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On the variation of SO2 emission from volcanoes
Journal of Volcanology and Geothermal Research, 33 (1987) 231-237
231
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
ON THE VARIATION OF SO2 EMISSION FROM VOLCANOES LAWRENCE L. MALINCONICO, Jr. Department o[ Geology, Southern Illinois University, Carbondale, IL 62901, U.S.A. (Received November 25, 1986)
Malinconico, L.L, Jr., 1987. On the variation of SO., emission from volcanoes. In: S.N. Williams and M.J. Carr (Editors), Richard E. Stoiber 75th Birthday Volume. J. Volcanol. Geotherm. Res., 33:231-237
Introduction In 1973, Stoiber and Jepsen used remote sensing techniques to measure the S02 flux from several volcanoes in Central America. Using these data, they estimated that the global contribution of SO2 from volcanoes was on the order of 7 × 10Gmetric tons per year. This initial estimate neglected sulfur in any form other than gaseous SO2 and did not include the contribution from erupting volcanoes. This number was subsequently modified by Stoiber et al. (1983b) and Stoiber et al. (1987, this volume ) to around 1.87× 107 tons per year and includes sulfate aerosols, and emission from erupting volcanoes. Another important application of remote sensing of S02 from volcanoes is forecasting of eruptions. This has proven feasible at Mt. Etna (Malinconico, 1979) and Mount St. Helens (Casadevall et al., 1981). For both of these applications, it is important to understand the long- and short-term patterns of SO2 emission from volcanoes. Continuous monitoring of all volcanoes is at best impractical. Only Kilauea and Mount St. Helens have nearly continuous records and 0377-0273/87/$03.50
these only since 1979 and 1980 respectively. However, as part of an on-going research program, the SOz flux from several volcanoes in Nicaragua, Guatemala, and Sicily have been measured on a fairly regular basis. The object of this paper will be to examine the results from some of those volcanoes and interpret how they might effect flux estimate and prediction techniques.
Instrumentation and techniques The instrument used to remotely measure the SOz flux is a correlation spectrometer (COSPEC) developed by Barringer Resarch in Toronto, Canada, primarily for environmental monitoring of SOz. The theory of the COSPEC operation has been well documented and interested readers are referred to the following papers: Newcomb and Millan (1970), Moffat and Millan (1971), Millan et al. (1976), Millan (1978, 1980) and Millan and Hoff (1978). The COSPEC provides straightforward techniques for remotely monitoring the SOz flux from volcanoes. The instrument is compact, durable, and can be used in a variety of modes.
© 1987 Elsevier Science Publishers B.V.
232
Since 1972, the measurement techniques have gradua41y evolved and been refined. Three different techniques can be used depending upon the available access to a given volcano. These include ground-based stationary and mobile techniques, and airborne techniques, and are discussed in detail in Stoiber et al. (1983a). The uncertainty of an individual SO2 measurement is an important consideration. This has also been discussed in Stoiber et al. (1983a) and will only be summarized here. The largest source of' uncertainty is introduced in the determination of wind speeds. Other sources of uncertainty include calibration cell determination, chart record reading error, and distance and location determination. Increasing pathlength between the plume and sensor sometimes has the effect of reducing the observed SO2 value, We have not been able to determine a factor to use to correct for increasing pathlength, so the values reported should simply be considered minimums. Other factors also effect the observed value. Plume opacity (caused by concentrations of ash in the plume ) tends to reduce the measured signal while the presence of water droplets in the plume tends to increase (by scattering) the radiation path through the plume thereby increasing the measured signal, The total uncertainty, not including signal attenuation, generally ranges from ± 15 to ± 25% and the worst case observed has been approximately ±45%. These estimates of uncertainty are important because it suggests that variations of the SO2 daily flux of 2 to 3 fold can be considered significant, SO2 v a r i a t i o n s a s p r e c u r s o r s to e r u p t i v e activity As stated earlier, one of the important applications of remotely monitoring SO2 emissions from volcanoes is to try to forecast when an eruption might occur. This has met with a fair degree of success for the eruptions that followed the catastrophic eruption of M o u n t St. Helens on May 18, 1980 (Casadevall et al.,
1981 ). The observation that the SO2 emission actually does increase before an eruption was first made at Mt. Etna by Malinconico (1979). Figure 1 is a summary of that work and shows how the emission rate increased at varying time intervals before eruptive activity. However, an important observation must be made concerning the use of such measurements for prediction purposes. This technique can only be effective if the non-eruptive background level and variability of S02 emission is known for each volcano. The data collected at Mt. Etna had no reference level until an eruption was actually monitored. At that point it was then possible to make the observation that, for Mt. Etna, if the SO2 flux reached a value equal to approximately 2 to 3 times the "background" levels (the open circles in Fig.1 ), there was then a high probability of eruptive activity. For the short interval monitored, those values seemed to be approximately 1 0 0 0 t d 1 ( metric tons per day ) for non-eruptive "background" levels (open circles) with a 2- to 3-fold premonitory increase before the eruption (stippled circles) and at least a 5-fold increase during the eruptive event (stars). Such information on the amount of flux increase associated with eruptions is probably only valid on a volcano-by-volcano basis. Figure 2 shows the variation in SO2 flux for Kilauea volcano for a three-month time period during the summer of 1979. Note that the scale of the SO2 flux is now 80 to 300 t d-1 versus the 0 to 5500 t d- 1 range for Mt. Etna. The average daily emission during this period was approximately 160 t d 1 ( + / _ 25 t d-1 ). The short-term variation in the flux apparently represents a normal variation during non-eruptive "quiet degassing" conditions of the volcano. However, around day 55, there was an increase from a background level averaging approximately 160 t d-1 to a daily average flux of almost 300 t d 1 for several days. This is almost a 2-fold increase in the flux levels and by M t . E t n a criteria this might have signaled that an eruption was possible. In fact, while
233
Mt. Etna
.--. ~
5500. 5000. 4500. 4000. 3500 ' 3000 '
SO 2Flux
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1
1500 • 1000 • 50O 0
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I
I
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10
15
20
25
30
35
~
"EruptiveDegassin~
•
"Premonitory Degassing"
O
"ActiveDegassing"
Day (beginning 7/25/77) Fig. 1. The daily average SO2 emissions for Mr. Etna volcano, Sicily for the time period from July 25 through August 27, 1977. Each symbol represents the average of from 2 to 8 measurements made during a given day. The different symbols correspond to different levels of emission. During these measurements, the general state of the volcano was considered to he "active". Only the measurements represented by the stars were obtained while the volcano was actually erupting.
