GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 17, PAGES 3227-3330, SEPTEMBER 1, 1998
Ground based millimeter-wave observations
of Arctic ozone depletion
during winter and spring of 1996/97
Björn-Martin Sinnhuber, Jens Langer, Ulf Klein, Uwe Raffalski,1
and Klaus Künzi
Institute of Environmental Physics, University of Bremen, Bremen, Germany
Otto Schrems
Alfred Wegener Institute for Polar Research, Bremerhaven, Germany
Abstract. Ground based millimeter-wave measurements of
Arctic stratospheric ozone in the winter 1996/97 are presented. The measurements have been performed at one of
the primary Arctic stations of the Network for the Detection
of Stratospheric Change (NDSC) in Ny-Ålesund, Spitsbergen (78.9◦N, 11.9◦E). Over the period 11 February to 26
April the measurements show an ozone mixing ratio decrease
of 1.3 ppm in the lower stratosphere. Correspondingly,
stratospheric ozone column densities decreased by more than
50 DU. Taking into account the transport of ozone due to
diabatic decent, we estimated chemical ozone loss rates of
22 ppb/day in February decreasing to 15 ppb/day in late
April 1997.
Introduction
Chemical depletion of lower stratospheric ozone in the
Arctic has been observed during the last years in winter
and spring [WMO, 1994; European Commission, 1997]. Although the extent of the Arctic ozone depletion is still less
than in the Antarctic Ozone Hole, the processes leading to
the ozone depletion are believed to be the same in both hemispheres. Polar stratospheric clouds (PSC) form at very low
temperatures and enable heterogeneous reactions to activate
chlorine which, in the presence of sunlight, can efficiently destroy ozone. However, dynamical processes usually lead to
a high ozone variability in the Arctic, that makes it difficult
to separate chemical ozone depletion.
Again in late winter and spring 1997 substantial ozone
loss has been observed in the Arctic [Manney et al., 1997;
Newman et al., 1997; Müller et al., 1997; Knudsen et al.,
1998]. The meteorology of the winter 1996/97 stratosphere
was unusual [Coy et al., 1997]. The polar vortex formed unusually late and was stable until the first days in May. Temperatures stayed low enough to allow the formation of PSCs
until the end of March. In this paper we derive chemical
ozone loss rates from ground based millimeter-wave ozone
measurements. The measurements have been performed at
1
Now at Swedish Institute for Space Physics, Kiruna, Sweden.
Copyright 1998 by the American Geophysical Union.
Paper number 98GL52488.
0094-8534/98/98GL-52488$05.00
one of the primary Arctic stations of the Network for the
Detection of Stratospheric Change (NDSC) in Ny-Ålesund,
Spitsbergen (78.9◦N, 11.9◦E). These continuous measurements of ozone profiles inside the polar vortex from February
to the end of April allowed the investigation of the chemical
ozone loss evolution during winter and spring 1997.
Ozone measurements
The Radiometer for Atmospheric Measurements (RAM)
is a ground based millimeter-wave radiometer operated continuously at Ny-Ålesund, Spitsbergen.
An ozone emission line is measured at 142 GHz over a
spectral band-width of 1.6 GHz. The high spectral resolution allows the retrieval of ozone profiles between 12 and
55 km from the pressure broaded line shape. The vertical
resolution is about 6–8 km. One to four ozone profiles per
hour are obtained all year round, largely independent of tropospheric weather conditions. Temperature profiles required
for the ozone profile retrieval are taken from analyses of the
National Center for Environmental Prediction (NCEP, former NMC). The precision of the measured ozone volume
mixing ratio (VMR) profiles is about ±0.1 ppm.
The RAM alternately measures ozone at 142 GHz and
chlorine monoxide (ClO) at 204 GHz. The ClO measurements of late winter-spring 1997 are presented in a companion paper by Raffalski et al. [1998]. For a detailed description of the RAM see Langer et al. [1997].
