## 1 Introduction  
Particulate matter (PM), often defined as atmospheric aerosol, is a general concept used for a mixture of solid or liquid particles suspended in the air, including organic and inorganic substances, volatile and non-volatile compounds, both water-soluble and insoluble, with different chemical, physical and thermodynamic properties [1; 2; 3]. These substances can originate from natural sources (erosion of rocks, soil resuspension, desert dust, volcanic eruptions, sea spray or biological aerosol) and/or anthropogenic activities (combustion of fossil fuels and biomass, waste incineration, high-temperature industrial processes, road abrasion, transport and municipal sources, etc.) [4] and many of them have a documented negative impact on human health [5; 6; 7], climate [8; 9], ecosystems [10] and materials [11]. The definition of primary PM include particles emitted directly into the atmosphere; whereas PM created as a result of reactions and chemical transformations of its precursors (sulphur and nitrogen compounds, volatile organic compounds) is called a secondary pollution [9]. According to the European Environment Agency (EEA), in addition to ozone (O\({}_{3}\)) and nitrogen dioxide (NO\({}_{2}\)), PM is recognized as the most problematic air pollutant in Europe [12]. Particular attention is being paid to the relationship between PM levels in the atmosphere and its impact on the human health, with many authors attributing the most detrimental effects for the smallest particles--PM\({}_{2.5}\), especially PM\({}_{1.0}\) (particles with aerodynamic diameter not greater than 2.5 and 1.0 \(\upmu\)m, respectively), because of their ability to reach the deepest parts of the lungs [5; 13]. Numerous studies indicate that ambient air in various areas usually contains a significant portion of fine particles [14; 15; 16]. In addition, they are often a carriers of various potentially toxic substances, such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals [16; 17; 18]. Moreover, it was found that fine aerosol particles are capable of long-term persistence in the air and can be transported over long distances-hundreds or even thousands of miles from the emission sources [8; 19]. Although epidemiological studies have associated chronic and acute adverse health effects with airborne particulate matter, our understanding of the exact causes and mechanisms of these effects remains limited [20]. Scientific research conducted in recent years indicates that among the physicochemical properties of the PM mainly size distribution, mass concentration and chemical composition are mentioned as those responsible for negative health effects and environmental impact [15]. However air quality standards have been defined exclusively based on PM mass concentration (PM load). Directive 2008/50/EC establishes limit value for PM\({}_{2.5}\) annual average concentrations--25 \(\upmu\)g\(\cdot\)m\({}^{-3}\)[21]. In comparison, more rigorous air quality guidelines (AQG), set by World Health Organization (WHO), accounts to 10 \(\upmu\)g\(\cdot\)m\({}^{-3}\) (regarding daily mean) and 25 \(\upmu\)g\(\cdot\)m\({}^{-3}\) (regarding annual mean) [6]. Moreover, the Ambient Air Quality Directive [21] also sets two additional standards for PM\({}_{2.5}\), relating to the exposure concentration obligation (20 \(\upmu\)g\(\cdot\)m\({}^{-3}\)) and national exposure reduction target (0-20%). Both standards depend on the average exposure indicator (AEI), set at national level, based upon measurements in urban background locations [12; 21]. In addition to the PM concentration and size distribution, chemical composition of particulates is another factor, that directly influences the reactivity, toxicity and the scale of PM impacts on the human body [5; 7; 20; 22; 23; 24]. The knowledge about PM chemical composition also help to understand a time and space variation in ambient particulate concentrations as well as source-receptor relationships [25; 26; 27].