Volcanic output of SO2 and trace metals: A new approach

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Geochimica d Cosmochimica Acfa Vol. 52, pp. 39-42 Q Papmon Journals Ltd. 1988. Mntcd in U.S.A.

Volcanic output of SO2 and trace metals: A new approach CLOAREC~~~ MADDALENA PENNISI

GBRARDLAMBERT,MARIE-FANCOISELE

Centredes FaiblesRadioactivit&s, LaboratoireMixte C.N.R.S.-C.E.A., Avenue de la Terrasse,91190 Gif Sur Yvette, France

(Received May 12, 1987; accepted in revisedform September 30, 1987) to a model of volcanic emission of gases and volatile& it was possible to normalize to *‘OPothe volcanic output of SOz, Pb, Bi and other trace as well as major metals. It appears that the results concerning SO*, Pb and Bi agree with previous estimates derived on a very different basis. The evaluation was extended to Cd, Cu, Zn, Al, Mg, Na and K. Moreover, it was observed that, even for poorly volatile major metals, the part of volcanic aerosols produced by evaporation is at least comparable to that which results from spattering.

Abstract-Owing

by LEHMANN and SITTKUS (1959) and BURTON and STEW-

INTRODUCTION

ART ( 1960):

SO1IS, AFI-ERCOz, the second major component of dry gases emitted from active volcanoes. However, the volcanic contribution to the atmospheric COz budget is negligible at a global scale, and the residence time of this gas in the atmosphere is rather long: both effects combine to explain that the CO2 concentration in a volcanic plume is Only several times the atmospheric background. In contrast the SO2 concentration in a volcanic plume can be hundreds or even thousands times that of the ambient atmosphere. This observation, and the optical properties of SOZ, make possible its remote sensing and a direct measurement of the flux emitted from an active volcano (HAULET et al., 1977; MALINCONICO, 1979; CARBONELLEet al., 1981; BANDYet al., 1982; CASADEVALL et al., 1983; STOIBERet al., 1983; MARTIN et al., 1986). The outputs of other trace elements can be then evaluated by normalization to the SO2 flux (BUAT-MENARD et al., 1978; CADLE et al., 1979; F’HELANet al., 1982; CASADEVALL et al., 1984). Therefore, it would be particularly valuable to have an accurate evaluation of the SO2 global output from active volcanoes. However, this emission represents only a few percent of the total source of atmospheric SO2 and therefore cannot be confirmed from a knowledge of the global sulfur cycle. The situation is entirely different in the case of the volcanic source of “‘PO, last radioactive nuclide of the z38U/Z26Ra series. In effect, LAMBERT et al. (1976) showed that the *“PO concentration in the plume of Mt. Etna was about 10’ times higher than in the usual atmosphere. LAMBERT et al. (1982a) found, owing to a global atmospheric budget of 222Rn and its decay products, that volcanoes represent about 50% of the global source of “opo to the atmosphere. It seems therefore fruitful to normalize the volcanic emissions of trace metals to *“?o rather than to SOz: that is the aim of this paper.

ABinBi -=APbh’b

ABi ABi +

As

n its concentration in atoms per kg (or m’) of air, X,is a scavenging constant equal to the inverse of the mean life 0 of submicronic aerosols on which these nuclides are fixed. It is generally accepted that 0 is close to 7 days, which corresponds to a 2’0Bi/2’oPbactivity ratio of about 0.5 in the whole troposphere, in good agreement with the results of MOOREet al. (1973) who measured 0.53 at different altitudes. However, the same value of 8 would give a 2”?o/2’(‘Pb activity ratio of about 1.7 X 10e2 instead of 1 X 10-l as actually observed. Therefore 5 atoms of “‘PO among 6 present in the troposphere must be ascribed to an extraneous source, different from the radioactive decay of the tropospheric 222Rn daughters, 2’oPb and 2’oBi. During years, this 2”?o excess was explained by the presence in the troposphere of stratospheric aerosols, whose residence time had been long enough to let 2’oPb and 2’oPo to reach the radioactive equilibrium. However, a more sophisticated treatment of the problem was made by LAMBERTet al. ( 1982b) who took into account these stratosphere-troposphere exchanges of long lived 222Rndecay products and, by calculating a 6.5 day mean residence time for the aerosols in the troposphere, needed the existence of an extrasource of 2’oPo, evaluated to 6.5 X lo4 Ci per year. After analysing the different possible components of this extrasource like coal burning, as suggested by JAWOR~WSKI et al. ( 1972), and more especially marine aerosols as suggested by TUREKIAN et al. (1974) and LAMBERT et al. (1974), the volcanic output of “‘PO was estimated to 5 X lo4 Ci per year, that is about 50% of its total injection into the atmosphere (LAMBERT et al., 1982a).

