J geology Observations of 1992 Crater Peak/Spurr Volcanic Clouds in their first few days of atmospheric residence William I. Rose, Gregg J. S. Bluth, David J. SchneiderColleen M. Riley, Lydia J



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June 29, 2000 draft, J Geology
Observations of 1992 Crater Peak/Spurr Volcanic Clouds in their first few days of atmospheric residence

William I. Rose, Gregg J.S. Bluth, David J. SchneiderColleen M. Riley, Lydia J. HendersonDepartment of Geological Engineering and Sciences, Michigan Technological University, Houghton, Michigan 49931


ABSTRACT

Satellite SO2 and ash measurements of Mt Spurr’s 1992 volcanic clouds are compared with ground-based observations to develop an understanding of the evolution of volcanic clouds which can be described as three stages. The three VEI=3 eruptions each reached the lower stratosphere (14 km asl) but were mainly dispersed at the tropopause by moder­ate to strong (20-40 m/s) tropospheric winds. Heavy fallout of large (>500 µmm) pyroclasts occurred during and immediately after the eruptions close to the volcano (<25 km from the vent; Stage 1). A much larger, highly elongated deposition region marked by a sec­ondary mass maximum occurred 150-350 km downwind in at least two of the Spurr events. This region was the result of aggregate fallout of a bimodal size distribution including fine (<25 µmm) ash which quickly depleted the volcanic cloud in fine silicates (during the first 18-24 hrs; Stage 2). The clouds continued to move through the upper tro­posphere but began decreasing in size and slowly disappeared as ash and SO2 were appar­ently removed by meteorological processes (Stage 3). Total SO2 in the Spurr clouds increased in the second day of atmospheric residence, possibly because of conversion of co-erupted H2S, or possibly because of the effects of sequestration by ice followed by sub­sequent SO2 release during fallout and dessication of ashy hydrometeors. SO2 and volca­nic ash travelled together in all the Spurr volcanic clouds. The initial (18-24 hrs) area expansion of the clouds and the subsequent several days of drifting could be mapped by both SO2 (UV) and ash (IR) satellite imagery.



1 Manuscript received July x, 2000; etc.

2 US Geological Survey, Alaska Volcano Observatory, Anchorage, AK

3 Centre for Environmental and Geophysical Flows, Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ UK

Introduction

The 1992 eruptions of Crater Peak, Mount Spurr, Alaska provided an opportunity to apply satellite measurement techniques to study the atmospheric residence and fallout of volcanic materials. Because of its proximity to Anchorage and the resources of the Alaska Volcano Observatory (USGS, State of Alaska and University of Alaska) and the National Weather Service (NOAA), an unusually thorough set of basic observations were collected. This paper integrates data on the Spurr eruptions from meteorological radar (Rose et al, 1995a), TOMS satellite observations (Bluth et al, 1995), AVHRR weather satellite data (Schneider et al, 1995; Shannon, 1996), ash sampling (Neal et al, 1995; Gardner et al, 1998) and a wide variety of other geophysical observations (Keith, 1995). The goal of the paper is to assemble this data in order to gain a better understanding of volcanic clouds.



Spurr Eruptions of 1992. The three 1992 Spurr eruptions were similar subplinian andesitic explosive events which resulted in significant airfall deposits and no pyroclastic flows. Table 1 lists basic characteristics of these three events. They are similar in intensity, duration, magma composition, and volume. Meteorological conditions for the events dif­fer, however. Each of the three eruptions penetrated the stratosphere, at least at the peak of the eruption. Each of the eruptions was recorded by a network of seismic stations (Power et al, 1995), and was observed by C-band radar at Kenai (80 km SE of Crater Peak) (Rose et al, 1995a). The volcano is located near a regular radiosonde measurement point (Anchorage airport, 125 km ESE) and the fallout deposit from each event was mapped and sampled (Neal et al, 1995). The Spurr events are typical of VEI=3 events which occur about 10 times/decade and which represent the most common type of volcano/stratosphere interactions, even though some VEI= 3 events (especially between 30N and 30 S latitude) do not actually reach the stratosphere, and many have only marginal stratospheric interac­tion.
Table 1: Basic comparison of eruption characteristics and environmental conditions, Cra­ter Peak eruptions of Mount Spurr, 1992.



27 June 1992

19 August 1992

17 Sept. 1992

Ref

Start time, UT

1504

0042

2003

1

Eruption peak, UT

1823

0055

2221

2

End time, UT

1907

0410

2339

1

Duration, minutes

243

208

216

1

Mean Column height, km asl

11.1

11.8

12.0

3

Max Column height,km asl

14.5

13.7

13.9

2

Mean Eruption rate, m3/s

820

1120

1160

3,4

Max Eruption rate, m3/s

3232

2333

2497

2,4

Clast density, kg/m3

1760

1500-1550

1530-1580

3

Wind speed, 0-3 km asl, m/s

5.8

6.5

5.2

5

Wind speed, 3-6 km asl, m/s

16.2

12.3

15.1

5

Wind speed, 6-9 km asl, m/s

18.6

18.6

29.3

5

Wind speed, 9-12 km asl, m/s

21.3

24.9

36.4

5

Wind speed, 12-15 km asl, m/s

13.9

10.9

32.3

5

Avg wind direction, 0-3 km

169

205

153

5

Avg wind direction, 3-6 km

190

290

239

5

Avg wind direction, 6-9 km

200

315

274

5

Avg wind direction, 9-12 km

202

308

272

5

Avg wind direction, 12-15 km

201

275

276

5

Tropopause height, km asl

11.7

10.7

12.2

5

Temp, 0 km asl, ¼C

10.2

15.2

4.3

5

Temp, 3 km asl, ¼C

-2.8

-7.2

-7.0

5

Temp, 6 km asl, ¼C

-19

-23

-16

5

Temp, 9 km asl, ¼C

-41.7

-47

-39.9

5

Temp, 12 km asl, ¼C

-57.3

-62.3

-62

5

Dew Point, 0 km, ¼C

9.3

8.0

-9.4

5

Dew Point, 3 km, ¼C

-3.8

-8.3

-18

5

Dew Point, 6 km, ¼C

-23

-34

-17

5

Fallout Volume,DRE(x106m3)

12

14

15

3

Sources: 1 McNutt et al, 1995, p. 165; 2 Rose et al, 1995a, p. 21; 3 Neal et al, 1995, p. 68; 4 Sparks et al, 1997, p. 118; 5 NOAA radiosonde information from Anchorage, Alaska














Satellite sensing data

There is a difficulty in getting to volcanic clouds with direct sampling equipment and a reluctance in taking piloted aircraft into them given their well-known hazards (Casadevall, 1994). Because of this, study of volcanic clouds has mainly been through remote sensing, using ground-based and satellite sensors. Remote sensing methods use radar, microwave, IR and UV multispectral methods which can both detect and map volcanic clouds and retrieve spatial information about them. Table 2 lists the main satellite-based techniques used in this study. We also used ground-based C band radar data, which detected the vol­canic clouds in their very early stages (until about 30 minutes after eruption), while they still contained coarse particles (2-20 mm) and mass concentrations ranging from <.01 - 1 g/m



Table 2: Satellite Remote sensing tools used in study of volcanic clouds.


Sensor

TOMS

AVHRR

GOES

Wavelengths

312-380 nm

10-12.5 mm

10-12.5 µmm

Orbit

Polar

Polar

Geostationary

Sensing Target

Silicate ash, SO2

Silicate ash

Silicate ash

Archive

1979-

1981-

1996-

Scenes/day

1

4-8

48

Each of the three Spurr eruptions was observed and measured for several days by both the TOMS and AVHRR satellite detectors and information about the clouds were retrieved from remote sensing algorithms (Table 3). In this study we consider and compare basic information about these satellite-based measurements for the three Spurr events (AVHRR data is in Tables 4- 6; TOMS data from Bluth et al, 1995).


Table 3: Retrieval data from volcanic cloud sensors, 1999. The references give details on the retrieval algorithms.


Sensor

Retrieval data

Resolution (at nadir)

Reference

TOMS

2D position, SO2 mass

50 km

Krueger et al [1995]

TOMS AI

2D position, Opt Depth (uv)

50 km

Krotkov et al [1999]

AVHRR & GOES

2D position, Opt Depth (IR);

Particle effective radius, mass in 1-15µmm range



~4 km

Wen and Rose [1994]


Table 4: AVHRR 2-band brightness temperature difference (BTD) retrieval data from the June 1992 Spurr eruptions (Shannon, 1996).

