HR: 10:40h
AN: V42B-02 INVITED     [Abstracts]
TI: SO₂ flux measurements at Masaya volcano, Nicaragua - apparent downwind depletion due to topography
AU: * Williams-Jones, G
EM: glynwj@sfu.ca
AF: Earth Sciences, Simon Fraser University, 8888 University Dr., Burnaby, BC V5A 1S6, Canada
AU: Nadeau, P A
EM: tnadeau@gmail.com
AF: Geological & Mining Engineering & Sciences, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931-1295, United States
AB: For more than three decades, Masaya volcano, Nicaragua has been a choice volcano for ground-based remote sensing of volcanic SO₂ and other gases and is an ideal location to investigate behaviour of boundary layer volcanic gas plumes. Recent advances in miniature UV systems have enabled improvement in techniques for determining the speed of volcanic plumes (previously the greatest source of uncertainty in volcanic SO₂ fluxes). This has now allowed for more in depth investigations of other important factors affecting remotely measured SO₂ flux. Over 700 SO₂ measurements at Masaya in 2005 and 2006 revealed that measurements 15 km downwind of the active vent were systematically 30-50% less than those measured only 5 km from the vent. Dry deposition of sulphur from the plume and conversion of SO₂ to sulphate aerosols within the plume are not sufficient to account for the apparent loss. The site 15 km downwind is located on a ridge over which local trade winds and the entrained plume accelerate. Given the geometry of the ridge, topographic modification of winds is the most plausible source of the 30-50% apparent loss, as the enhanced wind speeds lead to dilution of the plume along the axis of propagation. Thus, plume speeds must be measured at plume height at the location of the flux measurement in order to accurately determine SO₂ fluxes. This also has significant implications for accurate estimates of the volumes of magma degassed and necessitates that earlier data sets be re-evaluated. Clearly, now that plume speeds can be easily and accurately measured, it is crucial that all sources of uncertainty be fully evaluated prior to any SO₂ flux survey.
DE: 7280 Volcano seismology (8419)
DE: 8409 Atmospheric effects (0370)
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8430 Volcanic gases
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly