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

View of gas plume emitted by Mount St. Helens from inside an airplane during a COSPEC flight

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.

COSPEC from the air: in plane or helicopter

COSPEC mounted in airplane with periscope sticking through window

Mounted through

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.

COSPEC from the ground: in a vehicle

COSPEC mounted inside a vehicle, Kilauea Volcano, Hawai`i

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

COSPEC mounted on a tripod near Pu`u `O`o vent, Kilauea Volcano, Hawai`i

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

LI-COR infrared analyzer: measuring CO2 emission rate

Use of a small infrared carbon dioxide analyzer (LI-COR), has recently become a standard method for measuring carbon dioxide emission rates at restless volcanoes. The LI-COR is mounted in a small aircraft configured for sampling outside air. Traverses are then systematically flown through the plume at different elevations until the entire cross-section of the plume is analyzed. From these data, a carbon dioxide emission rate can be calculated. This technique was first employed by USGS scientists at Popocatepetl volcano in Mexico in 1995. More recently, it has been used at several domestic volcanoes.

LI-COR secured inside aircraft

GPS receiver, LICOR, and data logger in airplane for sampling carbon dioxide gas

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.
Close view of LICOR gas-sampling port below COSPEC periscope

Aircraft rigged to sample
gas near COSPEC periscope

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

Fourier transform infrared spectrometer (FTIR): measuring many volcanic gases

A third technique for measuring gases in volcanic plumes involves the use of a Fourier transform infrared spectrometer system (FTIR). The FTIR is capable of analyzing several gases simultaneously using an open-path or closed-path system. The open-path method uses an optical telescope to aim the FTIR at a target gas some distance away. The infrared light source is either natural solar light or light from a heated filament behind the target gas. The closed-path method involves delivering gas from a plume or fumarole to a gas cell within the FTIR. Recently, a prototype closed-path FTIR successfully measured SO2 at Kilauea Volcano in Hawai`i. The volcano's plume was sampled directly by an FTIR mounted in an aircraft in the same way that the LI-COR anlayzer is used to measure CO2.

FTIR instrument

Close view of FTIR interior

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

FTIR in vehicle next to fumaroles on Halema`uma`u crater, Kilauea Volcano, Hawai`i

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