Ozone measurements from the Total Ozone Mapping Spectrometer on the Nimbus 7 satellite are compared with ground-based measurements from five Dobson stations at northern latitudes to evaluate the accuracy of the TOMS data, particularly in regions north of 50 deg N. The measurements from the individual stations show mean differences from -2.5 percent up to plus 8.3 percent relative to TOMS measurements and two of the ground stations, Oslo and Longyearbyen, show a significant drift of plus 1.2 percent and plus 3.7 percent per year, respectively. It can be shown from nearly simultaneous measurements in two different wavelength double pairs at Oslo that at least 2 percent of the differences result from the use of the CC' wavelength double pair instead of the standard AD wavelength double pair. Since all Norwegian stations used the CC' wavelength double pair exclusively a similar error can be assumed for Tromso and Longyearbyren. A comparison between the tropospheric ozone content in TOMS data and from ECC ozonesonde measurements at Ny-Alesund and Bear Island shows that the amount of tropospheric ozone in the standard profiles used in the TOMS algorithm is too low, which leads to an error of about 2 percent in total ozone. Particularly at high solar zenith angles (greater than 80 deg), Dobson measurements become unreliable. They are up to 20 percent lower than TOMS measurements averaged over solar zenith angles of 88 deg to 89 deg

Barthel, K.

Heese, B.

Hov, O.

English

NASA Technical Reports Server

N95- 10641
TOMS TOTAL OZONE DATA COMPARED WITH NORTtIERN LATITUDE
DOBSON GROUND STATIONS
B. Heese
Alfred-Wegener-Institut, Bremerhaven, Germany
K. Barthel, O. Hov
University in Bergen/Nansen Environmental and Remote Sensing Center, Bergen, Norway
ABSTRACT
Ozone measurements from the Total Ozone Mapping
Spectrometer on the Nimbus 7 satellite are compared with
ground-based measurements from five Dobson stations at
northern latitudes to evaluate the accuracy of the TOMS data,
particularly in regions north of 50°N. The measurements
from the individual stations show mean differences from
-2.5% up to +8.3% relative to TOMS measurements and two of
the ground stations, Oslo and Longyearbyen, show a
significant drift of +1.2% and +3.7% per year, respectively. It
can be shown from nearly simultaneous measurements in two
different wavelength double pairs at Oslo that at least 2% of the
differences result from the use of the CC' wavelength double
pair instead of the standard AD wavelength double pair. Since
all Norwegian stations used the CC' wavelength double pair
exclusively a similar error can be assumed for Tromse and
Longyearbyen. A comparison between the tropospheric ozone
content in TOMS data and from ECC ozonesonde
measurements at Ny-/_lesund and Bear Island shows that the
amount of tropospheric ozone in the standard profiles used in
the TOMS algorithm is too low, which leads to an error of about
2% in total ozone. Particularly at high solar zenith angles
(>80°), Dobson measurements become unreliable. They are up
to 20% lower than TOMS measurements averaged over solar
zenith angles of 88 ° to 89 °.
INTRODUCTION
At high latitudes, errors in TOMS total ozone are possibly
larger than is generally assumed. The solar zenith angles are
higher, and there are generally more variations in total ozone
than at middle and low latitudes. When using TOMS data for
studies of the ozone climatology and for correlations with
synoptic meteorological parameters at northern latitudes it is
important to know their accuracy. The objective of this study is
to identify and possibly quantify errors in TOMS data by
comparison with Dobson measurements at five stations at
northern latitudes.
DATA AND METHOD
TOMS data exist in several versions, version 6 being the
most recent. Relative to version 5, the version 6 data have been
corrected for a drift caused by the degradation of the
instrument's diffuser plate using the internal "pair-
justification" technique. These data are in agreement within
the error limits with the average of 39 ground-based Dobson
stations [Herman et al., 1991]. The TOMS data were prepared
215
by the Ozone Processing Team of NASA Goddard Space Flight
Center: J.R. Herman, R. Hudson, R.D. McPeters and R.
