extinction coefficient at 20 km, (a2) 21 km, (a3) 22 km, (a4) 23 km; (b1) mixing ratio of nitric acid at 20 km, (b2) 21 km,(b3) 22 km, (b4) 23 km. Error bars for the ILAS data are shown in the figure. Solid circles represent the data identified as
PSCs. The red circles indicate the events consistent with the theoretical values of STS. Data numbered 1 to 4 come from theprofile observed on 19 January and those numbered 5 to 8 from the profile observed on 20 January. The theoretical curves
for STS (red lines), NAD (green line), and NAT (blue line) are shown in the figure. The thick solid red lines are based on theproper conversion factors depending on temperature. The thin dashed red lines are also shown for reference. See text about
more details. Ice frost points (Tice) are calculated using the total background amount of water vapor for each altitude level
Marti and Mauersberger [1993].

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SAITOH ET AL.: ARCTIC POLAR STRATOSPHERIC CLOUDS IN THE WINTER OF 1996/1997

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2

5

0
185

1

190

195
200
Temperature (K)

205

(b)

Figure 4. The same as Figure 2, but in early March, at 16– 18 km: (a1) 16 km, (a2) 17 km, (a3) 18 km,(b1) 16 km, (b2) 17 km, (b3) 18 km.
difficult to estimate the uncertainty of individual colocatedtemperature data. Pullen and Jones [1997] reported the
UKMO temperature has a 1.7 K positive bias and about
2 K of random scatter around TNAT. Manney et al. [1996]
derived much larger bias of 3.7 K for cold area (T < 200 K)
in January. Pan et al. [2002] showed that ILAS water vapor
data is consistent with a 3.5 K warm bias in the UKMO
data. Figures 2a1 – 2a4 also show ice frost points (Tice)calculated using the total background amount of water
vapor [Marti and Mauersberger, 1993]. If the temperaturebias is as large as 3 K, coexistence of ice would bepossible. There are some other reports of even larger bias,
depending meteorological conditions. Dörnbrack et al.
[2001] presented significant warm bias in synoptic temper-

ature analysis under active mountain-induced lee waves
over Scandinavia in January 1997.
3.4.2. Early March[24] In 1997, break up of the polar vortex occurred later
than usual, and it was reported that PSCs were observed by
the ILAS until early March of that year near 120E atrelatively low altitudes [Hayashida et al., 2000a, Plate 1c].
These were caused by the displacement of a cold air mass
following the movement of the polar vortex. Figure 4
shows the scatterplots of the ILAS extinction coefficientand nitric acid data at 16– 18 km in early March. Althougha few PSCs were observed in March, both the extinction
and nitric acid values suggest the existence of NAD or
NAT. After comparison with the theoretical values, most of

SAITOH ET AL.: ARCTIC POLAR STRATOSPHERIC CLOUDS IN THE WINTER OF 1996/1997

Table 1. Summary of the Ratio of the Number of Classified
Events in Each Category to the Total Number of PSC Events
Jan.
Feb.
Mar.

STS

NAD/NAT

Unclassified

All PSC Events

32.4
9.5
3.2

23.6
30.2
64.5

44.0
60.3
32.3

148
63
31

All the PSC events were categorized into specific groups based oncomparison of the ILAS extinction and nitric acid data with the theoreticalpredicted values. The ratios of the number of events for each category to thetotal number of PSC events in January, February, and March are indicatedin the left three columns. The unit is percent (%). The total numbers of PSC
events observed for each month are also shown in the rightmost column.

the events with enhanced extinction in the intense PSC
layers late in the PSC season could be categorized into
NAD or NAT (nitric-acid-containing solid PSCs).
[25] It is difficult to identify which composition is reliable
for some data plotted between the red (STS curve) and green(NAD curve) lines, when considering the temperature uncertainty. For NAT cases, however, discrimination between STS
and NAT could be easier, because the theoretical curves of
NAT and STS are far enough apart.
[26] On the contrary to the STS events discussed above,
the PSC events observed near 120E, shown in Figure 4,
have undergone relatively moderate temperature changes,
which will be discussed in the next section. It implies anassumption of an equilibrium state would be reasonable,
though there are many discussions on nonequilibriumtransition during solid particle formation. At any rate, it isapparent that the scatterplots in Figure 4 have quite different
feature from those in Figure 2.
[27] Table 1 summarizes the ratio of the number ofclassified events in each category to the total number of
PSC events in January, February, and March. Many STS
PSCs were observed at an earlier stage of the PSC season,
while the observed NAD/NAT ratio became higher at a laterstage. Some of the extinction coefficients and nitric acidvalues lying between the theoretically predicted values
could not be categorized into any. Those events are shownas ‘‘unclassified’’ in Table 1.

