More than 2000 ozone soundings and a large number of Dobson observations have been performed since 1967 in a unique procedure. The achieved very homogeneous data sets were used to evaluate significant long-term trends both in the troposphere and the stratosphere. The trend amounts to about plus 2 percent per year in the troposphere and to about minus 0.5 percent per year in the stratosphere. Extremely low ozone records obtained during winter 1991/92 are discussed in the light of the long term series. The winter mean of the ozone column is the lowest one of the series. The ozone deficit occurred mainly in the lower stratosphere. One cause may be the Pinatubo cloud. Even compared with the extreme winter mean following the El Chichon eruption the ozone content was lower. Additionally ozone was reduced by dynamical effects due to unusual weather situations

Claude, H.

Vandersee, W.

Wege, K.

English

NASA Technical Reports Server

N95- 10635
ON LONG-TERM OZONE TRENDS AT HOHENPEISSENBERG
It. Claude, IV. Vandersee, and K. Wege
DWD, Meteorologisches Observatorium Hohenpeissenberg, Germany
ABSTRACT
More than 2000 ozone soundings and a large number of
Dobson observations have been performed since 1967 in
a unique procedure. The achieved very homogeneous data
sets were used to evaluate significant long-term trends
both in the troposphere and the stratosphere. The trend
amounts to about +2% per year in the troposphere and
to about -0.5% per year in the stratosphere.
Extremely low ozone records obtained during winter
1991/92 are discussed in the light of the long term series.
The winter mean of the ozone column is the lowest one
of the series. The ozone deficit occured mainly in the
lower stratosphere. One cause may be the Pinatubo cloud.
Even compared with the extreme winter mean following
the El Chichon eruption the ozone content was lower.
Additionally ozone was reduced by dynamical effects due
to unusual weather situations.
1. I NTRODUC*I'ION
The main goal of this paper is to investigate the long-
term changes in ozone based on the l lohenpeissenberg
long-time series. These time series have been examined in
the past (Attmannspacher et :11. 1084, Bojkov et al. 1990,
Wege el al. 1980) and although the previously detected
trends have been repealedly confirmed, new investigati-
ons are nevertheless valuable. "Fhe data set has grown
longer and the strength of the resulting conclusions is
considerably more reliable. The statistical processes used
allow a refinement of the almost 25-year data set so that
the spatial and temporal course of ozone concentration
can he more precisely examined. Further, an estimation
of the low ozone concentration during winter 1991/92 is
only possible in view of this long data set. The initial
effects of the eruption of the Mount Pinatubo can certain-
ly be demonstrated in this case. llowever the data can
also be used to show that this volcanic eruption, the grea-
test of this century, cannot totally be responsible for the
reducti_m in ozone concentration.
2. DATASET
Measurements of the vertical ozone have been made since
1967 with electrochemical Brewer Mast sondes on every
Wednesday. Since 1977 the sounding frequency has been
twice a week during the summer and three times a week
during the winter. The careful pre-flight preparation of the
sondes has never changed and is described in Claude et
al. 1987. The measured ozone raw data has been correc-
ted for decreasing pump efficiency and other sources of
errors. The integrated column amount of ozone from each
sounding was ratioed with the column amount measured
by a Dobson spectrophotometer to calculate a correction
factor, which has been applied to all ozone values. The so-
called normalization is in any case an improvement of the
ozone sonde data and identifies totally erroneous soun-
dings. On the other hand a miscalibration of the Dobson
instrument has an influence on the sonde data. A change
in the absorption coefficients, for example, implemented
on January I, 1992, shifts the sonde data if this correc-
tion is applied. The Hohenpeissenberg data set is not
completely free of such influences either, but great care
has been taken in the past to minimize them. All total
column ozone data are based on "the old" absorption
coefficients according to V;groux, so that both the total
ozone data and the sonde data are very homogeneous.
Nevertheless, how much the applied Dobson normalizati-
on affects detected trends in an ozone data set requires
clarification, whereby each of the over 2000 profiles used
was an individual measurement. Figure 1 shows an inter-
comparison of two such data sets with the vertical distri-
bution of ozone trends 1967 - 1990 at tlohenpeissenberg.
It is obvious that within the troposphere there is almost
no difference between corrected and uncorrected data.
The relative tropospheric trend is approximately +2.2%
per year, significant at the 99.9% level. The largest diffe-
rence between the normalized and the raw data appears
within the lower stratosphere below the ozone maximum
