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SOLA, 2006, Vol. 2, 072 075, doi:10.2151/sola.2006 019

Variation in PSC Occurrence Observed with
ILAS-II over the Antarctic in 2003
N. Saitoh1, S. Hayashida2, T. Sugita1, H. Nakajima1, T. Yokota1, and Y. Sasano1
1
National Institute for Environmental Studies, Tsukuba, Japan
2
Faculty of Science, Nara Women’s University, Nara, Japan
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The Improved Limb Atmospheric Spectrometer (ILAS)II frequently observed polar stratospheric clouds (PSCs) in
the Southern Hemisphere (SH) throughout the winter of2003. Simultaneous observations of the aerosol extinction
coefficient (AEC) at 780 nm, nitric acid, and water vapor
data were analyzed to investigate the ambient thermodynamic conditions associated with observed PSCs. PSCs
were first observed with ILAS-II at the end of May, andobserved most frequently in August/September as temperatures cooled. At approximately 20 km late in the PSC
season, however, PSCs were less likely to occur, despite
cold temperatures, because of the lower concentration of
nitric acid due to denitrification caused by sedimentationof previously occurring PSCs. The probability of PSC occurrence and the probability of ambient temperaturescolder than nitric acid trihydrate (NAT) saturation temperature (TNAT ) were well correlated below 〜20 km
throughout the winter. In contrast, PSC frequency at 〜22km from late August to early September was low evenwhen temperatures were sufficiently colder than TNAT; thisis, at least, partly because of the decrease in backgroundaerosol particles in the atmosphere.

1. Introduction
Polar stratospheric clouds (PSCs) consist mainly of
nitric acid and water/ice and form in winter and early
spring in the lower stratosphere over both polar regions(e.g., Solomon 1999). PSCs play a crucial role in ozone depletion processes; they provide surfaces for heterogeneous reactions that convert inactive chlorine into active chorine(e.g., Solomon 1999) and cause denitrification (e.g., Fahey etal. 1990) and nitrification (e.g., H
ubler et al. 1990).
Activated chlorine causes severe ozone depletion, especially in the Southern Hemisphere (SH), which is known as
the “ozone hole”. The climatology of PSC frequency in the
SH was examined using aerosol data from theStratospheric Aerosol Measurement (SAM) II from 1978 to
1989 (Poole and Pitts 1994), data from the Polar Ozone andAerosol Measurement (POAM) II from 1994 to 1996
(Fromm et al. 1997), and data from multiple solar occultation sensors from 1978 to 2000 (Fromm et al. 2003).
Poole and Pitts (1994) examined the relationshipbetween PSC frequency and temperature and showed that
PSC formation in the SH is less likely later in PSC season
than earlier. Other studies (e.g., Watterson and Tuck 1989;
Hervig et al. 1997; Mergenthaler et al. 1997) have suggested
that PSC formation late in PSC season in the SH is strongly
affected by preceding denitrification and dehydration.
However, there have been few studies on the variability of
the relationship between PSC frequency and temperature
over the course of a PSC season that are based on extensive
observations of aerosols, nitric acid, and water vapor in theSH.
Against this background, we used data obtained by theCorresponding author and present affiliation: Naoko Saitoh,Center for Climate System Research, University of Tokyo,
General Research Building, 5-1-5 Kashiwanoha, Kashiwa-shi,
Chiba 277-8568, Japan. E-mail: snaoko@ccsr.u-tokyo.ac.jp.
©2006, the Meteorological Society of Japan.

Improved Limb Atmospheric Spectrometer (ILAS)-II over
the SH in 2003 to investigate the relationship between PSC
occurrence and temperature. ILAS-II is a solar occultation
sensor onboard the Advanced Earth Observing Satellite(ADEOS)-II. ILAS-II continuously observed vertical profilesof minor constituent gases and aerosols daily at 14 circumpolar points at high latitudes in each hemisphere (53.971.1°N, 63.6 88.0°S) in the stratosphere from April throughOctober 2003 (Nakajima et al. 2006). The ILAS-II Version
1.4 (V1.4) product (Yokota et al. 2006) used in this study
contains 2643 profiles for the SH. ILAS-II aerosol extinction
coefficient (AEC) data at 780 nm were used to compute thefrequency of PSC occurrence. The United Kingdom
Meteorological Office (MetO) stratospheric analyses data
were used to diagnose the ambient temperature. ILAS-II
nitric acid and water vapor data were used to calculate thenitric acid trihydrate (NAT) saturation temperature (hereafter referred to as “TNAT ”). TNAT has been widely applied toestimate PSC areal extents (e.g., Rex et al. 1999). This study
used TNAT as a reference for the existence of nitric acidcontaining particles such as NAT and super-cooled ternary
solution (STS). This study therefore considers variations inrelationships among PSC frequency, ambient temperature,
and TNAT at the ILAS-II measurement locations for theentire SH winter of 2003.

