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 CO2 injection
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 CO2 emission 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 CO2
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. The 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 (500 ton∙day-1) and ten (1000 ton∙day-1 times bigger than
nowadays but still no hazardous levels were predicted.

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