Carbon dioxide (CO2), one of the most abundant gases emitted by active volcanoes, is released primarily through the volcanic central vent and fissures, but large volumes of CO2 can also be passively emitted through volcanic soils. As the presence of atmospheric CO2 prevents the use of remote techniques, these so-called diffuse emissions remain unmeasured at many volcanoes, and labour-intensive field work is required to constrain them. Consequently, the global volcanic CO2 emissions of ~320 million tonnes per year are likely underestimated.
Diffuse CO2 emissions can be determined, but to do this they must be measured directly on or near the soil surface. The most common method is to use a portable CO2 flux meter, which consists of an analyser with an infrared sensor that measures the change in CO2 concentration in a chamber placed over the soil surface. Individual flux measurements are straightforward: place the chamber over the chosen spot, hold it stationary for up to 2 minutes while it collects data, and record the measurement. Although the measurement is simple, a flux survey can involve collecting more than 1,000 data points from one volcanic area, which takes weeks. From this, the amount of diffuse CO2 emitted over an area in tonnes per day can be estimated.
Studies estimating diffuse emissions using CO2 flux meters reveal that surprisingly high volumes of CO2 can be measured from the soils surrounding volcanic vents. For example, diffuse CO2 emissions could account for around half of the total emissions at the Mud Volcano site in Yellowstone, and a quiescent volcano in Italy exhibited diffuse soil fluxes similar to those from active volcanic craters. CO2 flux surveys have provided better constraints on global volcanic CO2 emissions, which alongside thermal, isotopic and geophysical information can give deeper insight into volcanic systems. However, CO2 flux surveys provide only a snapshot of diffuse emissions at one time, preventing longer-term information on temporal variability. Newer continuous soil CO2 flux sampling systems offer lower spatial resolution, but show promise for volcanic activity monitoring.
The author declares no competing interests.
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Rahilly, K. Measuring forgotten volcanic CO2. Nat Rev Earth Environ 1, 11 (2020). https://doi.org/10.1038/s43017-019-0006-5