2. PSC identification and TNAT calculation
PSCs were identified from ILAS-II AEC data over the
SH using methods similar to those for ILAS data in the NH(Hayashida et al. 2000). First, averages and standard deviations for ILAS-II AEC data at temperatures warmer than
200 K were calculated for each 10-day period and for each
altitude level from April to October in 2003. The mean plus
five times the standard deviation was defined as a PSC
threshold; aerosol data whose extinction exceeded thethreshold were then identified as PSCs. This method wasapplied to all ILAS-II data inside the polar vortex, theboundaries of which were determined using MetO data according to the method by Nash et al. (1996). This approach,however, sometimes could not determine a threshold,mainly in August and September, because all ILAS-II measurement points within a 10-day period had temperaturescolder than 200 K. If thresholds could not be determined,
then interpolated values were derived from thresholdvalues at the same altitude before and after the period withno threshold.
The ILAS-II V1.4 AEC data at 12 24 km in the SH usedin this study were validated by Saitoh et al. (2006). ILAS-II
AEC data have a negative bias (12 30% at 20 22 km)
relative to the Stratospheric Aerosol and Gas Experiment(SAGE) II Version 6.2 (V6.2) data. However, this negative
bias does not affect PSC identification because PSC thresholds adopted here were relative values determined from
ILAS-II AEC data themselves.
TNAT values were calculated at all ILAS-II measurement
locations inside the polar vortex using the empirical expression by Hanson and Mauersberger (1988). ILAS-II V1.4
nitric acid and water vapor data were used in the calculation. The nitric acid data were validated by Irie et al. (2006),
who showed that the precision of the nitric acid data was
13 14, 5, and 1% at 15, 20, and 25 km, respectively. The
accuracy was 13/+26% at 15 25 km, which suggests that

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Saitoh et al., ILAS-II Antarctic PSC Measurements in 2003
80

24

20

60
40

60

20

area 1
20
80

90

100

40

20 40

80
60

16

PPSC
60

60 40 20

(b)

7

80
5
4

20
40

90 0 0
8 6

60

20

16

HNO3 (ppbv)

640

Altitude (km)

8

20

3
2

PT(NAT)

1

0
-1

-1

-1

1
0

12
24

2

(c)

1

3. PSC distribution in the SH in 2003

2

0

-2

2

-1
0

2
1

-1

12
-60

-1

1

16

0
log10
-2.3
-2.4
-2.5
-2.6
-2.7
-2.8
-2.9
-3.0
-3.1
-3.2
-3.3
-3.4
-3.5
-3.6
-3.7
-3.8
-3.9
-4.0
-4.1
-4.2
-4.3
-4.4
< -4.5AEC (log10/km)-2

20∆T (T-TNAT)
(d)