K i l au ea
.~ --~
tOO
30O 280 26O 240 220 200
?
180 160 140 120 100
SO2 Flux 0 •
80
i
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10
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20
30
40
50
60
70
80
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90
"Active Degassing?" "Quiet Degassing"
I
100
Day (beginning 6/10/79) Fig. 2. The daily average SO2 emissions for Kilauea volcano, Hawaii for the time period from June 10 through September 21, 1979. Each symbol again represents the daily average of between 2 and 8 single measurements. These measurements were all made when the volcano was "quiet". Most of the variation probably represents the natural fluctuation of the system in response to environmental factors. However, the measurements shown by the open circles may represent a transition from "quiet degassing" to "active degaseing". Around day 55 there was harmonic tremor and a seismic swarm which were interpreted as an intrusive event in the SW rift zone.
there was no extrusive activity during this time,
volcano from "quiet degassing" to "active
harmonic tremor and increased seismic activity were recorded. This was interpreted as being caused by a deep intrusive event located in the SW Rift zone. Thus the increase in SO2 emission during this time, was probably in response to the intrusive event. What this may have represented is a change in the overall state of the
degassing". The main difference between the sets of data shown for Mt. Etna and Kilauea may be proximity in time of the volcano with respect to eruptive activity. The Mt. Etna data were obtained during a period of significant "activity", just after an eruptive event had stopped
234 and continuing through three other eruptive episodes. On the other hand, the Kilauea data were taken at a time when there was no evidence of eruptive activity, other than the slight harmonic tremor and seismic swarm ( intrusive event? ). The volcano, while emitting gases, was considered to be in a state of "quiet degassing". For Mt. Etna, emission levels during a "quiet degassing" period are probably significantly lower than the 1000 t d 1average shown in Fig. 1. The 330 t d ~ ( + / - 100) SO2 flux for Mt. Etna measured by Stoiber et al. (1975) probably represent emission levels during a noneruptive "quiet degassing" period, What is being suggested by these data is that there may be different levels of"non-eruptive" SO2 emission that vary in response to the proximityin time o f t h e volcano to eruptive activity, I have chosen to use the terms "quiet degassing", "active degassing", and "premonitory degassing" to describe the different levels. Note that the term "active degassing" is not used here to indicate emissions during eruptions, but simply a period when the volcano may be closer in time to an actual eruption. The measured SO,~ emissions from San Cristobal volcano in Nicaragua tend to support this concept of emission variability with time. Figure 3 shows the SO~ flux data for San Cristobal for the period 1971 through 1982. San Cristobal had been dormant since 1685 (Hazlett, 1977). In mid 1971, gas emissions began and the non-eruptive SO2 flux levels increased to a maximum in 1976 after which time they began to decrease. The high in the emission rate in 1976 corresponds to a time when the volcano was having small eruptions of ash and would be generally considered "active". ( None of the values plotted represent measurements made when the volcano was actually erupting). The values measured in 1972-1974 and 1980-1981 were obtained when the state of volcanic activity was "quiet". While Figs. 1 and 2 represent short-term fluctuations in SO2 emission, the data for San Cristobal represent long-term variations of the "non-eruptive" level of emission. These variations, are
obviously related to the state of activity of the volcano and can probably be used to provide a long-range indication of the probability of an eruption. In the case of San Cristobal volcano, eruptions may not be probable until the "noneruptive" emission levels reach at least 1500 to 2000 t d 1. This increase may occur a long time period before an eruption, thus providing a longterm signal of impending activity. Then, presumably, short-term "premonitory" variations like those observed at Mt. Etna, could be used to more precisely define the time of the eruption. The data shown in Figs. 1-3 represent three different kinds of non-eruptive SO2 flux variations from volcanoes. The first, as was observed at San Cristobal (Fig. 3), is a very long-term (on the order of months to years) variation in the "non-eruptive" emission from "quiet degassing" to "active degassing" periods. While not an immediate indication of a probable eruption, it may be that emissions have to reach a certain level ("active degassing") before an eruption is probable. Shorter-term increases to the "premonitory degassing" levels, as observed for Mt. E t n a (Fig. 1 ), possibly precede eruptions by hours or days. This short-term variability is superimposed upon the long-term increase and probably only occurs when the volcano is in the "active degassing" state. As suggested by a comparison of the data from Kilauea and Mt. Etna, the actual magnitude of the long- and short-term increase will have to be determined on a volcano-by-volcano basis. The general ( + / - 25 t d 1) variation shown in Fig. 2 could be interpreted as the natural fluctuation or "noise" of the curve in Fig. 3. The smallboxes in Figure 3 represent h o w t h e shortterm variations shown in Figs. i and 2 might be superimposed u p o n t h e longer-termvariations. G l o b a l e s t i m a t e s o f SO2 f l u x f r o m volcanoes Several investigators have estimated the annual atmospheric contribution of SO2 from volcanoes (Stoiber and Jepsen, 1973; Berres-
235 San Cristobal
~< = tOO
2000 1800 1600 1400 1200 1000 800 600 400 200 0 197(
