GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 8, 10.1029/2001GL014147, 2002

Satellite observation of dehydration in the Arctic
Polar stratosphere
L. L. Pan,1 W. J. Randel,1 H. Nakajima,2 S. T. Massie,1 H. Kanzawa,2 Y. Sasano,2
T. Yokota,2 T. Sugita,2 S. Hayashida,3 and S. Oshchepkov2
Received 27 September 2001; revised 10 January 2002; accepted 11 January 2002; published 18 April 2002.

[1] We report the first space-borne observation of dehydration in
the Arctic polar stratosphere. In January 1997, the Improved Limb
Atmospheric Spectrometer (ILAS) observed up to 3 ppmv water
vapor reduction during ice cloud formation and 2 ppmv
permanent removal of water vapor, mostly at altitudes between 23
and 26 km. In some cases, the dehydrated air was downwind frommountain wave induced Polar Stratospheric Cloud (PSC) events.
Furthermore, simultaneous observations of HNO3 and H2O showthat the gas phase reduction of HNO3 in the Arctic (January 1997)
was much smaller than that observed in the Antarctic (June 1997)
when a similar level of water vapor reduction occurred.
INDEX
TERMS: 0340 Atmospheric Composition and Structure: Middle
atmosphere—composition and chemistry; 0305 AtmosphericComposition and Structure: Aerosols and particles (0345, 4801);
3349 Meteorology and Atmospheric Dynamics: Polar meteorology

1. Introduction[2] Dehydration in the winter Antarctic polar stratosphere has
been observed for more than a decade by both in situ and satellite
sensors [Kelly et al., 1989; Santee et al., 1995; Vömel et al., 1995;
Nedoluha et al., 2000]. Due to the persistent low temperature in the
Antarctic winter, dehydration is a large-scale climatological feature[Nedoluha et al., 2000]. On the other hand, the occurrence of
dehydration in the Arctic polar region is sporadic and has only
been observed by in situ measurements in a few isolated incidents[Vömel et al., 1997; Hintsa et al., 1998]. In this letter, we report thefirst satellite observation of dehydration in the Arctic stratosphere.
Also reported here are simultaneous observations of HNO3 at the
beginning of dehydration season in both hemispheres, which show
a pronounced difference between HNO3 gas phase reduction in the
Arctic and the Antarctic, providing new information for investigating the PSC formation process.
[3] ILAS, onboard the Advanced Earth Observing Satellite(ADEOS), is a solar occultation instrument that has two spectrometers [Sasano et al., 1999]. The spectral range of the IR spectrometer covers 6 – 11 mm and the visible covers 753 – 784 nm.
ILAS retrievals provided multiple stratospheric trace gas species,
including ozone (O3), water vapor (H2O), nitrous oxide (N2O), andnitric acid (HNO3), as well as aerosol extinction profiles, from
November 1996 to June 1997. During this period, ILAS measurements sampled latitudes 50 – 70 degree N and 64 – 88 degree S, andprovided roughly 14 profile measurements in the Arctic polar
region and 14 measurements in the Antarctic polar region daily.
The vertical resolution is 1.6 km, spanning 10 – 50 km altitude.
The effective sampling length in the horizontal is about 230 km.

1

National Center for Atmospheric Research, Boulder, CO, USA.
National Institute for Environmental Studies, Tsukuba, Japan.
3
Nara Women’s University, Nara, Japan.
2

