1

Comparison of Empirically Derived Ozone Losses in the Arctic Vortex

N.R.P. Harris1 , M. Rex 2 , F. Goutail3 , B.M. Knudsen4 , G.L. Manney5 , R. Müller6 , and P.
von der Gathen2

1. European Ozone Research Coordinating Unit, Centre for Atmospheric Science,
University of Cambridge, Cambridge, UK
2. Alfred Wegener Institute, Potsdam, Germany
3. Service d'Aéronomie, Centre National de la Recherche Scientifique, Verrières le
Buisson, Paris, France
4. Danish Meteorological Institute, Copenhagen, Denmark
5. Jet Propulsion Laboratory, California Institute of Technology, CA, USA
6. Forschungzentrum Jülich, Jülich, Germany

Submitted:

February 2nd 2001

Revised:

August 17th 2001

Accepted:

August 22nd 2001

Short title: Comparison of Ozone Losses in the Arctic Vortex

SUBMITTED FOR PUBLICATION IN THE
FIRST SOLVE – THESEO 2000 SPECIAL SECTION

GAP numbers: 0300, 0340

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Abstract
A number of studies have reported empirical estimates of ozone loss in the Arctic vortex.
They have used satellite and in situ measurements and have principally covered the Arctic
winters in the 1990s. While there is qualitative consistency between the patterns of ozone
loss, a quantitative comparison of the published values shows apparent disagreements. In
this paper we examine these disagreements in more detail. We choose to concentrate on the
five main techniques (Match, SAOZ/REPROBUS, MLS, vortex average descent and the
HALOE ozone-tracer approach). Estimates of the ozone losses in three winters (1994/95,
1995/96 and 1996/97) are re-calculated so that the same time periods, altitude ranges and
definitions of the Arctic vortex are used. This recalculation reveals a remarkably good
agreement between the various estimates. For example, a superficial comparison of results
from Match and from MLS indicates a big discrepancy (2.0 ± 0.3 ppmv and 0.85 ppmv
respectively, for air ending at ~460K in March 1995).

However the more precise

comparisons presented here reveal good agreement for the individual MLS periods (0.5 ±
0.1 vs 0.5 ppmv; 0.4 ± 0.2 vs 0.3-0.4 ppmv; and 0.16 ± 0.09 ppmv vs no signficant loss).
Initial comparisons of the column losses derived for 1999/2000 also show good agreement
with four techniques giving 105 DU (SAOZ/REPROBUS), 80 DU (380-700K partial
column from POAM/REPROBUS), 85 ± 10 DU (HALOE ozone-tracer) and 88 ± 13 (400580 partial column from Match). There are some remaining discrepancies with ozone losses
calculated using HALOE ozone-tracer relations: it is important to ensure that the initial
relation is truly representative of the vortex prior to the period of ozone loss.

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Introduction
Long-term decreases in total ozone have been observed at high and mid-latitudes of both
hemispheres [e.g., Staehelin et al., 2001, and references therein]. The most dramatic losses
are observed over Antarctica during each Austral spring – in recent years, over 99% of the
ozone is removed from altitudes between 15 and 19 km during a period from mid August to
October. Total ozone measured at Halley Bay, Antarctica (69˚S) in October averaged 160
Dobson Units in the 1990s, 40% that of the long-term average [Jones and Shanklin, 1995].

The information about the long-term evolution of the ozone layer over the Arctic is less good
than over the Antarctic, a situation brought about by the lack of long-term measurements
within the Arctic circle (there is very little land mass there) and the larger interannual and
intraseasonal variability. However unusually low total ozone was observed in several Arctic
winters in the 1990s. For instance, low values of total ozone (up to 45% below the 19571992 average) were observed over the Canadian High Arctic (north of 70°N) in March 1996
and 1997 [Fioletov et al., 1997]. However, such low values are not seen in all Arctic
winters, as is now the case in the Antarctic, and unusually low values were also observed in
winters (e.g., 1967) during which chemical effects are expected to have been negligible
[Fioletov et al., 1997]. It is thus apparent that low ozone amounts in the Arctic result from
both chemical and dynamic factors which vary greatly from winter to winter, and whose
magnitude and relative importance are hard to quantify.

The first quantitative estimates of widespread chemical ozone loss in the Arctic lower
stratospheric vortex were for the 1988/89 winter [Proffitt et al., 1990; Schoeberl et al.,
1990]. Since then a number of studies have evaluated the chemical ozone loss that has
occurred in particular winters with increasing emphasis on its quantification. For the sake
of clarity, we use the term ‘ozone loss’ to mean a chemical change that has occurred in
defined air masses over a period of days or weeks, and the term ‘ozone trend’ to describe

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the long-term changes that have been observed over recent decades – of whatever cause.
These two terms are frequently used interchangeably and it seems useful to draw a
distinction. The results of studies based primarily on measurements that have produced
empirical estimates of the ozone loss have been quite diverse. It is hard to compare them.
In general (though not always) it seems that the empirical methods produce higher ozone
loss rates in the Arctic vortex than do the photochemical models [e.g., Becker et al., 1998,
2000; Guirlet et al., 2000; Kilbane-Dawe et al., 2001; Sinnhuber et al., 2000]. Given this
discrepancy, it is important to know how well the various empirical approaches agree.

Arctic vortex
The main feature of the Arctic winter stratosphere is the Arctic vortex, which, in conjunction
with the Aleutian high, dominates the high latitude circulation from November to March
[e.g., Schoeberl et al., 1992; Manney et al., 1995a]. The vortex forms in autumn as the
stratosphere cools radiatively. Air moving poleward is acted on by the Coriolis force and an
increasingly strong westerly jet stream develops.

The vortex extends from the lower

stratosphere to the mesosphere. The average temperatures inside the vortex are significantly
lower than in the surrounding air, but on any given day the lowest temperatures can either
be inside or straddle the edge of the vortex.

During each Arctic winter, the vortex is buffeted by a series of stratospheric sudden
warmings associated with planetary-scale waves which originate in the troposphere. During
these events the vortex is displaced off the pole, often by considerable amounts. In the case
of major warmings, it may break down temporarily resulting in a brief period of polar zonal
mean easterlies during winter. The final, springtime vortex breakdown in the Arctic is much
earlier than in the Antarctic, and it is often precipitated by wintertime warmings. The area of
the Arctic vortex, the strength of the sudden warmings and the timing of the final
breakdown vary enormously from year to year.

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The edge of the vortex can be defined using potential vorticity (PV). In ozone loss studies a
PV value typical of the edge region is often used, as this is found in practice to give
equivalent results to using the more general maximum gradient in PV. As is shown later, it
is important to be careful in defining the vortex edge when comparing empirically derived
ozone losses. PV is measured in PV units (1 PVU = 10-4 km 2 kg s -1) and the vortex edge
has to be defined by different PV values on different isentropic surfaces.