fig. 1
~
(~
SO2 Flux
© "Active S
fifig.2 1972
1974
' 1976
1978
1980
1982
. , 1984
Degassing" "Quiet Degassing"
Date Fig. 3. Average SO2 emissions for San Cristobal volcano, Nicaragua. Each symbol may represent the average of several readings taken over several days. The emission levels were obtained during different levels of activity of the volcano. The solid diamonds represent "quiet degassing" periods, while the stippled circles are considered "active degassing" periods. The change in the emission from quiet to active levels could possibly be used as a long-term forecasting tool. The small boxes superimposed upon the curve suggest how the variations oberved in Figs. 1 and 2 might fit into this long-term emission pattern.
heim and Jaeschke, 1983; Stoiber et al., 1983b). In these papers, it has been observed, that noneruptive degassing from volcanoes is possibly the largest component to the total contribution. However, very little, if any, consideration was taken of the possible variation of the SO2 emission rates for the simple reason that little information concerning the fluctuation was available. The estimates were made from values like those shown as single data points on Fig. 3. With the data presented in Figs. 1-3, it is now possible to get at least a rough,estimate of the variability of the emissions, The variability observed for the three-month period at Kilauea volcano (Fig. 2) will not effect the total estimate.This is because these variations probably represent a natural random fluctuation of the system possibly in response to other environmental factors. As can be seen in Fig. 2, the emission seems to be regularly variable about a mean of approximately 160 t d-1. Since the variation is random about the mean, an average of several different measurements will probably provide a representative average value. However, the variation shown for San Cris-
tobal volcano in Fig. 3 could significantly effect the estimates of total annual SO2 flux contributed by volcanoes. For example, both the initial paper by Stoiber and Jepsen (1973) and the paper by Berresheim and Jaeschke (1983) use an estimate of 360 t d - 1 SO2 emission for San Cristobal. This was the initial value measured by Stoiber and Jepsen in 1973, and subsequent measurements of the flux rate for San Cristobal have not made it into the literature. However, it is obvious from Fig. 3 that the emission rate for San Cristobal is not simply 360 t d - ~. In fact from the data available, the average over the ten-year period from 1972 to 1982 is probably in excess of 600 t d - 1, or almost twice the value used in the two estimates. While the contribution by San Cristobal to the total global estimate is admittedly very small (approximately 1% of the total), the point to consider is that the SOe flux from most volcanoes is probably variable and single point estimates (in time) may not accurately reflect long-term emission averages. Observations Two important observations can be made
236
concerning the use of SO2 flux measurements from volcanoes. The first is that, from the small amount of data now available, it is obvious that non-eruptive emission levels are not constant and probably vary with respect to proximity of the volcano ( in time) to eruptive activity. Variability from "quiet degassing" periods to "active degassing" periods may be as much as 5 to 10 fold. Long-term emission averages may be very different than "spot" measurements, and this could significantly effect estimates of global annual contribution of SO2 by volcanoes. The second observation is that these measured variations, in fact, may provide long- and short-term premonitory signs of pending volcanic eruptions. In the longer term, it seems that emission levels may need to reach a certain "threshold" level before eruptive activity is probable. This was observed at San Cristobal volcano (Fig. 3 ), where the non-eruptive emission levels increased from around 300 t d("quiet degassing") to almost 2000 t d-~ ("active degassing") before eruptive activity occurred and also Mt. Etna (Fig. 1) where the increase was possibly from 330 t d 1 ("quiet degassing") to around 1000 t d 1 ("active degassing"). Therefore, periodic monitoring, weekly or perhaps monthly, could provide a first-order indication of possible activity, There also appears to be a much shorter-term variation which could be used to more precisely forecast the actual onset of eruptive activity, This variability was observed at Mt. Etna ( Fig. 1 ), where premonitory increases of 2 to 2.5 times the "active degassing" levels were observed hours to days before the eruption occurred, A possible plan for monitoring volcanoes for premonitory increases in SO2 emission could be rather simple. Once the emission pattern of the volcano is known, monitoring would have to be done only on a periodic basis (weekly or monthly) to determine the background emission levels. When these levels reach a certain "threshold" value, monitoring could be increased to a daily basis to watch for short-
term increases which would signal the impending eruption. The problem with the rather simplistic monitoring plan proposed above is in the statement "once the emission pattern of the volcano is known...". It is obvious that each volcano will have to be treated individually and the emission patterns will have to be determined for each volcano. This will require almost continuous monitoring until at least one and preferably a sequence of eruptions can be monitored. In summary, remote sensing of SO2 emissions from volcanoes can be used to provide valuable information on the annual contribution of SO2 to the atmosphere and to possibly forecast volcanic events. However, it is obvious that we have only enough information to speculate. The collection of more data will be required in order to put these hypotheses to a significant test.
