## 3 Results and Discussion
### Identification of Combustion Products in Gas Cloud Generated from Toluene Combustion
This model predicts the gas expansion and transportation to the ground level-based on energy, momentum, and thermochemical equilibrium. When a fire or explosion happens, a generated massive heat immediately gets to be emitted for a short term forming a super high temperature under the high pressure. Then, secondary combustion called \"afterburn\" can occur causing the vertical elevation of the gas cloud with the end of the first explosion blast. Most incomplete or toxic compounds can be formed at this point. After quite a while, the gas cloud does not elevate upward until its gas density would be increasingly similar to the density of the air around. After emitting massive heat energy, the movement of the gas cloud becomes more influenced by wind flow while gas density in the gas cloud decreases. Conclusively, downdraft transports pollutants passively to reach the ground soil, as reported by [PERSON] and [PERSON] [6]. Moreover, the atmospheric condition has to be also considered. The rapidly cooled ground or inflow of chilly air right above ground at midnight makes it possible for airborne pollutants to reach the ground. After sunset, the ground cools down, and the air in touch with the ground becomes cooler increasingly. The relatively warmer air goes upward and the cooler one goes down to the ground [6; 7]. Due to this air condition, the inversion layer makes the high concentration of pollutants close to the ground because this inhibits the gas diffusion and elevation to the upper atmosphere and trap the contaminants such as combustion products below the inversion layer. Considering the actual toluene fire accident mentioned in this study, which happened at 2 a.m. in South Korea, it is reasonable that the inversion layer and cooled ground with downdraft can be formed at the accident site and contribute to ground contamination by the gas cloud at that time (Figure 2). In addition, internal boundary layers (IBLs) formation also can cause the gas cloud to reach the ground around the coastal industry complex [18]. Considering the actual chemical accident site was located near the coast, this theory would be much persuasive for assuming IBLs formation. Above all, the downdraft in the air must be an essential factor for highly ground pollution of airborne- contaminants such as combustion products in a gas cloud. Adsorption of combustion  
Figure 2: Modified image explaining soil pollution by gas cloud under the inversion layer, as suggested by [6], and schematic diagram showing the expectable combustion products with the formation mechanism proved by the following literature; 2-methylidene [10; 13], toluene to ethylbenzene (styrene) [13], toluene to ethylbenzene [14], toluene to ethylbenzene (phenylacetylene) [14], toluene to xylene (dimethylbenzene) [16], toluene to naphthalene, toluene to benzaldehyde [17]. products to the ground soil also has to be discussed. It has been reported that black carbon or soot, one of the incomplete combustion products, generated from the combustion of an organic chemical can be deposited in the soil [19]. There is a strong bond between the minerals in soil. [PERSON] et al. studied the sorption of volatile organic compounds (VOCs) onto soil minerals; they performed the experiment about the sorption isotherms of VOCs on soil, which has dissimilar properties, respectively, considering few parameters that include the mineralogical type and surface area [20]. Toluene, ethylbenzene, and xylene were verified to be adsorbed to soil porous media. The sorption mechanism is an ion-dipole interaction, sorption by pi(\(\pi\))-electron, and Van der Waals-London forces for aromatic molecules [20]. The batch experiment was also performed to identify whether combustion products can remain in the soil in this study, which reproduces the fire accident of toluene and transportation and adsorption of combustion products onto the soil. Toluene, ethylbenzene, 2-methylidene, out of suggested combustion products, were identified in the soil using GC-MS (Table 1).