The Brewer Ozone Spectrometer is being used in the World Ozone Network to monitor ozone and SO sub 2. SO sub 2 from natural as well as anthropogenic sources are measured. It has been demonstrated that SO sub 2 interferes with total ozone values as measured by the Dobson Spectrophotometer and the Total Ozone Mapping Spectrometer (TOMS). A small amount of manmade SO sub 2 is difficult to detect and quantify by TOMS because it is located near the surface. However, larger amounts of SO sub 2 injected into the stratosphere from volcanic emissions are detected by TOMS

Evans, Wayne J.

Kerr, James B.

English

NASA Technical Reports Server

COMPARISON OF GROUND BASED AND TOMS MEASUREMENTS OF SO2 
FROM VOLCANIC EMISSIONS 
James B. Kerr and Wayne J. Evans 
Atmospheric Environment Service 
The Brewer Ozone Spectrophotometer is being used in the 
World Ozone Network to monitor ozone and S02. SO2 from both 
natural as well as anthropogenic sources are measured. It has 
been demonstrated that SO2 interferes with total ozone values as 
measured by the Dobson Spectrophotometer and TOMS. A small 
amount of manmade SO2 is difficult to detect and quantify by TOMS 
because it is located near the surface. However, larger amounts 
of SO2 injected into the stratosphere from volcanic emissions are 
detected by TOMS. 
There have been some instances when the Brewer instrument 
was measuring SO2 from the ground at the same time TOMS was 
measuring it from space. Results of these comparative measure- 
ments are presented. 
Introduction 
The Brewer Ozone Spectrophotometer and TOMS measure column 
amounts of ozone and SO2 by ultraviolet spectroscopy. The Brewer 
measures both natural and anthropogenically produced SO2 to a 
precision of about 1 matmcm and TOMS measures stratospheric SO2 
which usually results from volcanic emissions. 
There have been three instances when TOMS detected SO2 from 
space on the same day that the Brewer was measuring it from 
below. These three occasions were after SO2 was injected into 
the stratosphere as a result of volcanic emissions. The three 
eruptions were Mount St. Helen's on May 18, 1980, El Chichon on 
March 28 - April 3, 1982 and Krafla, Iceland on September 5, 
1984. Results of the SO2 measured in these three volcanic clouds 
are presented here. 
Method 
Both ozone and SO2 have strong absorption bands at wave- 
lengths in the ultraviolet. The absorption spectra for these 
gases between 300 and 325 nm at a resolution of .5 nm is shown in 
Figure 1. The A and B short wavelengths for TOMS as well as the 
five operational wavelengths for the Brewer are indicated. 
The method to measure SO2 and ozone using the direct sun 
absorption technique with the Brewer Spectrophotometer has been 
described by Kerr et al. (1980). In this method,the absorption 
spectra of each gas have been carefully considered prior to 
selecting the five operational wavelengths. The measured solar 
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light intensities at the five wavelengths are used in two 
algorithms: one is insensitive to absorption by SO2 and the 
other is insensitive to ozone. Kerr et al. ( 1 9 8 4 )  have demon- 
strated that total ozone as measured by the Brewer is indeed 
unaffected by the presence of SO2 during pollution events at 
Toronto. 
I It is evident from Figure 1 that the absorption by SO2 at 
the TOMS A and B short wavelengths is about 2.5 times that of 
ozone. Therefore, the presence of 1 matmcm of atmospheric SO2 
causes an apparent increase of 2.5 matmcm ozone for both the A 
and B measurement. Because ozone affect the A and B 
between the two gases using the standard ozone inversion algo- 
rithms. Following the technique which is used to measure SO2 by 
the Brewer, an algorithm has been developed for discriminating 
between SO2 and ozone has been successfully used to extract 
stratospheric SO2 amounts from the TOMS irradiance data. 
Results 
There have been two occasions when significantly large SO2 
amounts from volcanic emissions were measured by the Brewer from 
the ground on the same days as TOMS measurements were made from 
space. shows the TOMS SO2 measurements for May 19-20,  
