## 3 Results and Discussion  
### Identification of Combustion Products in Gas Cloud Generated from Toluene Combustion  
A simulation test for toluene combustion was conducted in order to identify combustion products formed in a gas cloud and the compounds polluting the ground soil, using GC-MS. The detected compounds could be produced in the gas cloud after a fire accident occurs (Table 1). Total ion chromatogram is presented in Figure S1. Those were unburned methylbenzene (toluene), ethylbenzene, ethynylbenzene, benzaldehyde, 1-Phenyl-1-propylene, naphthalene, and 2-methylidene. The above-mentioned combustion products were obtained using GC-MS, which are expected to exist in the gas cloud generated during a chemical fire (Figure 2). A variety of research had already proved the mechanism of the formation of those products mentioned above. It is notable that naphthalene is a major pollutant in soil because naphthalene-related metabolites were discovered near the chemical accident site. The authors of [13] stated the principle of naphthalene and 2-methylindene formation from toluene combustion. According to [10; 13], naphthalene and 2-methylindene through radical reaction can be formed from indenyl molecules. If the indenyl molecule has a bond with the methyl group, it leads to methylindene. Accordingly, it seems that 2-methylindene can be formed if the methyl group attaches to the 2-carbon position of methylindene. Then, if one hydrogen atom is taken away from 2-methylindene or 2-methyl-2H-indene, it goes to 2-methylindenyl; in addition, one hydrogen abstraction from the former molecule leads to naphthalene production. There is also another pathway of naphthalene production. When 1-methylindene drops down one hydrogen atom, 1-methindenyl radical can be formed with the radical transition. Then, one hydrogen removal causes naphthalene production. The formation of naphthalene and 2-methylindene is described in Figure S2. Phenyl radical contributes to ethylbenzene formation through the Frenklach route [14]. C\({}_{8}\)H\({}_{7}\) radicals would be formed by the radical reaction of C\({}_{6}\)H\({}_{5}\) and C\({}_{2}\)H\({}_{2}\). If C\({}_{8}\)H\({}_{7}\) radical accepts one hydrogen atom, ethenylbenzene can be produced. Another report also proved that styrene production from toluene combustion with butanol flame [15]. The explained products generated from toluene combustion can be phenylacetylene (ethynylbenzene), styrene (ethenylbenzene), ethylbenzene, indenyl, indene, and naphthalene. Through this batch test, these combustion products have the potential to reach and contaminate the ground soil when a toluene fire accident occurs. Transportation and Adsorption of Combustion by-Products onto Soil. As for the diffusion and transportation of a gas cloud in the air, it is a general concept that most gases diffuse out and expand to the atmosphere around and become photodetgraded. However, the gas cloud can spread to the ground soil under specific atmospheric \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Combustion Product** & **Formula** & **Molecular Weight** & **t\({}_{\mathbf{R}}\) (min)** & **Selected Ions (m/z)** \\ \hline Methylbenzene & C\({}_{7}\)H\({}_{8}\) & 92.14 & 3.973 & 91,92,65 \\ Ethylbenzene & C\({}_{8}\)H\({}_{10}\) & 106.17 & 7.030 & 106,91,51 \\ Ethynylbenzene & C\({}_{8}\)H\({}_{6}\) & 102.13 & 7.869 & 102,76,50 \\ Benzaldehyde & C\({}_{7}\)H\({}_{6}\)O & 106.12 & 8.997 & 106,77,51 \\
1-Phenyl-1-propyl & C\({}_{9}\)H\({}_{8}\) & 116.16 & 9.926 & 116,115,63,89 \\ Naphthalene & C\({}_{10}\)H\({}_{8}\) & 128.17 & 11.218 & 128,127,102 \\
2-Methylindene & C\({}_{10}\)H\({}_{10}\) & 130.19 & 13.154 & 130,129,115 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of combustion products in a gas cloud using gas chromatography coupled with mass spectrometry (GC–MS). conditions. The gas diffusion moving downward to the ground level can contaminate the soil, as shown in Figure 2. This scenario is suggested by [6] about studying chlorobenzene fire and explosion accidents using the ADORA model.