SUMMARY
Carbon dioxide is a gas denser than air, and its point-source ground emission from natural systems or from areas impacted by CO2 injection underground may result in hazardous accumulation, especially in topographically-depressed sites. The use of atmospheric dispersion modelling helps predicting the dispersion of the CO2-enriched gas plume once emitted from underground and allows an accurate map of risk level through time under particular meteorological conditions. In this study, the accuracy of atmospheric dispersion simulations has been tested using a natural system of CO2 emission to atmosphere from underground in Solforata di Pomezia, in central Italy. Located in the Alban Hills region near the city of Rome and is called Solforata di Pomezia, and underwent volcanic activity during the Quaternary and is characterised by low permeability sedimentary formations that allow the accumulation of gas at shallow depths and surface. This site has been long investigated in terms of soil emission rates, which range from 44 to 95 ton·day-1.Using the TWODEE2 numerical code, a number of simulations were performed considering a set of combined CO2 soil flux emission and meteorological (wind, temperature) from literature. Results fit well in the range of measured CO2 concentration in air at distinct heights in the site. The model does not predict lethal gas concentration at heights 1 and 2 m above the ground based on actual soil emission rate (95 ton·day-1). Two probabilistic models were developed with emission rate five and ten (500 - 1000 ton·day-1) times bigger than nowadays but no hazardous levels were predicted.