JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D24, 8210, doi:10.1029/2001JD000615, 2002

Stratospheric ozone loss in the 1996/1997 Arctic winter:
Evaluation based on multiple trajectory analysis fordouble-sounded air parcels by ILAS
Yukio Terao
Institute of Geoscience, University of Tsukuba, Tsukuba, Ibaraki, Japan

Yasuhiro Sasano and Hideaki Nakajima
National Institute for Environmental Studies, Tsukuba, Ibaraki, Japan

H. L. Tanaka and Tetsuzo Yasunari
Institute of Geoscience, University of Tsukuba, Tsukuba, Ibaraki, Japan
Received 9 March 2001; revised 30 August 2001; accepted 3 October 2001; published 1 October 2002.

[1] Quantitative chemical ozone loss rates and amounts in the Arctic polar vortex for thespring of 1997 are analyzed based on ozone profile data obtained by the Improved Limb
Atmospheric Spectrometer (ILAS) using an extension of the Match technique. In this study,
we calculated additional multiple trajectories and set very strict criteria to overcome theweakness of the satellite sensor data (lower vertical resolution and larger sampling air mass
volume) and to identify more accurately a double-sounded air mass. On the average inside
the inner edge of the vortex boundary (north of about 70N equivalent latitude), the local
ozone loss rate was 50–80 ppbv/day at the maximum during late February between thelevels of 450 and 500 K potential temperatures. The integrated ozone loss during February
to March reached 2.0 ± 0.1 ppmv at 475–529 K levels, and the column ozone loss between
400 and 600 K during the 2 months was 96 ± 0.3 DU. Using a relative potential vorticity(rPV) scale, the vortex was divided into some rPV belts, and it was shown that themagnitude of the ozone loss increased gradually toward the vortex center from the edge. The
maximum ozone loss rate of 6.0 ± 0.6 ppbv/sunlit hour near the vortex center was higher
than near the vortex edge by a factor of 2–3. When we expanded the area of interest to
include all the data obtained inside the vortex edge (north of about 65N equivalent
latitude), the local ozone loss rate was about 50 ppbv/day at the maximum. This value isslightly larger than that estimated by the Match analysis using ozonesondes for the same
winter by 10 ppbv/day. Temperature histories of double-sounded air parcels indicated thatthe extreme ozone loss in the innermost part of the vortex was observed when the air parcel
experienced temperatures below TNAT during the two soundings and had experienced
temperatures near Tice in the 10 days prior to the first sounding. These facts suggest that thehigh ozone loss rate deep inside the vortex in the 1997 Arctic early spring correlates with the
INDEX TERMS: 0341 Atmosphericpresence of type Ia polar stratospheric clouds (PSCs).
Composition and Structure: Middle atmosphere—constituent transport and chemistry (3334); 0340
Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry; 0394 AtmosphericComposition and Structure: Instruments and techniques; 3360 Meteorology and Atmospheric Dynamics:
Remote sensing; 1610 Global Change: Atmosphere (0315, 0325)
Citation: Terao, Y., Y. Sasano, H. Nakajima, H. L. Tanaka, and T. Yasunari, Stratospheric ozone loss in the 1996/1997 Arctic winter:
Evaluation based on multiple trajectory analysis for double-sounded air parcels by ILAS, J. Geophys. Res., 107(D24), 8210,
doi:10.1029/2001JD000615, 2002.

1. Introduction[2] The possibility of ozone depletion due to chlorofluorocarbons (CFCs) was first reported by Rowland and Molina[1975], and the Antarctic ozone hole was discovered during
Copyright 2002 by the American Geophysical Union.
0148-0227/02/2001JD000615$09.00

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the 1980s [Chubachi, 1984; Farman et al., 1985]. Since
then, phenomenology, processes, and causes of the polarozone loss have been extensively investigated by manystudies on the basis of ground-based, airplane, and satellite
remote sensing observations and model simulations [e.g.,
WMO, 1999, and references therein], and the springtime
ozone loss has been proved to be a regular phenomenon of
the Antarctic stratosphere. A question of primary concern is

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TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

whether, and to what extent, the same or other chemical
processes are responsible for destroying ozone in the Arctic
polar vortex. Through investigations on the distribution ofozone and ozone-related gases in the Arctic, ozone depletion in the Arctic has been detected throughout the 1990s in
the same manner as in Antarctica [e.g., WMO, 1999, andreferences therein].
[3] During the Arctic winter to spring in 1996/1997, totalozone amounts in the northern polar region reached very low
levels [Newman et al., 1997]. The meteorological conditionsin that winter were different from those of the pervious years,
i.e., the late vortex formation and the extremely low temperature in the stratosphere, persisting until late spring [Coy etal., 1997; Pawson and Naujokat, 1999]. The chemical ozone
depletion inside the polar vortex in the 1997 Arctic spring
has been estimated both by observational studies [Manney etal., 1997; Müller et al., 1997; Knudsen et al., 1998; Sinnhuber et al., 1998; Hansen and Chipperfield, 1999;
Braathen et al., 2000; Bramstedt et al., 2000; Denis et al.,
2000; Schulz et al., 2000; Sasano et al., 2000] and bynumerical models [Lefèvre et al., 1998; Guirlet et al., 2000].
[4] Quantitative assessment of ozone changes by chemical processes separately from dynamical processes is crucial
to improve understanding of the polar stratospheric ozone
depletion. In the Arctic region, ozone concentration measured at a fixed location often shows large natural variability
due to significant dynamical influences, therefore any estimates of chemical ozone depletion ignoring transports arelikely to be affected by the dynamical effects. Althoughseparating the chemical and the dynamical influences on
ozone changes is a difficult task and requires careful treatment, a number of methods have been developed for thisseparation and used to estimate Arctic ozone loss. One suchmethod is known as ‘‘Match’’ [von der Gathen et al., 1995;
Rex et al., 1997, 1998, 1999; Schulz et al., 2000], which uses
pairs of ozonesonde profiles (‘‘matches’’) obtained at separated locations but identified by Lagrangian trajectory analysis to have traced the same air mass. By this approach, thedynamical effect in ozone changes can be neglected and only
chemical changes remain between the first and the second
observations of each matching pair, so that the chemicalozone loss rate and amount can be estimated quantitatively
in several Arctic winters.
[5] We applied a similar technique to the ozone profile
data obtained with the satellite-borne sensor, Improved
Limb Atmospheric Spectrometer (ILAS) on board theAdvanced Earth Observing Satellite (ADEOS) [Sasano
et al., 1999; Nakajima et al., 2002], and presented preliminary estimates of the ozone loss rates and amounts during
the Arctic spring of 1997 [Sasano et al., 2000] and the nitricacid change rates at the same time [Terao et al., 2000].
There are two advantages in using the ILAS data over the
Match analysis: (1) ILAS provides data not only regardingozone but also nitric acid, passive tracers, aerosol extinction
coefficient and so on and (2) ILAS can make measurementswith the same quality homogeneously over an entire narrowzone at high latitude (Figure 1). On the other hand, there is aproblem regarding conservation of air masses due to a
relatively low vertical resolution and a larger sampling air
mass volume of the satellite sensor than the ozonesondedata. In order to overcome this weakness of the satellite
sensor data, we introduce much stricter criteria than those of

ILAS measurement points [Feb 1 to Mar 31]

Equivalent latitude [ N]

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90
80
70
60
50
40
30
20
30

40

50

60

70

80

90

Date [day of the year 1997]
Figure 1. Location of ILAS measurement points on ageographic map (top panel) and a time-equivalent latitudesection of the measurements on a 475 K potential
temperature surface (bottom panel) in Northern Hemispherefrom 1 February to 31 March 1997. Thick and thin solid
curves in the bottom panel indicate the vortex edge and thevortex boundary region, respectively.
the sonde-Match, to identify more reliably double-sounded
air parcels.
[6] In this study we propose the Satellite-Match analysis
method and reanalyze quantitative chemical ozone loss rates
and amounts in the Arctic polar vortex for the spring of
1997. To determine the chemically induced ozone loss
accurately, we use the newest products of ILAS (version
5.20) and the improved analysis method for double-sounded
air parcels based on multiple trajectories. This study presents
vortex-averaged ozone loss rates and distribution of ozone
loss inside the vortex, and discusses the relationship between
the ozone loss and temperature history of the air masses.
[7] This paper consists of six sections. In section 2 we
describe the ILAS data and meteorological data used in thisstudy. Section 3 presents the method of the Satellite-Matchanalysis, and describes in detail the calculation of multiple
trajectories, criteria for selecting double-sounded air parcels,and algorithm for calculating chemical ozone loss rates.
Section 4 shows vortex-averaged ozone loss rates andchanges of ozone loss rate with respect to relative positionin the vortex, as well as temperature history along the
trajectory of a corresponding double-sounded air parcel. We
discuss the results in section 5, which includes a sensitivity
test of ozone change rate to the diabatic descent and to thecriteria for selecting double-sounded air parcels, the relation-

TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

ship between the ozone loss and low temperature experienced
by the air mass, and a comparison of the ozone loss rates withother studies. This study is summarized in section 6.

