# The Online Identification of the Behaviour of Pollutants inside the Tunnel Tube
## 1 Introduction  
Tunnels are constructed where it is necessary to overcome natural or artificial obstacles. They reduce the route length, improve its vertical and directional conditions, and preserve the environment above the tunnel in its original state. An essential role of tunnels is to increase transportation capacity and speed and reduce air pollution levels by appropriately locating the exits of ventilation units away from inhabited centres. When designing a tunnel, it is essential to make good use of technical means considering its long-term, trouble-free operation [1]. Technical conditions (TCs) and regulations provide solutions based on long-term experience for the design, engineering, implementation of technological equipment, and construction of road tunnels, ensuring that the tunnel is an economically and ecologically viable structure, guaranteeing a high level of safety for the general public [2]. European tunnels are equipped with all modern safety features required by EU Directive 2004/54 [3]. The tunnel has its own life, inevitably requiring smooth functioning and proper care. Increased attention to tunnel safety necessarily leads to the need for higher care and maintenance [4; 5]. The tunnel tube is a relatively small space that allows for the accumulation of gaseous and liquid substance-containing pollutants. These substances can be harmful to the human body as well as to the construction materials and technological equipment of the tunnel. Pollutants are typically not the primary cause of tunnel incidents. In the event of a ventilation malfunction, the tunnel must be closed. The primary safety of the tunnel focuses on minimizing accidents within the tunnel. However, secondary safety must also be given attention. Tunnel closure redirects traffic, increasing the risk of accidents, which can be mitigated via secondary safety measures. The reliability and quality of ventilation influence secondary safety. It is the reason for solving the optimization problem of the ventilation. Article [6] deals with rail transport applications' primary and secondary safety uses. In exceptional situations, such as accidents in tunnels, fire safety in road tunnels addressesventilation and traffic management during emergencies and crisis scenarios. It is important to note that this article does not cover fire safety and primary safety. The fire safety is explored in publications [7; 8; 9]. The exhaust gases from automobiles are a source of carbon monoxide (CO), a highly toxic combustible gas. It is formed during the incomplete combustion of carbon and other carbon-containing compounds. Another product that is formed in combustion engines of automobiles is nitrogen oxide. Nitrogen oxide molecules are reactive and unstable; they react with oxygen in the atmosphere, forming toxic nitrogen dioxide (NO\({}_{2}\)). NO\({}_{2}\) and nitric oxide (NO) are marked as a group of nitrogen oxides (NO\({}_{\rm X}\)). The units for the concentration of CO and NOx are ppm (parts per million). More information is available in [10; 11]. In light of these facts, the ventilation system is the most crucial component of the tunnel's technological equipment, greatly influencing its reliable and safe operation. The selected ventilation system, its design, the choice of specific types of fans, and their control affect the energy consumption of the ventilation system and, thus, the operational costs of the tunnel. In the case of ventilation based on physical parameters, fans are controlled according to the instantaneous values of carbon monoxide concentrations or visibility measured at individual points, for example, 200 m from the tunnel portal. The frequency of the ventilation system switches based on the immediate value of pollutant concentration, even if it is caused by a single vehicle with high emissions. The frequent and unpredictable activation of the ventilation system has an impact not only on increased energy consumption but also on the lifespan of the fans, which have a limited number of switching cycles.