Disgas, a new model for passive gas dispersion. Early applications for the warm gases emitted by Solfatara (Campi Flegrei, Italy)

Abstract

A model to describe the cloud dispersion of gas denser than air is presented here. The dispersion of heavy gas is basically governed by the gravity but, when the density contrast (gas vs air) is not important the dispersion is controlled by the wind and atmospheric turbulence (so-called “passive dispersion”). DisGas is a model for dense gases which are dispersed under passive conditions, based on the full solution of the advection-diffusion equations for the gas concentration (Sankaranarayanan et al., 1998). The wind field can be assumed with a horizontally uniform profile calculated in accord to the Monin-Obukhov similarity theory or it can be estimated by the so-called DIAGNO, a Diagnostic Wind Model (DWM) developed by the US Environmental Protection Agency, the latter option requiring topographic data, average wind and atmospheric stability information within the computational domain. The model is able to forecast gas dispersion over large and complex terrain. Following the study of Costa et al., (2005), we present here an application of DisGas on the gas dispersion from the crater of Solfatara (Campi Flegrei) which releases a large quantity of CO2 into the surrounding densely-inhabited areas. For the simulated cases, the soil CO2 flow rate was assumed to be about 800 ton/day, such as the average of twelve different surveys carried out in the period 1998 to 2008 (Chiodini et al., 2010). Local atmospheric dynamics (3-components of the wind, friction velocity, Monin-Obukhov length) were derived by a two-year period of observations with micrometeorological technique inside the Solfatara crater. Our main finding showed that the urban area of Naples is affected by the CO2 buildup above the normal air CO2 content for this “natural” contribution, particularly in the no windy nocturnal situation. Estimated values show the absence of any risk to the population safety at the present emission rate but suggest that volcanic CO2 contributes towards deteriorating and warming the urban air of Naples. DisGas model is able to simulate the dispersion of a heavy warm gas accounting for obstacles, topographic effects, variation of atmospheric conditions and wind direction. So, a possible application is the cloud dispersion of pollutant and/or greenhouse gases emitted by industrial chimney over urban or suburban environmental.