2. Data[8] ILAS is a solar occultation sensor and provides
vertical profiles of gas species (ozone, nitric acid, nitrogendioxide, methane, nitrous oxide, and water vapor) by infrared spectrometry and aerosol extinction coefficient by visible spectrometry [Sasano et al., 1999; Nakajima et al.,2002]. The present analysis uses the ozone mixing ratio
profiles obtained by the version 5.20 ILAS retrieval algorithm [Yokota et al., 2002]. Figure 1 (top panel) depicts thelocations of the ILAS measurements on a geographic map in
the Northern Hemisphere from 1 February to 31 March
1997, during which period 735 ozone profiles were obtained.
The ILAS measurements were made only between 63N and
70N but covered all longitudinal regions. Figure 1 (bottom
panel) shows the distributions of the ILAS measurements,
the vortex edge, and the vortex boundary region as functionsof date and equivalent latitudes [Nash et al., 1996] at a 475 K
potential temperature surface. The vortex edge and thevortex boundary region were defined on each day by asimilar method to that of Nash et al. [1996]. Throughout the
2 months ILAS made large numbers of observations inside
the poleward (inner) edge of, within the boundary region of,and outside the equatorward (outer) edge of the polar vortex.
[9] Although the quality of the version 5.20 ILAS data is
described by Yokota et al. [2002], we briefly summarize
them here. The estimated root-sum-square total uncertainties in ozone values are 14%, 9%, and 5% at 15 km, 20 km,
and 25 km, respectively, on the average in the Northern
Hemisphere from November 1996 to June 1997. Theseerrors are associated mainly with residuals of the nonlinear
least squares fitting of the simulated transmittance to theobserved transmittance in the retrieval. Measurement
repeatability, defined as a relative standard deviation ofozone values obtained during 3 – 5 days in quiescent periods(e.g., around the South Pole in summer), is 7% at 15 km and
3% at 20 km. These values are smaller than the root-sumsquare total uncertainties and should represent measurement
precisions. Note that the ILAS gas values could have beenbiased even after applying the ‘‘nongaseous correction’’[Yokota et al., 2002] when polar stratospheric clouds (PSCs)
existed at the same time. Numerical simulations show thatthe ozone values may have a very large positive bias of 0.90
ppmv (or a small negative bias of 0.16 ppmv) at 20 km if ice(or nitric acid trihydrate (NAT)) PSCs having such a large
extinction coefficient as 1  10 3 km 1 exist. Thus, careshould be taken especially when ILAS observed ice PSCs.
In this study we eliminated all the ozone data that may havebeen affected by PSCs occurrence, according to the information on PSCs detection provided by Saitoh et al. [2002].
[10] The version 5.20 ILAS ozone data have been validated against the ozone data obtained by ozonesondes,
instruments on board balloons and aircrafts, and groundbased instruments, as well as by the satellite sensors, Halogen
Occultation Experiment (HALOE), Stratospheric Aerosol
and Gas Experiment (SAGE) II, and Polar Ozone andAerosol Measurement (POAM) II [Sugita et al., 2002]. The
ILAS ozone data generally agree with them within ±10%.

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7-3

Focusing on the winter in the Northern Hemisphere, therelative differences in comparison with the ozonesonde dataare 14% at 11– 15 km and 8% at 16 –20 km during February,
and 7% at 11 –15 km and 5% at 16– 20 km during March.
[11] The following procedure is used in handling the
ILAS ozone mixing ratio profiles. Though the time of ILAS
measurements is precisely determined in the order of 10 ms,
we treat it in units of hour by rounding up to every hour for
convenience, because the time step of trajectory analysis is
1 hour (described later). Representative time and geometrical location of the observations are provided for thetangent height of 20 km. Note that changes in time andlocation of the measurement within each sounding are smallenough to be neglected. As the ILAS ozone data areprovided as a function of geometrical altitude with 1 km
intervals, the vertical coordinate is transformed to a potential temperature coordinate by using the United Kingdom
Meteorological Office (UKMO) assimilation data [Swinbank and O’Neill, 1994] and the ILAS ozone mixing ratios
are interpolated on potential temperature surfaces.
[12] The UKMO assimilation data are also used for
meteorological analysis and trajectory calculations. The data
are provided daily on a grid of 2.5 degrees in latitude and3.75 degrees in longitude on 22 Upper Atmosphere ResearchSatellite (UARS) standard pressure levels at 1200 UT.

3. Analysis Method[13] A double-sounded air parcel that the ILAS sounded
twice at different locations and at different times (match pair)is searched from the ILAS data set with 10-day forward
isentropic trajectories, and a change of ozone mixing ratio is
calculated from the two profiles of the air parcel taking the
diabatic descent effects [Knudsen et al., 1998] into consideration. The analysis is done at nine isentropic levels from
375 to 550 K at 25 K intervals, and 600 K.
3.1. Trajectory Analysis[14] Trajectory analysis is conducted with an isentropic
Lagrangian model which was supplied by UKMO (R.
Swinbank, private communication). The main specificationsof the trajectory model are as follows: The integration in
calculating trajectory is done by the fourth-order RungeKutta scheme with 1-hour interval. The time interpolationand the space interpolation with respect to the horizontal
direction are done linearly, and the vertical interpolation isdone linearly with respect to the log-P coordinate.
[15] Our previous analysis [Sasano et al., 2000] wasbased only on a single forward trajectory, which started
from each ILAS observation point (location 1) for searchinga matching observation (location 2). In the sonde-Matchanalysis, a single forward trajectory was used for searchinga match in the 1991/1992 winter [von der Gathen et al.,
1995; Rex et al., 1998] and six additional forward trajectories around each sonde measurement point were introduced
to check the accuracy of matches in the 1994/1995 winter[Rex et al., 1999]. Since satellite sensor data have arelatively low vertical resolution (ILAS has a verticalresolution of about 2 km) and larger sampling air mass
volume than the ozonesonde data, a strict criterion forassuring conservation of air masses needs to be introduced
to identify a double-sounded air mass more reliably.

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TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

Forward Trajectory

Backward Trajectory

Trajectories between 97/1/24 21UT and 97/1/30 18UTCenter trajectory (475 K)
Location #1
Location #2

Figure 2. Example of a single forward (left) and a single backward (right) trajectory on the 475 K
potential temperature surface between 24 and 30 January 1997. The first ILAS measurement point(location 1) is marked by a solid circle and the second point (location 2) by an open circle.
[16] For this purpose, we also calculate a backward
trajectory which starts from location 2. Figure 2 shows anexample of a pair of forward and backward trajectories on the
475 K potential temperature surface. As shown in Figure 2,
the forward trajectory which starts from location 1 matches
well with location 2, however, the backward trajectory fromlocation 2 never gets close to location 1. This type of caseshould be avoided when identifying double-sounded air
parcels.
[17] Furthermore, we consider a cluster of forward trajectories which start from around location 1 and a cluster ofbackward trajectories from around location 2. The initial
volume of the cluster is intended to represent a finite
sampling air mass volume of ILAS. Figure 3 presents aschematic picture of ILAS sampling air mass volume (1.6
km in height and 300 km in distance as representativevalues) and the corresponding multiple trajectories. We
calculate four additional trajectories which start from points

separated by ±150 km along the line of sight on the two
surfaces separated by ±0.8 km in altitude around each ILAS
observation point at its tangent height. In total, ten trajectories (a central single forward/backward trajectory and acluster of four forward/backward trajectories) are calculated
for each ILAS measurement at each level, as shown in
Table 1.
[18] Figure 4 illustrates an example of the cluster offorward/backward trajectories. The thick black curveshows the central trajectory on the 475 K potential temperature surface, and blue and red trajectories are at the upper
level (493 K) and at the lower level (457 K), respectively.
Locations 1 and 2 can be identified as a double-sounded
air parcel both by the forward and backward central
trajectories in this case. However, there are large differences between the upper and lower level trajectories, andthe locations of the end point of each trajectory are quite
different, which can be explained by a large vertical shear

Upper level trajectories
150 km
H +0.8 km
Tangent height= H km
SUN

ILAS

H -0.8km

Center trajectory

Lower level trajectories

Figure 3. Schematic picture of ILAS sampling air mass volume and a corresponding cluster oftrajectories.

TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

Table 1. Trajectory Calculation Levels
Potential
temperature, K

Approximate
altitude,a km+0.8 kmlevel,a K

0.8 kmlevel,a K

375
400
425
450
475
500
525
550
600

13.6
15.0
16.4
17.7
18.9
19.9
20.9
22.2
23.6

387.8
413.6
440.1
466.7
493.1
519.3
545.5
571.8
624.6

361.9
386.6
410.4
433.9
457.4
481.2
505.1
529.0
576.4

a

Three months mean during February, March, and April.

in the wind field. Also, as shown in the right-hand panel of
Figure 4, there are large differences between the two
trajectories on the same surface, which are caused by alarge horizontal shear in the wind field. As such, if four
multiple forward/backward trajectories diverge significantly, this pair of measurements should not be regarded
as a double-sounded air parcel. Figure 5 indicates anexample showing that all of the multiple forward andbackward trajectories are very compact during 7 days, so
we can reliably assume this is a double-sounded air parcel.
3.2. Identification Criteria of Double-Sounded Air
Parcel[19] We set several criteria for identifying reliable double-sounded air parcels.
1. The distance (referred to as a forward match radius)
between the position of location 2 and the central forward
trajectory which starts from location 1, which corresponds
to the distance between the large open circle and the tip of
the thick solid arrow in the left-hand panel of Figure 5,
should be less than 500 km.

