A Brewer Ozone Spectrophotometer has been in service at the Canadian ozone monitoring station at Stony Plain (53.55 deg N, 114.10 deg W), near Edmonton, Alberta, since 1984. During that time, the instrument has been operated in a fully automated mode that includes the collection of morning and evening Umkehr observations. Some 197 Umkehr observations have been analyzed to make an estimate of the temporal trend in ozone amount at high altitude over the station during the last 8 years. This work has shown that at 40 km the trend in the ozone concentration has been observed to be 0.14 plus or minus 0.10 percent per year

Hare, E. W.

Kerr, J. B.

Mcelroy, C. T.

English

NASA Technical Reports Server

N95- 10637
36ss57
OZONE TRENDS ESTIMATED FROM UMKEHR OBSERVATIONS
MADE AT EDMONTON, ALBERTA, CANADA
C.T. McElroy, E.W. Hare and J.B. Kerr
Atmospheric Environment Service
4905 Dufferin Street
Downsview, Ontario, M3H 5T4
ABSTRACT
A Brewer Ozone Spectrophotometer has been in
service at the Canadian Ozone monitoring station at
Stony Plain (53.55 ° N, 114.10 ° W), near Edmonton,
Alberta, since 1984. During that time, the
instrument has been operated in a fully automated
mode that includes the collection of morning and
evening Umkehr observations. Some 197 Umkehr
observations have been analyzed to make an estimate
of the temporal trend in ozone amount at high
altitude over the station during the last 8 years.
This work has shown that at 40 km the trend in the
ozone concentration has been observed to be
0.14±0.10 percent per year.
1 INTRODUCTION
Trends in the global ozone amount have been
estimated by several independent methods. These
have included the estimate of trends in high level
ozone using the Dobson Spectrophotometer Umkehr
results [DeLuisi and Nimira, 1979], the TOMS data
set [Stolarski et al., 1989], and values estimated
using the global total ozone data set [Bojkov and
Mateer, 1984]. The TOMS data set is nearly
independent of the ground-based measurements of
ozone in the Version 6.0 form, since the values
have been recalculated using an algorithm which
suppresses the effects of diffuser degradation
[Herman et al., 1991]. The Brewer Umkehr data set
offers an additional, independent method for the
estimation of ozone trends at high altitude.
The Brewer Ozone Spectrophotometer has been in
commercial production for i0 years. During that
time a large number of ozone observations have been
made at Canadian ozone monitoring sites. A Brewer
spectrophotometer has been in service at Toronto
since 1982, and at Edmonton since 1984. Umkehr
observations [Gotz, 1930; Mateer, 1984; McElroy,
1988] are made automatically as part of the daily
Brewer observation schedule. Since 1984 over 500
(see Table i) Umkehr observations have been
attempted at Edmonton (in the first quarter of each
year). Many of these were unsuccessful because of
the variability in light level which occurs during
cloudy weather, but a large number are available
for analysis.
The Edmonton data have been chosen to make an
ozone trend estimate for several reasons. The
Umkehr series from Edmonton is among the longest
Brewer data set available, the Edmonton weather
conditions are quite good, there is access to good
correlative data (ozonesondes), and the Edmonton
Dobson data have already been analyzed to determine
an ozone trend [Mateer, 1984].
199
2 DATA COLLECTION
The Umkehr data are collected by turning the
Brewer instrument sun director prism toward the
zenith and observing the intensity of light
scattered vertically downward by the sky at 8
wavelengths in the ultraviolet. The Brewer is
designed to make nearly simultaneous measurements
at 5 different wavelengths using time multiplexing
[Kerr et al., 1980]. Depending on the setting of
the spectrometer diffraction grating angle,
observations are made at 306.3, 310.1, 313.5,
316.8, 320.1 nm or 316.5, 320.1, 323.2, 326.4 and
329.5 run. Measurements are made alternately at