To make

comparison between different levels easier, the concepts of scaled and normalised PV are
used in studies described here. Scaled PV (defined as PV/g(dϑ/dp), where dϑ/dp is a
standard atmosphere value) is a relatively height-independent quantity. Normalised PV
(defined as 2.65 x 105 scaled PV) is also height-independent and is additionally chosen to
have the same numeric values (in s-1) as Ertel’s PV (in PVU) on the 475 K surface (see Rex
et al. [1999] for more detailed discussion).

The wave activity causes the Arctic vortex to be noticeably warmer and more dynamically
variable than its Antarctic counterpart. On average the vortex temperatures in the Arctic are
10-20 K higher than in the Antarctic [e.g., Manney et al., 1996a].

There is a large

variability in Arctic ozone on both short (day-to-day) and long (year-to-year) time-scales.
This variability in ozone is caused by the variability in the transport of air in the
stratosphere, in the tropospheric forcing, and by variations in the chemical ozone loss.

The chemical processes causing ozone loss in the Arctic lower stratosphere depend critically
on temperature, and they are basically the same as those causing the ozone loss in the
Antarctic (e.g., Ravishankara and Shepherd in WMO [1999] and references therein). At
temperatures below about 190-195K, heterogeneous chemical reactions can occur on the
surfaces of cold particles such as Polar Stratospheric Clouds (PSCs) that convert inactive
inorganic chlorine-containing species such as ClONO2 and HCl to active forms that can
catalytically remove ozone. The amount of ozone destroyed during each activation episode
is controlled by factors such as the size and location of the cold areas, the amount of

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sunlight present to drive the ozone loss catalytic cycles, and the rate of the subsequent
deactivation through reaction of ClO with NO2 to form ClONO2 . Deactivation is slower in
mid-winter when ambient NO2 concentrations are low (because the photolysis of HNO3 and
its reaction with OH are slow) or in air where HNO3 has been permanently removed through
sedimentation of particles (‘denitrified’).

The variability in the meteorology of the Arctic vortex has important chemical consequences.
The area in which the temperature is below that at which PSCs can potentially form varies a
great deal from year to year [Pawson and Naujokat, 1997, 1999]. Further, the timing of the
cold periods, the position of the cold areas within the vortex, and the position of the vortex
when they occur determine the amount of sunlight available to drive the chemical ozone loss
and the volume of air processed through cold regions where chlorine can be activated. The
combination of these factors means that the highly variable Arctic meteorology causes large
year-to-year variability in the amount of chemical ozone loss. In general more chemical
ozone loss is expected in colder winters than in warmer winters.

This variability underlies how important it is to know how much ozone there would have
been if no chemical ozone loss had taken place. The problem is complicated by the fact that
the magnitudes of the ozone changes resulting from the dynamic variability, the chemical
ozone loss, and the underlying seasonal cycle are similar. Studies of ozone loss need to be
able to separate these effects.

Methods of Estimating Ozone Loss
Empirical estimates of ozone loss in the Arctic vortex have been reported in a range of
studies based on a few measurements at a single site in one year to many measurements
made at many locations in several winters. In this section we describe the main types of
approach that have been used to estimate chemical ozone loss, concentrating on the five
techniques that have been used in several winters.

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The movement of air masses is clearly the most important natural influence on the ozone
fields at high northern latitudes. Measurement-based estimates of ozone loss must be able to
distinguish between these motions and photochemical loss. The fastest air motions are
horizontal and often reach 100 ms-1. On time-scales of a few days, these horizontal motions
occur on isentropic surfaces and are well captured in standard meteorological analyses. The
slower motion is vertical: local vertical velocities can be 1 ms-1, while the slow radiative
cooling of the air in the vortex (1 K/day), the diabatic descent, is equivalent to a downward
velocity of the order of 0.1 cm s-1.

These need to be taken into account in empirical

calculations of ozone loss.

Broadly speaking the various approaches can be split into two categories:
1)

studies where the effects of transport are calculated explicitly using transport
calculations driven by winds, temperatures, etc., based on meteorological analyses;
and

2)

studies where the effects of transport are implicitly allowed for by using
measurements of long-lived tracers.

These are now considered in turn.

Match
The Match technique has been developed to estimate chemical ozone loss rates in the lower
stratosphere as directly as possible [von der Gathen et al., 1995; Rex et al., 1997, 1998,
1999, 2001; Schulz et al., 2000, 2001]. Results from a similar approach using ozone
measurements by the ILAS satellite instrument have been reported for the 1996/97 winter
[Sasano et al., 2000]. Match is a pseudo-Lagrangian technique based on the identification
of air parcels whose ozone amount is measured twice within a ten day period.

Any

difference is ascribed to chemical loss. In practice a large number of such ‘matches’ are
required to obtain statistically significant results. The identification of these air masses is

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achieved by calculating 3D air mass trajectories with the horizontal (isentropic) component
based on winds from ECMWF analyses and with the vertical (cross-isentropic) motions
derived from the radiative cooling rates calculated by the SLIMCAT chemical transport
model (CTM) [Chipperfield, 1999] using the MIDRAD radiation scheme [Shine, 1987] with
ECMWF wind and temperature analyses, and climatological ozone values. During Match
campaigns, forward trajectories are calculated from ozone sonde flights until that air mass
can be sampled by a second ozone sonde (up to a limit of 10 days). The average length of a
Match trajectory is 5-6 days. An air mass is considered to be successfully intercepted if the
displacement between the end of a trajectory and the location of the second intercepting
ozone sonde is less than about 500 km. By performing a large number of such matched
measurements over a range of altitudes during a winter, the evolution of the vortex average
ozone loss can be reconstructed with high vertical (10 K or better) and temporal (2 week)
resolution.

The air masses inside the polar vortex can experience substantial strain and stirring. Match
depends on identifying those air masses that are well conserved, entailing a careful selection
procedure [Rex et al., 1999]. The criteria used include rejecting ozone sonde profiles which
show large changes over short vertical intervals; accepting limited dispersion within a cluster
of 6 companion trajectories calculated above, below and to each side of the central trajectory;
and limiting the PV variation along the trajectories. In order to determine whether a
measurement takes place inside or outside the vortex, a normalised PV is calculated along
the air mass trajectories. If it exceeds 36 PV units, the measurement is considered to have
been inside the vortex.

In each winter, the Match trajectories are examined to check that the vortex is sampled
homogeneously, so that the results reflect vortex averaged conditions. Ozone loss can occur
preferentially in some regions inside the vortex depending on the radiation and temperature
fields during a given winter. For example, in the 1996/97 winter the ozone loss rates were

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greater towards the centre of the vortex due to greater occurrence of low temperatures in this
region [Schulz et al., 2000].

MLS
Manney et al. [1996b] analysed the ozone loss during some time periods in winter 1994/95
by using MLS measurements of ozone and trajectory calculations to account for transport
effects. Horizontal winds from the UKMO data assimilation system and vertical velocities
from the radiation code MIDRAD driven by UKMO temperatures are used for the trajectory
calculations. Thus the MLS and Match approaches use the same radiation code, but with
different temperatures driving it. Reverse trajectory calculations are started at all points on
the gridded MLS data between 465 and 840K and run backward in time for a few weeks.
Previous MLS measurements are interpolated to the starting points of these trajectories.
This approach has been applied to measurements of long-lived tracers (N2 O and CH4 ) and
of ozone. Manney et al. [1995c] show that the advected tracer fields agree well with the
observations, suggesting that the trajectory calculations are sufficiently precise.