Residence,hrs eff radius,µmm Opt Depth ash mass,kT Area,km

4.3

8.3

1.9

317

15,000

20.80

12.5

8.2

0.86

438

47,000

9.24

23

6

0.19

258

167,000

1.55

26.3

6

0.13

194

140,000

1.38

31.3

5

0.16

166

120,000

1.38

36.3

6

0.03

214

173,000

1.24

107

6.5

0.16

183

134,000

1.37

118

5.7

0.07

122

205,000

0.60

128

6.8

0.1

128

137,000

0.93

142

6.5

0.1

102

114,000

0.90

146.3

6.6

0.09

94

121,000

0.78

152

6

0.1

110

134,000

0.82

___________________________________________________________________
Table 5: AVHRR 2 band BTD Retrieval data from the August 1992 Spurr eruption (Schneider et al, 1995).

Residence,hrs eff radius,µµµmm Opt Depth ash mass,kT Area,km

13.6

4.44

0.44

331

128,000

2.59

17.3

3.44

0.29

241

171,000

1.41

18.9

3.61

0.37

373

199,000

1.87

23.1

4.41

0.27

371

230,000

1.61

37.4

2.21

0.11

197

455,000

0.43

47.3

1.65

0.07

141

403,000

0.35

83.4

2.61

0.05

70

276,000

0.25



Table 6: AVHRR 2 band BTD Retrieval data from the September 1992 Spurr eruption (Schneider et al, 1995).

Residence,hrs eff radius,µµµmm Opt Depth ash mass,kT Area,km

3.7

2.99

1.27

240

34,000

7.13

8

4.43

0.89

423

79,000

5.39

13.7

5.07

0.71

412

87,000

4.71

26

3.23

0.21

254

232,000

1.09

35.7

3.88

0.3

224

131,000

1.71

49

3.89

0.22

262

216,000

1.22

57.9

3.23

0.15

153

200,000

0.76

70

2.2

0.08

118

307,000

0.38

Sequential TOMS and AVHRR data allow us to examine the dynamics of the Spurr clouds. Figure 1 shows how the 2D area, from the satellite perspective, changed for the three events. Both TOMS and AVHRR detected volcanic clouds of similar size and they also followed similar tracks. Separation of the SO2 and ash in the cloud, as noted in other eruptions (eg Schneider et al, 1999) did not occur in any of the Spurr eruptions. In all three cases the area of the clouds increased rapidly at first, and then after about 1-2 days began to decrease in size. Study of the trajectory of air parcels in conjunction with the sat­ellite data for the June eruption (Shannon, 1996) showed that areal growth of the cloud during its first few days was partly due to wind shearing and that the area decreases that occur after several days are largely the result of loss of the lower elevation portions of the volcanic clouds.

Estimates of the masses of SO2 and fine (diameters 1-25 µmm) silicate particles in the Spurr volcanic clouds are shown in figure 2. SO2 masses are higher in the second day of measurement for all three of the Spurr events. This difference cannot be explained by the continuing emission of SO2, because the first day's measurement occurred after the end of the eruption (with the exception of the June eruption, by about 30 minutes). It is unlikely to reflect an error in TOMS data analysis (Krueger et al, 1995) which would be far less than the observed difference. It is unlikely that the TOMS detector was saturated or sup­pressed by an interference from volcanic ash in the SO2 signal, because simulations of this effect would likely result in an overestimate the first day rather than an underestimate (Krueger et al, 1995). The favored explanation is that the mass increase is due to co-emis­sion and subsequent oxidation of H2S (Rose et al, 2000) but we are also investigating the possibility of temporary sequestration of SO2 by ice during the first day followed by sub­sequent release during the fallout and dessication of ashy hydrometeors (see discussion below).

The maximum fine ash masses detected in the Spurr volcanic clouds represent about 2% of the estimated total mass erupted in each eruption (Table 7), a fraction that is much higher than was found for several larger eruptions. We interpret this difference as reflect­ing the greater efficiency of ash removal for more intense eruptions, as a result of higher rates of particle reentrainment into the eruption column and more efficient removal by aggregation as predicted by Ernst et al (1996). The fine ash masses (Figure 2) decline at a more rapid rate during the first day of residence and at a slower rate, nearly parallel to the SO2 curves, thereafter.


