Volcanic Emissions - Plumes

How much gas is emitted from a volcano during a certain time period is directly related to the volume of magma that sits in the subsurface reservoir. Measuring the rate at which a volcano releases gas or degasses, typically reported in metric tons per days, allows scientists to get a glimpse of what is happening below the surface. Changes in gases like sulfur dioxide and carbon dioxide are important to monitor in active volcanic systems since they can be indicative of activity occurring in the volcano’s magma reservoir and hydrothermal system. Emission rates can be measured either from the ground or from an aircraft. Gas ejected high into the atmosphere during a volcanic eruption requires satellites to measure the emitted gas. 

Looking south across Halema`uma`u Crater at the gas plume rising from the Overlook vent. From USGS, HVO

Direct Sampling

The easiest, but perhaps the most dangerous way to collect a gas sample is by hand, placing a container directly in the gases. This technique is used to produce a detailed chemical analysis of a specific fumarole or vent, where a scientist can actually insert a tube into a hot opening. This method is ideal for long term study of volcanoes rather than for monitoring rapidly changing conditions. The technique requires days to weeks of laboratory analysis following sampling in order to get data.


Gas sampling from Baker, 1981


Direct sampling requires a scientist to insert a chemically inert and heat resistant tube into a hot opening like a fumarole or vent. It takes about 5 minutes for the tube to heat up to a point where any condensation within the tube has reached equilibrium with the escaping gases. Then, either by attaching an evacuated-sample bottle or a flow-through sample bottle to the collection tube, the gases will be gathered for analysis. 

USGS geologists collect gas samples around the dome of Mount St. Helens  


Evacuated-bottle method

The evacuated-bottle method is shown in the image to the right. The device includes a glass bottle with a sample port and a high vacuum-stopcock. Before arriving at the collection site, the bottle must be partially filled with concentrated aqueous sodium hydroxide (NaOH) that has been carefully weighted and evacuated with a vacuum pump. Once the tube is inserted into the fumarole or vent, the gases will bubble through the solution and gases like CO2, H2S, SO2, HCL and HF will dissolve into the liquid. Those gases that remain like N2, O2, H2, CO and He will rise further and collect in the headspace of the bottle.

This technique involves collecting the gases at the site where the gases are being emitted and then returning to the laboratory for analysis. This method is used because of its good analytical precision that stems from its ability to concentrate the gases in the solution and the headspace. Those gases that rise into the headspace are analyzed by gas chromatography. Those that dissolve into the liquid are analyzed by ion chromatography or traditional wet-chemical techniques.

USGS- http://www.global-greenhouse-warming.com/sampling-volcanic-gases.html


Flow-through bottle method

The flow-through bottle method is shown in this image to the left. The device includes a glass bottle but with a stopcock at each end and a hand-operated pump attached to the sampling tube. The purpose of the hand pump is to flush out the air while entraining the gases into the bottle. This method is not as precise as the evacuated-bottle method, but is utilized in situations where sampling must be done rapidly due to hazardous environments and conditions. 


USGS- http://www.global-greenhouse-warming.com/sampling-volcanic-gases.html


A detailed analysis has that advantage that it can provide the information necessary to reconstruct the conditions of the magma at depth, which is the source region for the emitted gases.



Additional References:

USGS website: Direct gas sampling and laboratory analysis 

Sutton, A.J., McGee, K.A., Casadevall, T.J., and Stokes, B.J., 1992, Fundamental volcanic-gas-study techniques: an integrated approach to monitoring: in Ewert, J.W., and Swanson, D.A. (eds.), 1992, Monitoring volcanoes: techniques and strategies used by the staff of the Cascades Volcano Observatory, 1980-90: U.S. Geological Survey Bulletin 1966, p. 181-188.

Remote Sensing

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 

Continuous Sampling

Continuous volcano monitoring stations can be used to gauge both short-lived degassing episodes that happen within minutes to hours as well as long-lived activity that happens over days to years. With advancing technology, scientists are able to set up a station to monitor gases from fumaroles, vents, soils, hydrothermal deposits, etc and transmit the data directly to an online directory or observation location. 

To monitor the activity at the Pu `u `O `o vent on the Big Island of Hawai’i, the Hawaiian Volcano Observatory (HVO) set up a monitoring station on the flanks of Kilauea’s east rift zone. Gas emissions as well as the wind speed and wind direction are periodically sampled and get transmitted every 10 minutes to HVO. This allows HVO to monitor degassing at the active vent almost instantaneously.