Copyright 2002 by the American Geophysical Union.
0094-8276/02/2001GL014147$05.00[4] The results reported here are based on version 5.20 H2O,
N2O, HNO3, and 780 nm aerosol extinction data, all validated bycorrelative in situ and coincident satellite measurements [JGR ILAS
special section, 2002]. IR channel aerosol extinction data are also
used in PSC type analyses using a spectral fitting approach[Oshchepkov et al., 2002]. Retrieval error analyses indicate that
ILAS HNO3 data are not significantly affected by the presence ofaerosols. N2O and H2O data, on the other hand, can potentially bebiased low in the presence of PSCs, due to the IR spectral overlap ofgas and particles. In the absence of PSCs, the precision of the H2O
measurement for the altitude range of concern is 10%. The
estimated magnitude of PSC induced biases depends on the particle
types. In particular, the PSC induced negative retrieval bias in H2O
data is not significant in the case of ice particles but can besignificant in case of nitric acid trihydrate (NAT) particles. A
quantitative discussion is given by Yokota et al. [2002]. The main
results presented here are based mostly on H2O data from thenorthern hemisphere (NH) in January 1997, when the majority of
PSC events were identified as supercooled ternary solutions (STS)or ice [Saitoh et al., 2002]. Extinction and HNO3 data have also been
carefully examined to ensure that the data points that lead to the main
conclusions of this paper are not observed in the presence of NAT.
[5] Figure 1 displays a sequence of ILAS water vapor
profiles for January 15 – 17, 1997. During each of these threedays, ILAS sampled both inside and outside the Arctic polar
vortex. The anomalously low water vapor events sampled inside
the vortex between 23 and 26 km altitude are readily apparent
in the Figure. The strong spatial correlation between H2O
minima and both temperature minima and enhanced aerosol
extinction, as demonstrated in the Figure, suggests the presenceof PSCs at the time of the observations. The presence of water
ice PSCs is inferred. The altitudes of these events are higher
than the typical altitude range of dehydration observed in the
Antarctic [Vömel et al., 1995; Nedoluha et al., 2000]. Anomalously low water vapor was continuously observed later in themonth from 17 to 26 km.

2. Observed H2O and HNO3 Gas
Phase Reduction[6] To search for dehydration events, we first identify all
events that show anomalously low water vapor mixing ratios
compared to the background values. In the absence of dehydration and PSC induced gas phase reduction, stratospheric H2O
forms a compact relationship with N2O. In principle, the background value of H2O can be inferred from the H2O – N2O
relationship in the absence of PSCs and prior to significant
dehydration [similar to Kondo et al., 2000 in deriving thebackground value of HNO3]. To eliminate data points that were
measured in the presence of PSCs, we select only the measurements with co-located temperatures greater than 200 K (wellabove the PSC formation temperature) as the background points.
Similar analyses are done for January NH and June southernhemisphere (SH) data for comparisons. Note that SH June is one
month earlier than NH January in the seasonal cycle, but it is the first

25 - 1

25 - 2PAN ET AL.: SATELLITE OBSERVATION OF DEHYDRATION IN THE ARCTIC POLAR STRATOSPHERE

Figure 1. ILAS NH water vapor cross-section of 37 profiles
along 65 degree N between January 15 and 17, 1997. Contours areco-located UKMO temperature field (top, 5 K increment) andaerosol extinction (measured by ILAS 780 nm channel) (bottom,
increase 5 steps per decadal range in 10 3 km 1).
month of the SH PSC season and the last month of the ILAS data
record. In both months, the temperature criterion we used eliminates
most data points that show a departure from the compact relationship. The background H2O (HNO3) – N2O relationship is thendetermined using a polynomial fit. Figure 2 displays H2O (HNO3)
versus N2O for all data points between 400 K and 600 K potential
temperature levels, as well as the polynomial fit to the backgroundpoints, for January (NH) and June (SH). A scaled potential
vorticity (sPV, in units of 10 4 s 1), derived from the UKMO
data [Manney et al., 1994] is used to indicate vortex and extra-vortex
air. The reason for using the entire month of PSC free data for the
background fit is to avoid introducing biases due to the seasonal
cycle related changes in H2O – N2O relationship. To shed more lighton the PSC processes involved, HNO3 anomalies are also examined.
The origin of the anomalies can be further examined using their N2O
values, since N2O has a well known stratospheric entry value in thetropics of 315 ppbv, and a vortex value less than 150 ppbv.
[ 7 ] Deviations from the background value, H 2 O or
HNO3, are defined as the difference between the measured
value and the fit curve. We consider H2O or HNO3 larger
than 2s as cases of significant departure from the background.
These cases are found in June (SH), January (NH) (Figure 2),
and February (NH) 1997 (not shown). Events showing large
negative H2O correspond to two different categories: 1)
observations made in the presence of PSCs where the loss ofwater vapor may be temporary; and 2) observations made in theabsence of PSCs where the loss of water vapor is permanent.
We refer to both categories as gas phase reduction events but
only to the latter as dehydration events.

