Continuous monitoring of carbon dioxide soil diffuse degassing in S. Miguel Island is being carried out since October 2001. The first station was installed at Furnas volcano caldera in the large degassing area that extends into the Furnas village. Two trachytic eruptions with explosive and effusive phases occurred in this volcano in historical times (1439-1443 and 1630). A second station was set up in February 2002 in the northern flank of Fogo Volcano, at the Pico Vermelho geothermal area. A major phreatoplinian eruption took place at Fogo caldera in 1563, immediately followed by a basaltic flank eruption.

Automatic continuous stations based on the accumulation chamber method are used to measure the CO₂ soil flux. The equipment includes additional sensors to record information regarding environmental parameters such as barometric pressure, air temperature, air humidity, wind speed and direction, precipitation, soil water content and soil temperature.

The influence of external factors on CO₂ flux was studied applying the multiple regression analysis and the obtained results show that the main influencing parameters don’t act in the same way on each monitoring site. At Furnas station barometric pressure, precipitation, soil water content, air temperature and soil temperature are the parameters that have statistical meaning on CO₂ flux fluctuations. At Fogo station wind speed emerge as an additional influencing parameter while precipitation is meaningless when all data set is considered.

The exact knowledge of how external parameters interfere with diffuse soil degassing at any particular monitoring site is crucial to discriminate changes due to deep processes.