Using atmospheric models to simulate backward certain gases that are the result of illegal activities and locate the source

Illegal activities such as the manufacturing of home-made explosives may result in the dispersion of certain type of gases in the atmosphere or water pollutants. Our study addresses the methodology that can be used to track the source location of these gases after they have been detected. Traditionally, tracking the source location of a gas release requires the knowledge of the emission maps, however, given the nature of the task, this is unknown in our case. Our methodology, does not involve inverse modeling, and only requires the use of a network of instruments on the ground or mounted on aerial vehicles. The FLEXPART-WRF Lagrangian dispersion model is used to backward simulate the dispersion and transport of the gases at 0.5-1 Km horizontal resolution. The numerical experiments take place at two different geographical regions, i.e. one with strong topography and another over plain terrain with no topography, and over a wide range of meteorological conditions. Our results indicate that 5 to 10 ground measurements are enough to locate the source with an accuracy of a few kilometers. Identifying the time of the release, on the other hand, is more challenging especially if the initial release of the gas does not take place instantly and is gradual.