Hence,

differences in ozone are attributed to chemistry. The differences in ozone are then averaged
over the polar vortex, which is defined by the 1.3 x 10-4 s -1 isoline of scaled PV (= 34 s-1
normalized PV), so that the results represent the vortex averaged ozone loss, even if the
ozone vmr or the ozone loss within the vortex is not homogenous. The same method has
been used to infer chemical ozone losses for some periods during the 1993/94 [Manney et
al., 1995a], 1995/96 [Manney et al., 1996a] and 1996/97 [Manney et al., 1997] winters.
Both this approach and one that is only slightly different (based on forward trajectories from
a regular grid and interpolation of the MLS measurements onto this grid) were used for
periods during the 1992/93 winter [Manney et al., 1995b].

Vortex average
A similar conceptual approach based on ozonesonde measurements was used for early 1997
by Knudsen et al. [1998] and for 1991/92 by Lucic et al. [1999]. Knudsen et al. calculate

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the trajectories on isentropic surfaces (350-675K) from ECMWF analyses. However rather
than following the large number of individual trajectories, Knudsen et al. [1998] calculate
the bulk vertical advection of the average ozone profile in the vortex from diabatic cooling
rates calculated using the ECMWF operational heating scheme with ozone mapped in PVtheta space and a constant 5 ppmv H2 O. Transport of ozone across the vortex edge through
mixing is calculated by tracking the trajectories in this region. The vortex edge is defined as
the maximum in the PV gradient with respect to equivalent latitude, smoothed over 3 days.

Observed vs CTM-calculated ‘passive’ ozone
3D transport models driven by meteorological analyses can be used to simulate what would
happen to ozone if no chemical loss took place (passive ozone). The difference between
measurements and the simulated passive ozone can be ascribed to chemical ozone loss. The
3D circulation model used in many of these studies is the REPROBUS (Reactive Processes
Ruling the Ozone Budget in the Stratosphere) CTM [Lefèvre et al., 1994; Lefèvre et al.,
1998]. It extends from the ground up to 10 hPa, with a horizontal resolution of 2° latitude
by 2° longitude. REPROBUS is driven by the 6-hourly ECMWF meteorological analyses.
Vertical motions are calculated directly from the analysed ECMWF residual vertical velocity
fields.

It is initialised in December of each winter, the exact date depending on the

formation of the vortex and the first cold period where activation could take place. The
ozone field is initialised using MLS data, usually version 4 (as used in Manney et al.
[1996b]). Comparisons with long-lived tracer measurements performed in the Arctic in
1995 [Goutail et al., 1999] and 1997 [A. Engel, personal communication] have shown that
despite the upper limit at 10 hPa, diabatic descent was reasonably well reproduced by the
ECMWF analysis and the semi-lagrangian transport scheme of REPROBUS for a 3-4
month period.

This approach has been used by Goutail et al. [1999] using the high latitude measurements
from the ground-based and balloon-borne SAOZ instruments which are inside the vortex at

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475K (PV > 42 PVU) and by Deniel et al. [1998] using ozone profile measurements from
POAM. The ozone losses derived from the two data sets are consistent. The principal
diagnostic of ozone loss produced using this technique is the vortex-averaged, column
ozone loss from the beginning of the integration, with the results usually presented as
percentage column losses. Vertical profiles of the ozone loss are derived using the POAM
and SAOZ balloon data.

A similar approach has been applied for ozone profile

measurements made at a single site [Hansen et al., 1997]. This is comparable to vortex
average studies as long as sufficient measurements are made inside the vortex as it moves
around to ensure that it is well sampled, or that the vortex is homogeneous.

HALOE tracer relations
The final technique considered here removes the effects of transport implicitly by the use of
tracers measured by HALOE [Müller et al., 1996, 1997ab, 1999, 2001]. In a homogeneous
air mass, a non-linear compact relation between ozone and an inert tracer will only change if
there is mixing with other air masses with different compositions or if there is any chemical
change in the ozone (see Müller et al. [2001] for full discussion). In the absence of mixing
into the vortex, any change in the relation between ozone and an inert tracer inside the vortex
can be ascribed to a chemical change in ozone. Initially this idea was used by Proffitt et al.
[1990, 1993], and it has been developed by Müller et al. [1996, 1997a,b, 1999] using
HALOE measurements of O3 , CH4 and HF to estimate ozone loss in several winters. There
is limited sampling of the vortex during early and mid winter as HALOE is a solar
occultation instrument and UARS has two measurement modes and covers the northern high
latitudes every other 35 days.

One potential confusion in the published results arises from the revision of the HALOE data
from version 17 to version 18 that affected the results of the early analyses [Müller et al.,
1996]. A reanalysis of the derived ozone loss for the first four winters based on version 18
data yielded larger calculated ozone losses than for version 17 data [Müller et al., 1997a].

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The reason for this is a combination of several smaller changes in the HALOE data, which
sum up to the observed effect. First, O3 mixing ratios are higher in version 18.

The

increase is larger at higher altitudes and for larger O3 mixing ratios, so that the estimated
ozone loss increases. Further, CH4 mixing ratios at lower altitudes (pressures greater than
about 50-70 hPa) have decreased from versions 17 to 18, which also leads to an increase in
the calculated ozone loss. Both effects are responsible for the fact that the ozone loss
calculated previously from version 17 data was underestimated [Müller et al., 1999]. The
current version of HALOE data is version 19. Owing to the much smaller change of the
HALOE O3 and CH4 data between versions 18 and 19 than between versions 17 and 18, the
ozone losses derived from the version 19 data are likely to deviate only slightly from losses
deduced from version 18 data. Preliminary analysis of the 1996/97 winter with version 19
data indicates that somewhat lower column losses will be derived than from version 18 data.

Comparisons of results
Comparisons of the results from the different approaches used to infer ozone loss in the
Arctic are hampered by the fact that the altitude range, horizontal extent (vortex definition)
and time periods used by the published works are different. These differences are partly
unavoidable due to the constraints of the data sets used.

However in many cases the

different data sets can be reanalysed for certain time periods and regions where they overlap.
Results from these reanalyses can be compared directly. Comparisons between pairs of
techniques which explicitly allow for transport are presented first, and these are then
compared to the tracer correlation technique where transport is implicitly accounted for.

Match - MLS
An example of how the relatively small differences in analysis criteria can affect the derived
ozone losses is shown in Figure 1.