Acknowledgments This section must obviously begin by acknowledging and thanking Dick Stoiber for the scientific insight and guidance that he provided to me and many other students and colleagues during our academic and personal development. Many individuals within and external to the Dartmouth Earth Sciences community have been a part of Dick Stoiber's research effort on remote monitoring of volcanic emissions. Many of these individuals have made COSPEC measurements at different volcanoes around the world and deserve to be acknowledged for their contributions. I will try to list as many of them as I can. Forgive me if I have left anyone out: Gary Malone, David Johnston, Tom Crafford, Rick Hazlett, Richard Birnie, Jeff Bratton, Chris Newhall, Stan Williams, Jerry Prosser, Bill Rose, Anders Jepsen, Tom Casadevall, Millan Millan and the many Dartmouth College "Stretch" field assistants. Thanks to you all for lugging the COSPEC with you.
237
References Berresheim, H. and Jaeschke, W., 1983. The contribution of volcanoes to the global atmospheric sulfur budget. J. Geophys. Res., 88: 3732-3740. Casadevall, T.J., Johnston, D.A., Harris, D.M., Rose, Jr., W.I., Malinconico, L.L., Stoiber, R.E., Bornhorst, T.J., Williams, S.N., Woodruff, L. and Thompson, J.M., 1981. SO., emission rates at Mount St. Helens from March 29 through December 1980. In: P.W. Lipman and D.R. Mullineaux (Editors), The 1980 eruptions of Mount St. Helens. U.S. Geol. Surv., Prof. Pap., 1250: 193-200. Hazlett, R.W., 1977. Geology and hazards of the San Cristobal volcanic complex, Nicaragua. Master's thesis, Dartmouth College, Hanover, NH (unpubl.). Malinconico, Jr., L.L., 1979. Fluctuations in SO2 emission during recent eruptions of Etna. Nature, 278: 43-45. Millan, M.M., 1978. Remote sensing of S02, a data processing methodology. Proc., 4th Joint Conference on Sensing of Environmental Pollutants, pp. 30-33. Millan, M.M., 1980. Remote sensing of air pollutants. A study of some atmospheric scattering effects. Atmos. Environ., 14: 1241-1253. Millan, M.M. and Hoff, R.M., 1978. Remote sensing of air pollutants by correlation spectroscopy - instrumental response characteristics. Atmos. Environ., 12: 853-864. Millan, M.M., Gallant, A.J. and Turner, H.E., 1976. The application of correlation spectroscopy to the study of
dispersion from tall stacks. Atmos. Environ., 10: 499-511. Moffat, A.J. and Millan, M.M., 1971. The application of optical correlation techniques to the remote sensing of SO2 plumes using sky light. Atmos. Environ., 5: 677-690. Newcomb, G. and Millan, M.M., 1970. Theory, applications and results of the long-line correlation spectrometer. IEEE Trans. Geosci. Electron., 8: 149-157. Stoiber, R.E. and Jepsen, A., 1973. Sulfur dioxide contributions to the atmosphere by volcanoes. Science, 182: 577-578. Stoiber, R.E., Malone, G.B. and Bratton, G.P., 1978. Volcanic emission of SO2 at Italian and Central American volcanoes. Geol. Soc. Am., Abstr. Programs, 10 (3): 148. Stoiber, R.E. Malinconico, Jr., L.L., and Williams, S.N., 1983a. Use of the correlation spectrometer at volcanoes. In: H. Tazieff and J.C. Sabroux (Editors), Forecasting Volcanic Events, Elsevier, Amsterdam, pp. 425-444. Stoiber, R.E., Williams, S.N. and Huebert, B.J., 1983b. Sulfur dioxide, hydrochloric acid, hydrofluoric acid and hydrobromic acid from volcanoes: annual atmospheric contribution. Geol. Soc. Am., Abstr. Programs, 15 (6): 698. Stoiber, R.E., Williams, S.N. and Huebert, B.J., 1987. Annual contribution of sulfur dioxide to the atmosphere by volcanoes. In: S.N. Williams and M.J. Carr (Editors), Richard E. Stoiber 75th Birthday Volume J. Volcanol. Geotherm. Res., 33: 1-8.