Stolarski, Z. Ahmad, X.Y. Gu, S. Taylor, and C. Wellemeyer.
Ten years of GRID-TOMS version 5 data (1979 - 1988) and 4
years of version 6 data (1986 - 1989) have been compared with
five Dobson stations at high latitudes: Oslo (59.5°N,10.5°E),
Troms¢ (69.4°N,18.6OE), Longyearbyen (78.1°N,15.4°E),
Reykjavik (64.1°N,21.5°W), and Resolute (74.4°N, 94.6°W).
The measurements at Troms¢ and Longyearbyen began again
in winter 1984 after a break in 1969, so that there are only 4
years for comparison. The 1979-1985 TOMS version 6 data
became available in spring 1991 when most of the work for this
study was done. Since the objective of this study is to find
reasons for the differences between TOMS and Dobson ozone
estimates and since the drift in the version 5 data for the
respective years is small (0.25% per year from November 1978
to June.1982 and 0.51% per year from July 1982 to December
1985) [Fleig et al., 1988a], the version 5 data for the years 1979 to
1985 are sufficient for this purpose and can be used together
with the version 6 data.
Errors of-+ 2% for the TOMS data [Fleig, 1988b] and -+3% for a
typical good Dobson measurement {Basher, 1982] are assumed,
so that the accuracy of this comparison is +_3.6% (rms).
RESULTS
Figures la and lb show the monthly mean relative
differences (Dobson - TOMS) / Dobson for Oslo and Reykjavik
calculated for ground and satellite measurements taken on the
same day during at least 13 days in the given month. The limit
of 13 days per month is chosen to obtain a sufficient number of
monthly mean values, particulary for the months around the
polar night.
Almost all Dobson measurements at the five stations show
greater annual mean ozone values than the TOMS total ozone
of the version 5 data. Only the first 7 years at the Reykjavik
station have differences less than or equal to zero (Table 1). If
direct-overpass TOMS data, which have a higher resolution of
about 50 km 2, are used instead of gridded TOMS data, their
relative differences to Dobson measurements do not change
significantly.
The relative differences at the stations with long
measurement series show a significant annual drift of +1.0%
at Oslo, +0.7% at Resolute, and +0.5% at Reykjavik, which is
based upon the slope of a linear regression through the data.
These individual drifts are comparable with the two-part
linear fit to the drift found by Fleig et al. [1988a].
Table 2 shows the annual mean percentage differences
between the ground measurements and the TOMS data of the
version 6. Since the drift should be corrected in these TOMS
https://ntrs.nasa.gov/search.jsp?R=19950004229 2020-06-16T11:28:04+00:00Z
data, no drift should occur relative to stable ground stations.
An observed drift in the relative differences at the ground
stations in Oslo and Longyearbyen of +1.2% and +3.7% per
year, respectively, or rather a shift from 1986 to 1987 of 2.5%
and a small drift of 0.6% the following years at Oslo and a
shiftfrom 1987 to 1988 of 2.I% and 7.7% from 1988 to 1989 at
Longyearbyen indicates a driftin these Dobson instruments or
a shift in the observing routine at these stations. Troms¢,
Reykjavik and Resolute show no significant drift, which is
indicated by a low square correlation coefficientr2.
-4_TOMS version5
-e-TOMS version6
2[} i , L , L , L i i J '
?°
-5
-LO , , ' ' ' '
years
a
_90
-e-TOMS version5
--e-TOMS version6
--4- ii_, ,l
15
°
_lL°
_ol o
-I0
79 80 Sl if2 83 84 85 86 B7 88 89 90
years
Fig. 1. Monthly mean relative differences (Dobson - TOMS) /
Dobson at the ground stations in (a) Oslo and (b) Reykjavik.
