Volcanic Gas Emissions: New Insights from New Instruments


A team of scientists at the University of Cambridge and Alphasense Ltd in Braintree, England has developed highly portable, low-power, low-cost instruments for volcanic gas detection and measurement. These new electrochemical sensor based instruments, with demonstrated capacity to characterise in situ volcanic gas plumes, present a technological advance in volcanic gas emissions monitoring.


volcano observatory team1


”Watch out for any eruptions and don’t fall in!” joked my lab-colleagues as I embarked on my first volcano gas-sensing field expedition to Mt. Etna, Italy, in 2006. Upon our arrival on the Italian island of Sicily, Etna’s Southeast Crater did in fact erupt, spewing a continuous flow of lava down the mountainside, accompanied by occasional and spectacular explosions that ejected gas, ash, and fragments of rock and molten lava into the atmosphere.


To me, as an atmospheric-scientist-


turned-rookie-volcanologist, the eruption was an added bonus, but for the 25% of Sicily’s population who live on Etna’s slopes, it was a cause for concern. The Italian , now in a state of high-alert, embarked on


round-the-clock monitoring of the hazard situation using a combination of seismic, thermal and gas sensors.


Our planned field-testing of instruments for volcanic gas detection was thus particularly timely. Volcanoes release a cocktail of toxic gases and aerosol, at concentrations that can exceed several 100s of ppmv (parts per million) close to the vent. This mixture of gases, at very high concentrations, and accompanied by highly acidic volcanic aerosol, presents a challenge for field measurement scientists: how to build an instrument that can accurately measure the complex mixture of acid gases in a volcano plume, that can withstand harsh, acidic environments, and can be deployed in remote and often very difficult to access regions?


To meet this goal, an interdisciplinary, industry-academia collaboration was estab- lished between scientists, a gas sensor company Alphasense2


instrument PDA-screen indicated it was functioning correctly, recording fluctuating concentration readings when in the plume, then returning to low, background values on encountering clean air. It was a promising start. We continued further into the nearby grounding plume where the high concentrations of ash and aerosol particles reduced visibility at times to a few tens of meters. Ahead, a group of volcanologists had gathered on Etna’s flank, close to the active lava-flow.


The lava turned out to be surprisingly viscous, owing to its high silica content, thus oozed downhill rather slowly, building lava-levées either side as it cooled at the edges. Standing on the levée and clad in a heat- reflective suit, one of the volcanologists attempted to extract chunks of fresh, hot (near 1000ºC) lava from the flow, for subsequent chemical analysis. By comparing the chemical composition of the lava matrix, the trapped volatiles (sulphur, chlorine, fluorine, and so on) within it, and the observed plume gas composition, we strive to form a better under- standing of the subsurface processes and establish links between gas emissions and volcanic activity. It is the gases, after all, that drive eruptions and trigger high-pressure explosions.


Back at our hotel, after a long day in the field, followed by a long shower to remove the acidic layer of grime deposited on my skin, I found time to analyse the field data from our new gas sensing instrument.


Both in-plume SO2 and H2S were detected at ppmv concentrations, and the 1 Hz data-logging of the rapidly fluctuating sensor output accurately recorded the plume heterogeneity. Thus a successful first in-plume testing of the instrument, and of me as a volcanologist.


The instrument has been deployed in subsequent field-campaigns worldwide, enabling studies of volcanic emissions from Kilauea (Hawaii), Erebus (Antarctica), Masaya (Nicaragua), Poas (Costa Rica), Aso (Japan) and Villarrica (Chile). These investigations repeated electro-


chemical measurements of volcanic SO2 and H2S but added several other volcanic gas species to the repertoire, including HCl, H2 and CO. Pre- and post-fieldwork calibrations demonstrate good stability of the


, and the Chemistry


and Geography Departments of the University of Cambridge. The team built an instrument that houses a suite of 6 Alphasense sensors connected to a palmtop computer data logger, recording the sensor output each second. Air is drawn over the sensors by a miniature pump with a filter on the inlet preventing volcanic aerosol and ash from entering the instrument. A sturdy outer-casing protects the electronic components from acid damage, whilst the electrochemical sensors inside measure the concentrations of plume


gases SO2, H2S, HCl, CO, H2 and NO2, all of which are detected simultaneously. The instrument is powered by a 12V battery pack and, being shoe-box sized, is highly portable, easily fitting into a backpack for a day-hike up a volcano.


Our first field-campaign in the plume of Mt. Etna in 2006 put the instrument’s durability and measurement capabilities to the test. Equipped with gas masks, sturdy boots and hard hats, and with a couple of prototype instruments on our backs, we ascended the mountain; admittedly the first 2900m by cable- car but the remaining 400m on foot over Etna’s rocky terrain.


After some minutes, we could see the actively-erupting vent, and not long after- wards we could smell it too; occasional whiffs of sulphurous gases with a hint of rotten-egg. This signalled it was time to strap on our gasmasks and power up the instruments. After a couple of false starts, in-field reading of the


sensors, despite the harsh conditions, and a linear response to the target gases. Due to the nature of the plume as a ‘cocktail of gases’, sophisticated analysis methods needed to be developed to achieve gas specificity which accounted for interferences that cannot always be removed through gas-filters on the sensors themselves. This was achieved through use of multiple simultaneous sensor outputs to extract the interferents, combined with individual sensor laboratory testing of cross-sensitivities.


May/June 2010


IET


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68