3. Evidence of Ice Cloud Formation
and Dehydration[8] To examine quantitatively whether some of the low water
vapor events during NH January were consistent with the presenceof ice clouds, we investigate the relationship of observed H2O
with that calculated from saturation vapor mixing ratio over ice.
Figure 3a displays H2O as a function of temperature with respect
to the frost point temperature (T), calculated for each individual
observation. Also shown in Figure 3a are corresponding saturation
mixing ratios over ice for these observations calculated using alaboratory-derived formula [Marti and Mauersburger, 1993](shown in Figure 3a as cyan points and curve). As expected,
thermodynamics predicts that the onset of negative H2O begins
only as the temperature drops to below the frost point. The data,
however, show that while negative H2O events exist for a wide
range of T (0 – 25 K), the vast majority of data are at thebackground level (within 2s) until the temperature drops to below
3 – 4 K above the frost point, according to the UKMO temperature data. Below this temperature, no data points were observed at
the background level. Retrieval uncertainty for these data points
was scrutinized and found not to have produced the observed
behavior. The most likely physical explanation to the data is that arange of warm biases, with a minimum of 3.5 K (suggested by
the point where all H2O drops to below the background value),
exists in the UKMO temperature data in these cold regions.
Although this warm bias is larger than typical uncertainty associated with the UKMO data, it is not uncommon for selected cold
regions [e.g. Manney et al., 1996]. Indeed, a wide range of warm
biases, as much as 10 K, was found in analyzed temperature data
(ECMWF data, which is comparable to the UKMO analyses forthis month) for this particular month and location by comparison
with radiosonde data and mesoscale model results [Dörnbrack etal., 2001]. We further show in Figure 3a that, if a simple 3.5 K
adjustment is applied to the UKMO data, the relation of negative
H2O with decreasing T at the low end of T is consistent withthe behavior of the saturation mixing ratio over ice. Furthermore,
spectral analyses of ILAS aerosol extinction data, using the IR andthe near visible channels, indicate that many of the events near theice saturation curve (after the 3.5 K adjustment) in Figure 3a areconsistent with the spectrum of ice particles.

Figure 2. H2O and HNO3 versus N2O for ILAS observations
between 400 K and 600 K. Solid curves represent a polynomial fit
to the background observations. Dashed lines represent ±2s fromthe fit. Colors are used to indicate vortex air (blue, sPV > 2), vortex
edge (green, 1 < sPV < 2) or extra-vortex air (red, sPV < 1). The
apparent shape difference between the January (NH) and June (SH)background curves of HNO3 – N2O reflects that January is later inthe vortex season and more air was sampled from higher altitudes,
where N2O has a shorter photochemical lifetime [Michelsen et al.,1998].

PAN ET AL.: SATELLITE OBSERVATION OF DEHYDRATION IN THE ARCTIC POLAR STRATOSPHERE

25 - 3∆T (K) (TUKMO - 3.5 K - Tice )
-5

0

5

10

15

20

3

(a)

∆ H2O (ppmv)

2
1
0
-1
-2
-3
-4
-5
-5

0

5

10

15

20

25

∆T(K) (TUKMO- Tice)(co-located temperature)∆ H2O (ppmv)(identified dehydration events)

1

(b)
0

-1

-2

-3
-5

0

5

10

15

20

25

∆ Tmin (K) (T min - T ice )(trajectory minimum temperature)

