Completo_DEF.qxd 1455 ANNALS OF GEOPHYSICS, VOL. 47, N. 4, August 2004 Key words volcano monitoring – volcano plumes – IR and UV spectoscopy 1. Introduction Volcanoes emit gases (principally H2O, CO2, SO2, HCl, HF and H2S) and aerosols to the atmosphere, both during and between erup- tions. Measurements of the chemistry, isotopic composition, and flux of these emissions, and their temporal variations are crucial in several applications (Oppenheimer et al., 2003). 1.1. «Imaging» volcanic plumbing systems and predicting volcanic activity Volcanic gas composition and flux relate to subsurface magmatic conditions and higher-lev- el fluid-rock interactions, providing information on magma composition, volume, storage, and transport. For instance, temporal variations in the ratios of emitted CO2/SO2 and HCl/SO2 have been used to infer changes in magmatic systems feeding volcanoes (e.g., Noguchi and Kamiya, 1963; Gerlach and Casadevall, 1986), whilst gas flux measurements have been used to constrain the masses of degassing magmas (Symonds et al., 1996). Such data are valuable for hazard assessment because volcanic activity is strongly controlled by the dynamics of de- gassing (Sparks, 2003). In the broader context of volcano monitoring efforts, the links between seismic, geodetic and gas geochemical signals can be strong. The degassing process may be closely associated with, even responsible for, observed seismicity and ground deformation. 1.2. Understanding atmospheric and radiative effects Quantification of volcanic volatile emissions is also important in order to evaluate their hemi- spheric to global scale atmospheric and climatic impacts. The sulfur emissions from major explo- Mailing address: Dr. Clive Oppenheimer, Cambridge Volcanology Group, Department of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, U.K.; e-mail: co200@cam.ac.uk Exploiting ground-based optical sensing technologies for volcanic gas surveillance Clive Oppenheimer and Andrew J.S. McGonigle Cambridge Volcanology Group, Department of Geography, University of Cambridge, U.K. Abstract Measurements of volcanic gas composition and flux are crucial to probing and understanding a range of mag- matic, hydrothermal and atmospheric interactions. The value of optical remote sensing methods has been recog- nised in this field for more than thirty years but several recent developments promise a new era of volcanic gas surveillance. This could see much higher time- and space-resolved data-sets, sustained at individual volcanoes even during eruptive episodes. We provide here an overview of these optical methods and their application to ground-based volcano monitoring, covering passive and active measurements in the ultraviolet and infrared spectral regions. We hope thereby to promote the use of such devices, and to stimulate development of new op- tical techniques for volcanological research and monitoring. 1456 Clive Oppenheimer and Andrew J.S. McGonigle sive eruptions can perturb the chemistry, dynam- ics and radiative transfer in both the troposphere and stratosphere, influencing surface climate (e.g., McCormick et al., 1995). Passively de- gassing volcano emissions must also influence tropospheric dynamics and chemistry, as recent investigations suggest that between 15 and 40% of the global tropospheric sulphate burden is volcanogenic (Chin and Jacob, 1996; Graf et al., 1998; Stevenson et al., 2003). These figures ex- ceed the percentage source strength of volcanic sulphur by a factor of 2-4, reflecting the gener- ally higher altitudes of atmospheric entrainment from volcanoes compared with biogenic or an- thropogenic sources, and highlighting the impor- tance of better understanding the tropospheric chemistry of volcanic emissions. 1.3. Evaluation of environmental and health impacts From local to regional scales, volcanic emissions can result in serious environmental and environmental health consequences, in- cluding destruction of agricultural crops, con- tamination of pasture, and human respiratory morbidity and cardiovascular mortality. Large lava eruptions, such as that of Laki (Iceland) in 1783/1784, which released ≈ 120 Tg of SO2, 7.0 Tg of HCl and 15 Tg of HF, have resulted in major pollution episodes responsible for re- gional-scale extreme weather, agricultural loss- es, and elevated human morbidity and mortali- ty (Thordarson et al., 1996; Witham and Op- penheimer, 2004). Individual passively de- gassing volcanoes can also represent major pol- lution sources. For example, Mt. Etna continu- ously emits of order 2 kg s–1 of HF, and > 8 kg s–1 of HCl (Francis et al., 1998). Its SO2 flux (≈ 70 kg s–1) is comparable to the total industrial S emission from France, and substantially ele- vates tropospheric sulphate in southern Italy (Graf et al., 1998). Adverse environmental and health impacts are observed downwind of many degassing volcanoes, including Masaya (Nicaragua), K1̃lauea (Hawai`i), Poás (Costa Rica), Miyakejima (Japan) and Popocatépetl (Mexico) (e.g., Baxter et al., 1982; Mannino et al., 1996; Delmelle, 2003; Fujita et al., 2003). At Masaya volcano, which has been degassing strongly for over a decade without significant eruption, boundary layer SO2 concentrations are elevated over a downwind area > 1000 km2, resulting in substantial economic impact from the loss of coffee crops, and exposing ≈ 50 000 people to levels exceeding WHO air quality standards (125 ppb over 24 h; 50 ppb over 1 yr). Downwind of Popocatépetl volcano, SO2 quadrupled, and sulphate concentrations dou- bled in Mexico City when fumigated by the volcanic plume (Raga et al., 1999). The aims of this paper are to review briefly some of the latest developments in ground based optical sensing of volcanic gas and aerosol emissions. We focus particularly on those in- struments that have come to the fore over the last ten years that are capable of augmenting or superseding the Correlation Spectrometer (COSPEC) device, which has seen widespread use over the last thirty years for volcanic SO2 flux measurements. We also discuss the challenges that remain in implementing gas-monitoring networks, and in interpreting their data streams. It is hoped that this article will stimulate devel- opment of further optical sensing technologies, characterised by low cost, low maintenance, low weight and bulk, high temporal resolution, and multi-component sensitivity, that will meet key volcanological requirements. 