COSPEC (Correlation Spectrometer)  

A correlation spectrometer or COSPEC was initially designed to measure industrial pollutants and now has been applied to the field of volcanology to measure volcanic gas emissions. The spectrometer is designed to measure the concentration of sulfur dioxide (SO2) in the volcanic plume that is emitted from the volcano. The device requires a standard, from which to analyze the ultraviolet light absorbed by the SO2 molecules in the plume.

Multiple measurements are made to acquire reliable results. This COSPEC is used either from the ground where it is mounted on a vehicle or tripod that scans the plume, or the device can be attached to an aircraft that traverses underneath the plume. The best quality measurements are obtained when an aircraft flies at right angles to the direction of plume travel acquiring data with each flight.

    

These images show various ways that a COSPEC can be set up-on a tripod, in a vehicle or on an aircraft.

Images from USGS.

 

 

 

Average daily SO2 emission rates from Mount St. Helens from 1980-1988. COSPEC data were retrieved using a COSPEC mounted on an aircraft.

For data, see Open-File Report 94-212.

 

 

 

Infrared Carbon Dioxide Analyzer (LI-COR) 

 

An infrared carbon dioxide analyzer or Li-COR has become a standard method for measuring carbon dioxide (CO2) emission rates. It is employed in a similar manner to the COSPEC but requires data from the whole plume in order to calculate a carbon dioxide emission rate. The aircraft that hosts the device flies systematically through the plume creating a cross-section analysis of the gas emissions at different elevations.

 

This photo to the right is taken while flying under the volcanic plume to measure SO2- photo from USGS. 

 

The LI-COR can also be used to measure soil efflux emissions. These soil emissions are typically in areas where volcanic gases rise from depth and remain in the soil directly beneath the surface. To measure the rate of gas emissions into the atmosphere, a accumulation chamber is set up on the soil surface and connected to a LI-COR instrument. The gas enters the chamber and is measured for increasing CO2 concentrations. A soil efflux for that specific location is calculated based on other parameters that include, pressure, temperature. Additional efflux values at various locations must be measured to acquire reliable measurements that are representative of a volcanic system, from which a map can be constructed showing the elevated soil CO2 values.

 

 

 

From USGS- Map of CO2 concentration- constructed from data near Horseshoe Lake and Mammoth Mountain, California from

 

Gerlach, T.M., Doukas, M.P., McGee, K.A., and Kessler, R., 2001, Soil efflux and total emission rates of magmatic CO2 at the Horseshoe Lake tree kill, Mammoth Mountain, California, 1995-1999: Chemical Geology, v. 177, Issues 1-2, pgs. 101-116. 

USGS- Measuring volcanic gases; soil efflux  

 

 

 

 

 

Fourier Transform Infrared Spectrometer (FTIR)

The FTIR or Fourier Transform Infrared Spectrometer can be used to measure dissolved volatile concentrations as described above or can be used to measure several gases emitted from a volcano simultaneously. The device can be used both as an open-path or closed-path system. The open-path system aims the FTIR at a plume using an optical telescope. The closed-path system delivers gas from a plume or fumarole to a gas cell within the FTIR.  

 

 

Additional References:

Gerlach, T.M., Doukas, M.P., McGee, K.A., and Kessler, R., 2001, Soil efflux and total emission rates of magmatic CO2 at the Horseshoe Lake tree kill, Mammoth Mountain, California, 1995-1999: Chemical Geology, v. 177, Issues 1-2, pgs. 101-116. 

McGee, K.A., and Casacdevall, T.J., 1994, A Compilation of Sulfur Dioxide Emission-Rate Data from Mount St. Helens During 1980-1988. U.S. Geological Survey Open-File Report 94-212, Version 1.0