these two settings during sunrise or sunset. Data
from three wavelength pairs are simultaneously
fitted in the analysis process.
The Brewer instrument is equipped with a
polarizer which is mounted in the foreoptics.
Measurements of the strong polarization component
of the zenith sky light are made by properly
orienting the instrument. The whole spectrometer
is rotated in azimuth under computer control to
maintain the direction of polarization during the
Umkehr period (60 to 90 degrees solar zenith
angle).
The data collected each day at the
observation site are stored on a floppy disk by the
instrument control software. Floppy disks are
filled and duplicated at the rate of 1 per week and
the copied data are sent to Toronto where they are
loaded onto the Brewer data base computer. The
results presented here were analyzed after
retrieval from the data base.
3 DATA ANALYSIS
The data collected by the Brewer are analyzed
using software based on the algorithm of Mateer
[1984] (the "short Umkehr method") which uses the
technique of Rodgers [1976] to make an optimal
estimate of the ozone profile based on both the
Umkehr data and a knowledge of ozone climatology
based on the analysis of a large ozonesonde data
set Mateer et al. [1980].
In the analysis, the data are first
preprocessed to produce interpolated intensity
values at the Mateer "standard Umkehr angles"
(60.0, 65.0, 70.0, 74.0, 77.0, 80.0, _3.0, _5.0,
86.5, 88.0, 89.0, 90.0 degrees solar zenith angle).
The processing includes correcting the data fo_
instrument dark count, non iinearity, and
interpolating by a least squares spline algorithm
at the standard angles. The data in the iongel and
shorter wavelength sets are interpolated to the
same angles {see Figure 1) . The prep_:-,cessing
stage also includes looking up the app_oD_iate
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So!or Zenith Angle (0 1° urn;is)
Vi_ze I This figure shows the fitted curve and
the raw data points output by the preprocessor
for the longest and shortest wavelengths observed
by the Brewer spectrophotometer.
Brewer Umkehr
6015O
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- 00.l 0 0 00.'1 0.12 (3'3
Averaging Kernels
1
0,4
Figure 2 The averaging kernels for the Brewer
Umkehr inversion algorithm. Note that the
performance of the algorithm is best between 20
and 40 km.
ozone value for each Umkehr curve from the total
ozone file for the observing station. All these
data are written out in a terse format for further
analysis by the Umkehr inversion software. A file
is written for each day and instrument, and
includes both a morning and evening Umkehr curve.
Quality control is done both at the
preprocessing stage and at the Umkehr analysis
step. The output from the preprocessor is examined
for those days where the spline fitting process
does not yield a good estimate of the observed data
because of large variations between successive data
points. If the RMS error between the observed and
the fitted data is excessive the Umkehr is
rejected.
If the curves cannot be inverted in 2 or 3
iterations of the inversion algorithm, the results
are also rejected. Excessive differences after the
second inversion iteration indicates that the
observations are not well described by the range of
curves represented by the climatology (perhaps due
to instrument problems or observing conditions).
4 RESULTS
Table i shows the statistics for the success
rate for Umkehr observations at Edmonton during the
first quarter of each year between 1984 and 1991
inclusive. Of the possible 1444 Umkehrs which
could have been observed, 518 were actually logged
in the data base. Of these, 327 passed the
preprocessing screening step, and 197 actually
produced acceptable, inverted profiles. This
success rate roughly parallels the "good data" days
for Edmonton based on the success rate for total
ozone observation attempts.
Table 2 shows the statistical report from the
