## 2 Methods
### The Chemical Accident Recognition Equipment
#### 2.1.1 Infrared System for Remote Chemical Detection (SIGIS-2)  
The SIGIS-2 was selected due to the type of target substances, detectable distance, and sensitivity of the IR-RCD. The SIGIS-2 is based on IR spectroscopy and comprises a single-element detector and a scanning mirror. The interferometer is a Michelson, which has a spectral resolution of approximately 0.5 cm\({}^{-1}\) and provides the capability to distinguish chemical compounds with similar spectra; the spectral range of the SIGIS-2 is 680 cm\({}^{-1}\) to 1500 cm\({}^{-1}\), and it includes a spectral library of 450 or more industrial chemical compounds [2; 22]. Thus, this system, which can track particle phases and detect low concentrations, can assist in the prompt monitoring of chemical leakages. This system sequentially measures the selected area using a scanning mirror and automatically analyzes the IR spectrum (in real time), which is then visualized by overlaying a chemical image onto a video image. The analysis algorithm of the SIGIS-2 calculates the coefficient of correlation between the measured spectrum and a reference spectrum. A chemical is identified as such if the coefficient of correlation and the measured signal intensity are above predefined thresholds. This process makes it possible to verify the identified chemical, as well as the position and size of a gas cloud [2; 22].  
Figure 2 shows the identification image and the coefficient of correlation image for an instance when ammonia was detected by the SIGIS-2 installed at the Jeongnyang site. For the region where ammonia has been identified, the coefficient of correlation is nearly white, i.e., it was close to 1.  
In the industrial complex chemical accident monitoring system, the SIGIS-2 program mode was set to \"automatic and repeated scanning\" for the entire industrial complex area. The head angle and array size were set mainly based on the location of the chemical facility in each section, which was divided by approximately 12\({}^{\circ}\), and camera rotation was minimized for rapid scanning. The scan time per unit section, set to 12\({}^{\circ}\) for pan and 30\({}^{\circ}\) for tilt, is about 1 minute. Figure 3 illustrates the program mode of the SIGIS-2 installed at the Jeongnyang site; a panorama was constructed by connecting the recorded sections (marked with orange squares in Figure 3).  
The SIGIS-2 scans the industrial complex in the order specified in the program mode; it simultaneously stores the optical camera images, chemical detection images, and original data by section in the network-attached storage (NAS). It shares the analysis of the results of the identified chemicals with the control software. Moreover, a function was developed to detect chemicals by rotating the SIGIS-2 to the corresponding section and scanning it when the LRVS or the AI model detects a chemical accident.  
Figure 2: The identification image (**left**) and the coefficient of correlation image (**right**) for the instance when ammonia was detected by the SIGIS-2 installed at the Jeongnyang site (Date: 21 September 2022 at 22:57 GMT+9).  
Thus, the IR-RCD (SIGIS-2), can detect and identify chemical leakages in real time. The shape of a gas cloud and the position of the accident are verified using meteorological data (temperature and wind direction). The meteorological data is measured every minute using a separately installed meteorological instrument and transmitted to the main server. Furthermore, the equipment can recognize chemical substances that are routinely generated in each section within the industrial complex through the accumulated observation data; it uses this information to improve the detection accuracy for chemical accidents.