## 3 Materials and Methods
Mass concentration of PM\({}_{2.5}\) was determined by the gravimetric method, according to the standard PN-EN 12341:2014-07 [64]. For analyses of elemental and organic carbon content in PM\({}_{2.5}\) thermal-optical method was used, whereas ion content was determined by ion chromatography. The details of analytical procedures could be found in the previous works for example, [31; 32]. Summarizing, the daily mean concentrations of PM\({}_{2.5}\), as well as the concentration of its related components, varied in wide range of values and exhibited spatial and seasonal variations. Regardless of the season, the share of total carbon in PM\({}_{2.5}\) exceeded 40% and was primarily determined by fluctuations in the share of OC, which was generally above 30% of the PM\({}_{2.5}\) mass. Ion composition of PM\({}_{2.5}\), from all measurement stations, was dominated by sulphates (SO\({}_{4}\)\({}^{2-}\)), nitrates (NO\({}_{3}\)\({}^{-}\)) and ammonium (NH\({}_{4}\)\({}^{+}\)). Combined share of above mentioned ions was (on average) \(\sim\)34%, \(\sim\)30% and \(\sim\)18% of PM\({}_{2.5}\) mass, respectively in Szczecin, Trzebinnia and Zloty Potok. The contribution of SIA was, in general, higher than SOC (secondary organic carbon) contribution. In addition, inorganic ions contents showed spatial variation with the highest levels recorded at urban background station in Trzebinnia, located in highly urbanized and industrialized area and lowest observed at rural background station in Zloty Potok. The obtained results revealed that SIA contribution in PM\({}_{2.5}\) mass was relatively constant in both periods (in contrast to SOC contribution), however the share of individual inorganic ions in PM\({}_{2.5}\) mass showed different seasonal variations. It was also found that the seasonal variation of PM\({}_{2.5}\) concentrations and concentrations of its major components was mainly due to changes in the intensity of emissions of PM\({}_{2.5}\) and its precursors from combustion sources, which increases in heating period, while clearly falls in the non-heating period. Secondary aerosol, whose presence is related to oxidation of gaseous precursors emitted from fuel combustion and biomass burning, had the largest contribution in observed PM\({}_{2.5}\) concentrations. In addition, the contribution of traffic sources together with road dust resuspension, was observed. The share of natural sources (sea spray, crustal dust) was generally lower. Obtained results and comprehensive bibliometric studies were the basis of the present work, which is addressed on the quantitative comparison of the chemical characteristics of PM\({}_{2.5}\) from different rural and urban background stations in Europe (Sections 4.1 and 4.2). It should be noted that information on concentrations of PM\({}_{2.5}\) and related chemical constituents is an important contribution to the knowledge of fine aerosol particles measured at ground level [47]. However, in order to obtain full information about the seasonal and spatial variability of the chemical composition of fine PM and the impact of different emission sources, it is also necessary to examine the relative content of compounds contained in it, in particular the secondary inorganic aerosol and carbonaceous matter, which is the dominant part of PM\({}_{2.5}\) and PM\({}_{1.0}\)[2; 32; 31]. Taking into consideration, the comparative data is presented in the following form:  
* tabular (Tables 1 and 2), where the data on concentrations of selected major components of PM\({}_{2.5}\) are summarized (Section 4.1);
* graphical (Figures 2 and 3), where the shares of secondary inorganic ions (SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)\({}^{-}\) and NH\({}_{4}\)\({}^{+}\)) and total carbon (TC = EC + OC) in PM\({}_{2.5}\) are shown (Section 4.2). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Location**} & \multirow{2}{*}{**Period**} & \multicolumn{1}{c}{**Concentration (\(\log\)m\({}^{-3}\))**} & \multicolumn{1}{c}{**Concentration (\(\log\)m\({}^{-3}\))**