-70

LatitudeFigure 1a shows daily mean MetO temperatures at
ILAS-II measurement locations inside the polar vortex(color shading) and PPSC values computed for each 10-day
period from June to October in the SH (hereafter referred toas the “PSC season”) (white contour lines). ILAS-II firstobserved PSCs in the SH at approximately 23 km on 30
May and then continuously observed them until ceasing
operation (24 October). PSCs were most frequent (morethan 〜80% of the time) in August and September. The dailymean MetO temperatures were coldest (colder than 〜185
K) in these two months.
At approximately 20 km in September, however, PPSC
values were not large despite cold temperatures. Figure 2ashows time-series of daily mean temperature (black) and
PPSC (dashed black) at 20 km. PPSC values increased from late
July through August, reaching as high as 〜80%. However,
PPSC dropped to 21% at the end of September, even thoughtemperatures were still as cold as 180 190 K. PSC formation is controlled not only by temperature but also by thepartial pressure of nitric acid, and daily mean nitric acidconcentrations, represented by color shading in Fig. 1b,
were lower in August/September (71.1 88.0°S) than in
June/July (65.3〜71.1°S). Figure 2a (gray line) shows thatthe daily mean nitric acid concentration at 20 km in midSeptember was 0.7 1.5 ppbv, while the concentration inearly July was as high as 5.5 9.3 ppbv. At a given pressure,
TNAT is lower for lower nitric acid concentration, so that
smaller PPSC can be expected at approximately 20 km in
September.
The white contour lines in Fig. 1b show the distributionof PT(NAT). PT(NAT) was well correlated with PPSC below 〜20 km.
The largest PT(NAT) , 80 90%, occurred in late September
between 15 17 km and corresponded to the largest PPSC ,also 80 90%. Most PSCs were observed at temperaturescolder than TNAT . The good correlation between PPSC andPT(NAT) thus suggests that PSCs occurred with high probability if the temperatures were sufficiently colder than TNAT .
However, the correlation between PT(NAT) and PPSC was not asstrong above 〜20 km as below 〜20 km. The discrepancy
between PPSC and PT(NAT) was significant at 〜22 km in lateAugust and early September (area 1). Figure 2(b) showsthat PT(NAT) was as high as 89% and 87%, while the corresponding PPSC was as low as 40% and 15% in late August andearly September, respectively. The discrepancy in June was
not as large, but it reached 〜30%. The reason for the discrepancies is discussed in the following section.
Several uncertainties exist in the computations of PPSC

>10
9

80

60

12
24

200
199
198
197
196
195
194
193
192
191
190
189
188
187
186
185
184
183
182

Temperature (K)

80

40

20

(a)

40

the nitric acid data are suitable for scientific purposes. The
V1.4 water vapor data were compared to coincident
Halogen Occultation Experiment (HALOE) water vapor
data (Version 19) (Harries et al. 1996) and SAGE II water
vapor data (V6.2) (Taha et al. 2004), both of which had been
already validated. ILAS-II water vapor data agreed with
HALOE and SAGE II data to within 10 15% below 25 km.
The following discussions consider two probabilities:
PPSC and PT(NAT). The probability of PSC occurrence (PPSC) andthe probability of temperature at the ILAS-II measurement
locations colder than TNAT (PT(NAT)) were defined as
NPSC
NT(NAT)PPSC (%) =× 100, PT(NAT) (%) =× 100,
NObs
NObs
where NObs is the number of all ILAS-II measurements, NPSC
is the number of data points identified as PSCs, and NT(NAT)
is the number of data points colder than TNAT . PPSC andPT(NAT) were calculated for every 10-day period at every
altitude level for all the ILAS-II measurement locations
inside the polar vortex.
The latitude of ILAS-II measurements shifted poleward
from June to September in the range of 23° (65.3 88.0°S;
Fig. 1d). Thus, the following time-series data include variability due to seasonal evolution and also due to latitudinalchanges.

-80-90Lat. Jun.

Jul.

Sep.
Aug.
Month of 2003

Oct.

Fig. 1. (a) Time-height cross section of daily mean MetO
temperature data at ILAS-II measurement locations (color
shading) and PPSC (white contour lines) inside the polar vortex;(b) time-height cross section of daily mean concentrations of
ILAS-II nitric acid (color shading) and PT(NAT) (white contour
lines) inside the polar vortex; (c) time-height cross section ofdaily mean values of ILAS-II AEC (color shading) and thedifference between ambient MetO temperature and corresponding TNAT (T) (white contour lines) inside the polar vortex; (d)latitude of the ILAS-II measurement locations in the SH. For(1) (3), 80% isopleth lines are dashed. Values of PPSC and PT(NAT)
were calculated for each 10-day interval at each altitude level.
See text for the definitions.

and PT(NAT) described above. Appling tolerable ranges forthresholds to identify PSCs did not cause any significant
change in PPSC. Irie et al. (2006) showed that ILAS-II nitricacid values are systematically large (14%) at 15 25 km
compared to balloon-borne measurements. PT(NAT) values
that considered the 14% positive bias differed from thevalues without the bias by less than 5%. Yokota et al. (2002)
and Yokota et al. (2006) suggested that the presence of
PSCs yields systematic errors in ILAS-II nitric acid andwater vapor data by the nongaseous contribution correction in the retrieval scheme. However, the systematic errors
had little effect on PT(NAT) values.
Uncertainties in MetO temperatures directly affectPT(NAT) values. Knudsen et al. (2002) suggested that MetO
temperatures have a slight positive bias of 〜0.3 K at 〜190
K. Here, 0.3 K warm bias was assumed, and PT(NAT) was thenrecomputed. The recomputed PT(NAT) values were slightly
larger, but the characteristics in Fig. 1b remained un-

