laptop computer and auxiliary optics required for flux measurements) is less than ... This approach enables fluxes to be measured with a mini- mum of logistical ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B9, 2455, doi:10.1029/2002JB002261, 2003
Sulphur dioxide fluxes from Mount Etna, Vulcano, and Stromboli measured with an automated scanning ultraviolet spectrometer A. J. S. McGonigle,1 C. Oppenheimer,1 A. R. Hayes,1 B. Galle,2 M. Edmonds,3 T. Caltabiano,4 G. Salerno,4 M. Burton,4 and T. A. Mather5 Received 22 October 2002; revised 8 April 2003; accepted 14 May 2003; published 30 September 2003.
[1] We report here SO2 flux measurements for the southern Italian volcanoes: Mount
Etna, Vulcano, and Stromboli made in July 2002 from fixed positions, using an automated plume scanning technique. Spectral data were collected using a miniature ultraviolet spectrometer, and SO2 column amounts were derived with a differential optical absorption spectroscopy evaluation routine. Scanning through the plume was enabled by a 45 turning mirror affixed to the shaft of a computer controlled stepper motor, so that scattered skylight from incremental angles within the horizon-to-horizon scans was reflected into the field of view of the spectrometer. Each scan lasted 5 min and, by combining these data with wind speeds, average fluxes of 940, 14, and 280 Mg d1 were obtained for Etna, Vulcano, and Stromboli, respectively. For comparative purposes, conventional road and airborne traverses were also made using this spectrometer, yielding fluxes of 850, 17, and 210 Mg d1. The automated scanning technique has the advantage of obviating the need for time-consuming traverses underneath the plume and is well suited for longer-term telemetered deployments to provide sustained high time resolution flux INDEX TERMS: 0394 Atmospheric Composition and Structure: Instruments and techniques; 8409 data. Volcanology: Atmospheric effects (0370); 8419 Volcanology: Eruption monitoring (7280); 8494 Volcanology: Instruments and techniques; KEYWORDS: volcanic gas monitoring, remote sensing, SO2 emissions, DOAS, ultraviolet spectroscopy Citation: McGonigle, A. J. S., C. Oppenheimer, A. R. Hayes, B. Galle, M. Edmonds, T. Caltabiano, G. Salerno, M. Burton, and T. A. Mather, Sulphur dioxide fluxes from Mount Etna, Vulcano, and Stromboli measured with an automated scanning ultraviolet spectrometer, J. Geophys. Res., 108(B9), 2455, doi:10.1029/2002JB002261, 2003.
1. Introduction [2] Measurements of volcanic gas emission rates (fluxes) are important parameters for understanding and predicting volcanic activity. Gas fluxes depend on several factors, including the geochemistry, pressure, and temperature of the source magma [e.g., Symonds et al., 1994], interactions with shallow hydrothermal systems, and the permeability of fracture and bubble networks in the magma column. In particular, the coalescence of gas bubbles, and their ascent velocity relative to magma (closed-system versus opensystem degassing) play important roles in determining eruption style and effusive-explosive transitions [Sparks, 2003]. Measurements of volcanic gas fluxes also support studies of the local environmental effects of volcanic degassing, volatile recycling at subduction zones [Fischer 1
Department of Geography, University of Cambridge, Cambridge, UK. Department of Radio and Space Science, Chalmers University of Technology, Gothenburg, Sweden. 3 Montserrat Volcano Observatory, Montserrat, West Indies. 4 Istituto Nazionale di Geofisica e Vulcanologia, Catania, Italy. 5 Department of Earth Sciences, University of Cambridge, Cambridge, UK. 2
Copyright 2003 by the American Geophysical Union. 0148-0227/03/2002JB002261$09.00
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et al., 2002a] and, particularly in the case of SO2 (which can form radiatively active sulphate aerosol), atmospheric and climatic impacts [e.g., Robock, 2000]. [3] Sulphur dioxide has been the species focused upon for such flux studies, because of its relative abundance (typically, third behind H2O and CO2, which are far more difficult to measure due to high ambient levels), and the various options for spectroscopic detection in the atmosphere. Thus measurements of fluxes of other volcanic species (such as CO2 and He) are typically reliant upon combination of measured SO2 fluxes with directly sampled gas composition data. For the last thirty years, most groundbased measurements of volcanic gas fluxes have been obtained using the Correlation Spectrometer, or COSPEC, which measures the absorption of scattered skylight by SO2 around 300 nm, and uses an internal mechanical correlation procedure to yield column amounts [Stoiber et al., 1983]. Typical COSPEC measurements involve traversing below a plume and multiplying the integrated SO2 cross section by estimated plume transport speed to yield fluxes. The ability of the COSPEC to operate remotely from volcanoes, providing data even throughout eruptive episodes, has resulted in its routine use by a number of volcano observatories and university-based groups, and the accumulation of an invaluable database of volcanic degassing worldwide [e.g., Andres and Kasgnoc, 1998]. However, because of