Forward Trajectories

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7-5

2. The distance (referred to as a forward cluster radius)
between the position of location 2 and all four forward
trajectories which start from around location 1, whichcorresponds to the distance between the large open circle
and each small solid or open circle in the left-hand panel of
Figure 5, should be less than 1600 km.
3. The same criteria are applied to the backward trajectory
analysis. Thus a backward match radius should be less than
500 km and a backward cluster radius less than 1600 km.
4. The difference between the maximum and theminimum value of potential vorticity (PV) along the central
forward trajectory, to which a 24-hour running mean wasapplied (referred as PV), should be less than 20%.
[20] These criteria were chosen on the basis of error
analysis similar to that by Rex et al. [1998, 1999].
Figure 6 shows a standard deviation of ozone mixing ratiochange as a function of forward match radius and backward
match radius, in which the ozone change is corrected by an
expected ozone change along the trajectory. The expectedozone change was calculated by multiplying the sunlit timealong the trajectory by the ozone change rate per sunlit time(the way of determining the ozone change rate by regression
analysis is described in the next section.). Thus Figure 6
indicates the dependence of a residual ozone change fromthe regression line, namely, an error, on the distancecriterion. If a larger distance criterion is used, the error
becomes larger, and conversely, if a smaller distance is used,
the number of double-sounded air parcels is too small to
calculate ozone change rates in a statistically meaningful
way. Therefore, 500 km was chosen as the criterion for thepresent analysis. The criteria of the forward/backward
cluster radius and PV were determined in the samemanner as the forward/backward match radius.
[21] Table 2 shows the total number of double-sounded
air parcels inside the inner edge of the vortex boundary
selected according to the criteria during February and March

Backward Trajectories

Trajectories between 97/2/12 16UT and 97/2/20 22UT
Center trajectory (475 K)
Upper trajectories (493 K)
Location #1
Location #2
Lower trajectories (457 K)

Figure 4. Example of a cluster of forward (left) and backward (right) trajectories between 12 and 20
February 1997. A thick black curve with an arrow shows a center trajectory on the 475 K potential
temperature surface. Blue and red trajectories are at the upper level (493 K) and the lower level (457 K),
respectively, and small solid circles show the end of each trajectory. See color version of this figure atback of this issue.

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TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

Forward Trajectories

Backward Trajectories

Trajectories between 97/3/7 20UT and 97/3/14 14UTCenter trajectory (475 K)
Upper trajectories (493 K)
Location #1
Location #2
Lower trajectories (457 K)

Figure 5. Same as in Figure 4, but between 7 and 14 March 1997. See color version of this figure atback of this issue.

∆O3* error [%]

10

5

5

0

0

28 66 127 190 262 331 404 497 602

200

400

600

800

1000

Forward Match Radius [km]∆O3* error [%]

10

5

7

0

0

25 59 96 142 182 219 257 284 310

200

400

600

800

1000

Backward Match Radius [km]Figure 6. Standard deviation of ozone mixing ratiochange, which is corrected by the expected ozone loss
along the trajectory, as a function of the forward match
radius (top panel) and backward match radius (bottom
panel). The data used here were obtained during 4 February
to 5 March on the 475 K potential temperature surface. The
figures at the bottom indicate the number of double-sounded
air parcels selected for each match radius criterion.

between 400 and 600 K potential temperature surfaces. If
we considered only forward match radius and PV, morethan one thousand double-sounded air parcels were obtained(row 1 in Table 2). When we added the backward match
radius criterion, the number of double-sounded air parcels
was approximately half. By considering all of the criteria
described above, several hundred double-sounded air parcels were identified for each month (row 3 in Table 2),
which, although they are less than one third of the number
determined only by the forward match radius, are enough tostatistically calculate ozone change rates. We cannot obtainenough double-sounded air parcels at the 375 K levels
inside the vortex, thus the following analysis was done forthe vertical range from 400 to 600 K surfaces.
3.3. Calculation of Ozone Change Rate[22] A change in ozone mixing ratio (referred as O3) iscalculated, by considering changes in potential temperatureby diabatic effects (referred as q), as O3 = O3(q +q)location 2 O3(q)location 1, where O3(q) means an ozone
mixing ratio at a q potential temperature level and thesubscript indicates a location of ILAS observation. Thus,
the ozone mixing ratio at location 2 is modified by potential
temperature changes due to diabatic cooling during the flighttime. The diabatic cooling rates averaged over the polar
vortex were taken from the work of Knudsen et al. [1998].
[23] A statistical treatment is applied to the subset of
double-sounded air parcels to calculate ozone change rates
for each day. Each subset consists of double-sounded air
parcels that are gathered within 7 days before and after each

Table 2. Total Number of Double-Sounded Air Parcels Between
400 and 600 K Potential Temperature Surfaces for Corresponding
Criteria During February and March
Criteria

FebruaryMarch

1 Forward match radius  500 km and PV  20%
2
1 + backward match radius  500 km
3 2 + forward and backward cluster radius  1600 km

1426
735
565

2179
891
654

Day 49 +7 at 475 K

200
0
-200

-6.0 +
1.0 p

-400
-600

pbv

/hou

(R 2=

0

20

40

60

0.70

r

)

80

Sunlit time [hour]
Figure 7. Scatterplot for ozone mixing ratio change as afunction of sunlit time along the trajectory. The data usedhere were obtained during 11 – 25 February on the 475 K
potential temperature surface. A regression coefficientestimated as 6.0 ± 1.0 ppbv/sunlit hour is referred to as
an ozone change rate on 18 February. The coefficient ofdetermination (R2) is 0.70.

target day. Assuming that ozone changes are linearly
proportional to the sunlit time along the trajectory, a proportional coefficient (ozone change against sunlit time) is
calculated using the least squares method from each subset
of data. Figure 7 is a scatterplot for ozone mixing ratiochanges of double-sounded air parcels as a function of sunlit
time along their trajectories. The regression line gives anozone change rate in ppbv per sunlit hour. The data used inthis figure were obtained inside the inner edge of the vortex
boundary region during 11 – 25 February on the 475 K
potential temperature surface, and the regression coefficient,
6.0 ± 1.0 ppbv/sunlit hour, is referred to as the ozone
change rate against sunlit time on 18 February.
[24] The present technique assumes that contributions bydynamical effects can be eliminated and only changes inozone due to photochemical reactions are derived [von derGathen et al., 1995]. To check this, we conducted abivariate regression analysis allowing ozone changes during
nighttime [Rex et al., 1998, 1999; Sasano et al., 2000]. The
results show that the ozone change rates from 4 February to
6 March on the 475 K potential temperature surface are
5.1 ± 1.9 ppbv/sunlit hour and 0.3 ± 1.0 ppbv/dark hour.
This shows that the ozone change during nighttime isalmost zero, and implies that the ozone change due todynamical effects can be neglected.

4. Results
4.1. Ozone Change Rate Averaged Inside the Inner
Vortex Edge[25] Figure 8 shows the location of double-sounded air
parcels as functions of date and PV (top panel), andequivalent latitude (bottom panel) on the 475 K potential
temperature surface. The PV change in each individual
double-sounded air parcel is shown to be conserved within
20% level because of the constraint by the match criterionof PV  20%. The equivalent latitude of each air parcel

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looks to change more significantly than the PV. This issimply because the maximum PV value at the center of thevortex is variable with time. Figure 8 also shows thevortex edge (thick line), and inner (poleward) and outer(equatorward) edges of the vortex boundary region (thinlines). In this subsection we use the double-sounded air
parcels (two crosses linked by a line in the figure) only
inside the inner edge of the vortex boundary, which was
located at around 70N equivalent latitude (corresponding
approximately to 45 PVU, 1 PVU = 10 6 km2 s 1 kg 1)
on the average. We define the ozone change rate derived
from this data set as a ‘‘vortex-averaged’’ value inside theinner vortex edge.
[26] The ozone change rate in ppbv per sunlit hour can beconverted to a quantity of ozone change per day by multiplying them with an averaged sunlit time (hours) for eachday. The sunlit time for each day can be calculated as theaverage of sunlit time experienced by the air parcels on
trajectories of interest. In addition, the net ozone change for2 months can be evaluated by integrating the ozone change
per day with time. In this calculation, the altitude change
due to diabatic descending motion of the air parcels is taken
into consideration.
[27] Figure 9a shows the ozone change rates (ppbv/day)averaged inside the inner edge of the vortex boundary as afunction of potential temperature and date, which is

Equivalent latitude [ N] Potential vorticity [PVU]

O3 change [ppbv]

TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

80
70
60
50
40
30
20

(a)

10

90
80
70
60
50
30

(b)
40

50

60

70

80

90

Date [day of the year 1997]
Figure 8. Location of double-sounded air parcels obtained
inside the inner edge of the vortex boundary as a function ofdate (day of the year 1997) and PV (top panel) andequivalent latitudes (bottom panel) on the 475 K potential
temperature surface. ILAS observation points are marked bycrosses, and two matching observations are connected by asolid line. Thick and thin solid curves in both panels show
the vortex edge and the poleward (inner) and equatorward(outer) edges of the vortex boundary region, respectively. A
dashed curve in the top panel shows the maximum PV in thevortex, which defines the center of the vortex.

