[1] We report here on the real-time measurement of CO 2 and SO 2 concentrations in the near-vent volcanic gas plume of Mount Etna, acquired by the use of a field portable gas analyzer during a series of periodic field surveys on the volcano's summit. During the investigated period (September 2004 to September 2005, the plume CO 2 /SO 2 ratio ranged from 1.9 to 10.8, with contrasting composition for Northeast and Voragine crater plumes. Scaling the above CO 2 /SO 2 ratios by UV spectroscopy determined SO 2 emission rates, we estimate CO 2 emission rates from the volcano in the range 0.9-67.5 kt d À1(average, 9 kt d À1 ) . About 2 kt of CO 2 were emitted daily on average during quiescent passive degassing, whereas CO 2 emission rates from Etna's summit were 10-40 times larger during the 2004-2005 effusive event (with a cumulative CO 2 release of $3800 kt during the 6 months of the eruption). Such a syneruptive increase, ascribed to the replenishment of the shallow (<6 km) volcanic plumbing system by CO 2 -rich (0.25 wt %) more primitive magmas, supports the potential of CO 2 output rates as key parameters for volcanic hazard assessment.

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Five years of gas monitoring from selected sites suggest that Mt Etna's plumbing system is much more extensive than previously reported. It extends at least 40 km SW from the volcano's boundary along the NE‐SW regional fault, where it discharges about 200 tons/day of gas, containing helium with mantle‐type isotopic composition. Synchronous variations of 3He/4He isotopic ratios in gas sampled at sites located 60 kilometers apart have allowed us to detect pulses of ascending magma in the plumbing system, thus providing a powerful tool for eruption forecasting. Following summer 2001 eruption, the still increasing trend of the 3He/4He ratios indicates that magma storage is even now occurring at a shallow depth. Hence, the volcano maintains a high capacity to re‐erupt within the next few months.

Mud volcanoes represent the largest expression of natural methane release into the atmosphere; however, the gas flux has never been investigated in detail. Methane output from vents and diffuse soil degassing is herewith reported for the first time. Measurements were carried out at 5 mud volcano fields around Sicily (Italy). Each mud volcano is characterized by tens of vents and bubbling pools. In the quiescent phase, methane emission from single vents ranges between 0.01 and 6.8 kg/day. Diffuse soil leakage around the vents is in the order of 102–104 mg m−2 d−1. An exceptional flux of 106 mg m−2 d−1 was recorded close to an everlasting fire. Soil CH4 flux is positive even at large distances from the mud volcano fields suggesting a diffuse microseepage over wider areas. A total of at least 400 tons CH4 per year can be estimated over the area investigated alone (∼1.5 km2).