Fig. 1 shows the RAM measurements of the ozone VMR
at the 475 K level (approximately 19 km altitude) between
mid-February and beginning of May 1997. A continuous
decrease from nearly 3 ppm in February to about 1.6 ppm
at the end of April can be seen, corresponding to an overall
reduction of 44 %. Diabatic decent would be expected to
increase the lower stratospheric ozone VMR over this time
period. The observed ozone decrease therefore has to be
attributed to chemical ozone depletion.
Potential vorticity (PV), derived from analyses of the European Centre for Medium Range Weather Forecast
(ECMWF), indicate that the ozone measurements during
this period were performed well inside the polar vortex, with
the exception of only a few days in mid April, when Ny-Ålesund was at the vortex edge as seen in the data by a slight
increase of the ozone mixing ratio. The vortex edge was determined using the PV gradient with respect to equivalent
latitude [Nash et al., 1996]. We defined the inner vortex as
3227
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SINNHUBER ET AL.: OBSERVATION OF ARCTIC OZONE DEPLETION
not homogeneous within the vortex. High horizontal variability of ozone within the vortex was also observed by the
Halogen Occultation Experiment (HALOE) [Pierce et al.,
1997]. Fitting a trend over 20 days averages out this horizontal variability. Measurements not made within the inner
vortex were excluded from the trend calculations. The observed ozone trends are shown in Fig. 3 (filled circels with
error bars).
To derive chemical ozone loss rates from the observed
ozone trends, we have to take into account how much of the
chemical loss is masked by the diabatic decent of air inside
the polar vortex. The ozone change on a given isentropic
surface due to diabatic decent is given as
3.5
2.5
2
1.5
1
FEB
60
MAR
APR
80
100
Day of Year 1997
MAY
∂O3
∂O3
= −Q (p0 /p)κ
.
∂t
∂θ
120
Figure 1. Ozone volume mixing ratio at the 475 K isentropic level as measured by the ground based millimeterwave radiometer at Ny-Ålesund in late winter and spring
1997. Each dot represents a single measurement. A continuing decrease of the ozone mixing ratio between mid of
February and end of April is evident.
the region of relatively low PV gradients, compared to the
high PV gradients at the vortex edge. Throughout March,
the inner vortex edge defined in this way corresponded approximately to the 48 PV units contour.
We performed domain filling trajectory calculations [Sinnhuber et al., 1996] to show that the inner vortex at 475 K
was well isolated from February to the end of April 1997. No
intrusion of outer-vortex air into the inner vortex occurred.
After April 26 the inner vortex moved away from Spitsbergen. Higher ozone VMRs of up to 2.5 ppm have been
measured in the vortex edge region between April 14 and 17
and after April 26. After May 10 measurements have been
made clearly outside the vortex, with typical mid-latitude
ozone mixing ratios of 2 ppm at 475 K.
Stratospheric ozone column densities above 12 km have
been computed from the RAM measurements. They show a
constant decrease between mid-February and the beginning
of April, see Fig. 2 (dots). Fig. 2 also shows UV/visible
total ozone measurements performed at Ny-Ålesund (asterisks, data provided by F. Wittrock). The RAM column
densities above 12 km are in reasonable agreement with the
UV/visible total ozone measurements, when a mean offset
of 66 DU accounting for the remaining ozone column below
12 km is added. Between the beginning of February and
beginning of April the ozone column densities decreased by
more than 50 DU. The decrease of the ozone column densities inside the polar vortex was also observed by the two
satellite-borne instruments Total Ozone Mapping Spectrometer (TOMS) [Newman et al., 1997] and Global Ozone Monitoring Experiment (GOME) [Bramstedt et al., 1997].