Figure 3. (a) Comparison of ILAS inferred H2O (red circles)
versus T (co-located temperature relative to the frost point) withthe calculated saturation vapor mixing ratios using co-located
UKMO T data (cyan dots and lower x-axis) and after anadjustment of 3.5 K in the T data (blue dots and upper x-axis).
Cyan and blue curves are fits to the points. The dashed line
(horizontal) marks 2s from the background value; points belowthat are considered candidates of dehydration events. Green filled
circles represent measurements of 970125. They are highlighted
for their large change from high T to low Tmin, as shown in (b).
(b) H2O, as in (a) for those identified as dehydration events, as afunction of Tmin (minimum T relative to the frost point during
the past 10 days).

[9] Among the local dry anomalies observed in January (datapoints below the 2s line in Figure 3a with N2O < 150 ppbv),
around 50% are identified as dehydration events using the temperature history analysis and aerosol extinction screening. Temperature
histories of the past 10 days are calculated for all January eventsusing UKMO data and a three-dimensional back trajectory model.
Dry air masses that experienced near frost point temperatures andwere observed in the absence of PSCs are identified as dehydrated.
A total of 40 events meet these criteria, showing up to 1.8 (±0.32)
ppmv permanent loss of water (0.32 is 1s from the backgroundvalue). Their H2O values as a function of past 10 day minimum
temperature relative to the frost point (Tmin) are given in Figure 3b.
Detailed examinations of data show that a number of dry anomalies
were observed at high Ts but experienced a low Tmin. A good
example is given by a group of measurements on January 25, 1997,
highlighted in Figure 3 (green filled circles). Most of thesedehydration events were observed between altitudes 20 – 26 km(20 – 35 mbar, 500 – 600 K). HNO3 values for these events range
from 2.3 to 1 (±0.98) ppbv, (0.98 is 1 s from background), nearly

Figure 4. The yellow line indicates the10 day back trajectory of adry event, observed on January 25 at 24 km altitude (31 mbar)
near 136 W, superposed on the UKMO temperature field on
January 22 at 31 mbar. The red dots mark the ILAS measurement
locations for January 22. The measurement location for the dry
event observed on the 25th is marked by ‘‘X’’. The solid contour
line inside the dark blue region marks the 190 K UKMO
temperature. The dashed contour lines mark 20 and 25 PVU in
modified PV, and are proxies for the vortex edge.

Figure 5. H2O and HNO3 for ILAS June SH (a) and January
NH (b) observations between 400 K to 600 K, color-coded using
the UKMO temperature (blue, T < 190 K, ice possible; green, 190
K < T < 196 K, PSC possible; red, T > 196 K, PSC free).

25 - 4PAN ET AL.: SATELLITE OBSERVATION OF DEHYDRATION IN THE ARCTIC POLAR STRATOSPHERE

all distributed within ±2s of the background HNO3 value. Notethat the scatter of these events as a function of Tmin around thefrost point in Figure 3b reflects the uncertainty of the temperature
data and the trajectory model calculations.
[10] This group of data measured on 25th of January also serves
as a good example that connects the observed dehydration with amountain wave induced low temperature event that was not
captured by the UKMO temperature data. This profile was measured over western Canada (136 W) and anomalously dry air was
found between 18 and 24 km altitudes. The trajectory analyses
show that these air masses were 3 days downwind from Scandinavia. The trajectory of one of the events is illustrated in Figure 4.
H2O values for these events, shown in Figure 3 (green filled
circles), are from 0.93 to 1.4 (±0.32) ppmv, when the co-located
UKMO temperatures ranged from 205 K to 208 K. ILAS aerosol
extinction values for these events are at background levels, indicating the absence of PSCs. Back trajectory calculations using UKMO
analyses indicate that these air masses experienced their temperature
minimum (190 K in UKMO analyses) over Northern Scandinavia
three days earlier (January 22). During this time period, a groundbased Lidar near Kiruna, Sweden (68N, 21E) observed intense
mountain-wave induced PSCs. Mesoscale temperature simulations,
supported by nearby radiosonde data, showed that the mountainwave induced temperature anomaly reached as low as 178 K (7 K
below local frost point temperature) at the 30 mbar level [Dörnbracket al., 2001]. The fact that dry air masses were measured three days
later at temperatures much higher than the frost point indicates thatthe ice particles grew large enough to sediment before total evaporation. These events probably resulted from the combination ofmountain-wave induced low temperature anomaly and the synoptic
scale low temperature pool inside the polar vortex.

