## 1 State of the Art  
Air is an essential environmental factor, and by changing its quality, negative effects can occur in ecosystems, at the same time generating an impact on human health. Thus, air pollution has become a pressing problem globally, and monitoring, maintaining, and even improving the air quality are topics of great interest to specialists because air can quickly transport certain amounts of pollutants, which lead to environmental damage and negative effects on human health [1]. Therefore, air quality standards aim to protect health and the environment through various regulations, including threshold limits for air pollutants [2]. At the European level, there are several laws and directives regarding the quality of the environment, targeting the monitoring presence of the polycyclic aromatic hydrocarbons, mercury, arsenic, nickel, and cadmium in the air (Directive 2004/107/EC; 2008/50/EC, Decision 2011/850/EU), and establishing a common international framework for air quality protection [3]. Additionally, there are various strategies to protect air quality, such as the \"Clean Air Policy Package\", published in 2013, which aims to significantly reduce air pollution in Europe by 2030 and includes a set of much stricter emission standards [4]. The Green Deal plan, presented in 2019, comprises a set of initiatives at the European Union (EU) level. The aim of this plan is to make Europe a neutral continent in terms of pollution and climate change by 2050 [5]. In Romania, the basis of the air quality legislative framework is strictly in accordance with the EU regulations, and the main legislation is Law no. 104/2011 [6]. At the international level, one example is the Clean Air Act, a specific US law that sets several standards for environmental quality and air pollutant emissions [7]. Comparing the EU and US air quality legislative framework, it can be observed that the EU's strategy is oriented to limit methane emissions, while the US is pursuing a green economy, but both are focused on climate change limitation. The European legislative framework, unlike that of the US, is based on the precautionary principle and pollution prevention, so that the ecological balance and human health can be assured [8]. To properly implement the pollution prevention principle, the air quality assessment process must include accurate information on air pollutants type, dispersion processes, and physical and chemical reactions, and the monitoring data used must be in accordance with the environmental standards. Thus, accurate data on air quality and air pollution levels can be delivered. There are several definitions of air quality monitoring activity, but the simplest indicates that this activity can be characterized by air sampling and analysis using analyzers or sensors [9]. Air quality monitoring methods are diverse and may include the Holo SpeckleNet tool [10], the improvement of some sensors [11], or the use of low-cost sensors and Internet of Things systems [12; 13]. Another topic of great interest in the world of research is the development of tools for modeling the dispersion of air pollutants. In this field, specialists try to combine various approaches to reach better results, such as combining the SINDy algorithm with Lasso regression [14] or using the Caliope system together with principal component analysis (PCA) [15]. Air quality modeling has evolved into a new discipline that includes various technologies and sciences from disciplines such as mathematics, environmental statistics, meteorology, physics, and air quality, which makes this system increasingly complex and advanced. Over time, air quality models have been classified into global, regional, and urban models, and knowing continuous advance, at the international level, small- and medium-scale regulated models such as AERMOD, ADMS, ISC3, etc., are currently used. Among the regional models are CMAQ, WRF-CHEM, CAMx, etc., and GEOS-CHEM is the best-known global model.