231
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
ON THE VARIATION OF SO2 EMISSION FROM VOLCANOES LAWRENCE L. MALINCONICO, Jr. Department o[ Geology, Southern Illinois University, Carbondale, IL 62901, U.S.A. (Received November 25, 1986)
Malinconico, L.L, Jr., 1987. On the variation of SO., emission from volcanoes. In: S.N. Williams and M.J. Carr (Editors), Richard E. Stoiber 75th Birthday Volume. J. Volcanol. Geotherm. Res., 33:231-237
Introduction In 1973, Stoiber and Jepsen used remote sensing techniques to measure the S02 flux from several volcanoes in Central America. Using these data, they estimated that the global contribution of SO2 from volcanoes was on the order of 7 × 10Gmetric tons per year. This initial estimate neglected sulfur in any form other than gaseous SO2 and did not include the contribution from erupting volcanoes. This number was subsequently modified by Stoiber et al. (1983b) and Stoiber et al. (1987, this volume ) to around 1.87× 107 tons per year and includes sulfate aerosols, and emission from erupting volcanoes. Another important application of remote sensing of S02 from volcanoes is forecasting of eruptions. This has proven feasible at Mt. Etna (Malinconico, 1979) and Mount St. Helens (Casadevall et al., 1981). For both of these applications, it is important to understand the long- and short-term patterns of SO2 emission from volcanoes. Continuous monitoring of all volcanoes is at best impractical. Only Kilauea and Mount St. Helens have nearly continuous records and 0377-0273/87/$03.50
these only since 1979 and 1980 respectively. However, as part of an on-going research program, the SOz flux from several volcanoes in Nicaragua, Guatemala, and Sicily have been measured on a fairly regular basis. The object of this paper will be to examine the results from some of those volcanoes and interpret how they might effect flux estimate and prediction techniques.
Instrumentation and techniques The instrument used to remotely measure the SOz flux is a correlation spectrometer (COSPEC) developed by Barringer Resarch in Toronto, Canada, primarily for environmental monitoring of SOz. The theory of the COSPEC operation has been well documented and interested readers are referred to the following papers: Newcomb and Millan (1970), Moffat and Millan (1971), Millan et al. (1976), Millan (1978, 1980) and Millan and Hoff (1978). The COSPEC provides straightforward techniques for remotely monitoring the SOz flux from volcanoes. The instrument is compact, durable, and can be used in a variety of modes.
© 1987 Elsevier Science Publishers B.V.
232
Since 1972, the measurement techniques have gradua41y evolved and been refined. Three different techniques can be used depending upon the available access to a given volcano. These include ground-based stationary and mobile techniques, and airborne techniques, and are discussed in detail in Stoiber et al. (1983a). The uncertainty of an individual SO2 measurement is an important consideration. This has also been discussed in Stoiber et al. (1983a) and will only be summarized here. The largest source of' uncertainty is introduced in the determination of wind speeds. Other sources of uncertainty include calibration cell determination, chart record reading error, and distance and location determination. Increasing pathlength between the plume and sensor sometimes has the effect of reducing the observed SO2 value, We have not been able to determine a factor to use to correct for increasing pathlength, so the values reported should simply be considered minimums. Other factors also effect the observed value. Plume opacity (caused by concentrations of ash in the plume ) tends to reduce the measured signal while the presence of water droplets in the plume tends to increase (by scattering) the radiation path through the plume thereby increasing the measured signal, The total uncertainty, not including signal attenuation, generally ranges from ± 15 to ± 25% and the worst case observed has been approximately ±45%. These estimates of uncertainty are important because it suggests that variations of the SO2 daily flux of 2 to 3 fold can be considered significant, SO2 v a r i a t i o n s a s p r e c u r s o r s to e r u p t i v e activity As stated earlier, one of the important applications of remotely monitoring SO2 emissions from volcanoes is to try to forecast when an eruption might occur. This has met with a fair degree of success for the eruptions that followed the catastrophic eruption of M o u n t St. Helens on May 18, 1980 (Casadevall et al.,
1981 ). The observation that the SO2 emission actually does increase before an eruption was first made at Mt. Etna by Malinconico (1979). Figure 1 is a summary of that work and shows how the emission rate increased at varying time intervals before eruptive activity. However, an important observation must be made concerning the use of such measurements for prediction purposes. This technique can only be effective if the non-eruptive background level and variability of S02 emission is known for each volcano. The data collected at Mt. Etna had no reference level until an eruption was actually monitored. At that point it was then possible to make the observation that, for Mt. Etna, if the SO2 flux reached a value equal to approximately 2 to 3 times the "background" levels (the open circles in Fig.1 ), there was then a high probability of eruptive activity. For the short interval monitored, those values seemed to be approximately 1 0 0 0 t d 1 ( metric tons per day ) for non-eruptive "background" levels (open circles) with a 2- to 3-fold premonitory increase before the eruption (stippled circles) and at least a 5-fold increase during the eruptive event (stars). Such information on the amount of flux increase associated with eruptions is probably only valid on a volcano-by-volcano basis. Figure 2 shows the variation in SO2 flux for Kilauea volcano for a three-month time period during the summer of 1979. Note that the scale of the SO2 flux is now 80 to 300 t d-1 versus the 0 to 5500 t d- 1 range for Mt. Etna. The average daily emission during this period was approximately 160 t d 1 ( + / _ 25 t d-1 ). The short-term variation in the flux apparently represents a normal variation during non-eruptive "quiet degassing" conditions of the volcano. However, around day 55, there was an increase from a background level averaging approximately 160 t d-1 to a daily average flux of almost 300 t d 1 for several days. This is almost a 2-fold increase in the flux levels and by M t . E t n a criteria this might have signaled that an eruption was possible. In fact, while
233
Mt. Etna
.--. ~
5500. 5000. 4500. 4000. 3500 ' 3000 '
SO 2Flux
2500. 2000" ~
1
1500 • 1000 • 50O 0
I
I
I
I
I
I
I
5
10
15
20
25
30
35
~
"EruptiveDegassin~
•
"Premonitory Degassing"
O
"ActiveDegassing"
Day (beginning 7/25/77) Fig. 1. The daily average SO2 emissions for Mr. Etna volcano, Sicily for the time period from July 25 through August 27, 1977. Each symbol represents the average of from 2 to 8 measurements made during a given day. The different symbols correspond to different levels of emission. During these measurements, the general state of the volcano was considered to he "active". Only the measurements represented by the stars were obtained while the volcano was actually erupting.