For the winter 1994/95, Manney et al. [1996b]

calculated the accumulated ozone losses in subsiding layers of air for three different time
periods (heavy black lines in Figure 1). In Rex et al. [1999] the ozone loss rates from the

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Match study were accumulated in several subsiding layers of air over one long time period
(dashed thin black lines in Figure 1). At first glance a comparison of loss rates seems
possible as one of the layers analysed in the original Match study (heavy blue line in Figure
1) is close to the three layers analysed by Manney et al. [1996b]. However, the vertical
offset in the region of large vertical gradients in the ozone loss rate prevents a meaningful
comparison of the results. Furthermore in the MLS analysis the vortex edge is defined 34 s1

nPV (given as 1.2 x 10-4 s -1 scaled PV in the paper), whereas the 36 s-1 isoline of nPV is

used in the Match study. If one were to ignore these facts and compare the techniques by
combining the MLS results to cover one long time period, one would find that Match gives
an accumulated loss of 2.0 ± 0.3 ppmv in that layer [Rex et al., 1999], while the combined
loss for the three MLS periods is 0.85 ppmv [Manney et al., 1996b]. This superficial
comparison indicates, wrongly, that there are considerable discrepancies between the
published results from the two techniques.

Here we present results from a reanalysis of the Match data set using the same definition of
the vortex edge, the same time periods and the same altitude regions which were used in
Manney et al. [1996b]. The results of the reanalysis of the Match data and the published
results from MLS are given in Table 1 and shown in Figure 2. Three time periods are
compared for the 1994/95 winter. For the latter two periods Match has been analysed for
exactly the same periods of time as in the MLS study and the results agree within the
experimental uncertainty. The overlap during the first period is not perfect: the starting date
for the first period in the MLS study is dictated by the date when the MLS instrument turned
south on 31 December 1994, while the Match results are available from 1 January 1995 on.
However due to the lack of sunlight during mid-winter it is not very likely that a significant
fraction of the accumulated ozone loss in the whole MLS period (21 December 1994 to 1
February 1995) occurred during the first nine days that are not included in the Match study.
This supposition is consistent with very low ozone loss rates found by Match in early
January, as shown in Figure 1. Overall the results from the MLS and the Match analyses

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agree well for the 1994/95 winter.

A similar procedure has been performed to compare the Match and MLS results for the
1995/96 and 1996/97 winters, with the Match results being reanalysed to match the time
periods, vertical regions and vortex edge definitions in which results are available from the
MLS study. The results of these reanalyses are shown with the MLS results in Figure 2 and
in Tables 2 and 3. The agreement for the 1995/96 winter when the MLS criteria could be
used exactly is very good. However for 1996/97 exactly the same criteria could not be
used. The time period used in the MLS study is from 26 February to 12 April 1997, with
the end date dictated by the start of the north viewing period of MLS. The Match results are
only available until 31 March 1997. Thus the last 12 days in the MLS study cannot be
included in the reanalysis of the Match results. However, the Match results for that winter
suggest that the ozone loss rates declined considerably by 31 March 1997 [Schulz et al.,
2000], so that the additional ozone loss in the last 12 days of the MLS study was probably
small. The good agreement between the ozone losses in Table 1 can be reasonably expected
to apply to the longer MLS period.

Match - vortex average ozonesondes
Estimates of the ozone loss in the 1996/97 winter have also been made using vortex
averaged ozone mixing ratios from ozonesondes coupled with isentropic trajectories with
adjustments for diabatic descent and mixing [Knudsen et al., 1998]. The Match results have
been reanalysed to cover the same time period. The same definition for the vortex edge
could not be used as Knudsen used the maximum gradient in the PV fields while in Match
the 36 s-1 isoline of nPV defines the vortex edge.

However during the time period

considered, the two definitions are found to be nearly identical.

The results of the

comparisons are given in Table 3 and Figure 2, and the agreement is generally good. The
Knudsen et al. study reports ozone loss which is slightly, but not significantly, higher than
the results from Match. These slight differences can be explained by differences in the

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diabatic cooling rates used to adjust for vertical transport. The accumulated ozone losses
that result from applying the diabatic rates used in Match to the Knudsen study are shown in
parenthesis in Table 3, and there is better agreement with the Match results.

Lucic et al. [1999] developed a similar technique (but with vortex averaged ozone mixing
ratios on isentropic surfaces) using ozonesonde measurements made in the 1991/92 winter.
The vortex was stable for the first three weeks of January 1992 after which there was a
major disturbance and in-flow of air from mid-latitudes. Their analysis of ozone loss was
thus limited to the first three weeks of January during which they report an ozone loss of
0.32 ± 0.15 ppmv between 475 and 550K. When the same time period is considered, the
ozone loss derived from Match is 0.3 ± 0.2 ppmv in good agreement with their results [Rex
et al., 1999]. The larger losses by Match for all of January [von der Gathen et al., 1995]
result from the significantly faster losses in the last part of January compared to the first
part. The altitude distribution of the ozone loss found by the two approaches is also similar,
with a small, statistically insignificant loss at 550K.

MLS - vortex average ozonesondes
The results from MLS [Manney et al., 1997] and the vortex averaged ozone [Knudsen et
al., 1998] approaches have also been compared for 1996/97 and good agreement was found
(Table 3 and Figure 2) despite the difference in the definition of the vortex edge.

Match – SAOZ/REPROBUS
Goutail et al. [1999] calculated the accumulated column ozone loss over the 1994/95 winter,
using the 42 PVU isoline of potential vorticity at 475 K potential temperature as the
definition of the vortex edge. This analysis has been repeated to match the time period and
vortex definition of the Match study. For the period from 1 January to 31 March 1995, the
total column loss derived from SAOZ is 117 DU while the partial column (380-600K) loss
derived from Match is 127 ± 14 DU. The agreement is good, well within the estimated

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uncertainty, and is consistent with the underlying assumption that the chemical ozone loss
all occurred within the partial column covered by Match. Apart from 1994/95, the only
winter where there is a sufficient number of individual matches to calculate column ozone
losses from Match is 1999/2000 [Rex et al., 2001]. Analysis shows similarly consistent
results in that winter, with SAOZ/REPROBUS finding an accumulate loss of 105 DU for
the whole column between 2 January and 25 March 2000, as against 88 ± 13 DU for the
partial column found by Match (Figure 3 and Table 4).

When POAM profile data for

1999/2000 are used in conjunction with the REPROBUS passive ozone instead of SAOZ
data, a loss of 80 DU is found for the partial column 380-700 K over the same period.

Comparison of results from tracer correlation studies with explicit
approaches
In this section results from work based on HALOE O3 /CH4 and O3 /HF relations are
compared with the results from the studies that account for dynamical processes by explicit
transport calculations. It is hard to make the same detailed comparisons with the tracer
correlation approach as the early winter measurements are so important in determining the
total loss in the winter and the time evolution of the ozone loss is less well defined than for
the other techniques. The problem is not the technique (see e.g., Richard et al [2001] for an
analysis of the 1999/2000 winter using in situ data), but the limited (not continuous)
sampling of HALOE measurements inside the vortex during the whole winter. However the
good agreement between the various approaches which explicitly allow for transport allows
us to concentrate on making comparisons between the total ozone losses derived from the
tracer correlation and SAOZ approaches. The results of these comparisons are summarised
in Table 4 for 1994/95, 1995/96, 1996/97 and 1999/2000 and are shown in Figure 3.

