## 1 Introduction  
Chemical hazards may cause severe damage to societies and the environment. Their effect on human beings, organic life, food and water supplies, and infrastructure [1, 2, 3] may be a result of a human or organizational activity [4], or natural disaster [5]. While defining chemical hazards, which can occur in the form of a solid, liquid or gas, the focus is on their toxicity--the amount or degree of a substance needed to be poisonous [6] (p. 482). Toxicity relates to durability, the time in which the chemical material retains its toxic qualities after use. Chemical hazards may be non-persistent, being volatile and evaporating quickly, from a few minutes to an hour. There are also semi-persistent chemicals, which remain in space from a few hours to one day, and persistent chemicals that often have dense and oily structures and can last from a few days to several weeks. Their persistence depends primarily on meteorological conditions, volume, and dissemination means. The abovementioned qualities of chemical hazards play a significant role, while environmental reconnaissance (detection, identification, and monitoring--DIM) is performed in the case of any unexpected or predicted release. It should be emphasised that the existence of the substance does not determine its effectiveness, but when misused, it can be turned into a weapon of mass destruction (WMD), which encompasses chemical warfare agents (CWAs) and a broad range of toxic industrial chemicals (TICs) [7] (p. 39). CWAs are \"extremely toxic synthetic chemicals that can be dispersed as a gas, liquid or aerosol or as agents adsorbed to particles to become a powder\" [8], while TICs are created and used in medical or industrial settings and when released to the environment may harm human health [9]. Their misuse can be related to either their properties or the potential to interact with the environment or other substances. The mitigation and reduction of threats resulting from the CWAs and TICs require quick and prompt reconnaissance activities. Human senses have limited capacity to detect chemical agents, even those which pose a risk to human health and life. Therefore, it is necessary to support the perception of human senses by instruments or sets capable of chemical threat reconnaissance. There are various devices for chemical reconnaissance [10; 11; 12]; they can rely on different methods of chemical detection and identification (for instance, ion mobility spectrometry--IMS, flame photometry, infrared spectroscopy--IR spectroscopy, surface acoustic wave, colorimetry, Raman spectroscopy) [13]. Usually, they are used as separate tools integrated within a checkpoint. However, with technological advancements, there are greater possibilities for taking more complex countermeasures against chemical attacks [14; 15; 16]. Artificial intelligence, space capabilities, big data, and high-performance computing are technologies that facilitate the development of solutions for keeping society and the environment safer and more secure [17]. Their use may allow for different functionalities integration and an increase in reconnaissance capabilities by providing fast, clear, and reliable data. In this article, an approach to chemical reconnaissance system development is proposed that relies on cutting-edge technologies, i.e., a novel network of integrated sensors that exploits advanced machine-learning, modelling algorithms, and contaminant dispersion models. The integration and synchronisation of different sensors increases the reconnaissance capabilities and reliability (mainly through a false alarm rate reduction) of the whole system. As a research approach, a systemic analysis of the case study concerning the system development is used. In particular, heterogeneous sensor nodes components, chemical agents to be detected, and EU-SENSE system architecture with its three layers are analysed. The system and its accompanying tools were created within the European Sensor System for CBRN Applications (EU-SENSE) project involving nine European partners.