## 4 Results and Discussion
### Chemical Composition of PM\({}_{2.5}\)--Contributions
Taking into account the results recorded at the rural background station in Zloty Potok, the total share of the secondary inorganic ions was generally lower in relation to the values recorded at the stations listed in Figure 2 (e.g., stations in Germany, the Netherlands, France and Greece). It was also lower compared to the share of SIA in PM\({}_{2.5}\) from Raciborz, Puszcza Borecka and Zielonka, with average (during calendar year) SIA/PM\({}_{2.5}\) ratios at the level of \(\sim\)26%, \(\sim\)34% and \(\sim\)27%, respectively. Almost the same content of SIA in PM\({}_{2.5}\) to the one found in Zloty Potok was recorded at the rural background stations in Harwell (UK) (_Ep_), K-Puszta (Hungary) and Ispra (Italy) (_Ep_). It should be noted that for the majority of the rural background stations (Figure 2), the share of SIA in PM\({}_{2.5}\) showed more or less pronounced seasonal variation (in contrast to Zloty Potok), with higher values recorded in the heating period (e.g., Puszcza Borecka (\(\sim\)41%), Zielonka (\(\sim\)31%), Harwell (\(\sim\)19%), Waldhof (\(\sim\)62%), Neuglobosw (\(\sim\)56%), Melpitz (\(\sim\)47-50%), Montseny (\(\sim\)18%)), less often in the non-heating period (e.g., Raciborz (\(\sim\)29%), Uto (\(\sim\)14%), Ispra (\(\sim\)20%)) (Figure 2). However, the situation observed in Zloty Potok (SIA: \(\sim\)17%) is nothing exceptional--the lack of clear seasonal variability of the SIA contribution in PM\({}_{2.5}\) was also noted at some other stations in Europe, for example, Kosetic (the Czech Republic) (\(\sim\)39%), Rise (Denmark) (\(\sim\)43%), Schauinsland (Germany) (\(\sim\)36-38%), Cabauw-Zijdeweg (The Netherlands (\(\sim\)51%), Puy-de-Dome (France) (\(\sim\)41%), Iskrba (Sloveria) (\(\sim\)15-16%) (Figure 2). It is also easily observable that in the case of many rural background stations, the secondary inorganic aerosol clearly dominates over the carbon matter (Figure 2). Exceptions are two stations located in Central Europe-Iskrba (Sloveria) (SIA: \(\sim\)15% and TC: \(\sim\)32%; _Ep_) and K-Puszta (Hungary) (SIA: \(\sim\)17% and TC: \(\sim\)22%; _Ss_) as well as stations located in Italy (Ispra, SIA: \(\sim\)17% and TC: \(\sim\)40%; _Ep_) and Spain (e.g., Montseny with similar contributions both for SIA and TC (\(\sim\)16%, _Ep_). The rural background stations in Poland also differ a lot from the majority of stations listed in Figure 2 by the strong dominance of TC over SIA. This situation also applies to the urban background station (Figure 3), however the difference in the shares of the above-mentioned main components is less visible. The contribution of TC in PM\({}_{2.5}\) from the rural background stations reached values ranging from \(\sim\)12% (all averaging periods) (Ayia Marina, Cyprus) to \(\sim\)59% (Puy-de-Dome, France) (Figure 2). Considering the measurement station in Zloty Potok (\(\sim\)40% (_Ep_)), the share of TC in PM\({}_{2.5}\) was close to the value recorded in Raciborz (\(\sim\)45% (_Ep_)) and clearly higher compared to the stations located in Puszcza Borecka and Zielonka (respectively \(\sim\)31% and \(\sim\)32%; _Ep_), which applies to all averaging periods. A comparable share of TC in the fine particulate matter was recorded only at the regional background station in Ispra (Italy) (\(\sim\)40% (_Ep_)), however without distinct seasonal variation as in the case of Zloty Potok. A clearly higher value of the share of TC in PM\({}_{2.5}\) was recorded at the regional background station in Puy-de-Dome (France) during summer season (\(\sim\)59%), which could be explained by the occurrence of an additional source of TC in this period--the emission of secondary organic aerosol from biogenic sources (location of the station in the agricultural and forest area) [40]. In contrast to the secondary inorganic aerosol, the contribution of TC in PM\({}_{2.5}\) from Zloty Potok showed a clear seasonal variation with a higher shares in the heating period (\(\sim\)44%) compared to the non-heating one (\(\sim\)32%) (Figure 2).