It is apparent that somewhat larger discrepancies exist between the results from the HALOE
tracer correlation study and the results from SAOZ than between the results in the other
comparisons.

In 1994/95 and 1996/97 the tracer correlation approach found smaller

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accumulated ozone losses than SAOZ. In 1995/96 the tracer study found larger losses than
the SAOZ approach. Because there is good agreement between the SAOZ and Match
approaches during the shorter period of comparison, there seems to be a larger uncertainty
in the HALOE tracer correlation approach. This method to deduce ozone loss from tracer
correlations is based on considering deviations from an early winter ozone-tracer reference
relation. A proper reference relation must be measured late enough in the winter that a stable
distinct vortex has already formed, and yet early enough that it is not already affected by
chemical ozone loss.

The importance of the use of the proper reference relation is emphasised by the example of
the winter 1996/97. In the original study Müller et al. [1997b] used a November HALOE
reference; however, this reference is influenced by mixing in of ozone rich air into the polar
vortex during late November/early December 1996.
December lie too far at the edge of the vortex.

Moreover HALOE observations in
However it is possible to derive a

representative reference function for early January 1997 from the ILAS satellite data set,
which measured inside the vortex throughout the whole winter 1996/97. In early January
1997 the ozone-tracer relation is not changing inside the polar vortex [Tilmes et al.,
Calculation of chemical ozone loss in the Arctic winter 1996/97 using ozone-tracer
correlations: comparisons of ILAS and HALOE results, manuscript in preparation]. The
more appropriate reference derived from the January ILAS data is characterised by larger
ozone values and leads to the deduction of larger ozone losses (about 20%) in comparison to
the use of the November relation in the study by Müller [1997b]. Using this reference, one
obtains a vortex averaged column ozone loss of 64 ± 16 DU and an average loss in the
vortex core (PV > 50 PVU at 475K) of 77 ± 17 DU, in better agreement with the loss
derived from SAOZ/REPROBUS [Goutail et al., 1998] (see Fig. 3 and Table 4).

Discussion
Detailed comparisons between the different studies that infer Arctic ozone loss from ozone

18

observations and transport calculations reveal a generally good agreement of the results.
This degree of agreement is not obvious from the published data, since the time periods and
regions considered differ widely. The agreement shows up only if the different approaches
are applied for precisely the same times and regions. While there are similarities between
the approaches that include explicit transport studies in that they use meteorological
information, they use largely different types of instruments (ozonesondes, ground-based
uv-visible spectrometers, satellite-borne microwave limb sounders and solar occulation
instruments) to measure ozone and largely different approaches (trajectories of varying
length and 3-D CTMs using different meteorological analyses) to separate the chemical loss
from dynamical effects. However, with the exception of REPROBUS, the models used to
calculate heating rates in these methods are similar, and in some cases the same. Diabatic
heating rates in the lower stratosphere are difficult to determine accurately [Olaguer et al.,
1992] and more work on the validation of heating rates in different winters is required. In
general, though, the good agreement between their results increases our confidence in our
ability to quantify Arctic chemical ozone loss. At the same time, the importance of precise
comparisons of ozone loss is clear, a point that needs to be borne in mind when comparing
models with observations.

The studies relying on HALOE tracer relations to remove dynamical effects show somewhat
larger discrepancies compared with the approaches that rely on transport calculations. These
discrepancies vary from year to year. There are three possible reasons for this:
(a) quality of the HALOE measurements;
(b) uncertainty in the early winter relation, given HALOE’s limited vortex sampling;
and
(c) impact of mixing on tracer relations.
There is no evidence that the quality of the HALOE v.18 or v.19 data adversely affect the
analyses of ozone loss. However it is important to use studies using these data rather than
the earlier HALOE v.17 data.

19

In practice it is hard to separate the effects of (b) and (c). The comparisons presented here,
along with the discussion in the original publications (particularly Müller et al. [2001]),
indicate that a large uncertainty arises from the selection of the early vortex profiles used to
determine the early winter ozone-tracer relations. With the ozone-tracer relation approach,
the reference function should be calculated from profiles inside the early vortex, if the vortex
is not influenced by significant mixing processes. If the baseline correlation function is
derived too early, or around the edge of the vortex, it could have a significant impact on the
overall derived ozone loss. For example, any mixing in December and early January could
have a significant impact on the overall ozone loss derived from HALOE when the baseline
correlations are derived from the late November measurements. Depending on the timing of
the mixing relative to the ozone loss such mixing can lead to an overestimation or, more
likely, an underestimation of the chemical loss. Great care, therefore, has to be taken to
ensure that the baseline ozone-tracer relation is truly representative of the vortex prior to
ozone loss in order for this approach to be valid. The influx of mid-latitude air into the
Antarctic vortex during Austral winter [Russell and Pierce, 2000] would cause errors in
ozone loss rates deduced there if the initial relation is taken inappropriately early.

A second possible reason for the larger discrepancies is that anomalous mixing in tracer
space takes place during the periods of ozone loss. These considerations are in line with the
arguments presented in Michelsen et al. [1998] and Plumb et al. [2000] which consider the
effects of the mixing of mid-latitude air into the vortex. However the HALOE early winter
relations are derived from measurements made inside the vortex that in general will lead to
an underestimate of ozone loss in the vortex during winters when mixing is significant
[Müller et al., 2001]. Horizontal mixing is not significant inside an isolated vortex such as
that in 1999/2000 [Ray et al., 2001]. The importance of its effect on the derived ozone loss
will vary from year to year and will depend on the stability of the vortex during the periods
of ozone loss. Given the preceding discussion about the importance of determining the

20

initial relation correctly, it seems likely that this mechanism (mixing during ozone loss) is
best considered as contributing to the increased uncertainty associated with the ozone-tracer
approach (rather than as the dominant cause).

A second sampling issue relates to the inhomogeneity of ozone loss inside the vortex, a
point which is true for all methods used to derive vortex average ozone losses. The largest
discrepancy (in 1996/97) is partially caused by the strong inhomogeneities inside the vortex
that make it difficult to compare vortex averaged ozone losses [Müller et al., 1997b;
McKenna et al., 2001; Schulz et al., 2000]. In general though the careful comparisons
presented here indicate that there is reasonably good agreement in the ozone losses derived
from measurements using the techniques discussed.

Acknowledgements
The authors acknowledge support from the Environment and Climate programme of the
Research DG of the European Commission through the THESEO O3 loss project (ENV4CT97-0510), the CASSIS (ENV4-CT97-0550) and the CRUSOE concerted actions (EVK21999-00252). N.R.P. Harris also acknowledges support from the Global Atmosphere
Division of the UK Department of Environment, Transport and the Regions. Work at the
Jet Propulsion Laboratory, California Institute of Technology was done under contract with
the National Aeronautics and Space Administration. We thank Simone Tilmes for her
helpful comments on the tracer analysis based on HALOE data for the winters 1996/97 and
1999/2000.