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TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

Column loss
[DU/day] Potential temperature [K][ppbv/day]

O3 change rate

-80 -40 -20 0

600

20 40

Integrated loss
600

550

550

500

500

450

450

400

(a)

0
-1
-2
-3
(c)
-4
30

(b)
-2

400
-1

0

[ppmv]

40

50

60

70

80

90

Date [day of the year 1997]
Figure 9. (a) Color-coded ozone change rates (in ppbv per day) inside the inner edge of the vortex
boundary (north of about 70 N equivalent latitude) as a function of potential temperature and date.
Vertical bars indicate the region with statistical significance less than 99%. Smooth thin curves show
potential temperature changes of air parcels (diabatic descent of air masses) (adopted from the work ofKnudsen et al. [1998]). (b) Integrated ozone changes from 30 January (day 30) to 31 March (day 90)
along the descending motion due to diabatic cooling. For the integration, ozone change rates arevertically interpolated by a cubic spline. For the uppermost two levels, days 51– 90 and days 37– 90 areused for the integration, respectively. Error bars represent 1 sigma. (c) Ozone column change rates (in DU
per day), which are obtained by integrating local ozone change rates (in number density per day) from
400 to 600 K. Results are plotted every 5 days. See color version of this figure at back of this issue.
derived from the set of double-sounded air parcels shownin Figure 8. Ozone losses take place at almost all altitudes
during February and March, with particularly large losses
during late February, where the local maximum ozone loss
rate is 50– 80 ppbv/day at 450– 500 K potential temperature levels. The local ozone loss rates as derived here are
10– 20 ppbv/day higher than those obtained by our preliminary analysis [Sasano et al., 2000]. The difference in
the ILAS product version (Sasano et al. [2000] used
Version 3.10 whereas the present study used Version
5.20) is relatively small for the ozone products. A probablecause for the higher ozone loss rates is the stricter criteria
used for identifying the double-sounded air parcels (discussed further in section 5.1). Vertical bars in the figure
indicate the region with statistical significance lower than
99% by the t-value test. Although some regions withozone gain (yellow color) appeared in early February
and late March, they have no statistical significance.
[28] Figure 9b shows the integrated ozone change from30 January (day 30) to 31 March (day 90) along thepotential temperature changes of air parcels which were
illustrated by the smooth thin curves in panel (a). Note thatthe integration for the uppermost two levels was done fordays 51 –90 and days 37 –90, respectively. The integrated
ozone loss reaches the maximum of 2.0 ± 0.1 ppmv on thesurface that reached the 475 K level on 31 March from the529 K on 30 January. This is about 55% of the initialvortex-averaged ozone concentration (3.6 ppmv on 30
January at the 529 K level).

[29] Furthermore, the column ozone change per day isshown in Figure 9c, where the ozone change rate in number
density per day at each level was integrated from 400 to
600 K. Monthly mean temperature and pressure profiles
inside the inner vortex edge were used to convert from mixing
ratio to number density in the unit of ozone change rate. The
column ozone loss during the 2 months is 96 ± 0.3 DU.
4.2. Ozone Change Rate as a Function of rPV[30] In order to investigate the relationship between ozone
loss and location relative to the vortex, we used a relative PV(rPV) scale [Rex et al., 1998, 1999; Schulz et al., 2000]. TherPV ranges from 0 corresponding to the center of the vortex,
where the PV value takes the maximum, to 1 corresponding
to the vortex edge. The rPV is scaled so that equal intervals
in rPV correspond to equal area fractions of the vortex eachday. The vortex is divided into rPV belts with their centers at
0.1, 0.2, 0.4, 0.6, 0.8, and 1.0, and with a width of 0.4. An
exception is a width of 0.2 for the rPV belt centered at 0.1.
Double-sounded air parcels which fall in each belt are
defined so that the rPVs at locations 1 and 2 are both situatedin that belt. Figure 10 shows the location of double-sounded
air parcels as a function of rPV. The colors discriminate therPV belts. For example, the 0.4 rPV belt contains the doublesounded air parcels whose rPVs both at location 1 and at
location 2 are between 0.2 and 0.6. The number of doublesounded air parcels inside the inner edge of the vortex
boundary (corresponding to about 0.6 rPV on the average)
became smaller than that for the vortex-averaged analysis

TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

edge 1.0

Relative PV

0.8
0.6
0.4
0.2
core 0.0
30

40

50

60

70

80

90

Date [day of the year 1997]
Figure 10. Location of double-sounded air parcels
obtained in each rPV belt. In the rPV scale, 0 refers to thecenter of the vortex where the PV value takes the maximum
and 1 refers to the vortex edge (see text). Red, magenta,orange, green, cyan, and blue indicate the double-sounded
air parcels in the rPV belts of 0.0– 0.2, 0.0– 0.4, 0.2– 0.6,
0.4– 0.8, 0.6– 1.0, and 0.8 –1.2, respectively. See color
version of this figure at back of this issue.
(section 4.1) because we selected only double-sounded air
parcels found just within each 0.4 rPV belt.
[31] Figure 11 shows ozone change rates (ppbv/sunlithour) as a function of rPV. The ozone change rate is calculated
by regression analysis using a subset of double-sounded air
parcels for each rPV belt from 4 February to 6 March at the
475 K level, where a large negative ozone change rate (loss)
was observed (Figure 9). The ozone loss increases with adecrease in rPV from the vortex edge toward the vortex
center, showing a step-like increase at 0.6 and 0.2 rPVs
(approximately 73N and 80N equivalent latitude, respectively). The maximum ozone loss is 6.0 ± 0.6 ppbv/sunlit
hour at 0.2 rPV near the vortex center. Even at the vortex edge
ozone loss occurred (2.7 ± 1.2 ppbv/sunlit hour). The results
indicate that the magnitude of ozone loss depends on rPV
within the vortex and that the ozone loss near the vortex
center is larger than near the vortex edge by a factor of 2– 3.
[32] By integrating ozone change rates from the vortex
center (0.0 rPV) to the inner vortex edge (0.66 rPVs), theozone change rate averaged inside the inner vortex edge canbe estimated as 5.0 ± 0.9 ppbv/sunlit hour, which can be
regarded as an average value from 4 February to 6 March at
the 475 K level. For the integration, ozone change rates areinterpolated with the rPV by a cubic spline. To compare thiswith the ozone change rate previously derived in section
4.1, we recalculated the ‘‘vortex-averaged’’ ozone change
rate with the same period at the same level using the data on
Figure 9. The ozone change rate inside the inner vortex edge
thus calculated is 45.0 ± 8.7 ppbv/day, which can beconverted to 5.1 ± 1.0 ppbv/sunlit hour. This is comparable to the value derived above although the ways ofaverage calculations are different.
4.3. Ozone Change Rate Averaged Within the Whole
Vortex[33] As shown in section 4.2, the magnitude of ozone loss
are strongly dependent on relative location to the vortex in

ILS

7-9

the winter 1996/1997. Thus the ‘‘vortex-averaged’’ ozone
loss rate may be influenced by the area of the vortex where
the average ozone loss rate is calculated. Figure 12 shows, forcomparison, two time series of ozone change rates (ppbv/day) at 475 K that are calculated with the method described in
section 4.1 using the data obtained inside the inner edge ofthe vortex boundary (squares), and inside the vortex edge
(diamonds). The former represents the ozone change rates as
averaged over north of about 70 N equivalent latitude, andthe later represents those over north of about 65 N equivalent latitude (approximately 37 PVU). Figure 12 shows theozone loss rates derived from the data inside the vortex edge
are 40– 50 ppbv/day during late February, and lower than
50– 60 ppbv/day derived from the subset of the data inside
the inner vortex edge. This is because the calculation of theozone loss rates inside the vortex edge include the data for theregion between 65 N and 70 N equivalent latitudes where
the ozone loss is relatively small. Inside the vortex edge, theintegrated ozone change during the 2 months is 1.6 ± 0.1
ppmv on the 475– 529 K surfaces and the column ozone loss
between 400 and 600 K surfaces during the 2 months is 72 ±0.4 DU. These results are also smaller than the analysis forinside the inner vortex edge.
4.4. Temperature History Along the Trajectory[34] In this section, we show two temperature histories
along the trajectories of a double-sounded air parcel: One isalong a central forward trajectory between location 1 andFigure 11. Ozone change rates (in ppbv per sunlit time) as
a function of rPV. Each data point is calculated by regression
analysis for double-sounded air parcels obtained in the rPVbelts of 0.0– 0.2, 0.0– 0.4, 0.2– 0.6, 0.4 – 0.8, 0.6– 1.0, and
0.8– 1.2, respectively, from 4 February to 6 March at 475 K.
Figures at the bottom indicate the number of double-sounded
air parcels used for the calculation. Vertical error bars
represent 1 sigma. A scale for approximate equivalent
latitude is also plotted. Vertical dashed lines show the vortex
edge (1.0 rPV) and the inner edge of the vortex boundary(0.66 rPV) on the average.