Table 7: Fine ash (effective radius 1-12 µmm) masses measured in volcanic clouds by sat­ellite (from Rose et al, 2000)


Volcano, Date

Total ash erupted

Max fine ash detected

%

Spurr 6/92

21.1 x 106 T1

.44 x 106 T

2.1

Spurr 8/92

21.3 x 106 T1

.42 x 106 T

2.0

Spurr 9/92

23.3 x 106 T1

.61 x 106 T

2.6

El Chichon, 4/82

910 x 106 T2

6.5 x 106 T2

0.7

Lascar 4/93

345 x 106 T3

4.8 x 106 T5

1.4

Hudson, 8/91

7600 x 106 T4

2.9 x 106 T6

0.04


1 Neal et al, 1995 4 Scasso et al, 1994

A measure of the area-averaged “burden” of the ash clouds can be estimated by dividing masses by areas in tables 4-6 (figure 3). The ash burdens for all three eruptions decline very rapidly in the first day, and quite slowly thereafter, while SO2 burdens show slow declines throughout. The ash burden estimates correlate well with optical depths (Figure 4) as would be expected.

The effective radius data for the three eruptions show that the June volcanic cloud had higher effective radius values (figure 5). We are unsure of the detailed meaning of these results. Higher effective radius values for the June case could reflect the greater influence of ice (see Rose et al, 2000; for more discussion of the role of ice). Except for the higher June values, the effective radius for all three Spurr events otherwise display a qualitatively similar evolution with time, consisting of decreasing size. This decrease was also observed in the El Chichon data (Schneider et al, 1999; figure 4, p. 4048). There are apparent minima in all three curves at ~36h, ~50h and ~24h, but we hesitate to interpret much from these at this point because effective radius retrieval is imprecise and affected by atmospheric water vapor (Yu et al, in prep).

Stages of Volcanic cloud evolution.

The Spurr clouds seem to have three stages of evolution (Table 8). First, during the eruption and for 1-2 hours following, they grow rapidly in size and are essentially opti­cally opaque to the infrared sensor. At this stage they resemble thunderstorms to the infra­red sensors and typically exhibit very cold temperatures. The core of these clouds is opaque in the infrared (optical depth ~4) and so infrared retrievals are impossible (Figure 6). During the first thirty minutes after the eruption stops, the C-band radar signal (propor­tional to the sixth power of the particle radius) falls rapidly as all coarse ash and lapilli-sized ejecta fall out from the ash cloud at high Re (Bonadonna, et al, 1998), accounting for much (>70%) of the total volume of fall materials. This material falls out (deposit mass/ area = 10,000-250,000 g/mIn the second stage of volcanic cloud evolution, which lasts no more than about one day, the cloud continues to grow, increasing its area by a factor of 2-5 (figure 1), but its optical depth and fine particle concentration density decreases very rapidly, by an order of magnitude or more. This period correlates with the time of anomalous fallout of aggre­gated fine ash in a settling regime characterized by particle Re number transitional between laminar and turbulent (Riley et al, 1999) and the formation of a secondary fallout maximum, which are well defined for both the August and September Spurr events (McGimsey, 2000). The deposit mass/area of fallout (100-2500 g/m



Table 8: Stages defined in volcanic cloud history, based on Spurr volcanic clouds