N.R.P. Harris, European Ozone Research Coordinating Unit, University of Cambridge, 14
Union Road, Cambridge, CB2 1HE, email: Neil.Harris@ozone-sec.ch.cam.ac.uk
M. Rex, Alfred Wegener Institute, Potsdam, Germany, email: mrex@awi-potsdam.de
F. Goutail, Service d'Aéronomie, Centre National de la Recherche Scientifique, Verrières le
Buisson, Paris, France, email: florence.goutail@aerov.jussieu.fr

21

B.M. Knudsen, Danish Meteorological Institute, Lyngbyvej 100, 2100 Copenhagen,
Denmark, Email: bk@dmi.dk
G.L. Manney, Jet Propulsion Laboratory, California Institute of Technology, CA, USA.
Also at Department of Physics, New Mexico Highlands University, Las Vegas, NM,
87701, USA. Email: manney@mls.jpl.nasa.gov
R. Müller, Institue für Stratosphärische Chimie, Fz Jülich, Jülich, Germany, email:
Ro.Mueller@fz-juelich.de
P. von der Gathen, Alfred Wegener Institute, Potsdam, Germany, email: gathen@awipotsdam.de

Figures
1. Ozone loss rates found by Match in the 1994/95 Arctic winter [Rex et al., 1999]. The
thick black lines show the subsiding air masses observed during the three periods for
which an accumulated ozone loss is reported using MLS data [Manney et al., 1996b].
The thick solid lines represent continuous periods of MLS observations, while the thick
dashed line represents the air mass linking the final measurements in one north looking
period of MLS with the early measurements in the next period. The solid blue line
indicates the subsiding air mass closest to the air masses observed by MLS, and the
dashed black lines indicate the paths of other subsiding air masses. In this study, the
Match ozone loss rates are integrated along the thick black lines to allow a direct
comparison with the MLS-derived losses. The isolines drawn with thin solid black lines
show the area below PSC existence temperatures.
2. Comparison of accumulated losses in ozone mixing ratio derived from MLS [Manney et
al., 1996a,b, 1997], Match [Rex et al., 1997, 1999; Schulz et al., 2000] and the vortex
average approach [Knudsen et al., 1998] at potential temperatures around 475K. In
each pair, the Match ozone loss rates have been integrated for the same time period and
in the same subsiding air mass to allow direct comparisons.

22

3. Comparison of column ozone losses derived from the SAOZ/REPROBUS [Goutail et
al., 1998, 1999]; POAM/REPROBUS [Deniel et al., 1998)]; Match [Rex et al., 1999,
2001]; and HALOE tracer correlation [Müller et al., 1996, 1997a, 1999; Müller et al.,
Chemical ozone loss and chlorine activation deduced from HALOE and OMS
measurements in the Arctic Winter 1999-2000, manuscript in preparation; Tilmes et al.,
Calculation of chemical ozone loss in the Arctic winter 1996/97 using ozone-tracer
correlations: comparisons of ILAS and HALOE results, manuscript in preparation]
approaches. The 1996/97 January estimate uses ILAS data for the initial relation, and
the 1999/2000 uses a mix of tracer data from the OMS remote and in situ balloon
payloads which flew in November 1999. In both years the March relations used were
found from HALOE data.

Tables
1) Ozone losses in the 1994/95 winter.
2) Ozone losses in the 1995/96 winter.
3) Ozone losses in the 1996/97 winter.
4) Column ozone losses.

References
Becker, G., R. Müller, D.S. McKenna, M. Rex and K. Carslaw, Ozone loss rates in the
Arctic stratosphere in the winter 1991/92: Model calculations compared with Match results,
Geophys. Res. Lett., 25, 4325-4328, 1998.

Becker, G., R. Müller, D.S. McKenna, M. Rex, K. Carslaw and H. Oelhaf, Ozone loss
rates in the Arctic stratosphere in the winter 1994/95: Model simulations underestimate
results of the Match analysis, J. Geophys. Res., 105, 15175-15184, 2000.

23

Chipperfield, M.P., Multiannual simulations with a Three-Dimensional Chemical Transport
Model, J. Geophys. Res., 104, 1781-1805, 1999.

Deniel C., J.P. Pommereau, R.M. Bevilacqua and F. Lefèvre, Arctic chemical ozone
depletion during the 1994-95 winter deduced from POAM II satellite observations and the
REPROBUS 3-D model, J. Geophys. Res., 103, 19231-19244, 1998.

Fioletov, V.E., J.B. Kerr, D.I. Wardle, J. Davies, E.W. Hare, C.T. McElroy and D.W.
Tarasick, Long-term ozone decline over the Canadian Arctic to early 1997 from groundbased and balloon observations, Geophys. Res. Lett., 24, 2705-2708, 1997.

Goutail, F., J-P. Pommereau, C. Phillips, C. Deniel, A. Sarkissian, F. Lefevre, E. Kyro,
M. Rummukainen, P. Ericksen, S. Andersen, B-A. Kaastad-Hoiskar, G. Braathen, V.
Dorokhov and V. Khattatov, Depletion of Column Ozone in the Arctic during the Winters
1993-94 and 1994-95, J. Atmos. Chem., 32, 1-34, 1999.

Goutail, F., J-P. Pommereau, E. Kyro, M. Rummukainen, P. Ericksen, S. Andersen, BA. Kaastad-Hoiskar, G.O. Braathen, V. Dorokhov, V. Khattatov, M. van Roozendael and
M. de Mazière, Total ozone reduction in the Arctic vortex during the winters of 1995/96 and
1996/97, Proc. 4th European Symposium on Polar Ozone, EC Air Pollution Research
Report No. 66, eds. N.R.P. Harris, I. Kilbane-Dawe and G.T. Amanatidis, European
Commission, 277-280, 1998.

Guirlet, M., M.P. Chipperfield, J.A. Pyle, F. Goutail, J-P. Pommereau and E. Kyrö,
Modelled Arctic ozone depletion in winter 1997/98 and comparison with previous winters,
J. Geophys. Res., 105, 22185-22200, 2000.

24

Hansen, G., T. Svenøe, M.P. Chipperfield, A. Dahlback and U-P. Hoppe, Evidence of
substantial ozone depletion in winter 1995/96 over northern Norway, Geophys. Res. Lett.,
24, 799-802, 1997.

Jones, A.E. and J.D. Shanklin, Continued decline of total ozone over Halley, Antarctica,
since 1985, Nature, 376, 409-411, 1995.

Kilbane-Dawe, I., N.R.P. Harris, J.A. Pyle, M. Rex, A.M. Lee and M.P. Chipperfield, A
comparison of Match and 3D model ozone loss rates in the Arctic polar vortex during the
winters of 1994/95 and 1995/96, J. Atmos. Chem., 39, 123-138, 2001.