4. Thermal History of PSC Particles[28] The 20-day isentropic backward trajectory was calculated by using the European Centre for Medium-Range
Weather Forecasts (ECMWF) TOGA Basic Level III temperature data to examine the thermal conditions at thesynoptic scale for the observed PSC particles. The Earth
Observation Research Center-Trajectory Analysis Model(EORC-TAM) developed by NASDA/EORC was used.
See Matuzono et al. [1998] for details of this model. The
backward trajectories were also calculated for four surrounding points, 100 km north, south, east, and west of
the measurement point, to investigate the variability ofconditions which may depend on small difference in location considering a rather large ILAS sampling volume. If theposition and temperature histories of the measurement point
and the four surrounding points diverged considerably overa few days, such trajectory clusters were excluded from theanalysis, since they likely had a lot of uncertainties. Figure 5
illustrates typical temperature histories of the PSC events

ILS

2-9

that suggest the formation of STS as described in section 3.
The thick black lines are the temperature histories from the
ILAS measurement locations, and the bars show the rangeof divergence of the four surrounding points. In the temperature scale, the solid black, dashed black, solid gray, anddashed gray lines indicate the sulfuric acid tetrahydrate(SAT) melting temperature (TSAT) [Zhang et al., 1993],
NAT saturation temperature (TNAT) [Hanson and Mauersberger, 1988], SAT deliquescence temperature (Tdel) [Koopand Carslaw, 1996], and ice frost point (Tice) [Marti andMauersberger, 1993], respectively. They are calculatedusing the background nitric acid and water vapor values
determined for each 10-day period.
[29] The feature of the temperature history of STS eventswas rapid cooling just before the measurements. In moredetail, they can be categorized into two different patterns, as
indicated in Figures 5a and 5b. The air mass shown in
Figure 5a experienced TSAT one day before measurement,with a subsequent decrease in temperature until it wasobserved. At the time of observation, the temperature wasstill above Tice; therefore, the observed PSCs would havebeen liquid (STS), as far as warm bias in ECMWF temperature is about 1.4– 1.6 K as reported by Knudsen [1996].
The temperature history in Figure 5b is that of PSC event
numbered 2 in Figure 3a. Although the air mass shown in
Figure 5b never experienced temperature exceeding TSAT
unlike the temperature history in Figure 5a, it almost always
experienced temperatures above TNAT until about two days
before the observation, then fell below TNAT, but never
below Tice. Therefore, liquid droplets (STS) would be mostplausible as observed PSC type [Tabazadeh et al., 1995].
[30] However, as already mentioned in the previous
section, PSC event numbered 2 was observed downwindof the Scandinavia. The shaded area labeled ‘‘S’’ in Figure 5
indicates the time when air mass passed over the mountainsof Scandinavia. As discussed above, the event seems tohave been influenced by gravity waves, and possibility ofice formation should not be excluded if local cooling causedby mountain lee waves was significant.
[31] NAT and NAD PSCs were observed over a verticalrange of several kilometers on 9 and 10 March, as shown in
Figure 4. Figure 6 shows the temperature histories of these
PSC events. The temperature history in Figure 6b is that of
PSC event numbered 1 in Figures 4a3 and 4b3, and thehistory in Figure 6c is that numbered 2 in Figures 4a3 and4b3. TSAT, TNAT, Tdel, and Tice are calculated using thebackground nitric acid and water vapor values determinedfor each 10-day period. The background nitric acid value of
9.2 ppbv based on the 10-day average of ILAS data at 20 km
for early March is much lower than the background in midJanuary (11.4 ppbv). The PSC events shown in Figure 6
clearly experienced temperatures below TNAT for a longer
period than those in Figure 5. They sometimes fell below
Tice during the 20-day period shown in the figure. Larsen etal. [1997] suggested that PSC particles might freeze if theyexperienced synoptic temperatures below TNAT for at least
1– 2 days, possibly accompanied by synoptic temperaturefluctuations, and this supports the freezing scenario of
Tabazadeh et al. [1995]. The nitric-acid-containing solid
PSC events observed in early March experienced temperatures below TNAT for more than several days, which isconsistent with the study of Larsen et al. [1997].