Volcanic Cloud stage

1

2

3

Duration, hrs after eruption stops

~1-2

18-24

24-96+

Ash fallout, km from volcano

<25

25-400

>400

Area of ash fallout, km2

<300

~5 x104

?,discontinuous

Ash fallout diameter range, mm

>0.5

0.5-<0.01

<0.01

Ash fallout rate, kilotonnes/hr

>104

<104-102

"very low

Fraction of fine ash(diam 1-25 mm), %

<1

10-50

>50

Cloud Area, km2

<104

104-106

106 ,decreasing

Cloud Area change, %/hr

>100

30-50

-10-+10

Mean Optical Depth, 11 µmm

>2

0.5-2

<0.3

Cloud ash burden, tonnes/km2

>25

3-10

<3

Fraction of ash mass suspended, %

100-~30

~30-3

<3

A third stage in volcanic cloud evolution lasts for several (3-5) days, during which the cloud moves thousands of kilometers, its ash concentrations and optical depths decrease very slowly, the masses of both SO2 and fine particles decrease steadily. During this stage fallout is very light and at low Re (Bonadonna et al 1998) and the mass of remaining sili­cate particles is only at most a few percent of the original volume. Finally after several days, both the infrared and ultraviolet detection of the cloud becomes difficult because the concentrations of SO2 and ash fall below the level of noise. Except for the June volcanic cloud, which traversed very cold Arctic regions that limited the sensitivity of the infrared detector from about 20-120 hours after the eruption, the positions, shapes and sizes of the SO2 and ash volcanic clouds were very similar throughout. This suggests that the SO2 and ash detected were part of the same air parcels.



Fallout Maps

The ash fallout from all three eruptions was sampled and mapped by McGimsey (2000). Highly symplified versions of the fallout maps from that study are found in Fig­ure 7. The June deposit was mapped and sampled in a more limited way (the map is unde­fined to the north), while the August and September deposits were followed for distances of up to 300 km . The latter two events show a clearly-defined secondary maximum in mass/area of fall deposits, located within the broad downwind region outlined. Size and shape determinations of distal Spurr fallout materials were made by Riley et al ( in prep). Several of the authors also independently modelled the dispersal data using trajectory models for single ash particles and aggregates of different sizes/porosities/densities falling from a 10-14 km high ashcloud. The results are discussed in detail elsewhere (Riley et al, 2000; Ernst et al, 2000). The conclusion from these dispersal modelling studies is that these materials fell as aggregates of fine ash which had diameters of 100-300 µmm, but con­tained a large majority of much finer particles, most in the 10-30 micron range. A size dis­tribution of one distal Spurr ash sample, which fell at Wells Bay (red X in figure 7) is shown in figure 8. It is bimodal, with peaks at about 18 and 90 µmm. Two modes with sim­ilar values were documented from equivalent locations relative to the secondary maximum for other medial deposits (eg Mount St Helens; May 18, 1980; Carey and Sigurdsson, 1982) but the detailed explanation of the two modes is unexplored. Riley et al (in prep) measured the terminal velocities of the individual particles in the Wells Bay sample and concluded that these very fine ashfall materials would have fallen out at distances about 5-10 times farther from the volcano if they remained as simple separate particles. The volca­nic clouds observed by AVHRR passed over the secondary maximum at times (4-15 hrs after eruption) which fall squarely in the stage 2 of the volcanic cloud. We conclude that the broad dashed line regions of the latter two Spurr events are closely associated with a period of particle aggregation that, based on position and travel times, correlates with stage two of cloud evolution. In analogy, the red lines in figure 6 outline the region of iso­mass contours of 5000 g/mThe end of volcanic clouds



After the first 18-24 hours Spurr’s volcanic clouds drifted along with relatively slow changes. They either remained about the same in 2D area, optical depth and ash particle density or slowly decreased in size (Figure 3). Detailed study of the June clouds by Shan­non (1996) showed that those clouds lost area from their more rapidly-drifting leading edges. This suggests that meteorological processes cause the dissapearance of volcanic clouds, as the fine ash in their lower parts acts as cloud condensation nuclei for a progres­sively moister atmosphere. We note that the decreases in SO2 mass during this period is much more rapid (e-folding only a few days) for these tropospheric volcanic clouds than that estimated for long-lived stratospheric clouds (Bluth et al, 1997), which suggests that SO2 may be removed by meteorological processes also.

Discussion

Volcanic cloud hazard to aircraft. The rapid decrease in ash mass during stage two of volcanic clouds is potentially significant to the issue of volcanic cloud hazards because nearly all seriously damaging aircraft encounters have occurred during the 24 hours after activity. The rapid ash burden (and inferred ash concentration) decrease in stage 2 of vol­canic clouds (figure 3) suggests that the processes accelerating the fallout of fine ash are efficient enough to remove a vast majority of the fine ash which resides in volcanic clouds within about the first day or day and a half. The burden (and concentration) of fine ash in volcanic clouds stage 3 are low and may be insufficient to cause engine failure, although damage to the aircraft would occur. Tests of the engine tolerance of ash may be needed to support this suggestion, which could serve to restrict the hazard of volcanic clouds to a 1-2 day period after eruption.

