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BY LIA
UNRAU
Rice News Staff
October 19, 2000
 Professor Frank Tittel perches on |
As researchers trek up to Masaya volcano, a
plume of gases stretches out across the valley and a faint sweet smell, similar to rotten eggs, fills the air. For the Rice Laser Science Group, led by Frank Tittel, the J.S. Abercrombie Professor in Electrical and Computer Engineering, that mixture of smelly gases is the target, and the group plans to capture it in sensitive measurements with its most recent prototype of a laser-based gas sniffer. The ability to accurately measure a variety of gases will help scientists in their efforts to predict volcanic eruptions. In collaboration with a research group from the University of Cambridge, England, Tittel, Rice doctoral candidate Dirk Richter and research associate Dirk Rehle set out for the Nicaraguan volcano to test their compact sensors earlier this year. Masaya has been venting continuously—about every 30 seconds—for the last 300 years. A region called the “death zone” spreads over the land underneath where the steady winds blow the volcano’s plume of gases. The emissions destroy the fertility of the soil and produce acid rains during the region’s rainy season. Civil defense and the impact of increased gas on health and agriculture are serious concerns in the area. Part of a national park, Masaya volcano sits about 30 miles south of downtown Managua, Nicaragua. It is the most active volcano in the region and an ideal place to study emissions because of the easy access by car to its rim. A mid-infrared diode laser-based gas sensor, developed by the Rice team, was set up next to the active volcano crater, marking the first time that this type of sensing system has been tested at a volcano. For 10 days, the team measured five different types of gases: carbon dioxide, sulfur dioxide, methane, hydrochloric acid and water vapor. The results of their field studies will be presented at the Optical Society of America (OSA) annual meeting in Providence, R.I., Oct. 22-26. Rice’s colleagues at Cambridge are Clive Oppenheimer, Hayley Duffell, Rodney Jones and Mike Burton. The Cambridge team studied the volcano’s gas emissions using a different type of sensing method, called Fourier transform infrared spectroscopy, and that system was on hand taking measurements. Armed with two types of sensors, one to focus on multiple types of gas and the other that zeros in on two gases, hydrochloric acid and methane, the simultaneous measurements allowed Tittel, Richter and Rehle to get a useful comparison between the two technologies. The advantage of the mid-infrared diode laser-based sensors is that they allow more sensitive, selective and real-time measurements. The researchers also were looking at the impact of gases on atmospheric chemistry. They found that Masaya was putting out 60 kilotons an hour of sulfur dioxide. In comparison, the total output of sulfur dioxide in all of the United Kingdom is 0.2 kilotons per hour. Volcanologists are interested in gas concentrations emitted from volcanos because by measuring the concentrations and ratio of hydrochloric acid and sulfur dioxide, they can deduce the de-gassing activity of the volcano and its patterns and look at the long-term trend of the volcano. If volcanologists are able to monitor sudden changes, they can predict whether an eruption is likely to occur. This provides further information for eruption predictions presently based on seismic activity measurements. For this reason, volcanologists would like an autonomous, unattended sensor to leave at the site for weeks or months at a time, which would allow them to monitor the emissions regularly and with high accuracy from a remote location. In spite of being checked as baggage on the airline, jostled around in a four-wheel drive vehicle and subjected to high humidity and temperature fluctuations, the diode lasers and fiber optics were not damaged and the sensor worked well, Tittel said. Another advantage over the Fourier transform system is that the sensor uses a simpler energy source—gas and batteries— and requires no consumables such as liquid nitrogen. As the prototypes continue to evolve, Tittel and Richter said the systems will be more compact, with separate modules for the energy source and the detector. At this point, the ultimate goal is to have a sensor the size of a shoebox. They also wish to apply the technology to different situations, such as fenceline monitoring around chemical plants and other industrial and manufacturing sensing and control applications. Richter, who is expected to complete his Ph.D. this month, will move to a position at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., in November. Masaya is one of 10 currently active volcanos along a ridge from Alaska to Chile. An eruption at Masaya in 4550 B.C. was one of the largest on Earth in the last 10,000 years. It has erupted at least 19 times since 1524, and the most recent explosive eruption was in 1993. The most recent Strombolian eruption occurred in 1997. Strombolian eruptions are characterized by the intermittent explosion or fountaining of basaltic lava from a single vent or crater and are caused by the release of volcanic gases. In 1981, sulfur dioxide was released from Santiago Crater at a rate of 500,000 tons per year. Three periods of similar gas activity occurred this century. More information and photos are available online at www.ruf.rice.edu/~lasersci. |