Umkehr analysis program for the data collected in
1991. For each year, all of the data collected in
the first quarter were analyzed together to produce
a mean profile, and an estimate of the variability
of the results for the quarter using the "good"
Umkehr curves. Since the same collection ;riteria
were used for all curves, the results for different
years should be comparable.
The amount of ozone in each Umkehr layer is
recorded as a function of year in Table 3. The
averaging kernels [Rodgers, 1990] shown in Figure
2 indicate the height range over which the Umkehr
method produces its best results. A profile
typical of those inverted from the 1992 Umkehr data
is shown in Figure 3. Also included in Figure 3 is
the climatological profile which was use([ in the
estimation of the inverted results. The calculated
rates of change of ozone based on these _ata are
presented in Table 4. These rates apply to the
actual observed ozone amounts and have not been
corrected for physical effects such as t-]e solar
cycle variation of the solar flux.
5 CONCLUSIONS
These results do not show a significant long-
term change in ozone concentrations _t high
altitudes above Edmonton, The SAGE results
presented elsewhere in this proceedings have
indicateda small, but statistically insignificant
trend [McCormick, this issue] which is consistent
with the results presented here. Howev{:r, they
demonstrate the _aality and quantity of data which
are available using the Brewer in its fully
automated mode. Based on the success of this
analysis, it is recommended that Brewer Umkehr
observations from all stations in the global
network be analyzed with the current a!qorithm and
published in Ozone Data for the World as soon as
possible.
200
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<
10
Brewer AM ii/01/91
50-
_ C_imalology
0
OE+O00 ' 2E+r012 4[+'012 ' 5E+'O12
Concentration (mol/cm 2)
Figure 3 This is a sample of the output of the
inversion algorithm presented as an altitude
profile of ozone. Both the climatological
"first-guess" and the retrieved profile are
shown.
YEAR Chances Obs Prepro Complete % Obs
1985 180 26 7 6 46.2
1986 180 12 1 1 8.3
1987 180 44 29 22 50.0
1988 182 48 30 19 22.9
1989 180 84 64 35 41.7
1990 180 76 56 40 52.7
1991 180 i00 77 41 41.0
1992 182 128 63 33 26.0
Total 1444 518 327 197 38.0
Table I. Observation statistics for Edmonton Umkehr
observations.
Solution Statistics for 41 Profiles
Ozone Obs.:0.3502±0.0438 Soi'n:0.3512±0.043
Avg. Residual = 0.52±0.11 Iterations = 103
Layer i0 9 8 7 6 5 4 3 2 1
AVG 1.5 4.0 ii 21 35 63 92 64 34 26
_._S .i .3 1 2 4 9 ii 13 9 2
% Err 19 17 15 Ii ii Ii Ii 13 16 19
Table 2. Statistics for the processing of 1991
Umkehr data from Edmonton.
Layer 1985 1986 1987 1988 1989 1990 1991 1992
i0
9
8
7
6
5
4
3
2
1
1.55 1.41 1.54 1.59 1.52 1.50 1.53 1.52
4.23 3.49 4.13 4.24 3.97 4.01 3.95 3.99
11.5 9.52 11.3 10.6 10.9 ii 5 i0 5 10.3
22.0 19 6 22.3 20.4 22.4 23 5 21 4 21.7
35.0 36 3 36.7 34.3 37.2 37 4 35 1 34.9
60.5 73 2 67.7 62.4 63.7 64 8 62 6 59.7
86.7 98 4 95.1 90.9 88.4 92 9 91 5 88.6
70.8 69 0 69.5 66.6 66.4 72 2 64 4 70.1
41.9 38 4 40.1 36.7 37.0 41 1 34 1 38.4
29.0 27 5 28.1 27.4 27.1 28 1 26 3 27.5
Number 6 1 22 19 35 40 41 33
Table 3. Edmonton ozone amounts (in Dobson Units
in each of the Umkehr layers (see Table 4 for
heights).
Layer Altitude Trend 1985-92 Trend 1987-92
(%/year) (%/year)
8 37.7 - 42.8 0.14±0.10 -0.87±0.10
7 32.8 - 37.7 0.69±0.07 0.15±0.i0
6 28.1 - 32.8 -0.06±0.05 0.51±0.12
5 23.6 - 28.1 -1.20±0.10 -1.70±0.10
4 19.1 - 23.6 -0.39±0.07 -0.82±0.06
3 14.7 - 19.1 -0.35±0.05 -0.09±0.10
Table 4. Ozone trends estimated using the data in
Edmonton Umkehr data.
ACKNOWLEDGEMENTS
The authors wish to express sincere appreciation of
the valuable comments contributed by the referees.
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Ozone trends estimated from Umkehr observations made at Edmonton, Alberta, Canada

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