Knudsen, B.M., N. Larsen, I.S. Mikkelsen, J-J. Morcrette, G.O. Braathen, E.Kyrö, H.
Fast, H. Gernandt, H. Kanzawa, H. Nakane, V. Dorokhov, V. Yushkov, G. Hansen, M.
Gil and R.J. Shearman, Ozone depletion in and below the Arctic vortex for 1997, Geophys.
Res. Lett., 25, 627-630, 1998.

Lefèvre, F., G.P. Brasseur, I. Folkins, A.K. Smith and P. Simon, Chemistry of the 199192 stratospheric winter: three dimensional model simulations, J. Geophys. Res., 99, 81838195, 1994.

Lefèvre, F., F. Figarol, K.S. Carslaw and T.Peter, The 1997 Arctic ozone depletion
quantified from three dimensional model simulations, Geophys. Res. Lett., 25, 2425-2428,
1998.

Lucic, D., N.R.P. Harris, J.A. Pyle and R.L. Jones, A technique for estimating polar
ozone loss: results for the northern 1991/92 winter using EASOE data, J. Atmos. Chem.,
34, 365-383, 1999.

25

Manney, G.L., R. Zurek, L. Froidevaux and J.W. Waters, Evidence for Arctic ozone
depletion in late February and early March 1994, Geophys. Res. Lett., 22, 2941-2944,
1995a.

Manney, G.L., R. Zurek, L. Froidevaux, J.W. Waters, A. O'Neill and R. Swinbank,
Lagrangian transport calculations using UARS data. Part II: ozone, J. Atmos. Sci., 52,
3069-3081, 1995b.

Manney, G.L., R.W. Zurek, W.A. Lahoz, R.S. Harwood, J.C. Gille, J.B. Kumer, J.L.
Mergenthaler, A.E. Roche, A. O'Neill, R. Swinbank and J.W. Waters, Lagrangian
transport calculations using UARS data. Part I: Passive tracers, J. Atmos. Sci., 52, 30493068, 1995c.

Manney, G.L., M.L. Santee, L. Froidevaux, J.W. Waters and R.W. Zurek, Polar vortex
conditions during the 1995-96 Arctic winter: meteorology and MLS ozone, Geophys. Res.
Lett., 23, 3203-3206, 1996a.

Manney, G.L., M.L. Santee, L. Froidevaux, J.W. Waters, R.W. Zurek, W.G. Read, D.A.
Flower and R.F. Jarnot, Arctic ozone depletion observed by UARS MLS during the 199495 winter, Geophys. Res. Lett., 23, 85-88, 1996b.

Manney, G.L., L. Froidevaux, M.L. Santee and J.W. Waters, MLS observations of Arctic
ozone loss in 1996/97, Geophys. Res. Lett., 24, 2697-2700, 1997.

McKenna, D.S. J-U. Grooß, G. Günther, P. Konopka, R. Müller, G. Carver and Y.
Sasano, A new chemical Lagrangian Model of the Stratosphere (CLaMS): Part II
Formulation of Chemistry – Scheme and Initialisation, J. Geophys. Res., in press, 2001.

26

Michelsen, H.A., G.L. Manney, M.R. Gunson and R. Zander, Correlations of
stratospheric abundances of NOy, O3 , H2 O, N2 O and CH4 derived from ATMOS
measurements, J. Geophys. Res., 103, 28347-28359, 1998.

Müller, R., P. Crutzen, J-U. Grooβ, C. Brühl, J.M. Russell III and A. Tuck, Chlorine
activation and ozone depletion in the Arctic vortex: observations by the halogen occultation
experiment on the upper atmosphere research satellite, J. Geophys. Res., 101, 1253112554, 1996.

Müller, R., P. Crutzen, J-U. Grooβ, C. Brühl, J.M. Russell III, D. McKenna and A.
Tuck, Severe chemical ozone loss in the Arctic during the winter of 1995-96, Nature, 389,
709-712, 1997a.

Müller, R., J-U. Grooβ, D. McKenna, P. Crutzen, C. Brühl, J.M. Russell III and A.
Tuck, HALOE observations of the vertical structure of chemical ozone depletion in the
Arctic vortex during winter and early spring 1996-1997, Geophys. Res. Lett., 24, 27172720, 1997b.

Müller, R., J-U. Grooβ, D.S. McKenna, P.J. Crutzen, C. Brühl, J.M. Russell III, L.L.
Gordley and A.F. Tuck, Chemical ozone loss in the Arctic vortex in the winter 1995-96:
HALOE measurements in conjunction with other observations, Ann. Geophys., 17, 101114, 1999.

Müller, R., U. Schmidt, A. Engel, D.S. McKenna and M.H. Proffitt, The O3 /N2 O
relationship from balloon-borne observations as a measure of Arctic ozone loss in 19911992, Q. J. R. Meteorol. Soc., 127, 1389-1412, 2001.

27

Olaguer, E.P., H. Yang and K.K. Tung, A reexamination of the radiative balance of the
stratosphere, J. Atmos. Sci., 49, 1242-1263, 1992.

Pawson, S. and B. Naujokat, Trends in daily wintertime temperatures in the northern
stratosphere, Geophys. Res. Lett, 24, 575-578, 1997.

Pawson, S. and B. Naujokat, The cold winters of the middle 1990s in the northern lower
stratosphere, J. Geophys. Res., 104, 14209-14222, 1999.

Plumb, R.A., D.W. Waugh and M.P. Chipperfield, The effects of mixing on tracer
relationships in the polar vortices, J. Geophys. Res., 105, 10047-10062, 2000.

Proffitt, M.H., J.J. Margitan, K.K. Kelly, M. Lowenstein, J.R. Podolske and K. Chan,
Ozone loss in the Arctic polar vortex inferred from high-altitude aircraft measurements,
Nature, 347, 31-36, 1990.

Proffitt, M. H., K. Aikin, J.J. Margitan, M. Loewenstein, J.R. Podolske, A. Weaver,
K.R. Chan, H. Fast and J.W. Elkins, Ozone loss inside the northern polar vortex during
the 1991-1992 winter, Science, 261, 1150-1154, 1993.

Ray, E.A., F.L. Moore, J.W. Elkins, D.F. Hurst, P.A. Romanshkin, G.S. Dutton and
D.W. Fahey, Descent and mixing in the 1999-2000 northern polar vortex inferred from in
situ tracer measurements, submitted to J. Geophys. Res., 2001.

Rex, M., N.R.P. Harris, P. von der Gathen, R. Lehmann, G.O. Braathen, E. Reimer, A.
Beck, M. Chipperfield, R. Alfier, M. Allaart, F. O'Connor, H. Dier, V. Dorokhov, H.
Fast, M. Gil, E. Kyro, Z. Litynska, I.S. Mikkelsen, M. Molyneux, H. Nakane, M.

28

Rummukainen, P. Viatte and J. Wenger, Prolonged stratospheric ozone loss in the 1995/96
Arctic winter, Nature, 389, 835-838, 1997.