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8 - 2 MCGONIGLE ET AL.: AUTOMATED SCANNING VOLCANO FLUX MEASUREMENTS
their antiquated technology and the fact that COSPECs are no longer routinely manufactured, these devices are becoming increasingly unreliable, requiring expensive repairs and causing loss of valuable data collection time [McGonigle and Oppenheimer, 2003]. [4] In accord with the growing consensus that a replacement technology for the COSPEC was timely, we recently demonstrated that a miniature ultraviolet (UV) spectrometer, based on developments in CCD array technology, performed equivalently to the COSPEC in side-by-side trials at Soufrie`re Hills Volcano, Montserrat [Galle et al., 2003]. Not only is this hand-held device less than 2% of the weight and 1% of the volume of the COSPEC, but its cost (including laptop computer and auxiliary optics required for flux measurements) is less than US$6,000, compared with US$60,000 for a reconditioned COSPEC. Indeed, this spectrometer is so small that it can readily be used by a single operator traversing beneath a volcanic plume on foot, where local conditions permit [McGonigle et al., 2002]. Unlike the COSPEC, the miniature UV spectrometer outputs spectra, from which column amounts can be determined using a differential optical absorption spectroscopy (DOAS) evaluation routine (i.e., differencing target spectra against a reference spectrum, usually obtained contemporaneously but outside the plume). Edner et al. [1994] and Weibring et al. [1998] earlier used a UV spectrometer operating on the same principle, but of considerably larger size and weight, to make flux measurements by profiling the plumes of the southern Italian volcanoes from a research ship. [5] Occasionally, fluxes have been measured from fixed positions, by placing the COSPEC on a rotating tripod head, and manually scanning its field of view (FOV) through the plume [e.g., Stoiber et al., 1983; Kyle et al., 1994; Andres et al., 1993; Hirabayashi et al., 1995; Fischer et al., 2002b]. This approach enables fluxes to be measured with a minimum of logistical support, and at targets that lack downwind water bodies or roads, without the high costs associated with airborne measurements. Recently, we performed manual scanning measurements, using this miniature UV spectrometer to measure the flux of Masaya volcano, Nicaragua [McGonigle et al., 2002], finding good agreement with fluxes contemporaneously obtained via car traverses. Here we present a fully automated development of this technique in which the field of view (FOV) of the stationary miniature UV spectrometer is scanned through the plume by means of a 45 turning mirror, mounted to a computer controlled stepper motor, thus eliminating the scan angle uncertainty associated with manual measurements. By outputting spectra, the miniature UV spectrometer offers the advantages over the COSPEC of being able to apply a radiative transfer model to accommodate variations in air mass factor in the scan data evaluation routine and to account for multiple scattering effects that can cause errors of more than 30% in these passive techniques [Edner et al., 1994]. The miniature UV spectrometer is particularly suitable for such a configuration given its sensitive detector that allows far smaller diameter light collection optics than those of the COSPEC (25 mm versus 80 mm), reducing demands upon the motors required to rotate the scanning optics. [6] Traverse flux data streams typically have far poorer temporal resolutions than those of volcano seismic and geodetic data (e.g., four measurements per day versus
sampling frequencies of 1 Hz for seismometry), limiting the utility of these data to analysis of longer-term (>1 day) volcanic phenomena, due to the finite times taken to traverse underneath plumes, and the need for someone to transport and operate the spectrometer. This limitation has contributed to geochemical data being considered as of subsidiary importance for volcano monitoring and eruption prediction than the two geophysical time series, despite a number of studies that have demonstrated the links between seismicity, ground deformation and degassing [e.g., Stix et al., 1993; Fischer et al., 1994; Watson et al., 2000]. It is noteworthy that various recent models of degassing-driven magma dynamics [e.g., Navon et al., 1998; Melnik and Sparks, 1999; Voight et al., 1999; Sparks, 2003] are based on theoretical and numerical treatments rather than gas flux time series. [7] The automated scanning technique introduced here offers the significant advantage of being able to deliver high time resolution degassing data (every few minutes for a scan), providing modelers the possibility of making the most detailed corroborations yet between degassing, geodetic and seismic data. Such models, in parallel with sustained high time resolution scanning flux monitoring, offer considerable promise for predicting volcanic behavior. Because of the low power requirement of the miniature UV spectrometer (