2. General methodological background and challenges The conventional way to measure volcanic emissions is by direct sampling, either by close- range collection of samples from fumarole vents and active lava bodies using «Giggenbach bottles», filter packs and condensing systems, or within atmospheric plumes from aircraft us- ing various kinds of sampling apparatus and on- board analysers. A range of spectroscopic, gravimetric, isotopic and chromatographic techniques is available to determine chemical concentrations in real time or subsequently in the laboratory (Symonds et al., 1994). Whilst direct sampling is capable of delivering very detailed and accurate analyses, it is difficult to sustain routine surveillance in this way, or to 1457 Exploiting ground-based optical sensing technologies for volcanic gas surveillance provide data of sufficient temporal resolution to compare meaningfully with seismic and geo- detic data streams. Arguably, the primary rea- son for this is the risk involved in the field, which generally restricts investigations to low temperature, subordinate vents (that probably do not characterize the bulk emission or repre- sent the parts of the magmatic-hydrothermal system most sensitive to change in the event of renewed magmatic activity). Additionally, chemical reactions between the container mate- rial or reagents and the collected gas sample may mask the original chemical composition. Whilst the use of telemetered electrochemical sensors for continuous gas monitoring avoids many of the above complications (McGee and Sutton, 1994), this approach is not yet wide- spread and it suffers the problem that sensors may be destroyed in the event of an eruption. In this section, we look at the general back- ground to volcanic gas sensing using optical methods, highlighting some of the key issues in adapting and developing remote sensing instru- ments for volcanological purposes, and interpret- ing the data they yield. Much of the discussion applies to volcano monitoring devices in general. 2.1. Spectroscopy background The sensing methods described in section 3 are based on the spectroscopic observation of molecular species of interest from their finger- print rotational, vibrational and electronic tran- sitions, usually seen in absorption (i.e., the at- tenuation of a source of radiation behind the gas cloud; fig. 1) but sometimes in emission. Spe- cific gases can be identified by their character- istic absorption spectra, and their abundances derived from the strength of the absorption, fol- lowing the Beer-Lambert formula: I(λ) = I0 (λ)exp(– σ(λ)NL) (2.1) where I(λ) is the observed intensity of radiation at wavelength λ, I0(λ) is the original intensity of radiation, before interaction with the sample, σ(λ) is the absorption cross section, of the ab- sorbing molecule at wavelength λ, and N is the mean concentration of the species over the path- length L of the sample. Measurements of more than one species provide gas ratios, in an analo- gous manner to direct sampling; traverses or scans of volcanic clouds combined with plume Fig. 1. Diagram of radiative transfer problem in absorption spectroscopy. From Horrocks (2001). 1458 Clive Oppenheimer and Andrew J.S. McGonigle transport speeds can be used to derive fluxes of gases. The remote sensing measurements may be considered «passive» if the source of radiation is natural, and «active» if it is artificial. Exam- ples of passive measurements include use of natural terrestrial radiation (e.g., from active la- va surfaces), diffuse sky radiation, or direct so- lar radiation (i.e., occultation measurements). Active sources include lamps (usually used for broad spectral band observations), and lasers (sometimes multi-wavelength or «tuneable» for differential spectral measurements). The spec- trometers may be dispersive, if the incoming light is separated spatially into its component wavelengths, or non-dispersive if the wave- length discrimination is achieved by other means (e.g., by interferometry). Passive meas- urements are invariably «open-path» observa- tions through the free atmosphere. Active meas- urements may be open-path, or «closed-path» wherein the gas sample is admitted to a cell, of given path-length, through which the beam is passed. An advantage of closed-path observa- tions is that the cell’s internal pressure may be lowered to reduce pressure-broadening effects on the spectral lines, permitting high-spectral- resolution measurements of trace species even in noisy regions of the spectrum. Multi-path cells of modest dimensions (e.g., < 1 m) can simulate substantial atmospheric paths (e.g., > 100 m) via multiple internal reflection. For quantitative applications, it is usually necessary to separate out the spectral features of the volcanic gas species of interest from those of the background atmosphere (which are dominated by spectral lines originating from H2O and CO2, and molecular scattering at shorter wavelengths). This can be achieved by modelling the total atmospheric path (e.g., us- ing a radiative transfer code, atmospheric and meteorological data, and known absorption co- efficients for all relevant species), or ratio-ing spectral observations acquired for the same path with and without the volcanic plume pres- ent (Iλ and I0,λ , respectively). From inspection of (2.1), it can then be seen that the column amount of a given species (i.e., the product of concentration and path length) can be obtained if σ(λ) is known. 