2 - 10 SAITOH ET AL.: ARCTIC POLAR STRATOSPHERIC CLOUDS IN THE WINTER OF 1996/1997

30

220

S

S

TSAT

210

20

10

Temperature (K)Hydrate saturation ratio

(a)

200

TNAT
Tdel
Tice

190

0

180
-20

30

220

-15

-10
Day

-5

0

S
(b)
TSAT

210

20

10

Temperature (K)Hydrate saturation ratio

ILS

200

TNAT
Tdel
Tice

190

0

180
-20

-15

-10
Day

-5

0

Figure 5. Typical temperature histories of PSC events suggesting the formation of STS PSCs: (a) at 22km on 19 January, (b) at 21 km on 19 January. The thick black lines indicate the temperature histories
from the ILAS measurement locations. The bars show the range of divergence of the four surrounding
points. In the temperature scale, the solid black, dashed black, solid gray, and dashed gray lines indicate
the sulfuric acid tetrahydrate (SAT) melting temperature (TSAT) Zhang et al. [1993], NAT saturation
temperature (TNAT) Hanson and Mauersberger [1988], SAT deliquescence temperature (Tdel) Koop andCarslaw [1996], and ice frost point (Tice) Marti and Mauersberger [1993], respectively. The 10-day
background amounts of nitric acid and water vapor are used in the calculations. The shaded area labeled‘‘S’’ indicates that the air mass passed over Scandinavian mountains. The NAT and NAD saturation ratios
in solution calculated along with the trajectory using the 10-day background amounts of nitric acid andwater vapor are depicted by thin black and gray lines, respectively.

SAITOH ET AL.: ARCTIC POLAR STRATOSPHERIC CLOUDS IN THE WINTER OF 1996/1997 ILS

30

220

(a)
TSAT
20

10

Temperature (K)Hydrate saturation ratio

210

200

TNAT
Tdel
Tice

190

0

180
-20

30

220

-15

-10
Day

-5

0

(b)
TSAT
20

10

Temperature (K)Hydrate saturation ratio

210

200

TNAT
Tdel
Tice

190

0

180
-20

30

220

-15

-10
Day

-5

0

(c)
TSAT
20

10

Temperature (K)Hydrate saturation ratio

210

200

TNAT
Tdel
Tice

190

0

180
-20

-15

-10
Day

-5

0

Figure 6. The same as Figure 5, but for nitric-acid-containing solid PSCs observed in early March: (a)at 19 km on 9 March, (b) at 18 km on 10 March, (c) at 18 km on 10 March.

2 - 11ILS

2 - 12 SAITOH ET AL.: ARCTIC POLAR STRATOSPHERIC CLOUDS IN THE WINTER OF 1996/1997[32] In contrast to the event in Figure 5b, all the events
shown in Figure 6 were observed over Siberia (120E),
not downwind of high mountainous areas. Therefore, they
are unlikely to have been influenced by a mountain-induced
lee wave. This suggests that their formation processes
should be explained from their synoptic scale temperature
histories.
[33] The hydrate saturation ratios of liquid were calculated along with the trajectory. The amounts of nitric acid,water vapor, and sulfate used in the calculations were thebackground values for each 10-day period. In Figures 5
and 6, the thin black and gray lines depict the NAT and
NAD saturation ratios, respectively. The PSC events shownin Figure 6 maintained relatively high nitric acid hydrate
saturation ratios for 20 days in contrast to those in Figure 5.
A higher saturation ratio produces a higher homogeneous
nucleation rate of nitric acid hydrates [Salcedo et al., 2001],
and therefore, these solid PSCs would freeze homogeneously [Salcedo et al., 2001; Tabazadeh et al., 2001].
[34] Although the homogeneous nucleation theory seems
to explain the formation process of the observed solid
particles, we cannot neglect the possibility of heterogeneous
nucleation on SAT for those events. As shown in Figure 6,
the air masses were exposed to temperatures below TNAT fora long time, and sulfuric aerosols might have formed SAT.
The temperature occasionally dropped to T del , when
ECMWF warm bias is into consideration. Under the usual
stratospheric conditions, SAT deliquesces into ternary solution at Tdel upon cooling. However, SAT does not completely deliquesce into ternary solution above Tice in adenitrified air mass [Martin et al., 1998]. Kondo et al.
[2000] reported significant denitrification in an analysis of
ILAS nitric acid data in February and March 1997. Therefore, in this case, SAT might have remained frozen until
observed.
[35] Some PSC events with a significantly low nitric acidvalue were observed from mid-February to early March.
Their decrease in nitric acid cannot be explained by any
theoretical predictions. Close examination of the temperature histories of these PSC events makes it clear that they
had experienced temperatures around Tice or lower for a
fairly long time before the measurements. Exposure to such
low temperatures would generate large particles, leading todenitrification. The PSC events with low gaseous nitric acidmight be observed as ongoing denitrification. Actually, thenitric acid values inside the polar vortex in March were
smaller than in January, as mentioned above. Even if thetemperature fell low enough to generate PSCs, particle
growth would be limited by the insufficient amount of nitricacid, in such denitrified air masses. In this case, theextinction is not as high as expected from theory, with very
low gaseous nitric acid.