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SOLA, 2006, Vol. 2, 072 075, doi:10.2151/sola.2006 019

194

8

80

7

70

6
192
5
190
4
188
186
184

daily mean T
daily mean HNO3PPSC
PT(NAT)

182
200

(b)

198

194

50
40
30

2

20

1

10

0
10

0
100

9

90

8

80

7

70

6
192
5
190
4
188

60

3

22 km

196
Temperature (K)90

HNO3 (ppbv)

Temperature (K)

196

100

9

60
50
40

3

30

186

2

20

184

1

10

182

0

0

Jun.

Jul.

Aug.
Month

Sep.

PPSC, PT(NAT) (%)

20 km

10

PPSC, PT(NAT) (%)

(a)

198

HNO3 (ppbv)

200

Oct.

-4

10
AEC (km-1)

AEC (without PSCs)
corresponding altitude

25
24
23
22

-5

21
20

N2O=15-25 ppbv
bv

10

mid-Aug.

late Aug.

early Sep.

Altitude (km)Fig. 2. Time-series of daily mean temperature (solid black line),
daily mean nitric acid concentration (solid gray line), PPSC
(dashed black line), and PT(NAT) (dashed gray line) inside the polar
vortex. Values of PPSC and PT(NAT) depicted by circles were calculated for each 10-day interval at each altitude level; (a) 20 km
and (b) 22 km.

mid-Sep.

Date

Fig. 3. Time-series of the daily mean ILAS-II AEC (thick black)and corresponding altitude (dashed black) at the iso-N2O levelof 15 25 ppbv inside the polar vortex from mid-August to midSeptember. A 10-day running mean was applied. The PSC data
identified were excluded from the AEC averages.

changed. The reported temperature bias had little impact
on the results presented above.

4. Discussion
Several satellite sensors have observed PSCs in the SH,such as SAM II (Poole and Pitts 1994), POAM II (Fromm etal. 1997), and POAM III (Nedoluha et al. 2003), which hadlatitudinal coverages similar to the coverage by ILAS-II. All
observations, including those by ILAS-II, showed thelargest PPSC values in August and September. The PPSC of 40
50% in June 2003 from ILAS-II was large compared to somepast measurements; for example, the 10-year ensemble
average of PPSC in June between 1978 and 1989 ranged from
5 to 20% (Poole and Pitts. 1994; Fig. 2a) and PPSC values inJune from 1994 to 1996 were 15 to 20% (Fromm et al. 1997;
Fig. 10), although PPSC in June 2002 was exceptionally highat 〜60% (Nedoluha et al. 2003).
The above results show that PPSC was not as large at approximately 20 km in September even though temperatures at the ILAS-II measurement locations were still cold.
In other words, PSC formation was less likely late in the
PSC season, which is consistent with the results reported