2.2. Instrumental specifications: data acquisition Initial considerations in the design or ex- ploitation of a gas sensor include the species to be targeted, and the dynamic range and sensi- tivity of the measurements. This requires, first- ly, an appreciation of the molecules and/or iso- topes worth observing from the perspective of what volcanologically-useful information they can provide, and the purpose of the investiga- tion (volcano monitoring for activity predic- tion, quantification of concentrations of harm- ful gases, etc). The mixing of volcanic gases with the atmosphere and subsequent plume transport and dispersion act to dilute concentra- tions of species, making it particularly difficult to detect and measure the two principal vol- canic gas components, H2O and CO2. The sig- nificant and rapid changes in humidity of the background atmosphere add to complications of sensing volcanic H2O. H2O and CO2 also hamper spectroscopic observations of other gas species because of their abundant absorptions across the fingerprint region of the infrared spectrum, which can mask more subtle features of other trace gases. Because of the budgetary constraints on volcano observatories and institutes, it is cer- tainly true that it helps if instrumentation is comparatively cheap. Decisions always have to be reached in respect of costs of personnel, field vehicles and running costs, as well as equipment. However, given the potential cost benefits to society of effective volcano moni- toring and accurate activity forecasts, volcanol- ogists should be prepared to defend even ex- pensive equipment purchases where a case can be made that the instrumentation will signifi- cantly enhance capabilities. Observatories are often stretched not only in financial terms but also because of the many demands on the time of their personnel. This is especially the case during volcanic crises when operations are increased, and other parties (lo- cal officials, the public, the media, etc.) demand access to information. Field equipment should therefore be, generally speaking, easy to oper- ate and maintain; data should be available in re- al time or near-real time, preferably with high 1459 Exploiting ground-based optical sensing technologies for volcanic gas surveillance time-resolution, and ideally with minimal post- processing required. (Recording data from the COSPEC instrument – see Section 3.1.1 – has tra- ditionally been achieved using chart recorders, demanding time-consuming, and potentially in- accurate, visual processing of the rolls of paper. This can often result in data not being processed for several days, despite their poten- tial importance for hazards assessment). Inter- net-ready, digital data-streams have obvious ad- vantages for remote access, and multi-parame- ter data interpretation. Equipment should be robust enough for sus- tained operation, even in hostile conditions (acid gases, ash deposition, etc.), and generally low in weight in bulk, especially if its use relies on being moved around regularly and transport- ed in vehicles (clearly, autonomous operation is the ideal since it frees up personnel for other ac- tivities). Power requirements should be modest as grid supplies are seldom an option on remote volcano locations. 2.3. Error budgets and error reduction In order to interpret measurements, and the significance of any observed changes, it is cru- cial to understand sources and magnitudes of er- rors in the data. A number of data fidelity issues can be addressed in an automated fashion through various error checking regimes. Instru- ment response functions should be monitored regularly where necessary, with calibration and intercalibration issues addressed as appropriate. This may include careful examination of radia- tive transfer models, assumptions concerning at- mospheric profiles, plume distribution (especial- ly in the vertical), and Mie scattering processes. While budgeting and reducing errors are es- sential exercises, they should be approached in the context of the necessary accuracy and pre- cision for the volcanological task in hand, and the non-volcanological influences that can modify observations. For example, imagine a volcano where instantaneous SO2 gas fluxes at source may change by an order of magnitude on timescales of days. It may be sufficient to dis- criminate changes in flux of a factor of 5 in or- der to identify reliably the source signal, and therefore be unnecessary to go to great lengths to reduce errors. In respect of gas flux meas- urements, considerable attention has to be giv- en to the estimation of plume transport speeds. It is widely acknowledged that uncertainty in plume speed (typically 20-40%) represents the major contribution to errors in measurements of SO2 and other gas fluxes. A consistent approach to constraining plume speed is therefore essen- tial. If plume speeds are poorly constrained there is little point in engineering spectrometer sensitivity to improve retrievals by only a few per cent. Absorption-correlation methods using spatially distributed instruments provide one means to track plume velocity accurately (Williams-Jones et al., 2003). 2.4. Data interpretation Having secured a geochemical data-stream, the next job is to interpret it. The basic tasks are to identify volatile sources, magma-hydrother- Less soluble volatiles exsolve and form separate vapour phase More soluble volatiles exsolve Fresh input Cooling and crystallisation Fluid-rock reactions; sealing; water-soluble gases scavenged in hydrothermal system Diffuse (soil) degassing H2O, CO2 and SO2 Hydrofracturing Gas escape if magma and/or conduit walls permeable Meteoric water Seawater Rn, CO2 D ecom pression Fig. 2. Potential physical and chemical processes oc- curring in a magmatic-hydrothermal system, includ- ing the influence of magma dynamics in the chamber- conduit plumbing system, and interactions between magmatic fluids and the crust. These can strongly modulate the speciation and flux of various magmatic components emitted into the atmosphere, complicat- ing the interpretation of geochemical measurements of surface emissions (from Oppenheimer et al., 2003). 