The role of ice in volcanic clouds. The 1994 Rabaul eruption (Rose et al, 1995b) has raised our awareness of the role of hydrometeors, especially ice, in volcanic clouds. Work in the application of eruption column models that include microphysical processes (Her­zog et al, 1998; Textor, 1999) has further emphasized the possible role of ice. The source of H2O for the formation of hydrometeors comes from entrainment (Woods, 1993: Glaze et al, 1997) and in the case of Rabaul and Soufrière Hills (Mayberry et al, 2001) from the ocean. Even though there was no interaction with the ocean, the Spurr volcanic clouds may have also contained ice, either from entrainment or possibly from melting of glacial ice by the magmatic heat. One of the principal roles of the ice could be to accelerate the fallout of fine pyroclasts by enhancing or driving the aggregation. Icy pyroclasts may be more likely to stick to each other than non-icy ones. Once aggregates start to form they will have a high internal surface area relative to their mass (specific surface area) and could rapidly fill up with ice by deposition of water vapor and heterogeneous nucleation on the aggregate. In the Spurr case the dispersal data is consistent with aggregates of 200 microns and 60% porosity (accretionary lapilli-like particles), ie with a density of 1025 kg/mCould volcanic cloud SO2 increases be due to ice evaporation? As explained above, the favored explanation for the increases observed in SO2 mass in the Spurr volca­nic clouds is the co-eruption of H2S and its conversion after atmospheric emplacement (Rose et al, 2000). There is no direct data supporting this hypothesis, however. An alter­native explanation is possible if there could be a sequestration of SO2 in ice within the vol­canic cloud. This hypothesis was invoked by Rose et al (1995b) to explain the extremely low SO2 mass in the volcanic clouds of Rabaul, a volcano whose vent was at sea level, and which was readily accessible to seawater. In the case of the Spurr events all of the H2O vapor in the volcanic cloud would have to come from entrainment of the moist tropo­spheric air. This may be consistent with the relatively minor suppression of SO2(~25%) and its restriction to the first day. During the fallout of ash in stage two, ice evaporates and SO2 is released to the atmosphere, which explains the second day rises. At this point we offer this possible explanation as a speculation.

Human health effects of fine volcanic ash. Our observations of volcanic clouds are important with respect to the fallout of fine ash which is a potential hazard to human health. Moreover the presence of silica phases such as cristobalite (Baxter et al, 1999) or even the small size of silicate ash (Norton and Gunter, 1999) is potentially harmful to health because of the respirable characteristics of fine ash (less than 10 mm in diameter and especially that < 2.5 µmm in diameter). The data we have presented on the Spurr clouds shows that a lot of fine ash fell out over Alaska in the stage two of the Spurr clouds and examination of the fallout materials (Fig 9) shows that abundant fine ash is present within the materials of the distal ash blankets. As studies in Idaho (Norton and Gunter, 1999) have shown, volcanic ash can be a dominant component of the respirable dust (PM10) for many years after the eruption is over, even in an area which has experienced only light ashfall. Our work suggests that respirable dust studies in distal ash fall areas would be of interest in assessing the human health hazards. The area of the distal fall blankets and in particular the secondary maximum of fallout thickness should be a target of such future studies.

Radar detection of aggregation. Rose et al (1995a) demonstrated that C-band radar observations of the Spurr clouds were limited to stage one and were the result of particle sizes of at least 2 mm. It may be that new generation radar such as NEXRAD (Krohn et al, 1994) which is now (after the Spurr events) installed across the USA will enable detec­tion of stage two aggregation, which likely involves individual particle diameters in the range of 1-100 mm (but up to perhaps 1500mm) forming aggregates 100-500 mm (but with maximum diameters up to perhaps 2000 mm (see Table 1 of Carey et al, 1982; and figure 2 of Bonadonna et al, 1998). NEXRAD radar measurements could potentially establish the heights and locations for the aggregation and should help clarify its nature and cause.