TOMS version 5 (solid circles) and version 6 (open circles),
markers indicate new year.
For each station the relative mean difference to TOMS for the
years 1986 to 1989 has been calculated from all monthly mean
differences. It is as high as +6.9% and +8.3% at Oslo and
Longyearbyen but lower at the other stations and within the
accuracy of ±3.6% for this comparison: Troms¢ has a mean
difference of +2.9%, and Resolute has a mean difference of
+1.2%, while Reykjavik is the only station with a negative
mean difference of -2.5%.
TABLE 1.: Annual Mean Relative Differences Between
Satellite and Ground-Based Measurements of Total Ozone:
(Dobson-TOMS) / Dobson, (Version 5)
;_-__.----_[o-- _m ;f - -_ ;_; --_-y-__- -_-_l-_-_--
1979 3.1 -1.2 0.04
1980 2.9 -1.7 0.9
1981 3.8 -2,3 0.6
1982 4.3 -1.8 1.2
1983 5.6 -0.3 1.3
1984 5.0 -0.3 1.0
1985 6.5 7.4 9.8 0.02 4.6
1986 7.9 6.6 7.6 1.0 4_2
1987 10.9 5.7 8.4 0.5 32,
1988 12.6 8.0 132, 2,8 6.7
TABLE 2.: Annual Mean Relative Differences Between
Satelliteand Ground-Based Measurements ofTotal Ozone :
(Dobson-TOMS) / Dobson, (Version6)
1986 4.5 4.0 5.3 -1.8 1.7
1987 7.0 2.4 5.7 -3.1 0.1
1988 7.5 1.8 8.8 -2.3 2.3
1989 8.2 3.2 16.5 -2.8 0.5
...............................................
All stations show individual differences relative to the
TOMS version 6 data. While Troms¢, Reykjavik, and
Resolute are within the error range of +3.6% for this
comparison, the difference between TOMS and Dobson at Oslo
and Longyearbyen is greater than 3.6% and increases from
1986 to 1989, indicating a drift in these ground-based
instruments. Some reasons for these differences have been
studied for both the Dobson and TOMS measurements.
Since the relative differences are larger for all Norwegian
stations, a common source of error may be present in the
observation technique. Dobson measurements with the AD
wavelength-pair at direct sun are standard, since they are
least dependent on atmospheric scattering. The C wavelength
pair is to be used together with the C' wavelength pair (332 nm,
453 nm) when clouds are present. Additionally, the pathway of
the radiation through the atmosphere may be longer, which is
quite important for observing stations in the north according to
the relatively high solar zenith angles. However, the ozone
values have to be taken from empirical charts derived from
comparisions with AD measurements, since the algorithm is
valid only for AD measurements [Basher, 1982]. These values
may be less accurate, especially if the charts are not quite
actual. All Norwegian stations measured exclusively with the
CC' wavelength double pair until 1990 to obtain a homogeneous
time series, while the other two stations also used the AD double
pair if weather conditions permited. This choice of the
wavelength pair seems to produce errors.
Simultaneous measurements with the C wavelength pair (311
nm, 332 nm) and the AD double pair (305 nm, 325 nm; 317 nm,
339 nm) taken at direct sun in Oslo by S.H.H. Larsen,
Physical Institute, from May until October 1990 show
systematically higher monthly mean values in observed total
ozone from the C wavelength pair between 9.1 and 21.1 DU.
(Figure 2). These differences amount to 2% - 7% of the observed
total ozone column. So the measurements with the CC'
wavelength pair at the Norwegian stations cause an error of at
least 2% in total ozone.