1460 Clive Oppenheimer and Andrew J.S. McGonigle mal system interactions, the dynamics of de- gassing, and changes in these through time. Un- fortunately, interpretation of the observations is far from straightforward because of the many factors that control magmatic volatile content (mantle melting, slab contributions, wall-rock assimilation, etc.), exsolution and gas separa- tion of different volatile species from magma, and the subsequent chemical and physical inter- actions of the exsolved fluids, for example, with crustal rocks and hydrothermal fluids, as they ascend to the surface (fig. 2). Although thermodynamical codes enable «restoration» of observed gas analyses to equilibrium composi- tions (e.g., Gerlach, 1993; Symonds et al., 1994), identifying precise magmatic and hy- drothermal conditions remain real challenges. Indeed, a casual perusal of the literature will re- veal conflicting interpretations of similar obser- vations - for instance, decreasing SO2 fluxes could be due to i) depletion of volatiles in a magma body, or ii) a decrease in the permeabil- ity of the plumbing system. Process i) might in- dicate decreased eruption likelihood, while ii), perhaps induced by sealing of bubble networks, which would act to increase overpressure (e.g., Edmonds et al., 2003c), could increase the chance of an explosive eruption. Thus the same observation can be interpreted in different ways with contradictory hazard implications. Advances in this area will benefit from de- velopment and validation of comprehensive physico-chemical models for volcanic de- gassing based on the integration of results from experiments on the controls on distribu- tion of volatiles in synthetic and natural melts, analysis of dissolved volatiles preserved in melt inclusions, and observed volcanic gas geochemistry. Ultimately, such models can be applied to integrated geophysical, geodetic and geochemical monitoring data to support eruption forecasting. 3. Instrumentation Over the last thirty years, ground-based op- tical remote-sensing techniques have been in- creasingly used for volcanic gas (and aerosol) monitoring (table I). Such techniques possess many of the advantages of seismic and defor- mation monitoring over direct sampling, such as the ability to obtain measurements in rea- sonable safety, allowing semi-continuous mon- Table I. Overview of spectroscopic methods for ground-based optical sensing of volcanic gases. Instrument Transport methods Volcanic gas species detectable Flux measurements (Yes/No) COSPEC car, aircraft, boat SO2 Yes UV grating spectrometers (e.g., Ocean Optics USB 2000) on foot, car, aircraft, boat SO2, H2S, BrO Yes FTIR (e.g., Brucker OPAG 22, MIDAC AM series) car CO2, CO, OCS, CH4, SO2, H2O, HCl, HF, SiF4 Yes (with sun-tracker) Other NDIR (e.g., LI-COR CO2 analysers) on foot, aircraft CO2, H2 O Yes (by plume profiling or ground flux surveys) DIAL (i.e., using atmospheric backscatter to return signal) truck, ship SO2, other species feasi- ble Yes Laser spectroscopy (short path, extractive) car as FTIR plus isotopes No 1461 Exploiting ground-based optical sensing technologies for volcanic gas surveillance itoring even through violent eruptive periods. Additionally, gas concentrations may be re- trieved non-invasively in near real-time, obvi- ating the need for subsequent laboratory analy- sis, and eliminating the possibility of sample contamination. Because remote sensing tech- niques measure integrated gas concentrations through cross sections of the plume, in contrast to in situ sampling, they can potentially yield a more representative picture of bulk plume composition and flux. In the following subsections, we briefly re- view some recent developments in optical sens- ing applications to volcanic gas surveillance. With the exception perhaps of the ASTER sen- sor on NASA’S Terra platform, spaceborne re- mote sensing is unlikely, in the near future, to provide a capability for frequent measurements of weaker tropospheric volcanic plumes, e.g., sustained by quiescent degassing. We focus, therefore, on ground-based approaches that are suitable for routine monitoring applications. For a fuller review of the field, including spec- troscopic determination of volcanic aerosol concentrations and properties, see McGonigle and Oppenheimer (2003). 3.1. UV spectroscopy 3.1.1. Correlation mask instruments: COSPEC Arguably, the most widely applied instru- ment for ground-based remote-sensing of vol- canic plumes is the Barringer Research COSPEC, which was originally developed to measure in- dustrial SO2 and NOx emissions (Moffat and Millán, 1971). Over the last thirty years, open- path ultraviolet (OPUV) COSPEC SO2 measure- ments have been performed at numerous vol- canoes worldwide (e.g., Casadevall et al., 1984; Stoiber et al., 1986; Caltabiano et al., 1994; Gerlach et al., 1998, and references therein). The COSPEC is typically operated by measuring the absorption of ultraviolet (UV) zenith skylight, by overhead SO2 (Millán and Hoff, 1978). This is achieved by dispersing the collected skylight with a grating and then im- aging this spectrum on to the radial length of a spinning disk, behind which is a detector. The disk has segments with etchings at radii that block light «correlating» either to wavelengths of low or high absorption (in the spectral re- gion 300-315 nm), such that the output signal from the detector is modulated according the absorption of overhead SO2. (A development of this form of correlation spectroscopy is to mount cells containing the gases of interest on a rotating chopper disk, in an analogous fash- ion to the COSPEC’s etched masks. This gas correlation filter spectrometry method has been implemented to sense volcanic CO and OCS concentrations but operating in the in- frared spectral region (Stix et