K i l au ea
.~ --~
tOO
30O 280 26O 240 220 200
?
180 160 140 120 100
SO2 Flux 0 •
80
i
0
10
'
I
|
I
I
I
I
I
20
30
40
50
60
70
80
I
90
"Active Degassing?" "Quiet Degassing"
I
100
Day (beginning 6/10/79) Fig. 2. The daily average SO2 emissions for Kilauea volcano, Hawaii for the time period from June 10 through September 21, 1979. Each symbol again represents the daily average of between 2 and 8 single measurements. These measurements were all made when the volcano was "quiet". Most of the variation probably represents the natural fluctuation of the system in response to environmental factors. However, the measurements shown by the open circles may represent a transition from "quiet degassing" to "active degaseing". Around day 55 there was harmonic tremor and a seismic swarm which were interpreted as an intrusive event in the SW rift zone.
there was no extrusive activity during this time,
volcano from "quiet degassing" to "active
harmonic tremor and increased seismic activity were recorded. This was interpreted as being caused by a deep intrusive event located in the SW Rift zone. Thus the increase in SO2 emission during this time, was probably in response to the intrusive event. What this may have represented is a change in the overall state of the
degassing". The main difference between the sets of data shown for Mt. Etna and Kilauea may be proximity in time of the volcano with respect to eruptive activity. The Mt. Etna data were obtained during a period of significant "activity", just after an eruptive event had stopped
234 and continuing through three other eruptive episodes. On the other hand, the Kilauea data were taken at a time when there was no evidence of eruptive activity, other than the slight harmonic tremor and seismic swarm ( intrusive event? ). The volcano, while emitting gases, was considered to be in a state of "quiet degassing". For Mt. Etna, emission levels during a "quiet degassing" period are probably significantly lower than the 1000 t d 1average shown in Fig. 1. The 330 t d ~ ( + / - 100) SO2 flux for Mt. Etna measured by Stoiber et al. (1975) probably represent emission levels during a noneruptive "quiet degassing" period, What is being suggested by these data is that there may be different levels of"non-eruptive" SO2 emission that vary in response to the proximityin time o f t h e volcano to eruptive activity, I have chosen to use the terms "quiet degassing", "active degassing", and "premonitory degassing" to describe the different levels. Note that the term "active degassing" is not used here to indicate emissions during eruptions, but simply a period when the volcano may be closer in time to an actual eruption. The measured SO,~ emissions from San Cristobal volcano in Nicaragua tend to support this concept of emission variability with time. Figure 3 shows the SO~ flux data for San Cristobal for the period 1971 through 1982. San Cristobal had been dormant since 1685 (Hazlett, 1977). In mid 1971, gas emissions began and the non-eruptive SO2 flux levels increased to a maximum in 1976 after which time they began to decrease. The high in the emission rate in 1976 corresponds to a time when the volcano was having small eruptions of ash and would be generally considered "active". ( None of the values plotted represent measurements made when the volcano was actually erupting). The values measured in 1972-1974 and 1980-1981 were obtained when the state of volcanic activity was "quiet". While Figs. 1 and 2 represent short-term fluctuations in SO2 emission, the data for San Cristobal represent long-term variations of the "non-eruptive" level of emission. These variations, are
obviously related to the state of activity of the volcano and can probably be used to provide a long-range indication of the probability of an eruption. In the case of San Cristobal volcano, eruptions may not be probable until the "noneruptive" emission levels reach at least 1500 to 2000 t d 1. This increase may occur a long time period before an eruption, thus providing a longterm signal of impending activity. Then, presumably, short-term "premonitory" variations like those observed at Mt. Etna, could be used to more precisely define the time of the eruption. The data shown in Figs. 1-3 represent three different kinds of non-eruptive SO2 flux variations from volcanoes. The first, as was observed at San Cristobal (Fig. 3), is a very long-term (on the order of months to years) variation in the "non-eruptive" emission from "quiet degassing" to "active degassing" periods. While not an immediate indication of a probable eruption, it may be that emissions have to reach a certain level ("active degassing") before an eruption is probable. Shorter-term increases to the "premonitory degassing" levels, as observed for Mt. E t n a (Fig. 1 ), possibly precede eruptions by hours or days. This short-term variability is superimposed upon the long-term increase and probably only occurs when the volcano is in the "active degassing" state. As suggested by a comparison of the data from Kilauea and Mt. Etna, the actual magnitude of the long- and short-term increase will have to be determined on a volcano-by-volcano basis. The general ( + / - 25 t d 1) variation shown in Fig. 2 could be interpreted as the natural fluctuation or "noise" of the curve in Fig. 3. The smallboxes in Figure 3 represent h o w t h e shortterm variations shown in Figs. i and 2 might be superimposed u p o n t h e longer-termvariations. G l o b a l e s t i m a t e s o f SO2 f l u x f r o m volcanoes Several investigators have estimated the annual atmospheric contribution of SO2 from volcanoes (Stoiber and Jepsen, 1973; Berres-
235 San Cristobal
~< = tOO
2000 1800 1600 1400 1200 1000 800 600 400 200 0 197(
fig. 1
~
(~
SO2 Flux
© "Active S
fifig.2 1972
1974
' 1976
1978