-20
-40
-60
17 38 29 18 22 22 23 31 33 25
29 72 68 42 37 39 51 68 55 62
-80
30
40
50
60
70
80
90

Date [day of the year 1997]
inside the inner vortex edge
inside the vortex edge
Figure 12. Time series of ozone change rates (in ppbv perday) on the 475 K potential temperature surface. Solidsquares give the ozone change rates as calculated using thedata obtained inside the inner edge of the vortex boundary(north of about 70 N equivalent latitude), and open
diamonds give the ozone change rates using the data inside
the vortex edge (north of about 65 N equivalent latitude).
Figures near the bottom show the number of double-sounded
air parcels inside the inner edge of the vortex boundary (row1) and inside the vortex edge (row 2). Results are plotted
every 6 days. Diamonds are shifted horizontally for the sakeof clarity. Error bars denote 1 sigma.

600
550
500
195

450
200

400
30

50

40

60

70

80

90

Date [day of the year 1997](b) 10-day backward trajectory
600
550
500
195

location 2 (called TH-F) and the other is along a 10-day
central backward trajectory which started from location 1(TH-B). When the ozone loss is observed in the doublesounded air parcel, TH-F indicates the temperature historyof the air parcel while the ozone loss is occurring, and TH-B
indicates that before the ozone loss occurs.
[35] Figure 13 shows the minimum temperatures found in
TH-F (a) and TH-B (b) for the air parcels inside the inneredge of the vortex boundary (corresponding to the trajectories used to calculate the ozone change rates in Figure 9).
The minimum temperatures were searched for both on TH-F
and TH-B for each double-sounded air parcel, and wereaveraged within 7 days before and after each target day in asimilar way to that used for calculating the ozone change
rate. The distribution patterns of the minimum temperaturesfrom TH-F and TH-B are very similar to each other, but theminimum temperature on TH-B is by about 5 K lower thanon TH-F. This difference may result from the geographicaldifferences in the route of the trajectories (described later).
Focusing on the regions between 450 and 550 K potential
temperatures during late February, where the large ozone
loss was observed, the temperature drops below 195 K on
TH-F and below 190 K on TH-B.
[36] Figure 14 plots the minimum temperatures (solid
square) and the maximum temperatures (open diamond)from TH-F (a) and TH-B (b) as a function of the rPV. The
frost temperature of ice (Tice) [Marti and Mauersberger,1993], the equilibrium temperature of NAT (TNAT) [Hanson

Minimum Temperature(a) between the location #1 and #2

450

19
5

0

and Mauersberger, 1988], and the melting temperature ofsulfuric acid tetrahydrate (TSAT) [Middlebrook et al., 1993]
were calculated using the monthly mean and standard
deviation of water vapor and nitric acid mixing ratios at
475 K measured by ILAS inside the vortex during February(5.5 ± 0.3 ppmv H2O and 10.5 ± 0.2 ppbv HNO3). Tice, TNAT,and TSAT vary within ±0.5 K corresponding to the standard
deviations in the water vapor and nitric acid mixing ratios.
Furthermore, the UKMO temperatures at the NAT point
were reported to be higher than the radiosonde temperatures
by 1.7 K at 475 K with a standard deviation of about 2 K,
from comparisons of the UKMO lower stratospheric temperatures with those obtained by the radiosondes during thewinter of 1994/95 [Pullen and Jones, 1997]. If the same
positive bias exists in the 1996/1997 winter, it also may be acause of uncertainty in the following analysis.
[37] The ozone loss increases from the vortex edge to thevortex center (Figure 11) while the minimum temperatureson the temperature history (TH-F and TH-B) decreases(Figure 14). Furthermore, a stepwise structure at 0.2 rPV
is seen both in the ozone change rate and in the temperature
history. In the vortex core region (0.1 and 0.2 rPV belts),
where the ozone loss is large, the minimum temperature on
TH-B drops near Tice and that on TH-F drops well below
TNAT. This means that the large ozone loss is observed in the
deep part of the vortex when the double-sounded air parcels

190

475 K

Potential temperature [K]O3 change rate [ppbv/day]

20

TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

Potential temperature [K]7 - 10

ILS

400
30

40

50

60

70

80

90

Date [day of the year 1997]

Figure 13. Minimum temperatures along (a) a central
forward trajectory between locations 1 and 2 and (b) a 10day central backward trajectory, which started from location
1 for double-sounded air parcels inside the inner edge of thevortex boundary as a function of potential temperature anddate. The contour interval is 1 K.

TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH(a) between the location #1 and #2
220

Day 35-65
475 K
215

220

Day 35-65
475 K
215

minimum temp.
maximum temp.

Temperature [K]

TSAT

TSAT

210

210

205

205

200

200

TNAT

190
185
0.0
core

TICE
8 23
0.2

25
0.4

41
0.6

41
0.8

Relative PV

7 - 11(b) 10-day backward trajectory

minimum temp.
maximum temp.

195

ILS

29
1.0
edge

TNAT

195
190
185
0.0
core

TICE
8 23
0.2

25
0.4

41
0.6

41
0.8

Relative PV

29
1.0
edge

Figure 14. Minimum temperatures (solid square) and maximum temperatures (open diamond)experienced along (a) a central forward trajectory between locations 1 and 2 and (b) a 10-day central
backward trajectory, which started from location 1 as a function of rPV. Horizontal lines show the SAT
melting temperature and the NAT and ice PSC forming temperatures (see text). Each data point iscalculated as the average for double-sounded air parcels obtained in the rPV belts of 0.0– 0.2, 0.0– 0.4,
0.2– 0.6, 0.4– 0.8, 0.6– 1.0, and 0.8– 1.2, respectively, from 4 February to 6 March at 475 K. Verticalerror bars represent 1 sigma.
undergo a temperature below TNAT between two matchingsoundings and experienced a temperature near Tice in thepast 10 days. In the middle part of the vortex (0.4 and 0.6
rPV belts), the minimum temperature on TH-B is just below
TNAT and never reaches Tice, and that on TH-F is near TNAT.
On the contrary, the less ozone loss is derived near thevortex edge (0.8 and 1.0 rPV belts), in which the air parcels
experience the minimum temperature near or slightly above
TNAT on TH-B and above TNAT on TH-F. We can also see
differences between the interior of the vortex and the vortex
boundary region in terms of the maximum temperature: themaximum temperature is always below TSAT on both TH-F
and TH-B from 0.1 to 0.6 rPV belts, while near the vortex
edge the maximum temperature is well below TSAT on TH-F
but near or above TSAT on TH-B.
[38] The lower minimum temperature and the higher
maximum temperature on TH-B than those on TH-F may
result from the geographical differences in the route of theirtrajectories and the differences in the duration for theirtrajectories. Figures 15a and 15b show the forward trajectories between locations 1 and 2 (associated with TH-F) andthe 10-day backward trajectories from location 1 (associatedwith TH-B), respectively, on the 475 K surface during
February. The top, middle, and bottom panels show the
trajectories for the double-sounded air parcels that are
obtained in the vortex core region, in the middle part ofthe vortex, and in the vortex boundary region, respectively.
For the double-sounded air parcels near the vortex coreregion and in the middle part of the vortex, a large numberof backward trajectories pass through near the center of thevortex, the lowest temperature region, whereas the forward
trajectories do not cross the vortex core region so often withsome exceptions. This may be a possible reason for the
lower minimum temperature on TH-B than on TH-F. One of

the causes for the difference of the trajectory routes is thatthe backward trajectory can pass anywhere, whereas theforward trajectory must end with the location 2 which islocated at almost the same latitude as the location 1. Thus,
the trajectory crossing the center of the vortex between thetwo matching soundings may be rare because the prevailingwind is zonal.
[39] Another possible reason for the lower minimum
temperatures experienced on TH-B than those on TH-F
may be that the 10-day backward trajectory is always longer
than the forward trajectory between the two matchingobservations. On the average the duration time of the
trajectory between matches (TH-F) is about 3 days in thiscase. A possibility that the trajectory travels over either alower temperature region or a higher temperature region
increases with its duration time. The longer duration time of
TH-B could also cause the higher maximum temperature on
TH-B than on TH-F. In fact, the 10-day backward trajectories pass even over the lower latitude region, which is ahigher temperature region, in contrast to the forward trajectories. Figure 15 also suggests that the trajectories
between the matching pairs (TH-F) may not provide a
reasonable ‘‘geographical’’ coverage of the entire vortex
and hence a poor sampling of cold temperatures, although
they may give a reasonable sampling of rPV values.