al., 1996). Internal electronic processing yields SO2 concentrations (in parts per million meters - ppm m; see Gerlach, 2003, for discussion of the problems inherent in use of these units), and calibration is achieved by placing quartz cells containing known amounts of SO2 in the internal optical path of the spectrometer. Flux- es are obtained by traversing underneath the plume, approximately perpendicular to its ax- is, in a road vehicle, boat or aircraft, and recording SO2 concentration as a function of position (from a GPS receiver). By integrating the concentrations across the plume and multi- plying by plume speed, SO2 fluxes are derived (typically expressed in tonnes per day, t d−1, or kg s−1). The COSPEC possesses many advantages as a tool for volcanic surveillance. As this de- vice operates using scattered skylight, align- ment is trivial and measurements are possible in overcast conditions. However the accuracy of the derived fluxes is very dependent on the ad- equacy of the plume speed data (Stoiber et al., 1983). Typically, the wind speed is obtained from distant radiosonde data, visual observa- tions of the moving plume, or ground based anemometry. More accurate wind data can be obtained by videography, or from aircraft navi- gational equipment in the case of airborne trav- erses. A further source of error is that the dif- fuse skylight can be scattered into the COSPEC’s field of view from above, below or within the plume (Millán, 1980; Moffat and Millán, 1971), in contrast to the flux calculation’s as- sumption that light passes vertically through the entire plume. Scattering effects in ash-laden 1462 Clive Oppenheimer and Andrew J.S. McGonigle plumes can introduce further scattering related errors (Andres and Schmid, 2001). The COSPEC was originally applied in a vol- canological context to assess whether changes in SO2 gas fluxes could be associated with changes in eruptive activity of open conduit volcanoes (i.e. rising or falling of magmas, and sealing or opening of magma chambers). Posi- tive correlations of increasing SO2 flux with ac- tivity were observed during measurements at Mt. Etna (Malinconico et al., 1979). In 1991, the SO2 flux of Mt. Pinatubo was observed to increase by an order of magnitude over two weeks, in parallel with seismic unrest, provid- ing evidence for a shallow intrusion of magma, and prompting a civil evacuation (Hoff, 1992; Daag et al., 1996). Decreasing COSPEC SO2 fluxes in parallel with decreasing post-eruptive activity have also been observed on many vol- canoes, notably at Mt. St. Helens from 1980 to 1988 (McGee, 1992), following the 1980 erup- tion. Through COSPEC a catalogue of fluxes from actively and passively degassing volca- noes worldwide has been obtained, from which the total volcanic SO2 flux to the atmosphere has been estimated at ∼ 20 Tg yr–1 (Stoiber and Jepsen, 1973; Berresheim and Jaeschke, 1983; Stoiber et al., 1987; Andres and Kasgnoc, 1998). COSPEC measurements have also re- vealed that many volcanoes emit SO2 in excess of levels that could be sustained by degassing of their erupted magmas, highlighting the so- called «excess sulfur» issue (Wallace, 2001). However, interpreting COSPEC data can be complicated. For instance, in the past both in- creasing and decreasing flux signatures have been found to pre-empt changes in volcanic ac- tivity (Symonds et al., 2001). Furthermore, SO2 fluxes are modulated by the action of hy- drothermal systems (Doukas and Gerlach, 1995; Oppenheimer, 1996; Symonds et al., 2001) and during transport in the atmosphere through deposition and chemical transforma- tions (Malinconico, 1979; Oppenheimer et al., 1998a). This is partly why monitoring gas ra- tios can be so informative. A final, more practi- cal problem is that the COSPEC is no longer in routine production, and servicing and sourcing replacement parts is becoming increasingly costly and difficult. 3.1.2. Broad band measurement and spectral analysis A potential drawback of the COSPEC is its «black box» nature. The original design of the COSPEC was very much engineering-oriented, with the goal to build a system capable of mini- mizing all «noise» (i.e., other atmospheric ab- sorptions, etc.) to deliver information on just one species (e.g., SO2). The result is an instrument that yields an estimate of the column amount of SO2 in the field of view. While this greatly sim- plifies data retrieval and processing, it makes it difficult to assess potential errors that arise from wavelength shifts, scattering and solar elevation effects, thermal and mechanical distortions, etc. Also, the COSPEC response depends on the mask used and the concentration of gases present. An alternative approach is to measure broad band spectra with sufficient spectral resolution to be able to model trace gas concentrations. This ap- Fig. 3. Simple configuration of instrumentation for OPUV measurements using mimniature spectrome- ter (a) connected by fibre optic cable (b) to telescope (c). GPS receiver (d) provides continuous tracking to locate all spectra saved to laptop computer via USB cable. The ensemble could be further reduced in size and weight by replacing the laptop computer with a palm top unit. A reflective screen is desirable for vis- ibility and enhanced battery life. a) b) c) d) 1463 Exploiting ground-based optical sensing technologies for volcanic gas surveillance proach is often termed differential optical ab- sorption spectroscopy (DOAS; Platt, 1994). The first volcanological DOAS observa- tions were performed at sea from 1992 to 1997 (Edner et al., 1994; Weibring et al., 1998), to measure the SO2 emitted by Etna, Stromboli and Vulcano. In general, the scattered skylight is collected using a vertically pointing tele- scope, and coupled into the spectrometer with an optical fibre, i.e., the same OPUV approach used for COSPEC. Direct solar UV (i.e., Sun oc- cultation) is also used in DOAS observations, which has the advantage of simplifying the ra- diative