1980 .  Here the SO2 which was emitted by the May 18 eruption of 
Mount St. Helen's is clearly evident. SO2 values as large as 100 
matmcm were observed on May 20  with typical values of about 60 to 
70 matmcm. The leading edge of the cloud was located about 3 0 0  
km southwest of Toronto (indicated by x) at about 17:OO GMT when 
the TOMS map was made. The ground based Brewer SO2 measurements 
for May 20-23, 1 9 8 0  are shown in Figure 3. At about 19:OO GMT on 
May 2 0  the SO readings began increasing and peaked about 70 
matmcm at 23:OO 6 MT. The following day SO2 values remained 
between 30 and 40  matmcm before returning to near normal values 
near the end of the day. In general, the results of the TOMS and 
Brewer SO2 measurements are in good agreement with regard to both 
the measured time of arrival of the cloud in the Toronto area as 
well as the measured values of SO2 within the cloud. 
The second occasion when both the Brewer and TOMS measured 
significant values of SO2 on the same day was on September 7, 
1 9 8 4  over Norrkoeping, Sweden. This was two days after an 
eruption of the Krafla volcano in Iceland. Figure 4 shows the 
drift of the SO2 cloud from September 5 to 7 as measured by TOMS. 
This map suggests that the cloud passed over Norrkoeping (indi- 
cated by N) sometime between September 6 and 7. SO2 values 
within the cloud are typically between 30  and 40  matmcm. 
The SO2 values as measured by the Brewer for September 7 are 
shown in Figure 5. Observing conditions on this day were very 
good with a clear sky all day. For comparison SO2 on September 6 
was typically .4  matmcm and on September 8 it was about 1.0 
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matmcm. The values in excess on 40 matmcm on September 7 are 
attributable to the Krafla eruption. The ground based measure- 
ments suggest that there are at least two times during the day 
that the SO2 column exceeded 40 matmcm: once in the early 
morning and later in the mid afternoon. When the ground based 
record is compared with the TOMS measurements, the early morning 
large values are consistent with the passage of the cloud as 
measured by TOMS. However, there is no evidence of the second 
(mid afternoon) cloud on the TOMS record. Airmass trajectory 
studies suggest that the SO2 in the later cloud is at pressures 
between 500 and 700 mb in the troposphere. It is quite likely 
that the early morning peak is the stratospheric SO2 which 
arrived over Norrkoeping a few hours ahead of the tropospheric 
S02. Also, it appears that the lower level SO2 was "hiding" and 
was not detected by TOMS. 
Following the eruption of El Chichon, measurements were made 
of SO2 in the remnants of the volcanic cloud. Measurements were 
made five weeks after the initial eruption when the SO2 had 
dispersed significantly. On this occasion, the Brewer made 
measurements from a NASA CV-990 aircraft which flew under the 
volcanic debris between 20" and 50° N latitude at a longitude of 
about 125O W. Figure 6 shows the TOMS measurements for May 6, 
1982 with the flight path of the Brewer indicated. There is 
evidence of slight traces of S02, however, for the most part, the 
measurements are within the noise of TOMS measurements. Figure 7 
shows the underpass measurements made from an altitude of 35,000 
ft. There was an abrupt increase from background values of SO2 
to about 5 matmcm at 35" N. The value of matmcm SO2 is quite 
likely to be below the detection limit of TOMS. 
The TOMS figures were provided by Arlin J. Krueger, GSFC. 
References 
J.B. Kerr, C.T. McElroy and R.A. Olafson, Measurements of ozone 
with the Brewer ozone spectrophotometer, Proceedinqs of the 
International Ozone Symposium, Boulder, CO, pp 74-79, 1980. 
J.B. Kerr, C.T. McElroy, D.I. Wardle, R.A. Olafson and W.J. 
Evans, The automated Brewer ozone spectrophotometer, Proceedinqs 
of the Quadrennial Ozone Symposium, pp 396-401, Halkidiki, 
Greece, 1984. 
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Comparison of ground based and TOMS measurements of SO2 from volcanic emissions

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