Computation of ozone loss rates
From the RAM ozone measurements at 475 K loss rates
have been determined by fitting a linear trend over periods of 20 days. A 20 day period was chosen because our
measurements show some variability on time-scales of a few
days, indicating that the horizontal distribution of ozone is
(1)
Where O3 is the ozone VMR, Q the diabatic heating rate, p
and p0 are pressure and reference pressure respectively, κ =
2/7 is the ratio of of the dry air gas constant to the specific
heat at constant pressure, and ∂O3 /∂θ is the ozone gradient
with respect to the potential temperature [Braathen et al.,
1994, ]. The heating rates include solar heating through
the absorption of shortwave radiation and cooling through
longwave emission into space.
The longwave cooling rate calculations have been performed with a narrow-band model with a resolution of
10 cm−1 [Shine, 1991]. The computation included ozone,
water vapor and carbon dioxide. Daily profiles of the RAM
ozone measurements at Spitsbergen have been used for the
heating rate calculations. Since no routine measurements of
stratospheric water vapor were available, a typical late winter vortex profile was assumed with a minimum of 3.5 ppm at
400 K increasing to 6 ppm above 675 K [Ovarlez and Ovarlez, 1994]. However, water vapor has only a small impact on
both longwave and shortwave stratospheric heating rates.
The carbon dioxide VMR was assumed to be 360 ppm.
Temperature profiles averaged over the inner vortex have
been taken from the ECMWF analyses. The error through
the use of vortex averaged temperatures for the heating rate
400
350
Ozone column [DU]
Ozone VMR [ppm]
3
300
250
200
150
FEB
60
MAR
APR
80
100
Day of Year 1997
MAY
120
Figure 2. Ozone column densities above 12 km as
measured by the ground based millimeter-wave radiometer (dots) and total ozone above sea level from UV/visible
measurements (asterisks) at Ny-Ålesund in late winter and
spring 1997.
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SINNHUBER ET AL.: OBSERVATION OF ARCTIC OZONE DEPLETION
The analysis of the millimeter-wave ozone measurements
shows substantial chemical ozone loss at the 475 K isentropic
level until the begin of April 1997. During this period we also
measured high levels of ClO [Raffalski et al., 1998]. Ozone
loss rates computed from the ClO measurements agree well
with the observed ozone loss. The still high ozone loss rates
during March and the begin of April are partly due to the
high amount of sunlight available inside the vortex at that
time. Fig. 4 shows the estimated ozone loss rates per sunlit
hour (open diamonds). They have been derived from the loss
rates shown in Fig. 3 divided by the average hours of sunlight
per day available inside the vortex during the corresponding
periods. Between February and March, the ozone loss rates
per sunlit hour at 475 K decreased by more than a factor
two. For comparison the area of temperatures below 195 K
at the 475 K level, where PSCs may exist, is also shown
in Fig. 4. It shows that the reduction of the possible PSC
existence area between February and March is correlated
to the vortex averaged chemical ozone loss rates per sunlit
hour.
Ozone change [ppb/day]
10
0
-10
-20
-30
FEB
60
MAR
80
Day of Year 1997
APR
100
Figure 3. Observed ozone change at 475 K derived from
the millimeter wave measurements at Ny-Ålesund (filled circles with errorbars). The solid line shows the computed
ozone change due to vortex averaged diabatic decent of
ozone. Estimated chemical ozone loss rates are indicated
by open diamonds (see text).
The ozone loss rates of 15 ppb/day in the second half of
April 1997 – three weeks after temperatures rose above the
approximate PSC existance temperature – are about a factor of two or three higher than we would expect under this
condition from usual gas-phase chemistry without chlorine
activation, i.e. due to the NOx- and HOx-cycles alone [Lary,
1997]. Catalytic chlorine cycles on the other hand would require a vortex averaged chlorine activation of about 0.5 ppm
ClO to explain the observed ozone loss in the second half of
April, as calculations with the Bremen Atmospheric Photochemical Model indicate [Raffalski et al., 1998]. The cause
for the unexpected large ozone loss in late April is unclear
at present. The possibility that there may be chlorine ac-
12
PSC Area [10^6 km^2]
Discussion and Conclusion
20
O3 loss [ppb/hr]
calculations, Q(T ), instead of averaging the heating rates
after the computations, Q(T ), is estimated to be less than
0.05K/day.