transfer problem. Light is generally spectrally dispersed using a grating, and spectra are collected using a CCD array (or photomul- tiplier tube and scanning mechanism) outside the plume, through the plume, and with light blocked from entering the spectrometer (to read the «dark current»). In the retrieval process the latter spectra are subtracted from the former two types in order to reduce instrumental noise effects. All the «plume spectra» are then divid- ed by an out-of-plume «background» spectrum, in order to reduce interferences caused by back- ground atmospheric absorption and the solar spectral structure (Fraunhofer lines). The loga- rithm of the result is taken (along with high- and low-pass filter stages), following Beer’s law (2.1), and then SO2 concentrations are de- rived by scaling a reference spectrum of known column amount over the ∼ 303-315 nm fitting region to match the observed spectrum. The instrument used on the Italian volcanoes was comparatively bulky. More recently, in 2001, volcanological field tests were carried out with a commercially available, miniature ultravi- olet spectrometer (fig. 3; Galle et al., 2003). This instrument is considerably smaller, lighter, cheaper, and lower in power consumption, than the COSPEC (Galle et al., 2003) but the perform- ance is comparable. Side-by-side intercompar- isons with a COSPEC at Soufrière Hills Volcano (Montserrat) and Mt. Etna revealed good corre- spondence between the two instruments (Galle et al., 2003; McGonigle et al., 2003). Given the favourable characteristics of the smaller device, it is already attracting interest in the volcanolog- ical research community as a replacement tech- nology for COSPEC. Because of its genuine porta- bility (rather than «transportability», which bet- ter describes many other devices sold on the market with claims to be «portable»), SO2 flux measurements can even be obtained by travers- ing beneath the plume on foot (fig. 4). This opens up the possibility of measurements at vol- 0 50 100 150 200 250 300 350 400 450 500 1:59 2:01 2:04 2:07 2:10 2:13 2:16 2:19 UT /hh:mm 1 2 3 Cell 3 4 5 6 Fig. 4. Example of raw data collected in real-time by walking traverse at Meakan-dake volcano, Japan, using equip- ment shown in fig. 3. The first excursion shows a SO2-filled quartz cell placed in front of the telescope. Six back- and-forth traverses beneath the plume, a few tens of metres from the source, follow. The timescale is subsequently corrected using the GPS log to a distance scale perpendicular to the plume transport direction. This provides the col- umn cross section of SO2, which is then multiplied by plume speed to yield flux. In this case the SO2 emission amounts to around 40 g s-1, highlighting the capability to measure very low fluxes from individual fumaroles. 1464 Clive Oppenheimer and Andrew J.S. McGonigle canoes that lack suitable roads or other forms of vehicular access, with a minimum of logistical support, and avoiding the high costs of airborne operation (McGonigle et al., 2002). The miniature UV spectrometer can be read- ily adapted for scanning measurements, whereby the field of view is manually turned through the plume (e.g., from horizon to horizon; McGo- nigle et al., 2002. It has also been configured for automated fixed position flux measurements, by way of a 45º turning mirror rotated by a stepper motor (Galle et al., 2002; McGonigle et al., 2003; Edmonds et al., 2003a). A particularly promising development is the installation of a number of automated and telemetered plume scanners by the Montserrat Volcano Observatory (MVO) for Soufrière Hills Volcano. The use of multiple instruments in different locations helps in observing the plume under varying wind di- rections, and to permit crude, tomography-style retrieval of range-resolved SO2 concentrations. This system is now delivering high temporal res- olution (every few minutes) sustained SO2 flux data during daylight hours (Edmonds et al., 2003a). Such data will provide unprecedented opportunities for the cross-correlation of time- stamped geodetic, seismic and geochemical data streams, promising new volcanological insights into degassing and magma dynamics. By virtue of recording spectra, broadband UV spectroscopy has the capability of detecting multiple gas species. Measurements of H2S/SO2 ratios have recently been accomplished at Vul- cano, Italy using the miniature UV spectrometer in a closed-path configuration (i.e., with a fixed path-length cell and artificial UV source; O’D- wyer et al., 2003). The same instrument, applied to sky OPUV measurements at Montserrat, yielded SO2/BrO ratios in the volcanic plume (the BrO being the oxidation product of vol- canic HBr; Bobrowski et al., 2003). 3.2. Non-dispersive infrared (NDIR) spectroscopy As discussed, it is desirable, in many cases, to measure not only SO2 but other volcanic gas species. This has been one of the great benefits of the conventional direct sampling approach to gas geochemistry, since many species can be measured down to ppt levels using laboratory analytical techniques. Of particular relevance, the availability over the last ten years or so of field portable, robust Fourier transform infrared (FTIR) spectrometers has extended the capabil- ities of remote sensing of volcanic gases in this direction. These devices operate across the so- called fundamental region of the infrared spec- trum, providing access to the rotation-vibration absorption features of, among other species, HCl, H2O, SO2, HF, CO2, SiF4, OCS and CO. FTIR spectrometers are based on Michelson interferometers, in which incoming light is split into two beams using an optical beam-splitter, which also recombines these beams after they are reflected at mirrors. One of the mirrors in scanned back and forth along the axis of its beam, introducing a variable path difference be- tween the two beams, resulting in a time-vari- able signal from the single broad-band detector due to interference