1980
1982
. , 1984
Degassing" "Quiet Degassing"
Date Fig. 3. Average SO2 emissions for San Cristobal volcano, Nicaragua. Each symbol may represent the average of several readings taken over several days. The emission levels were obtained during different levels of activity of the volcano. The solid diamonds represent "quiet degassing" periods, while the stippled circles are considered "active degassing" periods. The change in the emission from quiet to active levels could possibly be used as a long-term forecasting tool. The small boxes superimposed upon the curve suggest how the variations oberved in Figs. 1 and 2 might fit into this long-term emission pattern.
heim and Jaeschke, 1983; Stoiber et al., 1983b). In these papers, it has been observed, that noneruptive degassing from volcanoes is possibly the largest component to the total contribution. However, very little, if any, consideration was taken of the possible variation of the SO2 emission rates for the simple reason that little information concerning the fluctuation was available. The estimates were made from values like those shown as single data points on Fig. 3. With the data presented in Figs. 1-3, it is now possible to get at least a rough,estimate of the variability of the emissions, The variability observed for the three-month period at Kilauea volcano (Fig. 2) will not effect the total estimate.This is because these variations probably represent a natural random fluctuation of the system possibly in response to other environmental factors. As can be seen in Fig. 2, the emission seems to be regularly variable about a mean of approximately 160 t d-1. Since the variation is random about the mean, an average of several different measurements will probably provide a representative average value. However, the variation shown for San Cris-
tobal volcano in Fig. 3 could significantly effect the estimates of total annual SO2 flux contributed by volcanoes. For example, both the initial paper by Stoiber and Jepsen (1973) and the paper by Berresheim and Jaeschke (1983) use an estimate of 360 t d - 1 SO2 emission for San Cristobal. This was the initial value measured by Stoiber and Jepsen in 1973, and subsequent measurements of the flux rate for San Cristobal have not made it into the literature. However, it is obvious from Fig. 3 that the emission rate for San Cristobal is not simply 360 t d - ~. In fact from the data available, the average over the ten-year period from 1972 to 1982 is probably in excess of 600 t d - 1, or almost twice the value used in the two estimates. While the contribution by San Cristobal to the total global estimate is admittedly very small (approximately 1% of the total), the point to consider is that the SOe flux from most volcanoes is probably variable and single point estimates (in time) may not accurately reflect long-term emission averages. Observations Two important observations can be made
236
concerning the use of SO2 flux measurements from volcanoes. The first is that, from the small amount of data now available, it is obvious that non-eruptive emission levels are not constant and probably vary with respect to proximity of the volcano ( in time) to eruptive activity. Variability from "quiet degassing" periods to "active degassing" periods may be as much as 5 to 10 fold. Long-term emission averages may be very different than "spot" measurements, and this could significantly effect estimates of global annual contribution of SO2 by volcanoes. The second observation is that these measured variations, in fact, may provide long- and short-term premonitory signs of pending volcanic eruptions. In the longer term, it seems that emission levels may need to reach a certain "threshold" level before eruptive activity is probable. This was observed at San Cristobal volcano (Fig. 3 ), where the non-eruptive emission levels increased from around 300 t d("quiet degassing") to almost 2000 t d-~ ("active degassing") before eruptive activity occurred and also Mt. Etna (Fig. 1) where the increase was possibly from 330 t d 1 ("quiet degassing") to around 1000 t d 1 ("active degassing"). Therefore, periodic monitoring, weekly or perhaps monthly, could provide a first-order indication of possible activity, There also appears to be a much shorter-term variation which could be used to more precisely forecast the actual onset of eruptive activity, This variability was observed at Mt. Etna ( Fig. 1 ), where premonitory increases of 2 to 2.5 times the "active degassing" levels were observed hours to days before the eruption occurred, A possible plan for monitoring volcanoes for premonitory increases in SO2 emission could be rather simple. Once the emission pattern of the volcano is known, monitoring would have to be done only on a periodic basis (weekly or monthly) to determine the background emission levels. When these levels reach a certain "threshold" value, monitoring could be increased to a daily basis to watch for short-
term increases which would signal the impending eruption. The problem with the rather simplistic monitoring plan proposed above is in the statement "once the emission pattern of the volcano is known...". It is obvious that each volcano will have to be treated individually and the emission patterns will have to be determined for each volcano. This will require almost continuous monitoring until at least one and preferably a sequence of eruptions can be monitored. In summary, remote sensing of SO2 emissions from volcanoes can be used to provide valuable information on the annual contribution of SO2 to the atmosphere and to possibly forecast volcanic events. However, it is obvious that we have only enough information to speculate. The collection of more data will be required in order to put these hypotheses to a significant test.