Rex, M., P. von der Gathen, N.R.P. Harris, D. Lucic, B.M. Knudsen, G.O. Braathen, H.
De Backer, R. Fabian, H. Fast, M. Gil, E. Kyro, I.S. Mikkelsen, M. Rummukainen, J.
Staehelin and C. Varotsos, In-situ measurements of stratospheric ozone depletion rates in
the Arctic winter 1991/92: a Lagrangian approach, J. Geophys. Res., 103, 5843-5853,
1998.

Rex, M., P. von der Gathen, G.O. Braathen, S.J. Reid, N.R.P. Harris, M. Chipperfield,
E. Reimer, A. Beck, R. Alfier, R. Kruger-Carstensen, H. De Backer, D. Balis, C. Zerefos,
F. O'Connor, H. Dier, V. Dorokhov, H. Fast, A. Gamma, M. Gil, E. Kyro, M.
Rummukainen, Z. Litynska, I.S. Mikkelsen, M. Molyneux and G. Murphy, Chemical
Ozone Loss in the Arctic Winter 1994/95 as Determined by the Match Technique, J. Atmos.
Chem., 32, 35-39, 1999.

Rex, M., R.J. Salawitch, N.R.P. Harris, P. von der Gathen, G.O. Braathen, A. Schulz,
H. Deckelmann, M. Chipperfield, B.-M. Sinnhuber, E. Reimer, R. Alfier, R. Bevilacqua,
K. Hoppel, M. Fromm, J. Lumpe, H. Küllmann, A. Kleinböhl, H. Bremer, M. von
König, K. Künzi, D. Toohey, H. Vömel, E. Richard, K. Aikin, H. Jost, J.B. Greenblatt,
M. Loewenstein, J.R. Podolske, C.R. Webster, G.J. Flesch, D.C. Scott, R.L. Herman,
J.W. Elkins, E.A. Ray, F.L. Moore, D.F. Hurst, P. Romashkin, G.C. Toon, B. Sen, J.J.
Margitan, P. Wennberg, R. Neuber, M. Allart, B.R. Bojkov, H. Claude, J. Davies, W.
Davies, H. De Backer, H. Dier, V. Dorokhov, H. Fast, Y. Kondo, E. Kyrö, Z. Litynska,
I.S. Mikkelsen, M.J. Molyneux, E. Moran, T. Nagai, H. Nakane, C. Parrondo, F.
Ravegnani, P. Skrivankova, P. Viatte and V. Yushkov, Chemical depletion of Arctic ozone
in winter 1999/2000, this issue, 2001.

29

Richard, E.C., K. Aikin, A.E. Andrews, B.C. Daube, Jr., C.Gerbig, S.C. Wofsy, P.A.
Romanshkin, D.F. Hurst, E.A. Ray, F.L. Moore, J.W. Elkins, T. Deshler and G.C. Toon,
Severe Chemical Ozone Loss Inside the Arctic Polar Vortex during Winter 1999-2000
Inferred from in situ Airborne measurements, Geophys. Res. Lett., 28, 2197-2200, 2001.

Russell, J.M. and R.B. Pierce, Observations of Southern Polar Descent and Coupling in the
Thermosphere, Mesosphere and Stratosphere Provided by HALOE, in Atmospheric Science
Across the Stratopause, Geophysical Monograph 123, American Geophysical Union, 2000.

Sasano, Y., Y. Terao, H.L. Tanaka, T. Yasunari, H. Kanzawa, H. Nakajima, T. Yokota,
H. Nakane, S. Hayashida and N. Saitoh, ILAS observations of chemical ozone loss in the
Arctic vortex during early spring 1997, Geophys. Res. Lett., 27, 213-216, 2000.

Schoeberl, M.R., M.H. Proffitt, K.K. Kelly, L.R. Lait, P.A. Newman, J.E. Rosenfield,
M. Loewenstein, J.R. Podolske, S.E. Strahan and K.R. Chan, Stratospheric constituent
trends from ER-2 profile data, Geophys. Res. Lett., 17, 469-427, 1990.

Schoeberl, M.R., L.R. Lait, P.A. Newman and J.E. Rosenfield, The structure of the polar
vortex, J. Geophys. Res., 97, 7859-7882, 1992.

Schulz, A., M. Rex, J. Steger, N.R.P. Harris, G.O. Braathen, E. Reimer, R. Alfier, A.
Beck, M. Alpers, J. Cisneros, H. Claude, H. De Backer, H. Dier, V. Dorokhov, H. Fast,
G. Hansen, H. Kanzawa, B. Kois, Y. Kondo, E. Kosmidis, E. Kyro, Z. Litynska, M.
Molyneux, G. Murphy, H. Nakane, C. Parrondo, F. Ravegnami, C. Varotsos, C. Vialle,
V. Yushkov, C. Zerefos and P. von der Gathen, Match observations in the Arctic winter
1996/97: High stratospheric ozone loss rates correlate with low temperatures deep inside the
polar vortex, Geophys. Res. Lett., 27, 205-208, 2000.

30

Schulz, A., M. Rex, N.R.P. Harris, G.O. Braathen, E. Reimer, R. Alfier, I. KilbaneDawe, S. Eckermann, M. Allaart, M. Alpers, B. Bojkov, J. Cisneros, H. Claude, E.
Cuevas, J. Davies, H. De Backer, H. Dier, V. Dorokhov, H. Fast, S. Godin, B. Johnson,
Y. Kondo, E. Kosmidis, E. Kyro, Z. Litynska, I.S. Mikkelsen, M. Molyneux, G.
Murphy, H. Nakane, F.O. Connor, C. Parrondo, F.J. Schmidlin, P. Skrivankova, C.
Varotsos, C. Vialle, P. Viatte, V. Yushkov, C. Zerefos and P. von der Gathen, Arctic
ozone loss in threshold conditions: Match observations in 1997/98 and 1998/99, J.
Geophys. Res., 106, 7495-7503, 2001.

Shine, K.P., The middle atmosphere in the absence of dynamical heat fluxes,
Q.J.R.Meteorol. Soc., 113, 603-633, 1987.

Sinnhuber, B.M., M.P. Chipperfield, S. Davies, J.P. Burrows, K-U. Eichmann, M.
Weber, P. von der Gathen, M. Guirlet, G. Cahill, A.M. Lee and J.A. Pyle, Geophys. Res.
Lett., 27, 3473-3476, 2000.

Staehelin, J., N.R.P. Harris, C. Appenzeller and J. Eberhard, Ozone trends: a review, Rev.
Geophysics, 39,231-290, 2001.

von der Gathen, P., M. Rex, N.R.P. Harris, D. Lucic, B.M. Knudsen, G.O. Braathen, H.
De Backer, R. Fabian, H. Fast, M. Gil, E. Kyrö, I.St. Mikkelsen, M. Rummukainen, J.
Stähelin and C. Varotsos, Observational evidence for chemical ozone depletion over the
Arctic in winter 1991-92, Nature, 375, 131-134, 1995.

WMO, Scientific Assessment of Ozone Depletion: 1998, WMO Global Ozone Research and
Monitoring Project, Report No. 44, WMO, Geneva, 1999.