of recombined beams. Ap- plication of an inverse Fourier transform to the temporal signal yields spectra, which are analysed (using radiative transfer models or by ratio-ing in-plume and out-of-plume spectra) in order to determine the concentrations of vol- canic gases absorbing in the optical path. A range of IR light sources: direct sunlight, fire fountains, artificial IR lamps, and hot rocks have been used in volcano FTIR surveillance highlighting the flexibility of the approach to adapt to the circumstances of activity, access and terrain. Love et al. (1998, 2000) have shown it is also possible to measure volcanic gases in emission (i.e., their emission lines rather than their absorption lines) against a cold sky back- ground. At Popocatépetl volcano, Mexico, Love et al. (1998) observed a steady increase in SiF4/SO2 ratio prior to an eruption in February 1997, followed by a tenfold decrease within a few hours. These results suggested a cooling of the gas prior to the eruption, attributed to adia- batic gas expansion on release of a conduit plug. The first FTIR volcanic gas spectroscopy was carried out in 1991 at Asama volcano, Japan, (Notsu et al., 1993), by Japanese re- searchers who have subsequently reported meas- urements at Unzen, Japan (SO2 and HCl; Mori et al., 1993), Aso, Japan (CO, OCS, CO2, SO2 and 1465 Exploiting ground-based optical sensing technologies for volcanic gas surveillance HCl; Mori and Notsu 1997), and Vulcano, Italy (SO2 and HCl; Mori et al., 1995). The CO/CO2 ratio obtained at Aso was used to constrain fu- marole temperatures. Further developments have been undertaken by a UK-based group (Francis et al., 1995; Francis et al., 1996; Oppenheimer et al., 1998c, 2002; Francis et al., 2000). The last of these works permitted plume temperature esti- mates based on HF/SiF4 ratio obtained at Vul- cano. Later work on Mt. Etna resulted in the first FTIR spectroscopy of volcanic plumes by solar occultation (Francis et al., 1998). Since 1998, this group has carried out annu- al campaigns at Masaya volcano, Nicaragua, where high volcanic gas concentrations permit- ted measurements of volcanic CO2 and H2O, in spite of these species’ high ambient concentra- tions (Burton et al., 2000). Measurements at Masaya indicated consistent SO2/HCl and HCl/HF molar ratios of 1.6 and 5, respectively, during 1998-2000, indicating steady-state, open system degassing (Horrocks et al., 1999). In contrast, in 2001 a SO2/HCl ratio of 4.5 was found, coinciding with reduced SO2 fluxes, de- scent of the magma column in the vent on the crater floor, and preceding a small explosive eruption on the 23rd April (Duffell et al., 2003). Traverse flux measurements of HCl have also been obtained by operating the FTIR spectrom- eter with a solar tracker, in contrast to the usual method of combining OPUV-derived SO2 flux- es with SO2/HCl ratios from FTIR spectroscopy or direct sampling (Duffell et al., 2001). FTIR measurements at Soufrière Hills Vol- cano, Montserrat have indicated that HCl/SO2 molar ratios of 1-5 typify dome building episodes, and that lower ratios (down to 0.1) characterise non-eruptive periods (Oppen- heimer et al., 1998d, 2002; Edmonds et al., 2001, 2002, 2003b). This behaviour has been explained in terms of an andesitic HCl source that exsolves on ascent from the magma cham- ber, and a deeper SO2 reservoir (probably de- rived from intruded mafic magma) that degasses to the atmosphere discontinuously, depending on the plumbing system’s permeability. Based on this interpretation, Edmonds et al. (2002) have identified the potential degassing signals that might herald the end of this eruption. Op- penheimer et al. (1998b) demonstrated the ap- plication of FTIR spectroscopy from a helicop- ter at distances of ≈ 100 m from the lava dome. The first routine FTIR spectroscopic vol- cano surveillance programme has been running since April 2000, at Mt. Etna, under the aus- pices of the Istituto Internazionale per Geofisi- ca e Vulcanolgia (INGV; Burton et al., 2003). The results obtained to date serve as an excel- lent advertisement for the value of FTIR spec- troscopy in complementing other surveillance efforts (e.g., geodesy, seismology and petrolo- gy), and in supporting monitoring efforts dur- ing volcanic crises (Calvari, 2001). In particu- lar, a doubling of the SO2/HCl ratio was ob- served prior to the 2001 Mt. Etna eruption and clear geochemical trends in SO2/HCl, CO2/SO2, and HCl/HF for different active vents at differ- ent elevations through the course of the erup- tion. These kinds of data would have been im- possible to collect in real time by any conven- tional technique. While the operation of FTIR spectrometers is relatively straightforward, pro- cessing the spectra and retrieving gas column amounts requires some expertise. Representing another class of NDIR spec- trometer in volcanological use are the instru- ments manufactured by LI-COR. These are generally dual-wavelength closed-path analy- sers, and have been used to measure both dif- fuse CO2 emissions from the ground (McGee et al., 2000), and CO2 fluxes by in-plume sam- pling (Gerlach et al., 1998). A third application of LI-COR is based on eddy correlation or co- variance, which involves measurements of ver- tical windspeed and CO2 concentration (e.g., Anderson and Farrar, 2001). 