Monitoring:  | Gas | Ground Deformation | Hydrologic | Remote Sensing | Seismicity |

Measuring volcanic gases: emission rates
of sulfur dioxide and carbon dioxide in volcanic plumes

Flying beneath volcanic gas plume to measure SO2 Photograph by S.R. Brantley on June 16, 1982	Like this plume rising from the crater of Mount St. Helens, a typical plume of gas rises to some height above a volcano where it reaches equilibrium with the atmosphere and is bent over and blown away. By measuring both the amount of a specific gas in the plume and the wind speed, scientists can calculate the emission rate or discharge of the gas. Several methods are used to measure the amount of specific gases in a volcanic plume.
The amount of sulfur dioxide gas (SO2) in a plume is measured with an optical correlation spectrometer (COSPEC) by moving the instrument beneath the plume in an aircraft or along the ground. The amount of carbon dioxide gas (CO2) in a plume is measured with a small infrared analyzer (LI-COR) by flying the instrument through the plume several times so that it can continuously sample an entire cross section of the plume. A third technique for measuring gases in volcanic plumes involves a Fourier Transform infrared spectrometer system (FTIR) that also continuously samples gas in a volcanic plume. Coupled with this instrumentation, Global Positioning System (GPS) technology is now routinely used by USGS scientists to map airborne traverses through volcanic plumes. GPS data is simultaneously collected along with chemical data so that accurate plume cross sections and flight paths can later be accurately constructed.

Correlation spectrometer (COSPEC): measuring SO2 emission rate

The correlation spectrometer (COSPEC) has been in use for more than two decades for measuring sulfur dioxide emission rates from various volcanoes throughout the world. Originally designed for measuring industrial pollutants, the COSPEC measures the amount of ultraviolet light absorbed by sulfur dioxide molecules within a volcanic plume. The instrument is calibrated by comparing all measurements to a known SO2 standard mounted in the instrument. Although the COSPEC can be used from the ground in a vehicle or on a tripod to scan a plume, the highest quality measurements are obtained by mounting a COSPEC in an aircraft and flying traverses underneath the plume at right angles to the direction of plume travel (sketch of traverse).

	COSPEC from the air: in plane or helicopter
	Mounted through window	Airborne SO2 measurements are made by flying below and at right angles to a volcanic plume with the upward-looking COSPEC. Typically, 3-6 traverses are made beneath the plume in order to determine the average SO2 concentration along a vertical cross section of the plume. Wind speed is determined during flight either by GPS or by comparing true air speed, flying with and against the wind, with true ground speed. Details.
	COSPEC from the ground: in a vehicle
	| med | large | View inside vehicle	Ground-based SO2 measurements can be made from a vehicle by traversing directly beneath a volcanic plume. Because the location of roads around a volcano are fixed, however, such a configuration is not always possible. For the measurements to be successful, the plume must pass directly over a road. Wind speed is usually determined by using a hand-held anemometer; this method is less accurate than wind speeds determined aloft with an aircraft.
	COSPEC from the ground: from a stationary tripod
	| med | large | On tripod near vent	The COSPEC can also be mounted on a tripod near a volcanic vent to scan horizontally or vertically so that the light coming into the instrument first passes through the plume. Wind speed is determined by using a hand-held anemometer or from a portable meteorological station.

Examples of COSPEC Data Mount St. Helens, 1980-1988 Kilauea Volcano, 1979-1997: Open-File Report 98-462 Cook Inlet volcanoes, 1990-1994: Open-File Report 95-55

	LI-COR secured inside aircraft
	| med | large | Inside aircraft	A typical set-up inside an aircraft includes the LI-COR carbon dioxide analyzer and flow control unit (instruments in middle), GPS receiver (left), and laptop computer (right) for running data-acquisition software.
	Aircraft rigged to sample gas near COSPEC periscope
	| med | large | Sampling tube for LICOR	During flight, air and volcanic gases are sampled through a small tube (black tube beneath the COSPEC periscope). The air intake is located well upstream of an aircraft's engine exhaust to avoid contamination.

Example of LI-COR Data Mammoth Mountain, Long Valley caldera, California

	FTIR instrument
	| med | large | Interior of FITR	Interior view of an closed-path FTIR showing gas cell, Michelson interferometer, globar light source, helium-neon laser, and MCT detector. The detector is cooled to operating temperature (77 K or -196° C)  through the use of a closed-cycle stirling engine microcooler.
	FTIR in vehicle
	| med | large | Kilauea Volcano, Hawai`i	USGS scientists use a closed-path FTIR mounted in a vehicle to measure gases being discharged from fumaroles within the caldera of Kilauea Volcano, Hawai`i.

Example of FTIR Data Pu`u `O`o vent, Kilauea Volcano, Hawai`i: 19 September 1995

Other methods of monitoring volcanic gases

• Measuring gas-emission rates in volcanic plumes
• Direct gas sampling and laboratory analysis
• Continuous on-site gas monitoring
• Soil-efflux measurements