where X is the radioactive constant of a given nuclide,

*“PO VOLCANIC OUTPUT It is well known that the 226Ra decay product 222Rn is a noble gas which is continuously injected into the atmosphere from the soil of the continents. Assuming a steady state in the troposphere, the 222Rn long lived decay products 2’oPb, 2’% and 2’opo should verify the equations initially written

SO2 FLUX EVALUATIONS

A global SO2 flux evaluation was first attempted by KELD (1972) from an estimation of the volume of emitted lavas and their likely SO2 content, and an output of 0.75 et al.

39

40

ti. Lambert. M.-F. le Cloarec and M. Pennisi

X IO6 tons per year was proposed. Later on. FRIEND (1973) and CADLE(I 975) made similar calculations by using revised data and found 4 X IO6 and 7.5 X 10” tons per year. respectively. STOIBERand JEPSEN(1973) measured SO2 emissions from seven Central American volcanoes, during several days, and extrapolated their results on a basis of 100 volcanoes simultaneously active throughout the world, and found an output of 7 X lo6 tons per year, that is in good agreement with CADLE (1975). However, LE GUERN (1982) pointed out that viscous lavas associated with American subduction areas are characterized by fluxes significantly smaller than those from volcanoes whose lavas are very fluid, such as Niragongo, Etna or Kilauea. He proposed, therefore, a revised figure reaching 10 X IO6tons per year. More recently BERRESHEIM and JAESCHKE(1983) made an evaluation taking into account a classification of the eruptive volcanic activities as well as pre- or post-eruptive periods for 18 volcanoes and by integrating a large number of flux measurements. found a global SO2 emission of 15.2 X lo6 tons per year. This evaluation is probably the best one attainable by extrapolation of direct SO* measurements in volcanic plumes. In order to normalize this flux to that of 2’oPo, simultaneous measurements of SOz and “‘PO in volcanic gases and aerosols were conducted on several active volcanoes in different parts of the world. On Mt. St. Helens, in 198 1, and Momotombo, in 1983, water vapour and acid gases were condensed in a vessel, using ethyl ether as a coolant, according to the technique of CIONI and CORAZZA ( 198 1). The SO2 contents were measured using gas chromatography by LE GUERN ( 1987) for Mt. St. Helens, and with the soda bottle method for Momotombo by ALLARD (1986). In both cases “‘PO was analysed through the method described by 1.~ CLOAREC ~‘1 al. ( 1986). On Mt. Etna, aerosols were collected on PoelmannSchneider blue cellulose filters, at a flow rate of 15-20 m3 per hour according to the technique described by POLIAN and LAMBERT (1979) and filters activities of “‘PO were determined by gross alpha counting. Each sample corresponded to about 15 m3 of air. SO2 was simultaneously measured either by calorimetry on Zn acetate filters by FAIVRE-PIERRET (1983), or directly with a flame photometry detector. All the results are shown in Table I. It appears that the 2’oPo/S02 ratio varied on Mt. Etna between 1983 and 1985, from 0.2 to 0.6 mCi per ton of S02. The values obtained for Mt. St. Helens and Momotombo are of the order of 2 mCi per ton. It seems that an acceptable value could be of the order of 1 mCi per ton of SO*. This figure gives a global volcanic output of SOz of 50 X lo6 tons per year. This value is about 3-fold that proposed by BERRESHEIM and JAESCHKE (1983), which seems to be a good agreement

when we consider the difference in the methods ofevaluation. However, this represents 7-fold the values initially proposed by STOIBER and JEPSEN (I 973) and CADLE ( 1975). ‘This discrepancy suggests that either the 2’oPo volcamc output and/ ortheS02/2’oPo ratio used inourcalculationscould bcslightlh overestimated. LEAD FLUN EVALUATION In a model of radium daughters emissions from Mt. Etna. (1985/86) postulated that the emanation coefficient of polonium, tpu, which is defined as its partition between gaseous and magmatic phases, is 1. In other words, they considered that all polonium atoms present in a flowing lava are volatilized. This assumption was based on the lack of 2’oPo in fresh lava samples (BENNETT et a/., 1982: LE CLOAREC et al., 1984; GILL et ~11..1985). In contrast, LAMBERT ef al. (1985/86) calculated an emanation coefficient for lead (cpb) of I.5 X 10.-2. This determination was based on the assumption that no fractionation could occur between ‘lOPb and common lead. However. it was observed by PENNISI et al. (1987) that the specific activity of 2’0Pb is close to 1 dpm per pg of lead in Mt. Etna aerosols, when it is close to 0.4 in the corresponding lavas. This leads to values of t possibly different for “‘Pb and common lead which could be, by following the same calculations as LAMLAMBERT et al.

BERT d al. (1985/86),

in the range of0.6

In this paper all the calculations intermediate

X 10e2 to 1.5 X 10m2.

will be conducted

with an

value of I X IOm2for EPb, and an uncertainty

of a factor two. A budget of 5 X lo4 Ci per year of 2’oPo therefore corresponds to 5 X lo2 Ci per year of “‘Pb. To evaluate the volcanic output of common lead from this “‘Pb flux, it is necessary to know a global average specific activity of 2’0Pb per mg of common lead. Unfortunately, very few data are available, because the lead content and the activity of either 2’0Pb equal acor 226Ra (which are supposed to be approximately cording to KRISHNASWAMI et al., 1984, and CONDOMINES et al., 1987) must be measured in lhe samesample. In contrast,

the numerous 238Umeasures are not useful, due to the general disequilibrium of this nuclide with 226Ra and consequently 210Pb in lavas (SOMAYAJULU et al., 1966; OVERSBY and GAST, 1968; NISHIMURA, 1970: CHEMINEE, 1973; CAPALDI tY al.

(1976). We measured specific activities of “?b per mg of common lead in several lava samples. Measurements were made according to the technique described by LE CLOAREC et ul. (1984). It may be observed in Table 2 that most of the values are between 0.23 and 0.59 dpm per mg of lead, except Merapi Table 2.

“‘Pb

/Pb

in

lavas per

(desincegratian

ug

of

per

mn and

lead) ---

Mt St Helens 1981 2.0

Momotomba 1983 2.4

Mt Erna september 22 23 25 mean

1983 0.18 0.25 0.11 0.10 0.13 0.25

value-O.17

“‘Pb dpn/g

Mt Eltna April 1985 26

0. ‘30

27

O.>9 0.78 0.66 0.65

29 mean

va1ue=0.60

MC ~ameroun it st Helens Piron de la Nerapi Merapi it “t “t Ht

~tna Etna Etna Erna

I978 1982 1976 1983 1984 1985

1981 1981 Fournaise

r’“Pb

/Pb

UP/P

dpmhl: __..