by Poole and Pitts (1994). This is due to the low concentration of nitric acid caused by a permanent denitrification in
August, as suggested by trajectory analyses coupled with
ILAS-II nitric acid and AEC data (Sugita et al. 2005). Our
results show that PSC formation at approximately 20 km in
the late PSC season is inhibited because of nitric acid depletion attributable to denitrification caused by sedimentationof previous PSCs (e.g., Watterson and Tuck 1989;
Mergenthaler et al. 1997).
Most PSCs were observed where temperatures werecolder than TNAT . According to the definition of PPSC andPT(NAT), the ratio of PPSC and PT(NAT) (PPSC/PT(NAT)) can be expected
to be close to the probability of PSC occurrence where temperatures are colder than TNAT . PPSC /PT(NAT) below 〜20 km
was almost 0.8〜0.9, which suggests that PSCs occurredwith high probability if the temperatures were colder than
TNAT . However, at 〜22 km in June, PPSC was not large
despite relatively large PT(NAT). This is because the temperature there was only slightly cooler than TNAT . Figure 1c
depicts the averaged differences between temperatures at
ILAS-II measurement locations inside the polar vortex andthe corresponding TNAT as white contour lines of T. Temperatures at 〜22 km in June were 0 2 K warmer on
average than TNAT. It is well-known that STS droplets format 3 5 K colder than TNAT (e.g., Carslaw et al. 1994), and
NAT particles would also form at temperatures several
degrees colder than TNAT (Tabazadeh et al. 2001) or at evenlower temperatures (Carslaw et al. 1998). Therefore, a highprobability of PSC formation (PPSC) cannot be anticipated inJune even though PT(NAT) was relatively large.
Discrepancies between PPSC and PT(NAT) were much larger
in late August and early September than those in June at
the same altitude region. As shown in Fig. 1c, temperatures
were more than 1-2 K colder on average than TNAT in lateAugust and early September. A possible explanation for
low PPSC values despite such low temperatures is that background aerosol particles that had the potential to nucleate
PSCs had already disappeared at these altitude levels. Colorshading in Fig. 1c represents daily mean values of ILAS-II
AEC; ILAS-II AEC values at 〜22 km were much smaller
after mid-August than before. This suggests a decrease of
background aerosol particles with radii of 0.4 0.5 µm towhich ILAS-II extinction measurements are most sensitive.
The decrease in AEC could have been caused both bystrong descent motion of upper air with low aerosol concentration inside the polar vortex and “cleansing effect”
due to sedimentation of PSCs that had formed previously(e.g., Thomason and Poole 1993). Atmospheric subsidence
inside the polar vortex was evident in ILAS-II N2 O andCH4 data (Ejiri et al. 2006; Fig. 1, 2). To clarify the effect ofparticle sedimentation, time-series of the AEC at the isoN2O level (15 ppbv < N2O < 25 ppbv) is shown in Fig. 3. The
AEC distinctly decreased from mid-August through earlySeptember; this decrease can be attributed to “cleansing”
caused by sedimentation of PSCs that occurred earlier inthat altitude region. Once “cleansing” occurs, aerosol concentration in the atmosphere cannot increase at high altitudes without aerosol loading from low latitudes coupledwith the breakup of the polar vortex (Thomason and Poole
1993). The above results indicate that PSC formation would
be inhibited because of a shortage of background aerosol
particles in the atmosphere even when ambient temperatures were sufficiently colder than TNAT.

5. Conclusion
ILAS-II frequently observed PSCs from 30 May throughOctober 2003 in the SH. Qualitative agreement was foundbetween cold regions and high PPSC (probability of PSC occurrence) computed from ILAS-II AEC data. Generally, PPSC
correlated with low temperatures; PPSC was largest in
August/September when temperatures were coldest.
However, at approximately 20 km in September, PPSC was
not as large even though temperatures were still cold. PSC
formation late in the PSC season was less likely due to a

Saitoh et al., ILAS-II Antarctic PSC Measurements in 2003
decrease in nitric acid in the atmosphere caused bydenitrification. PPSC and PT(NAT) (probability of ambient temperature colder than TNAT ) calculated from ILAS-II nitricacid and water vapor data correlated well below 〜20 km.
PSCs existed in that region with high probability whentemperatures were sufficiently colder than TNAT . However,
PPSC at 〜22 km from late August to early September was
small even when PT(NAT) was large, in part because of “clean
sing effect” that removed background aerosol particles
from the atmosphere. It is suggested that PSC frequency
depends on the degree of denitrification and/or the amountof background aerosols that can be nuclei of PSCs.
Therefore, TNAT cannot be always a good reference for PSC
occurrence, especially above 20 km in late winter.

Acknowledgments
We thank all members of the ILAS-II Science Team andValidation Experiment Team, and their associates. We are
grateful to the United Kingdom Meteorological Office(MetO) for supplying the stratospheric analyses data. The
ILAS-II data retrieval processing was conducted at the
ILAS-II Data Handling Facility (DHF) at the National
Institute for Environmental Studies (NIES). The ILAS-II
project was funded by the Ministry of the Environment ofJapan (MOE). A part of this research was supported by the
Global Environment Research Fund provided by the MOE.

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Manuscript received 11 November 2005, accepted 17 April 2006
SOLA: http://www.jstage.jst.go.jp/browse/sola/