Acknowledgments This section must obviously begin by acknowledging and thanking Dick Stoiber for the scientific insight and guidance that he provided to me and many other students and colleagues during our academic and personal development. Many individuals within and external to the Dartmouth Earth Sciences community have been a part of Dick Stoiber's research effort on remote monitoring of volcanic emissions. Many of these individuals have made COSPEC measurements at different volcanoes around the world and deserve to be acknowledged for their contributions. I will try to list as many of them as I can. Forgive me if I have left anyone out: Gary Malone, David Johnston, Tom Crafford, Rick Hazlett, Richard Birnie, Jeff Bratton, Chris Newhall, Stan Williams, Jerry Prosser, Bill Rose, Anders Jepsen, Tom Casadevall, Millan Millan and the many Dartmouth College "Stretch" field assistants. Thanks to you all for lugging the COSPEC with you.
237
References Berresheim, H. and Jaeschke, W., 1983. The contribution of volcanoes to the global atmospheric sulfur budget. J. Geophys. Res., 88: 3732-3740. Casadevall, T.J., Johnston, D.A., Harris, D.M., Rose, Jr., W.I., Malinconico, L.L., Stoiber, R.E., Bornhorst, T.J., Williams, S.N., Woodruff, L. and Thompson, J.M., 1981. SO., emission rates at Mount St. Helens from March 29 through December 1980. In: P.W. Lipman and D.R. Mullineaux (Editors), The 1980 eruptions of Mount St. Helens. U.S. Geol. Surv., Prof. Pap., 1250: 193-200. Hazlett, R.W., 1977. Geology and hazards of the San Cristobal volcanic complex, Nicaragua. Master's thesis, Dartmouth College, Hanover, NH (unpubl.). Malinconico, Jr., L.L., 1979. Fluctuations in SO2 emission during recent eruptions of Etna. Nature, 278: 43-45. Millan, M.M., 1978. Remote sensing of S02, a data processing methodology. Proc., 4th Joint Conference on Sensing of Environmental Pollutants, pp. 30-33. Millan, M.M., 1980. Remote sensing of air pollutants. A study of some atmospheric scattering effects. Atmos. Environ., 14: 1241-1253. Millan, M.M. and Hoff, R.M., 1978. Remote sensing of air pollutants by correlation spectroscopy - instrumental response characteristics. Atmos. Environ., 12: 853-864. Millan, M.M., Gallant, A.J. and Turner, H.E., 1976. The application of correlation spectroscopy to the study of
dispersion from tall stacks. Atmos. Environ., 10: 499-511. Moffat, A.J. and Millan, M.M., 1971. The application of optical correlation techniques to the remote sensing of SO2 plumes using sky light. Atmos. Environ., 5: 677-690. Newcomb, G. and Millan, M.M., 1970. Theory, applications and results of the long-line correlation spectrometer. IEEE Trans. Geosci. Electron., 8: 149-157. Stoiber, R.E. and Jepsen, A., 1973. Sulfur dioxide contributions to the atmosphere by volcanoes. Science, 182: 577-578. Stoiber, R.E., Malone, G.B. and Bratton, G.P., 1978. Volcanic emission of SO2 at Italian and Central American volcanoes. Geol. Soc. Am., Abstr. Programs, 10 (3): 148. Stoiber, R.E. Malinconico, Jr., L.L., and Williams, S.N., 1983a. Use of the correlation spectrometer at volcanoes. In: H. Tazieff and J.C. Sabroux (Editors), Forecasting Volcanic Events, Elsevier, Amsterdam, pp. 425-444. Stoiber, R.E., Williams, S.N. and Huebert, B.J., 1983b. Sulfur dioxide, hydrochloric acid, hydrofluoric acid and hydrobromic acid from volcanoes: annual atmospheric contribution. Geol. Soc. Am., Abstr. Programs, 15 (6): 698. Stoiber, R.E., Williams, S.N. and Huebert, B.J., 1987. Annual contribution of sulfur dioxide to the atmosphere by volcanoes. In: S.N. Williams and M.J. Carr (Editors), Richard E. Stoiber 75th Birthday Volume J. Volcanol. Geotherm. Res., 33: 1-8.