Atmospheric dispersion modelling of CO2 emission in the Colli Albani volcanic district (Central Italy)

Carbon dioxide is a gas denser than air, and its point-source ground emission from natural systems or from areas impacted by CO2injection underground may result in hazardous accumulation, especially in topographically-depressed sites. The use of atmospheric dispersion numerical models helps predicting the dispersion of the CO2-enriched gas plume once emitted from underground and allows an accurate map of hazard level through time under particular meteorological conditions. In this study, the accuracy of atmospheric dispersion simulations has been tested using a natural system of CO2emission to atmosphere from underground in an area called Solforata di Pomezia, near the city of Rome in central Italy. This area is located in the Alban Hills, which underwent volcanic activity during the Quaternary, and is characterised by low permeability volcanic and sedimentary formations that allow the accumulation of gas at shallow depths and below surface. This site has been long investigated in terms of soil CO2emission 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 CO2soil flux emission and meteorological (wind, temperature) from literature. The results fit well in the range of measured CO2concentration 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 (500 ton∙day-1) and ten (1000 ton∙day-1times bigger than nowadays but still no hazardous levels were predicted.