Thus one reason for the systematically higher Dobson ozone-
values could be found in the permanent use of a different
wavelength pair than the standard AD wavelengths. Since
216
1990 the Norwegian stations have also used the AD wavelength
double pair if weather conditions permit (S.H.H. Larsen,
personal communication, 1991) so this error should decrease
in the measurements aRer 1989.
o_
40O
_0
360
340
320
3OO
28O
260
"e-AD-wavelength
-e-C-wavelength
i i l i i i
month
Fig. 2. Monthly mean total ozone from Dobson measurements
at Oslo with the AD wavelength double pair (solidcircles)and
C wavelength pair (open circles)in 1990.
A second possible error source lies in the correction ofTOMS
measurements to tropospheric ozone, which is measured only
partially because the effective reflecting surface for the
backscattered radiance liesin the upper troposphere. So the
amount below this layer, i.e.,most of the tropospheric ozone,
depends on the standard ozone profiles in the TOMS
algorithm. These standards are fixed climatological, annual
means. Thus seasonal and day to day variations up to 50% in
tropospheric ozone and a long term trend of 0.5 - 1.0% per year
observed by sonde measurements at several stations in the
northern hemisphere [Bojkov, 1988] will not be measured by
TOMS.
To study the resultant error, the ozone amount in the two
lowest layers (up to 240 hPa) of the TOMS standard profiles
were compared with the amount in the same layer measured by
electrochemical concentration cell (ECC) ozonesondes at Ny-
Alesund and on Bear Island in 1989. The layers from ground
up to 240 hPa represent the troposphere only roughly but allow
for an approximation of the error size. ECC sonde frequency
was irregular, in particular at Ny-Alesund, and some sondes
at both stations were destroyed at low altitude. They are
omitted, and the remaining number of sonde measurements
are 25 for Ny Alesund and 28 for Bear Island. The sonde data
have been made available by the German Alfred Wegener
Institute (AWl), the Norwegian Institute for Air Research
(NILU), and the Norwegian Meteorological Institute (DNMI).
Figure 3 show that the tropospheric ozone of almost allECC
measurements at Ny-Alesund are higher than in the TOMS
standard profiles, especially in summer. The same was found
for Bear Island. This shows that the seasonal variations of
tropospheric ozone, i.e., the higher values in summer, were not
measured by TOMS.
The resultant error in total ozone can be seen from the
relative differences between TOMS and ECC sonde
measurements (Figure 4). Averaged over all days with
observations, the mean difference is +2.3% at Ny-._lesund and
+0.8% at Bear Island. At these two stations, roughly 1 to 2%
more total ozone is measured by the sondes than by TOMS. To
find the part of this difference caused by the standard ozone
profiles in the TOMS algorithm, the tropospheric ozone in
TOMS totalozone isreplaced by the value measured by the ECC
sondes. Then the averaged relative difference in total ozone
changes significantly at both stations by about -2% (Figure 4).
The ECC measurements are quite good in the troposphere,
about ±2% error (Komhyr, 1985) Hence, the TOMS total ozone
measurements are at least 2% too low and show seasonal
variations because of the dependence of the tropospheric ozone
amount in the standard ozone profiles in the algorithm.
80
70
r_
60
o_
f_
_d3
_J
"F.
40
o
o_ 3o
20
• trop.ozoneTOMS
o trop.ozoneECC
, i i L i
o
"o
o1_ °o o
e_, e
• • e_
o •
0 flO 120 180 240
julian days
o
Oo
QII
i i
a
Fig. 3. Tropospheric ozone from TOMS standard profiles and
ECC sonde measurements in Ny Alesund in 1989.
25
15
-5
-15
-25
• original
O trop. ozone corrected
i J i i i
o
0
o •
o
a
, i r , j
julian days
Fig. 4. Relative differences in total ozone between TOMS and
ECC sonde measurements (ECC - TOMS) / ECC in Ny
A]esund. Original TOMS measurements (solid circles) and
TOMS with tropospheric ozone corrected (open circles),
217
Another possible error source is the high solar zenith angle
(SZA) at the time of observation. The relative differences
between TOMS version 6 and Dobson measurements for the
years 1986 to 1989 versus the TOMS SZA (Figure 5) show lower
values as well as larger scatter at SZA greater than 80 °. The
larger scatter is probably caused by the higher dependence of
TOMS total ozone on the standard vertical profile, since the
reflecting surface moves upward into the ozone layer at high
SZA [Dave and Mateer, 1967; Pommerenu, 1989] and the path
length of UV radiance through the atmosphere becomes
greater.