3.3. Laser techniques To date, the most commonly applied laser technique for volcano measurements is lidar, in which a pulsed laser beam is directed towards the plume. Recording the temporally varying intensity of backscattered light provides infor- mation about the atmospheric composition as a function of propagation distance along the beam’s path. lidar has been used to measure concentrations and fluxes (via traverses) of sulfate aerosol (Casadevall et al., 1984; Edner 1466 Clive Oppenheimer and Andrew J.S. McGonigle et al., 1994; Porter et al., 2002), and ash (Hobbs et al., 1991). Parallel gas sampling and aerosol measurements can enable estimation of gas to particle conversion rates (e.g., for SO2 to SO4 2- Stith et al., 1978; Radke, 1982; Rose et al., 1986). Although studies of anthropogenic plumes indicate SO2 to SO4 2- conversion rates of a few % h–1 in the lower troposphere (Eatough et al., 1994), very few comparable investigations of volcanic plumes (which can have widely varied gas, ash and liquid water contents) have been undertaken (e.g., Oppen- heimer et al., 1998a; Rose et al., 2001; Hor- rocks et al., 2003). A variation on lidar, known as differential ab- sorption lidar (DIAL) involves rapid switching the frequency of laser pulses on- and off-reso- nance of an absorption feature of the gas of in- terest. By dividing the lidar curves (returned sig- nal versus height) obtained at the two wave- lengths and applying the Beer-Lambert law, range-resolved gas concentrations (ppm) may be derived providing 2D or 3D plume structure, in contrast to the pathlength integrated concentra- tions (ppm m) obtained from FTIR, COSPEC and DOAS. The technique has been applied to the Southern Italian volcanoes using UV lasers (Ed- ner et al., 1994), in parallel with COSPEC and UV- DOAS instruments, revealing SO2 concentra- tions up to 50% higher in the former case as a consequence of scattering-induced errors in the passive techniques. However, this DIAL appara- tus was costly, heavy and bulky. Whilst DIAL of- fers unique capabilities for volcanology, it re- quires further innovation in order to become a suitable tool for routine observatory use. Alternative sensing strategies using near and mid-infrared diode based lasers (Gianfrani et al., 2000; De Natale et al., 2001; Richter et al., 2002) have been evaluated by monitoring the laser’s absorption following numerous tran- sits of a multipass cell, into which the volcanic gas sample is pumped. Due to the very narrow linewidths lasers can provide, the most promis- ing application of these techniques is in spec- trally resolving between isotopes to provide in- field isotope ratios. Whilst only gas ratio meas- urements have been realised to date in the field, Richter et al. (2002) and Weidmann et al. (2003) have described a mid-IR laser system capable of measuring all isotopes of CO2, while Gianfrani et al. (2003) have presented a diode laser spectrometer able to measure water iso- topes in the near-IR. An additional laser based technique that shows potential for field isotope measurements is photoacoustic spectroscopy, in which the sound waves, generated by resonant laser light exciting the target species, are meas- ured (Meyer and Sigrist, 1990). 4. Concluding remarks Surveillance of gas composition and flux are essential for interpretation of volcanic ac- tivity, since the nature of degassing exerts a strong control on eruption style, and is closely associated with volcano seismicity and ground deformation. New optical remote sensing tech- niques are emerging for the monitoring of vol- canic emissions such as the miniature ultravio- let spectrometers described by Galle et al. (2003). These tools and their data-streams have the potential for full automation and telemetry, fast processing and evaluation, closer integra- tion with other monitoring data-streams (i.e., seismic, geodetic, etc.), and more sophisticated modelling and interpretation. While instrument developments are always to be encouraged, we believe that the state of the art is already adequate to justify the prolif- eration of both IR and UV spectroscopic tools amongst the volcano observing community. This is beginning to happen, with FTIR spec- troscopy routinely applied to monitoring of the Southern Italian volcanoes by INGV, and UV methods used on Montserrat by MVO. In addition to the immediate monitoring and haz- ard evaluation goals of such institutes, sus- tained surveillance programmes promise the most significant science gains in the future, as geochemical, geodetic and seismic data- streams are further integrated, cross-correlat- ed, and modelled. As we introduce tools that will replace or supplement other methods, it is crucial that we maintain continuity between old and new data- sets. This demands the direct comparison and intercalibration of the various gas geochemical tools via laboratory and field experimentation, 1467 Exploiting ground-based optical sensing technologies for volcanic gas surveillance as carried out periodically by the IAVCEI Com- mission on Volcanic Gases. Additionally, it helps to conform to internationally recognized data standards. One attractive development in this respect is the «WOVOdat» concept of the World Organisation of Volcano Observatories, which aims to establish standardised units and formats for surveillance data so that they may be brought together in a unified, digital data- base, accessible via the Web and linked with the Smithsonian Institution’s unique database of historical eruptions. A further important benefit of proliferating optical sensing techniques ca- pable of gas flux measurements is that this will lead to more accurate, and time-resolved esti- mates of the global emission of volcanic volatiles to the atmosphere. Acknowledgements We gratefully acknowledge research support from the Gruppo Nazionale per la Vulcanologia (project «Development of an integrated spectro- scopic system for remote and continuous moni- toring of volcanic gas»), the European Com- mission 5th Framework programme (projects «MULTIMO; http://earth.leeds.ac.uk/~aj/Mul- timo/» and «DORSIVA»), and the UK Natural Environment Research Council project «Field laser spectroscopy of volcanic gases and their isotopes» and Research Fellowship awarded to AJSM. This work originates from the tremen- dous Workshop-Short Course on Volcanic Sys- tems held in Seiano (Napoli) in September 2002 «Geochemical and Geophysical Monitoring: melt inclusions: methods, applications and problems» organized by B. De Vivo, R.J. Bod- nar, E. Boschi and G. Macedonio. 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