The integration of the solar heating rates over the vortex
has been performed by computing the heating rates for every 10 degrees of latitude. All calculations have been done
for clear sky conditions only. Knudsen et al. [1998] found
that tropospheric clouds increase lower stratospheric cooling
rates only by a few percent. The effect of PSCs on stratospheric cooling rates is negligible [Rosenfield et al., 1994].
The resulting vortex averaged net heating rates at 475 K
decrease from about 0.3 K per day in February to nearly
zero at the end of March. There is slightly less cooling during February and March 1997 than found by Rosenfield et
al. [1994] for February and March 1992. Our calculations
showed no significant diabatic heating during the period
considered in 1997. This is a result of the unusually low
ozone mixing ratios in April. Since the solar heating in
the lower stratosphere depends almost exclusively on ozone
absorption, higher ozone VMR would have caused diabatic
heating in April.
From the diabatic heating rates and observed vertical
ozone gradients, the diabatic transport rates are computed
using Eq. (1) (Fig. 3, solid line). They show that in the
absence of any chemical ozone depletion, the diabatic decent
would have increased ozone by about 5 to 15 ppb/day during
February and March.
Estimates on the chemical ozone depletion rates, given
as the difference between the observed ozone decrease and
the computed increase due to the diabatic decent are indicated in Fig. 3 (open diamonds). Significant chemical
ozone loss rates are observed throughout the whole period.
The chemical loss rates stayed nearly constant at about
20 ppb/day throughout February and March and decreased
to 15 ppb/day in the second half of April. They are in
good agreement with results of the 1997 MATCH campaign,
which are available until the beginning of April (P. von der
Gathen, personal communication, see also von der Gathen
et al. [1995]). The estimated overall chemical ozone loss derived from the RAM measurements at 475 K between February 11 and April 26 is 1.6 ppm or 55%.
6
0
2
4
FEB
60
MAR
80
Day of Year 1997
APR
100
Figure 4. Estimated chemical ozone loss rates per sunlit
hour at 475 K (open diamonds). For comparison the possible
area of PSC existence at 475 K is displayed, approximated
by the area of temperatures lower than 195 K within the
vortex (see text).
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SINNHUBER ET AL.: OBSERVATION OF ARCTIC OZONE DEPLETION
tivation with ClO mixing ratios as high as 0.5 ppm during
this period is disscussed by Raffalski et al. [1998].
The millimeter-wave observations performed at Ny-Ålesund showed that substantial ozone loss occurred again in
winter and spring 1997, in agreement with other observations. The late break-up of the polar vortex and record low
temperatures during March and early April led to a chemical
ozone loss that lasted longer than in previous years. However, the overall ozone loss at 475 K for the winter 1996/97
was lower than that reported for the previous two years [European Commission, 1997; Rex et al., 1997].
Acknowledgments. We thank the staff at Ny-Ålesund
for their support. Keith P. Shine is gratefully acknowledged
for providing the radiation model for cooling rate calculations.
Meteorological data were provided by the European Centre for
Medium-Range Weather Forecast and by the National Center for
Environmental Prediction. We thank Folkard Wittrock for the
use of the UV/visible data prior to publication. Part of this work
was supported by the Commission of the European Community,
by the German Bundesministerium für Bildung und Wissenschaft,
and the Alfred Wegener Institute for Polar Research, Germany.
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B.-M. Sinnhuber, J. Langer, U. Klein, and K. Künzi, Institute
of Environmental Physics, University of Bremen, PO Box 33 04
40, D-28334 Bremen, Germany. (e-mail: [email protected])
U. Raffalski, Institute for Space Physics, PO Box 812, S-98128
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O. Schrems, Alfred Wegener Insitute for Polar and Marine Research, PO Box 12 01 61, D-27515 Bremerhaven, Germany.
(Received February 20, 1998; revised May 27, 1998;
accepted June 3, 1998.)