4. Discussion of HNO3 Gas Phase Reduction in
the Arctic[11] In addition to the water vapor anomalies, Figure 2 shows astrong contrast of HNO3 anomalies between the Arctic and theAntarctic. Although it is well known that the perturbation of gas
phase HNO3 is in general much smaller in the Arctic [e.g. Santee
et al., 1995], it is somewhat surprising that only 3 – 5 ppbv of
HNO3 gas phase reduction was observed during the occurrence ofice cloud formation and dehydration. The different characteristicsof HNO3 versus H2O between the Antarctic (June) and theArctic (January) are further highlighted in Figure 5. Note that theseH2O and HNO3 values are not equivalent to dehydration anddenitrification, but represent gas phase reduction, many of which
may be due to the presence of PSCs. The likelihood of PSCs is
indicated by the colors in Figure 5, based on their co-locatedtemperatures. It is apparent that while the inferred H2O arecomparable for these two months, the HNO3 in the NH (January)
are much smaller (35% of the HNO3 background value in the NH
compared to over 90% in the SH).
[12] In light of recent studies showing that the growth of solid
nitric acid containing particles requires 6 – 7 days [Fahey et al.,2001], and that the growth rate peaks in a narrow temperature
window (190 – 192 K at 500 K altitude) [Tabazadeh et al., 2001],
we examined the parcels’ temperature histories. The results show
that, typically, the Antarctic air masses experienced a stable lowtemperature (below critical temperature for NAT) for several
days, whereas the Arctic trajectories show a more rapid coolingand warming (reflecting the fact that the Arctic cold pool issmaller and asymmetric in shape). On one hand, the muchsmaller HNO3 produced under these conditions are consistent
with the fact that these Arctic parcels did not stay long enough inthe temperature window that is optimal for solid nitric acidcontaining particles to form. On the other hand, it brings out a
new aspect that the freezing of water is a faster process than theformation of nitric acid containing particles. In principle, this canlead to the situation where dehydration may occur before
denitrification, or in some cases, without denitrification. These
results also show that without persistent low temperatures, thepresence of ice particles does not necessarily produce efficient
denitrification. Furthermore, the fact that only around 3 – 5 ppbv
of HNO3 gas phase reduction (likely due to STS, [Saitoh et al.,2002]) was observed under conditions of ice cloud formation
challenges the current understanding of STS formation. To
understand the future changes in Arctic stratospheric ozone, these
new aspects need to be investigated by theoretical modeling andlaboratory experiments.

[13] Acknowledgments. We thank A. Dörnbrack, D. Hanson, B.
Ridley and A. Tabazadeh for helpful discussions. This work is supported
in part by the National Science Foundation through its support to the
University Corporation for Atmospheric Research, by the NASA Upper
Atmosphere Research Satellite guest investigator program, and by theNASA Atmospheric Chemistry Modeling and Analysis Program.

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L. L. Pan, W. J. Randel, and S. T. Massie, National Center for Atmospheric Research, Boulder, CO 80307, USA.
H. Nakajima, H. Kanzawa, Y. Sasano, T. Yokota, T. Sugita, and S.
Oshchepkov, National Institute for Environmental Studies, Tsukuba, Japan.
S. Hayashida, Nara Women’s University, Nara, Japan.