at 22 kin. Within the stratosphere this is the range with
the most pronounced trend amounting to approximately
-0.5% per year.
19O
https://ntrs.nasa.gov/search.jsp?R=19950004223 2020-06-16T11:27:33+00:00Z
NORMALIZ[D IDOBSON CORRECT[D)
DATA 1967-1990
.... MOT NORMALt2[D
RAW OATA 1%7-1990
-o.e_ o.oo o. ,s,o t.oo t, .e,O 2.00 _.SO
rELATivE TREND I.I. PErYE^rl
Mean trend proJ_les based on balloon sonde dat_
The evaluation of the raw data reveals a very similar
trend profile with a slightly stronger ozone decrease in
that range. This examination shows that the fundamental
trends in the Hohenpeissenberg sonde data are practically
independent of the Dobson normalization. The following
investigations are based on the normalized data.
3. TROPOSPHERIC OZONE
Time series of tropospheric ozone have been investigated
repeatedly in the past (Logan 1985; Bojkov 1988; Staehelin
and Schmid 1991). Almost without exception a trend of
increasing ozone at rates of about 1% per year or more
was found, most pronounced within the lower troposphere.
Figure 2 shows annual means at 2, 4, 6, and 8 kilometers
a.s.I, above Hohenpeissenberg with corresponding regressi-
on lines from 1967 - 1990. The calculated correlation co-
efficients of ozone versus time are highly significant at
between 0.94 and 0.96. The regression analysis reveals a
significant increase of more than 2% per year throughout
the whole troposphere. Compared with other stations this
is one of the strongest tropospheric trends but it is in
remarkable agreement with the + 2.2% per year observed
at the summit of the Zugspitze (42 km south of Hohen-
peissenberg, 2964m a.s.I.) for the period 1978 - 1989
(Schneider, 1992).
" __T .+_:
_ I_4° --- B U. .--
+e
+.I :'l "_i," "+,4" "_" "_l' "lu + "ca" _i_'--'iE+_+K_'-_
_oue
Figure 2: Annual means of ozone at different altitudes within
the troposplnere with regression lines.
About 50 years ago Ehmert (1949/50) described ozone
measurements on an aircraft over Bavaria using Regener's
(1938) potassium iodide method. The partial pressures of
that time were between 2 and 9 nbar throughout the
whole troposphere. Figure 3 shows an attempt to combi-
ne these observations with our tropospheric ozone records.
The extrapolated regression lines of the corresponding
present data agree with the very low ozone data of that
time. Even if such intercomparisons are problematical
because of uncertainties they point out that our strong tro-
pospheric ozone increase might be the effect of a marked
change in human activities within the last decades.
,+e-
s-
I.
_mR
i _,,i,++ t _sa lime i s_l +s_,_ is _o i _ _ l +,+m L,mm. l_P.m
v+_
Figure 3: Course of tropospheric ozone at Hohenpeissenberg
with regression lines extrapolated back to the fourties when
Ehmert performed measurements on aircraft.
4. STRATOSPHERIC OZONE
Figure 1 reveals the vertical trend distribution. Besides the
tropospheric increase mentioned above, a stratospheric de-
crease clearly dominates. It is significant only between 17
and 25 kin. In regard to the long term mean this -0.7 nbar
per year corresponds to approximately -0.5% per year
(Wege et al., 1989).
191
To gain more insight into the temporal and spatial course
of the ozone trend, a dynamic trend analysis was perfor-
med (fig. 4). It was carried out by shifting a 10-year time
span :dong the entire period year by year. The trend was
calculated for each of the_ 10-year windows. Hatched and
cross-hatched areas represent time-altitude ranges where
trends are statistically significant at the 2 and 3 o-level
respectively. It reveals that stratospheric regions with
significant trends are exposed to marked spatial and tem-
poral changes. During the first years the largest decreases
were observed below 26 km, prevailing at around 20 km
and between 12 and 15 km.
_T -<i0"_:,_<,. v__
"" _+ 5... " "-0S ................... -0S ..... "
",..'U_+.,S+".:- ::;(, '........ .-,+": ,:+ +:.':uL_.-.
lilt IOIIAOI$1
Figure 4: Time-height cross-section of the stratospheric ozone
trends at ttohenpeissenherg. The units on the contours are
nhar,/vear.
This decrease became smaller in the following years and
reached significance only at the 2 o-level in the period
from 1977 to 1986 at approximately 18 kin. The centre of
ozone decrease seems to be shifted to higher altitudes at
levels :lbove 30 kin. l Iowever, it must be considered that
the sonde reliability decreases generally at those altitudes.
5. TOTAL COLUMN OZONE
The long-term series of total ozone measured with the
Dobson 104 (fig. 5), exhibits a relative change of approxi-
mately -0.1% per year, i.e. almost no trend. Significance
tests prove negative here. The reason for this lies in the
counteraction of stratospheric ozone decrease by tropo-
spheric ozone increase, which amounts to about 45% since
1967. That is, there has been a shift of the proportion of
ozone between the stratosphere and the troposphere.
Whereas in 1967 the troposheric ozone made up only
about 7% of the total ozone, the value today is 10%.
In addition figure 5 also displays a very pronounced year