~__._

1.58 .27 0.74 4.46

6.67 5.46 1.62 15.80

0.28 0.23

3.18 5.0

36.02 9.22

5.0 3.63 3.75

12.20 6.17 9.94

0.09 0.54 O.Ll 0.59

I

1983

Pb

_________

0.46 0.28

0.38 __-

_-

41

Volcanic SO*and trace metal output in 1982. It is clear that the 2’oPb activities mentioned in this table, which agree very well with CONWMINES et al. (1987), are significantly higher than many published data concerning volcanic rocks (KRISHNASWAMIet al., 1984, GILL et al., 1985), but this does not imply that the corresponding specific activities of 2’oPb per mg of common lead should also be significantly different, as shown by OVERSBY and GAST ( 1968), who found from 0,14 to 0,25 in trachyandesites from Tristan da Cunha; 0,22 and 0,43 in alkali basalts from Faial (Azores); and 1.19 only in a nepheline basalt from Mt. Vesuvius, particularly rich in “‘Pb 34 dpm per g. Therefore it seemed reasonable to use an intermediate value of 0.4 dpm of ‘r”Pb per mg of lead, as observed in lavas from Mt. Etna. This corresponds to 5 tons of lead per Ci of “‘Pb. This ratio yielded a global volcanic lead output of 2500 tons per year. This figure has a confidence interval of about a factor of 3, which is essentially due to the uncertainty in the emanation coefficient of lead as well as in the 2’oPb specific activity. This result is quite comparable to the previous estimates of 4200 to 96000 t per year by NRIAGU (1979). PATTERSON and SETTLE(1987)* used a weighted Pb/S ratio of about 7 X 10m5which would correspond to 1750 t per year with our SO2 flux, but only to 500 t per year by using the estimation of BERRESHEIMand JAESCHKE( 1983). However, it must be emphasized that, as for S02, the way of evaluation used in the present paper is completely different. FLUXES OF OTHER EVAPORATED TRACE METALS The volcanic output of the other metals can be related to the flux of lead through the equation:

where @is the flux of a metal, in tons per year, C is its average concentration in basalts as proposd by TAYLOR( 1964), and c its emanation coefficient. &+ was calculated above. Emanation coefficients were evaluated in Mt. Etna aerosols by PENNISI et al. (1987) for 8 metals and are listed in Table 3. These authors pointed out some variations of 6, particularly in the case of volatile metals. However, most of these variations are in the range of a factor of 2, and therefore do not change significantly the uncertainty of such evaluations. It may be reasonably concluded, as a first approximation, that the average volcanic value oft should not be very different. The results of the calculations are shown in Table 3. Similar evaluations were made by NRIAGU (1979) using a different approach. This author started from an evaluation of the total mass of volcanic particles presented by ELL~AESSER(1975), and corrected them by means of an average enrichment factor characteristic of each element. It is remarkable that the flux evaluations presented herein fall in the range (yather broad, indeed) of those of NRIAGU, which are, in lo3 tons per year, 300 to 7800 for Cd; 2500 to 33000 for Cu; 4600 to 105000

Table 3. Flux of metals Av.Basalt" (mm) Pb Bi Cd CU Zn Al “g Na K

?Phennisi

5 0.15 0.2 100 Iml 87600 45000 19400 8300

2.5 1.5 I 15 5 88 45 194 250

et al (1987)

=faylor (1964)

for Zn. The value given in this paper for Bi agrees with the figure of 1200 tons per year proposed by PATTERSONand SETTLE(1987).* It is worthy noting that all these fluxes are relative to metals emitted as vapours and subsequently condensed in the cold plume as aerosols. It was shown by LAMBERTet al. (1987) that, for volatile metals, almost all the volcanic aerosols are produced by this process, the part of spattered materials being negligible. CONCLUSION It seems very fruitful to normalize volcanic emissions to “‘PO, half of whose atmospheric budget can be ascribed to volcanoes. The values obtained by this method for the volcanic sources of SOz, lead and bismuth agree well with previous estimates derived independently and on a different basis, and therefore confirm their results. However, the accuracy of these evaluations remains in the range of a factor of 3, essentially due to the difficulty of determining a global mean value for the elemental concentration ratios in volcanic plumes. Moreover, an accurate tracing of common lead by 2’oPb seems to be questionable. The main feature of the method followed here is probably the possibility to extend volcanic flux estimates to a large number of chemical species. Acknowledgements-The authors are grateful to all people having participated in this hard work: sampling on the rims of the craters. This work was supported by the “programme interdisciplinaire de recherche pour la prevision et la surveillance des eruptions volcaniques” du C.N.R.S. C.F.R. contribution 880. Editorial handling: J. D. Macdongall

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