5. Summary[36] The ILAS successfully observed the profiles ofaerosol extinction, nitric acid, and water vapor in both poleregions from November 1996 through June 1997. We
identified about 250 events in 65 profiles as PSCs.
[37] This study compared the ILAS extinction and nitricacid data with theoretically predicted values assuming theexistence of STS, NAD, and NAT. In mid-January, both theextinction coefficient and nitric acid level of some of theobserved PSC events showed better agreement with thetheoretical values for STS than with those of NAT or NAD.
They showed that the enhanced volume corresponds to thedecrease in the ambient nitric acid, suggesting the uptake of
nitric acid into particles as STS forms. The ILAS observed
STS PSCs over a wide vertical range on 19 and 20 January.
On the other hand, a corresponding decrease in water vapor
was reported by Pan et al. [2002]. Mixture of STS and ice
in a large sampling volume of ILAS would be a reasonableinterpretation to explain the synchronous decrease in nitricacid and water vapor. It would be also possible that theparticle growth was limited in dehydrated air. Although a
few PSCs were observed in March, most of the characteristic PSC events in the late PSC season were categorized as
NAD or NAT. The result of PSC classification was summarized in Table 1.
[38] The 20-day isentropic backward trajectory was calculated using ECMWF TOGA Besic Level III temperature
data to examine the thermal conditions in synoptic scale forthe observed PSC particles. The temperature histories of the
STS events indicate that no sulfuric hydrate or ice particles
suitable as nuclei for solid PSCs could have existed in theambient air before the measurements, and therefore, liquid
droplets (STS) could form. However, more detailed analysisof mesoscale cooling is needed to understand ice formation
under the perturbed conditions by mountain-induced lee
waves.
[39] The nitric-acid-containing solid PSC events observedin early March experienced temperatures below TNAT for
more than several days, and sometimes fell below Tice during
the 20-day period, which is consistent with the study by
Larsen et al. [1997]. They had not passed over typicalmountainous area before their measurements, so the formation mechanisms of these solid particles should be explained
from their synoptic scale temperature histories, without
considering lee waves. They maintained relatively highnitric acid hydrate saturation ratios along their trajectory,
which suggests their homogeneous nucleation.
[40] From mid-February to early March, some PSC eventswith significantly low nitric acid values were observed;
temperatures had been around Tice or lower for a fairly longtime before the measurements. In these likely denitrified air
masses, the shortage of nitric acid would limit particle
growth, even if the temperature did fall low enough togenerate PSCs.
[41] Acknowledgments. We wish to express our sincere thanks to allof the ILAS science team members and their associates. We thank Sergey
Oschepkov for the helpful discussion and information on PSC discrimination using the ILAS aerosol extinction data. We are also grateful to
Richard Swinbank for supplying the UKMO stratospheric assimilation data.
We also thank NASDA/EORC for providing the EORC-TAM trajectory
software programs. We obtained the OPC data from the ILAS Correlative
Measurements DataBase (ILAS-CMDB). The ILAS retrieval data processing was carried out at the ILAS Data Handling Facility (DHF) at NIES.
Financial support was partly provided by NASDA.

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S. Hayashida and N. Saitoh, Faculty of Science, Nara Women’s
University, Kita-uoya Nishi-machi, Nara 630-8506, Japan. (sachiko@ics.
nara-wu.ac.jp; naoko@leo.ics.nara-wu.ac.jp)
L. L. Pan, Atmospheric Chemistry Division, National Center for
Atmospheric Research, Boulder, CO 80307, USA. (liwen@acd.ucar.edu)
Y. Sasano, National Institute for Environmental Studies, Tsukuba, Ibaraki
305-0053, Japan. (sasano@nies.go.jp)