5. Discussions
5.1. Sensitivity of Ozone Change Rate to Analysis
Parameters[40] Errors are introduced randomly and systematicallyduring the analysis. Random errors from any cause appear
on each matching pair of observations and give the uncertainties in the ozone change rates. The statistical uncertainty

ILS

7 - 12TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

Figure 15. (a) Forward trajectories between locations 1 and 2 and (b) 10-day backward trajectories fromlocation 1 for the double-sounded air parcels obtained around the vortex center (top panels), in the middle
part of the vortex (middle panels), and in the vortex boundary region (bottom panels) during February at 475
K. Figures in the parentheses indicate the range of rPV of the double-sounded air parcels in each panel.
of ozone changes in individual double-sounded air parcels(O*3 in Figure 6) was estimated to be about 7% during
February at 475 K. As main causes of systematic error, we
discuss the dependence of ozone change rate on several
parameters used in the analysis in this section.
[41] One of the possible sources of systematic error is the
diabatic descent rate. Figure 16 shows the ozone change
rates (ppbv/sunlit hour) averaged inside the inner vortex
edge at the 475 K level as calculated by using 0%, 50%, and150% cooling rates of the ones used in section 4. The
magnitude of diabatic cooling rate influences the ozone
change rate significantly only in early February, but muchless after mid-February even though the given cooling wasextreme. The lower sensitivity to the cooling rate implies
that the vertical gradients of the ozone mixing ratio obtainedby ILAS near the 475 K level were relatively small. Matcheswith a strong vertical ozone gradient would be more sensitive to diabatic descents and to small-scale vertical diffusion

processes [Rex et al., 1998]. Above 475 K, a significant
difference in the ozone change rate was also found occasionally during late February and March (not shown here),
but the magnitudes of difference were similar to those as
seen in early February at 475 K. Hence, the influence on thelocal ozone change rate by the diabatic cooling rate is notimportant in this study.
[42] On the other hand, the uncertainties in the diabatic
cooling rate may affect the amount of ozone loss as integrated along the descending motion of the air masses during
a long period (Figure 3b). However, the effect was verysmall for the potential temperature surface on which the
maximum ozone loss was observed. For example, even if we
consider very large biases of ±50% cooling, the integrated
ozone loss from 30 January to 31 March is 2.0 ± 0.1 ppmv
(529– 475 K, original cooling rate), 1.9 ± 0.1 ppmv (556–475 K, 150% cooling rate),and 2.1 ± 0.1 ppmv (501– 475 K,
50% cooling rate).

TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH[43] The more essential factor that may affect theuncertainties with respect to the ozone change rates is how
the double-sounded air parcels are defined. As described in
section 3.1, the present study introduced ten forward andbackward trajectories in total for searching a matchingobservation, while our preliminary study [Sasano et al.,2000] and the Match 91/92 [von der Gathen et al., 1995;
Rex et al., 1998] used only a single forward trajectory.
Although six additional forward trajectories were introduced in the sonde-Match analysis from 1994/95 winteronwards [Rex et al., 1999], there was little discussion on
the effect of this match criteria on derived ozone change
rates. We investigated sensitivity of the ozone change rate
to the different criteria in defining double-sounded air
parcels.
[44] Figure 17 demonstrates that the ozone loss rates asderived using the multiple forward and backward trajectories (squares) are significantly higher than those using thesingle forward trajectory (diamonds) by a factor of approximately 2 in some cases. It is shown that the differences inozone change rates at the 475 K level are relatively small
between using the present criteria (forward/backward match
radius 500 km, forward/backward cluster radius 1600
km, and PV  20%; squares in the figure) and using themuch stricter criteria (match radius 400 km, cluster radius1200 km, and PV  20%; circles in the figure). On theother levels we found some cases that the ozone loss rates ascalculated under the stricter distance criteria become higher
than those under the present criteria (not shown). However,note that the ozone change rate as determined under thestricter distance criteria is often statistically much lesssignificant mainly because of the smaller number of double-sounded air parcels.
[45] To investigate the reason for the higher ozone loss
rate found in the multiple trajectory analysis, we focused on
the differences in the scatterplots for ozone mixing ratiochanges in individual double-sounded air parcels between
the single trajectory and the multiple trajectory analyses.
Figure 18 depicts the differences of scatterplots for ozone
mixing ratio changes of double-sounded air parcels between
the two different criteria for defining the air parcels. The
closed circles represent the data as selected by the criteria inthis study, and the open circles the data selected by thesingle forward trajectory without the multiple trajectory
analysis. By adding the criteria associated with the multiple
trajectories, the data points which give a small change inozone mixing ratio in spite of a long sunlit time (opencircles after 30 sunlit hours in the figure) are filtered out,
and a higher ozone loss rate is derived. Most of the filteredout data are located in the region from 70N to 75N inequivalent latitude, near the vortex boundary region. Thus,
the multiple trajectory analysis serves to screen out the data

O3 change rate [ppbv/sunlit hour]

Figure 16. Time series of ozone change rates (in ppbv per
sunlit hour) inside the inner edge of the vortex boundary on
the 475 K potential temperature surface for different
diabatic descent rates. Solid squares represent the ozone
change rates as calculated with the cooling rates originallyfrom the work of Knudsen et al. [1998], which are used inthis paper. Open circles and open triangles depict the ozone
change rates with 50% and 150% of the cooling rates,
respectively. Solid diamonds show the ozone change rates
without cooling. Figures at the bottom show the number ofdouble-sounded air parcels used for the regression analysis.
Results are plotted every 6 days. Diamonds, circles, andtriangles are shifted horizontally for the sake of clarity. Error
bars denote 1 sigma.

7 - 13

ILS

5

475 K
0

-5

-10
30

40

50

60

70

80

90

Date [day of the year 1997]
fwd match radius <500 km
fwd/bk radius <400 km, cluster <1200 km
fwd/bk radius <500 km, cluster <1600 km

Figure 17. Time series of ozone change rates for different
criteria associated with double-sounded air parcel selection.
Solid squares show the ozone change rates as determined by
the five criteria, i.e., both forward and backward match
radius  500 km, both forward and backward cluster radius 1600 km, and PV  20% (the same as the squares in
Figure 16). Open circles depict ozone change rates ascalculated with stricter distance criteria than above: bothforward and backward match radius  400 km, bothforward and backward cluster radius  1200 km, and PV 20%. Solid diamonds give the ozone change rates ascalculated with only two criteria: the forward match radius
 500 km and PV  20%.

7 - 14

O3 change [ppbv]

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TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

Day 49 +7 at 475 K

200
0
-200
-400
-600

0

20

40

60

80

Sunlit time [hour]
fwd match radius <500 km
fwd/bk radius <500 km, cluster <1600 km
Figure 18. Differences of scatterplots for ozone mixing
ratio change between two different criteria for selecting
double-sounded air parcels. Closed circles depict the ozone
mixing ratio change as selected by the criteria of theforward/backward match radius  500 km, the forward/
backward cluster radius  1600 km, and PV  20%, andthe solid line indicates the ozone change rate derived fromthe subset of closed crosses (the same as Figure 7). Opencircles depict the additional data, which satisfy only thecriteria of the forward match radius and PV, and thedashed line indicates the ozone change rate derived from alldata points ( 3.9 ± 0.6 ppbv/sunlit hour). Conditions are
the same as in Figure 7 in every other respect.
near the vortex edge selectively, possibly because the clusterof trajectories tends to diverge near the vortex edge where
the horizontal and vertical wind shear are larger than in theinterior of the vortex. In this situation, the backward
trajectories were also hardly able to return to near the firstsoundings. The multiple trajectories thus help to identify
accurately the double-sounded air masses.
[46] As described in section 2, we eliminated the ozone
data that may have been affected by PSCs occurrence
because the ILAS gas values could have been biased evenafter applying the nongaseous correction when PSCs existed
at the same time. A total of 30 (17) double-sounded air
parcels was discarded in February (March), which are 3 – 5%of the total number of the air parcels (see Table 2). If the
double-sounded air parcels that included PSCs were added
to the analysis, the ozone change rates averaged inside theinner vortex edge became 50– 70 ppbv/day at the maximum
during late February at 450 – 500 K levels, which wereslightly smaller than the present study (50 – 80 ppbv/day).
The same analysis shows that the integrated ozone loss at
475 K during 2 months was 1.9 ± 0.1 ppmv and the column
ozone loss was 94 ± 0.3 DU. These are very slightly smaller
compared to the analysis in section 4.1 (2.0 ± 0.1 ppmv and96 ± 0.3 DU, respectively).
5.2. Relationship Between Ozone Loss and Low
Temperature[47] The 1996/1997 Arctic winter to spring polar vortex
was very strong and symmetric about the North Pole, andthe stratospheric temperature was low enough for PSCs toexist [e.g., Coy et al., 1997; Pawson and Naujokat, 1999].
Such low temperature likely forms PSCs, and then rapidly
produces reactive chlorine through heterogeneous reactions
on the surface of PSCs. The reactive chlorine destroys
catalytically ozone under sunlit conditions [Solomon etal., 1986]. The identification of PSCs from aerosol extinction coefficient profiles at 780 nm of the ILAS observations
confirmed that PSCs existed in the Arctic stratosphere
during January, February, and early March [Hayashida etal., 2000; Saitoh et al., 2002]. The large ozone loss
happened to coincide with the occurrence of PSCs [Sasano
et al., 2000] and the very low temperature that appeared on
the temperature histories (Figure 13), which supports thegeneral idea that the very low temperature and consequently
the PSCs caused the large ozone losses during early spring.
[48] The temperature history can be regarded as anindicator of PSCs formation and phase transition. PSC types
are critical to ozone losses because only solid particles cancause denitrification which prolongs the chemical ozone
destruction [e.g., Salawitch et al., 1993; Rex et al., 1997]
and because the heterogeneous reaction rates vary depending on the chemical composition of the particles [e.g., Peter,
1997]. One view of temperature history in the PSC particles
cycle is briefly as follows [Koop et al., 1997; Peter, 1997;
Larsen, 2000]: Liquid sulfate aerosol stake up nitric acidand water vapor with decreasing temperature and turn into
supercooled ternary solutions (STS) as type Ib PSCs below
TNAT. Then, water freezes below Tice, hence type II PSCs
are formed, and the nucleation of NAT takes place on the ice
by deposition. As the temperature rises above Tice, ice will
evaporate and only NAT will remain (type Ia PSC), which
will evaporate above TNAT and turn into SAT. Once SAT
particles are produced, they exist until the temperature rises
above about TSAT, or they turn into NAT and type II PSCs if
the temperature decrease again.
[49] According to this scenario, Figure 14 suggests twopoints. The first point is that type Ia PSCs could exist
between some of the two matching observations in the deeppart of the vortex (0.1 and 0.2 rPV belts), because thetemperature within 10 days prior to the matches drops near
Tice and the temperature between the matches is almost
below TNAT. Although the above mechanisms require thetemperature to be 2 – 3 K below Tice, the statistical analysis
of synoptic temperature histories of PSCs indicated thatsynoptic temperatures below Tice are not necessary for typeIa PSCs formation [Larsen et al., 1997]. Recent studies
present other NAT formation scenarios including homogeneous nucleation of NAT near 190 K [Tolbert and Toon,
2001]. Fahey et al. [2001] observed very large NAT
particles (‘‘NAT rocks’’) where no ice clouds were observedor predicted. The trajectories of NAT rocks shows that theseparticles grow above Tice by a few degrees. These studies
support that the minimum temperature near Tice within 10
days prior to the matches might be low enough for solid
PSCs to form. Actually, the ILAS nitric acid data demonstrated denitrification during mid-February to early Marchat this altitude [Kondo et al., 2000], and the magnitude of
denitrification was larger at a higher equivalent latitude withthe maximum at 80 equivalent latitude [Irie et al., 2001].
The microwave Limb Sounder (MLS) instrument on board
the UARS illustrated that enhanced ClO filled a smaller

TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

portion inside the vortex and the largest ClO values
appeared near the vortex core region around the 475 K
level [Santee et al., 1997]. These evidences strongly suggestthat the double-sounded air parcels obtained in the innermost part of the vortex were exposed to type Ia PSCs, andexperienced denitrification and extreme ozone losses.
[50] The second point is that type Ib PSCs could exist
between the two matching observations near the vortex
boundary region (0.8 and 1.0 rPV belts), because the temperature within 10 days prior to the matches experiences below
or near TNAT but never falls below Tice, and the temperature
between the matches is almost above TNAT by a few degrees.
Type Ib PSCs may cause smaller ozone loss without denitrification, compared to near the vortex core region. In the
middle part of the vortex (0.4 and 0.6 rPVs), the formation of
NAT particles between the two matching observations may
have been difficult because the minimum temperatures on
trajectories seem too high, even if homogeneous nucleation
or NAT rocks scenario is taken into consideration. The
formation of type Ib PSCs between matches may be possible
also in the middle part of the vortex.
5.3. Comparison With Other Studies[51] Several other studies investigated the stratosphericozone loss during the 1996/1997 Arctic winter, though here
we discuss mainly the results which give purely chemicalozone loss rates and amounts in 1996/1997 winter – spring.
In this study we presented two kinds of estimation forvortex-averaged ozone change rates, i.e., the average inside
the inner edge of the vortex boundary region (section 4.1)
and the average inside the vortex edge (section 4.3). Most ofother studies used the data inside the vortex edge, rather
than the inner vortex edge, for calculating vortex-averagedozone change rates. Therefore, the ozone change rates as
averaged inside the vortex edge are employed here forcomparison with other studies.
[52] Schulz et al. [2000] presented the chemical ozone loss
rates as determined by the 1996/1997 Match experiment.
The pattern of ozone changes by them is similar to the one
from the present study. They showed that the vortex-averaged ozone loss rates were from 25 to 45 ppbv/day at the
maximum, the column ozone loss (mainly from 400 to 520 K
between 1 February and 26 March) was 43 ± 9 DU, and theintegrated ozone loss was 0.9 ± 0.2 ppmv on the 450 K
surface. The ozone change rates inside the vortex edge in thisstudy are 20– 50 ppbv/day at 475 K (diamonds in Figure 12).
As for the column ozone loss, the time and range of thecolumn for integration were adjusted to those of Schulz et al.
and the column ozone loss was reevaluated from this study,
resulting in 56 ± 0.3 DU. The integrated ozone loss between
1 February and 26 March was 1.3 ± 0.1 ppmv on the 450 K
surface. These results shows that the vortex-averaged ozone
losses derived from the present study are a little larger thanthose from the sonde-Match analysis. The definition of thevortex edge (about 37 PVU at 475 K) is similar to that usedin the sonde-Match analysis (36 PVU at 475 K). The changeof ozone loss rate with the relative location to the vortex
were qualitatively consistent with each other. Their ozone
loss rate of about 6 ppbv/sunlit hour at the vortex core region(from 10 February to 10 March at 450– 500 K) quantitatively
agrees with our result in Figure 11 (from 4 February to 6
March at 475 K). On the other hand, for the other part of theILS

7 - 15vortex, the ozone loss rates in our study are higher than thatof Schulz et al.
[53] There are some differences between the ILAS-Matchand the sonde-Match not only in the data source (ozonesonde or satellite measurement) but also in the details of theanalysis method (e.g., trajectory calculation tool, match
criteria, definition of the vortex boundary, and diabatic
cooling rate). One of the reasons for the differences in thevortex-averaged ozone loss may have arisen from therelative position of the matches to the vortex. As discussed
previously, the multiple trajectory analysis in this study
selectively eliminated the unreliable matches near the vortex
edge, thus making the vortex-averaged ozone loss rate
higher. In addition, we used more matching observationsin the deep part of the vortex than the sonde-Match analysis(compare Figure 8 and Figure 10 in this paper with Figure 3
in the work of Schulz et al. [2000]) to calculate the vortexaveraged ozone loss rate. In this study a lot of sampling
existed near the vortex core region during February andMarch, while for the sonde-Match analysis only few
matches were obtained in the vortex core region during
March. In fact, the ozone loss rates, as calculated inside thevortex edge but without near the vortex core region (i.e.,from 1.0 to 0.4 rPVs, which is a similar range used in thesonde-Match), are 15 – 20 ppbv/day in March, which iscomparable to the sonde-Match results. Another possible
factor for the differences between the ILAS-Match and thesonde-Match approach is solar zenith angles (SZAs) on
trajectories. The differences in the geographical location of
the trajectories may produce differences in the SZAs andalso in the photolysis rates.
[54] Müller et al. [1997] identified chemical ozone
depletion using the correlation between ozone and longlived tracer gases observed by HALOE on UARS, andestimated the column ozone loss as 50– 80 DU between
350 and 550 K, varying with the location inside the vortex.
When we put an upper limit of the vertical integration to550 K and a lower limit to 375 K in our analysis, thecolumn ozone loss becomes 81 ± 0.4 DU, which is close to
the result by Müller et al. They also suggested that the larger
ozone losses tended to occur toward the vortex interior. Thisin homogeneity is qualitatively consistent with our results.
[55] Manney et al. [1997] used MLS observations andtransport estimations to deduce the degree of chemicalozone depletion. The ozone losses between 20 and 26
February were 1.3%/day (34 ppbv/day) at 465 K. Theirresult is in good agreement with the vortex-averaged ozone
loss rates of 32 ppbv/day at 465 K from our analysis, asaveraged over the same period.
[56] Knudsen et al. [1998] calculated vortex-averagedozone depletions on several potential temperature surfaces
using ozonesonde data and reverse domain-filling trajectory
calculations, taking the effect of transport into account. The
integrated chemical ozone loss from 30 January to 31 March
was 1.0 ppmv at 400 K and 1.3 ppmv at 450 K. The
integrated ozone loss in the present study, 1.1 ppmv at
400 K and 1.6 ppmv at 450K, is slightly higher than the
results of Knudsen et al., possibly because of the differencein the distribution of the sounding location between theozonesonde and the ILAS. Their average column ozone lossof 79 DU (from 375 to 550 K) were comparable to our
results, 81 ± 0.4 DU (the same vertical region).