At the Reykjavik station, three February mean relative
differences in the 4 years of the version 6 data were as low as
-6.7%,-8.6%, and-6.3% (Figure lb). These negative
difference may partly be caused by the relatively high number
of measurements at high SZA in February. Large increase in
TOMS data relative to Dobson for measurements at high SZA
were found especially at Reykjavlk (Figure 5) and Resolute,
When averaged over 1 ° intervals of the SZA, the Dobson
measurements at Reykjavik are up to 20% smaller than TOMS
at SZA of 88 o to 89_.
This is contrary to the results reported by Pommerenu [1989],
who found that the TOMS version 5 ozone content decreases
relative to SAOZ (Systbme d'Analyse par Observation
Z_nithale) measurements at Dumont d'Urville and
Greenland with increasing SZA. The SAOZ is constructed to
measure at high SZA, so that these measurements should be
more accurate and the error should lie in the TOMS data.
Pommereau has found that the differences are largely reduced
when using the TOMS version 6 data because of the additional
standard profiles in the new algorithm for polar conditions
(J.P. Pommereau, personal communication, 1991).
Since Dobson measurements are taken at SZA higher than 80 °
at the five stations, where they become unreliable [Komhyr,
1980], the results from Pommereau and from Figure 5 indicate
that the Dobson measurements at SZA higher than 80 ° are
particularly inaccurate.
40
3)
20
10
0
-I0
-20
-3O
-40
3O
SZA-TOMS, °
Fig. 5. Relative differences in total ozone at Reykjavik
versus the solar zenith angle (SZA) at the time of TOMS
measurements.
SUMMARY
The comparison between total ozone measurements from
satellite (TOMS) and ground-based (Dobson) measurements
show significant differences between the ground stations.For
the years 1986 to 1989 the relative mean difference to TOMS
can be calculated for each station. It is as high as +6.9% and
+8.3% at Oslo and Longyearbyen but lower at the other stations
and within the accuracy of +3.6% for this comparison: Troms_
has a mean difference of +2.9%, and Resolute has a mean
difference of +1.2%, while Reykjavik is the only station with a
negative mean difference of-2.5%.
A drift of +1.2% per year in eels and +3.7% per year in
Longyenrbyen is found in the relative differences. Since there
is no drift in the TOMS data from the algorithm version 6, the
observed driR may be due to some drift in the Dobson
instrument at the ground stations.
It can be shown from nearly simultaneous measurements in
two different wavelength double pairs in Oslo that at least 2%
of the difference results from the use of the CC' wavelengths
instead of AD wavelengths. Since all Norwegian stations
have used the CC' wavelengths exclusively, a similar error
can be assumed for Troms_ and Longyearbyen. Since 1990 the
Norwegian stations have also used the AD wavelength double
pair if weather conditions permit (S.H.H. Larsen, personal
communication, 1991) so this error should decrease in the
measurements after 1989.
Another error source is mainly caused by the use of standard
ozone profiles in the TOMS algorithm. The comparison
between the tropospheric ozone amount in these standard
profiles and from ECC ozonesonde measurements shows that
the tropospheric ozone in the standard profiles used in the
TOMS algorithm is too low, and this leads to an error of about
2% in total ozone.
Solar zenith angles in northern latitudes are higher than 80°
during a significant part of the year. The relative differences
between TOMS and Dobson totalozone show a higher scatter at
large SZA, so that daily ozone values, in particular, may be in
error.
_CES
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TOMS total ozone data compared with northern latitude Dobson ground stations

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