to year variability. For example the years 1975, 1983, and
1990 exhibit very conspicuous minima. The minima in
1975 and 1983 can almost certainly be connected with the
volcanic eruptions of Fuego and El Chichon.
MIlL ll"
I¢1,
311"
_O
Itll _7_ y _lA R _ TWJ
Figure 5: Course of total ozone based on annual means.
This connection can also be identified in the longer total
ozone series from Arosa (Di_tsch, 1984). The counterac-
tions between volcanic products and ozone are not yet
completely understood, possible causes for the reduction
in ozone concentration could be:
1. direct ozone destruction due to injected chlorine.
2. heterogeneous processes involving long-living stra-
tospheric aerosols.
3. dynamic processes due to the warming of the aero-
sol-loaded stratosphere, associated with a shift in
the circulation pattern.
6. RECORD OZONE LOWS FOLLOWING THE
MOUNT PINATUBO ERUPTION
During the whole of the winter of 1991/92 very low ozone
values were measured at Hohenpeissenberg. This led, as
figure 6 shows, to the lowest total ozone winter mean of
the series, a decrease of about -9.7% compared with the
long-term winter mean, and -1.6% lower than the minimal
values of the winters of 1974/75, 1982/83, and 1989/90.
m]
II1'
161-
Iti-
1/t-
z
% % % % % %%
Figure 6: Course of total ozone winter means with marked
volcanic eruptions.
The low values measured in the winter of 1989/90 and
indeed the whole of 1990 (fig. 5) can hardly be directly
related to a large volcanic eruption, unlike the other "mi-
nimal" years. It is doubtful whether the two weak volcanic
events indicated have any appreciable influence.
192
qo 2o_o o _ zo,_,',.
"1
i
_25_
_ ,
_ lsj
10_ 88
(x --, ..... WINTER MEAN lqeT/e]
\_ _ WINTER MEAN tS91/gZ
0 3 [NBAR]
)0
i" WINTER N[AW 1957-88
_ .... WtNV[iMEtN190_/a3
-_ -_ - Jo -_ ._o -io 4'0
"1EMPERATURE [oC]
F_o_re 7." Mean ozone profiles for tlze wbzter montl,s De-
cenzher - February and relative difference between winter
91/92 and 82/83.
Figure 7 shows the mean ozone profiles for several win-
ters..As well as the long-term mean for 1967-88, the me-
ans for the winter of 1982/83 after the El Chichon erupti-
on, and that for the past winter were chosen.
h is obvious that almost the complete ozone profile bet-
ween O and 33 krn for the I:[s! winter lies well below ttlat
of the long-term mean profile. Even compared with the
extreme El Chichon winter, the ozone concentration was
vastly reduced at most attitudes. ']'his is pointed out by the
relative difference profile A %, i.e., in the lower stratos-
phere between 11 and 16 km an especially great reduction
of 20-30% compared to 1982-83 occured.
Moreover, the altitudes above the main maximum show a
conspicuous decrease compared to the winter of 1982/83.
Because of the height this can hardly be attributed to a
direct effect of Pinatubo aerosols. Flere again the question
arises as to other additional causes.
The soundings show, besides the extremely low ozone
values, very low temperatures in almost the whole stra-
tosphere. Figure 8 shows the temperature profiles analago-
usly to the last figure. |t can be seen that the past Euro-
pean winter was very unusual. Just as in the ozone pro-
files, the temperatures at altitudes between 9 and 33 km
were very low, so that at an altitude of approximately
26 km a reduction in temperature of 5 K occured. This is
very extreme for a 3-month average.
Fig_re 8: Mean temperature profiles for the winter months
December- February.
193
Therefore, it must be assumed that, besides the direct
ozone reduction by volcanic products, other ozone-redu-
cing processes are involved, whereby the dynamic proces-
ses must be given primary consideration. The past winter
has been distinguished by almost persistent pressure and
temperature anomalies over vast areas of Europe (Geb
and Naujokat, 1991/92). Because of the superimposition
of the Pinatubo effects and the dynamic processes the
individual causes cannot be separated or determined
quantitatively. In addition it must be remembered that the
long-term downwards trend in the stratosphere favours the
minima. This trend might be the anthropogenic compo-
nent of the observed stratospheric ozone loss over Europe
during the last winter.
7. CONCLUSIONS
The long-term sounding data from llohenpeissenberg ex-
hibit pronounced ozone trends. Both increase in tropos-
pheric ozone and decrease in stratospheric ozone are
highly statistically significant. However, the long-term
record of total column ozone shows practically no trend.
The past winter is distinguished by record low values in
both total column ozone as well as stratospheric ozone
profiles.
The causes of this are recognized, however, the size of the
effect of each cause cannot be determined. Nevertheless,
it is certain that the eruption of Mount Pinatubo and
abnormal dynamic processes played the dominant roles.
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194


On long-term ozone trends at Hohenpeissenberg

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