Why is there no separation of SO2 and ash in the Spurr clouds? The lifetime and mass loadings of the Spurr volcanic clouds were such that ash-gas clouds separation would have been expected if the driving mechanism was a fluid dynamical instability as suggested by Holasek et al (1996). The fact that it was not observed in any of the Spurr events strongly suggests that this mechanism is unlikely to be the main cause in other cases, when it has been observed (eg El Chichon; Schneider et al, 1999). We propose that when such separation does occur that it is related instead to the presence of a separate SO2-rich vapor phase which surges out ahead of the more dense ash cloud. The existaence of a separate vapor phase in the magma chamber of arc volcanos is suggested by other studies (eg Gerlach et al, 1996). This vapor phase is very hot and would be more buoyant than the following ashcloud and would therefore rise higher, in agreement with observa­tions (Rose et al, 2000, Table 6). The absence of separation at Spurr suggests that the vol­ume of a separate vapor phase was insufficient to be detected by TOMS. If the hypothesis of Holasek et al (1996) were correct, the Spurr volcanic clouds should have developed into a multilayered system during the first 1-2 days. Moreover we have studied the work by Holasek et al (1996) and found that its experiments cannot be conclusive since the effects of the tank walls must have forced segregation in the experiments by arresting the spread­ing gravity currents, thereby preventing the flow from behaving as a suspension (as mod­elled in that paper) on a timescale shorter than the one expected for the modelled instability to take place. Thus the possibility of this instability remains but must be inves­tigated with a larger tank experiment. One possibility that may prevent its occurrence is that aggregation and particle removal is occurring fast enough to preclude the accumula­tion of particles at the cloud base.

Conclusions

Three eruptions of Crater Peak, Mt Spurr in 1992 were similar in duration, intensity, volume and atmospheric conditions. All reached the stratosphere, but were mainly dis­persed in the upper troposphere. The volcanic clouds were mapped and measured for sev­eral days by both TOMS and AVHRR satellite sensors. Each of the three volcanic clouds had more SO2 mass in its second day than in its first day of atmospheric residence, sug­gesting that some of the magmatic sulfur release was in the form of H2S, or that SO2 was temporarily sequestered in ice within the first day of the volcanic cloud and emerged as ice evaporated during fallout. In all three eruptions the fallout of fine ash (<25 mm in diame­ter) was very rapid in the first 24 hours of cloud residence, an observation that cannot be explained by fallout as simple, separate particles.

The ash fallout blanket for each eruption was highly elongated, and at least two had a prominent secondary mass maximum located 150-350 km downwind. Fallout at these secondary maxima had bimodal fine-skewed size distributions which reflect aggregate fallout processes. The Spurr volcanic clouds showed similar patterns of atmospheric resi­dence in 3 stages: 1. The first hour of atmospheric residence was dominated by rapid fall­out of large (>500 µmm in diameter) ash and lapilli which resulted in heavy fallout near (<25 km) the volcano, affecting a small area (<~300 km

AcknowledgementsWork on the Spurr eruptions began in 1992 and was greatly aided by the cooperation of the USGS through References Cited

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Figure Captions:

Figure 1: 2D areas for Spurr clouds and infrared (10.8 mm) optical depths for the three Spurr eruptions. Data from tables 4-6.
Figure 2: Masses of SO2 and fine (1-25 mm diameter) ash in Spurr volcanic clouds. Data from tables 4-6.
Figure 3: Ash Burdens of 1992 Spurr Volcanic clouds. Data from Tables 4-6.
Figure 4: Plots of covariations between optical depth and Ash Burden. Data from tables 4-6.
Figure 5: Effective radius of silicates in Spurr Volcanic clouds. Data from tables 4-6.
Figure 6: AVHRR image at 331 GMT on August 19, 1992, during the eruption of 19 August 1992. 6a is a band 4 brightness temperature image which shows the thunderstorm like cold image. 6b is a band 4 - band 5 brightness temperature difference image in which the opaque (optical depth ~ 4) core of the cloud shows no signal, but the transparent fringe is brightly outlined. These “stage 1” volcanic clouds have opaque cores with optical depths of 4 or more, preventing retrievals of the entire cloud. See Schneider et al (1995) for more details.
Figure 7: Simplified fallout maps of the three 1992 ash blankets. The dashed line outlines the total area where fallout was detectable. The red line outlines the region where mass of ash exceeded 5000g/m

Figure 8: Grain size distribution as determined by two different laser diffraction devices (Microtrac and Malvern) for a distal ash sample located by the red x in figure 6. (from Riley et al, 2000). The Malvern instrument uses a blue laser which apparently more accu­rately detects particles smaller than about 1 mm.


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