CO₂ and CH₄ soil fluxes from the Occhibolleri cold degassing site, Monti Livornesi, Tuscany, Italy

Data acquisition was conducted during four field campaigns in July 2024, September 2024, October 2024, and July 2025 in the area near the Occhibolleri gas manifestations.. Soil gas fluxes were measured using a portable fluxmeter equipped with a LI-COR LI-850-3 nondispersive infrared (NDIR) CO₂ analyser (detection limit: 0.002 mol m⁻² d⁻¹), a tunable diode laser absorption spectroscopy (TLDAS; WS-CH4-TLD) CH₄ detector (detection limit: 0.0005 mol m⁻² d⁻¹), and a cylindrical accumulation chamber with an open base area of 314 cm² and a volume of 0.00277 m³, connected to a palmtop computer. The fluxmeter system was calibrated in the laboratory, whereby known CO₂ and CH₄ fluxes were imposed using a mass flow controller. The study area is characterised by well-developed vegetation, primarily consisting of woodlands, Mediterranean shrubland, and grassland. Soil gas flux measurements were carried out using the accumulation chamber method. As no previous soil CO₂ flux data were available for the area, an initial reconnaissance survey was performed over a relatively large area (~50,000 m²), with a sampling grid of approximately 20 × 20 m. Subsequently, measurement density was increased around two cold gas manifestations (Occhibolleri vents). The spatial distribution and density of measurement points were locally constrained by rocky and irregular terrain and, in some areas, by dense Mediterranean shrub cover. Measurements were conducted under stable meteorological conditions and avoided periods of heavy rainfall.