ILS

7 - 16TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH[57] Braathen et al. [2000] also calculated ozone depletion from the time series of the ozone mixing ratio observedby the ozonesonde, which was corrected by diabatic coolingrate but ignoring the transport effects. The local ozone
change rates were 15– 30 ppbv/day during late February
and early March at 475 K, and the integrated ozone depletion
was about 1.6 ppmv during 2 months. Using the same
method as Braathen et al. [2000], the Global Ozone Monitoring Experiment (GOME) ozone profiles indicated ozone
depletion of about 1.5 ppmv during the same period and at
the isentropic surface [Bramstedt et al., 2000]. Although thetechnique used in these studies could not derive purely
chemically induced ozone loss because dynamical ozone
changes caused by the horizontal advection of air masses
were ignored, these estimations give relatively similar ozone
losses to the present study.
[58] Three-dimensional Chemistry Transport Models(CTM) also provided estimates of the Arctic ozone loss.
The integrated ozone loss as simulated by SLIMCAT [Guirlet et al., 2000] is 36% at 480 K on 31 March over north of75N equivalent latitude, which underestimates the integrated ozone loss averaged inside the inner vortex edge
(north of 70N equivalent latitude), 55% at 475 K in thepresent study. As for the column ozone loss, our result iswithin the estimations by the Reprobus CTM, which varyfrom 60 to 120 DU loss with the location inside the vortex[Lefèvre et al., 1998].

6. Summary[59] In this paper we evaluated quantitative chemicalozone loss rates and amounts in the Arctic stratosphericvortex for the spring of 1997 using the ILAS newest ozone
profile data (Version 5.20) and the Satellite-Match analysis.
[60] The analysis method is an extension of the Matchtechnique. The advantages of using the ILAS data are that
the ILAS measurements intensively cover all longitudinal
regions in the high latitude zone, ILAS is performed daily allwith the same quality, and ILAS provides not only ozone
data but also data on nitric acid and passive tracers so thatfurther comprehensive analysis [Terao et al., 2000] is
possible. In order to overcome the disadvantages of satellite
sensor data, low vertical resolution and larger sampling air
mass volume, the analysis method was improved and very
strict criteria assuring conservation of air masses wereintroduced to identify a double-sounded air mass morereliably. For this purpose, we calculated multiple trajectories
for each ILAS observation, including central forward/backward trajectories and a cluster of four forward/backward
trajectories (Figures 2, 4 and 5). Based on the multiple
trajectory analysis, several hundred double-sounded air
parcels were identified between 375 and 600 K potential
temperature surfaces during February and March (Table 2).
We statistically calculated chemical ozone change rates byregression analysis on the subset of double-sounded air
parcels taking into account the diabatic descent. The statistical uncertainty of ozone changes was estimated to beabout 7% in this analysis. In the analysis the derived ozone
change rates were less sensitive to uncertainties in diabatic
descent rate.
[61] Using the double-sounded air parcels obtained inside
the inner edge of the vortex boundary (corresponding tonorth of about 70N equivalent latitude), vortex-averagedozone loss rates were evaluated (Figure 9). The result showsthat the local maximum ozone loss rate was 50– 80 ppbv/day
during late February at 450 – 500 K levels. The integrated
ozone loss during February and March was 2.0 ± 0.1 ppmv at475– 529 K levels, and the column ozone loss between 400
and 600 K during the 2 months was 96 ± 0.3 DU. We alsostudied the relationship between the ozone loss rate and thelocation relative to the vortex (Figure 11). The ozone change
rates as a function of rPV demonstrated that the ozone loss
increased toward the vortex center with a step-like increase
at 0.2 rPV (about 80N equivalent latitude). The magnitudeof ozone loss is strongly dependent on relative location to thevortex in this winter. The higher ozone loss in the deeper partof the vortex is consistent with other studies [Müller et al.,
1997; Manney et al., 1997; Lefèvre et al., 1998; Guirlet et al.,
2000; Schulz et al., 2000]. The maximum ozone loss rate of
6.0 ± 0.6 ppbv/sunlit hour near the vortex core region was also
quantitatively in agreement with the sonde-Match analysis.
[62] When we expanded the area of interest to include all
the data obtained inside the vortex edge (north of about 65
N equivalent latitude), the local ozone loss rate was about
50 ppbv/day at the maximum. The integrated ozone change
during 2 months is 1.6 ± 0.1 ppmv on the 475 – 529 K
surfaces and the column ozone loss between 400 and 600 K
during the 2 months is 72 ± 0.4 DU. The vortex-averagedozone loss inside the vortex edge is in agreement with thoseof several other studies [Manney et al., 1997; Müller et al.,
1997; Knudsen et al., 1998; Lefèvre et al., 1998], but isslightly larger than the 96/97 Match results [Schulz et al.,2000] and our previous analysis [Sasano et al., 2000] by
about 10 ppbv/day. One of the reasons for the differences inthe vortex-averaged ozone loss may have arisen from therelative position of the matches to the vortex. The multiple
trajectory analysis selectively eliminated the case of matching observations near the vortex edge. Thus the vortexaveraged ozone loss rate as determined in this study likely
reflects the higher local ozone loss deep inside the vortex. In
addition, we obtained more matching observations in the
deep part of the vortex in March than the sonde-Matchanalysis. In fact, the ozone loss rates, as calculated in asimilar vortex range used in the sonde-Match, are comparable to the sonde-Match results.
[63] Temperature histories of a corresponding doublesounded air parcel indicated that large ozone losses in the
innermost part of the vortex were observed when the airparcels reached a temperature below TNAT between the two
matching soundings and had experienced a temperature near
Tice within the 10 days prior to the soundings. Considering
the phase transition cycle of PSCs [Koop et al., 1997; Peter,
1997; Larsen, 2000; Fahey et al., 2001; Tolbert and Toon,
2001], this temperature history suggests that type Ia PSCs
could have existed deep inside the vortex during the period
when the extremely large ozone loss occurred.
[64] Acknowledgments. The authors express their sincere thanks to
Richard Swinbank for supplying the UKMO assimilation data and trajectory programs, Bjørn M. Knudsen for providing his diabatic cooling ratedata, and Naoko Saitoh and Sachiko Hayashida for providing their PSCs
analysis data. We are also grateful to Peter von der Gathen, Takafumi
Sugita, and the anonymous referees for their helpful comments anddiscussions. The ILAS data were processed at the ILAS/ILAS-II Data
Handling Facility (DHF), National Institute for Environmental Studies

TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH(NIES). The ILAS data were made available by the tremendous efforts of
the ILAS project staff, to whom the authors are greatly obliged.

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H. L. Tanaka, Y. Terao, and T. Yasunari, Institute of Geoscience,
University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan. (terao@luft.geo.
tsukuba.ac.jp)
H. Nakajima and Y. Sasano, National Institute for Environmental Studies,
Tsukuba, Ibaraki 305-0053, Japan.

TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

Forward Trajectories

Backward Trajectories

Trajectories between 97/2/12 16UT and 97/2/20 22UT
Center trajectory (475 K)
Upper trajectories (493 K)
Location #1
Location #2
Lower trajectories (457 K)

Figure 4. Example of a cluster of forward (left) and backward (right) trajectories between 12 and 20
February 1997. A thick black curve with an arrow shows a center trajectory on the 475 K potential
temperature surface. Blue and red trajectories are at the upper level (493 K) and the lower level (457 K),
respectively, and small solid circles show the end of each trajectory.

Forward Trajectories

Backward Trajectories

Trajectories between 97/3/7 20UT and 97/3/14 14UTCenter trajectory (475 K)
Upper trajectories (493 K)
Location #1
Location #2
Lower trajectories (457 K)

Figure 5. Same as in Figure 4, but between 7 and 14 March 1997.

ILS

7 - 5 and ILS

7- 6

TERAO ET AL.: 1996/1997 ARCTIC WINTER OZONE LOSS EVALUATED BY ILAS-MATCH

Column loss Potential temperature [K][DU/day][ppbv/day]

O3 change rate

-80 -40 -20 0

600

20 40

Integrated loss
600

550

550

500

500

450

450

400

(a)

0
-1
-2
-3
(c)
-4
30

(b)
-2

400
-1

0

[ppmv]

40

50

60

70

80

90

Date [day of the year 1997]
Figure 9. (a) Color-coded ozone change rates (in ppbv per day) inside the inner edge of the vortex
boundary (north of about 70 N equivalent latitude) as a function of potential temperature and date.
Vertical bars indicate the region with statistical significance less than 99%. Smooth thin curves show
potential temperature changes of air parcels (diabatic descent of air masses) (adopted from the work ofKnudsen et al. [1998]). (b) Integrated ozone changes from 30 January (day 30) to 31 March (day 90)
along the descending motion due to diabatic cooling. For the integration, ozone change rates arevertically interpolated by a cubic spline. For the uppermost two levels, days 51– 90 and days 37– 90 areused for the integration, respectively. Error bars represent 1 sigma. (c) Ozone column change rates (in DU
per day), which are obtained by integrating local ozone change rates (in number density per day) from
400 to 600 K. Results are plotted every 5 days.

edge 1.0

Relative PV

0.8
0.6
0.4
0.2
core 0.0
30

40

50

60

70

80

90

Date [day of the year 1997]
Figure 10. Location of double-sounded air parcels obtained in each rPV belt. In the rPV scale, 0 refers
to the center of the vortex where the PV value takes the maximum and 1 refers to the vortex edge (seetext). Red, magenta, orange, green, cyan, and blue indicate the double-sounded air parcels in the rPVbelts of 0.0– 0.2, 0.0– 0.4, 0.2 – 0.6, 0.4 –0.8, 0.6– 1.0, and 0.8 – 1.2, respectively.

ILS

7 - 8 and ILS

7- 9