## 4 Results and Discussion
### Chemical Composition of PM\({}_{2.5}\)--Contributions
Similar results were also found at other rural background stations in Poland (except for Puszcza Borecka, TC: \(\sim\)31% for all averaging period) and on some European stations (e.g., Kosetic (Czech Republic) (\(\sim\)30% (_Ws_) and \(\sim\)26% (_Ss_)) and Ispra (Italy) (\(\sim\)41% (_Hp_) and \(\sim\)37% (_NHp_)). However, in general the contribution of TC in PM\({}_{2.5}\) from the rural background stations listed in Figure 2 remained relatively constant over the year or was higher in the warm period, for example, Schauinsland (Germany) (\(\sim\)22% (_Hp_) and \(\sim\)28% (_NHp_)) and Puy-de-Dome (France) (\(\sim\)14% (_Ws_) and \(\sim\)59% (_Ss_)). Urban and urban background stations were typically characterized by higher share of TC in PM\({}_{2.5}\), which ranged from \(\sim\)14% (Dunkerque (France), _Ss_) to \(\sim\)49% (Trzebinia (Poland), _Hp_) (Figure 3). Considering the entire period of research, the share of TC in PM\({}_{2.5}\) from Szczecin (\(\sim\)41%) and Trzebinia (\(\sim\)43%) was generally much higher in relation to data from the majority of stations listed in the Figure 3(e.g., Saint-Omer (France) (\(\sim\)16%), Huelva (Spain) (\(\sim\)20%)). Relatively high TC/PM\({}_{2.5}\) ratios were also recorded at the measurement stations in Marseille (France) (\(\sim\)41%), Prague (the Czech Republic) (\(\sim\)44%, \(H\!p\)), Athens (Greece) (\(\sim\)38%) and Bern (Switzerland) (\(\sim\)39%). In the case of urban and urban background stations, there is no such visible difference in the share of SIA and TC in PM\({}_{2.5}\) (Figure 3). A higher share of SIA was found, among others, at many stations in Spain (e.g., Valencia, SIA: \(\sim\)35% and TC: 30%), Duisburg (Germany) (SIA: \(\sim\)39% and TC: 31%), Thessaloniki (Greece) (SIA: \(\sim\)27% and TC: 21%; \(Ep\)) and Genoa (Italy) (SIA: \(\sim\)39% and TC: 29%). A clear dominance of SIA over TC contribution characterized especially the PM\({}_{2.5}\) composition in France (e.g., Saint Omer, SIA: \(\sim\)50% and TC: 16%), Zurich (Switzerland) (SIA: \(\sim\)46% and TC: 26%) and Limassol (Cyprus) (SIA: \(\sim\)43% and TC: 19%). The total carbon had the highest share of analysed chemical components in the case of measurement stations in Prague (the Czech Republic) (SIA: \(\sim\)36% and TC: \(\sim\)44%), Rotterdam (the Netherlands) (SIA: \(\sim\)23% and TC: \(\sim\)31%), Marseille (France) (SIA: \(\sim\)28% and TC: \(\sim\)41%), Bern (Switzerland) (SIA: \(\sim\)30% and TC: \(\sim\)39%), Budapest (Hungary) (SIA: \(\sim\)30% and TC: \(\sim\)35%) and Istanbul (Turkey) (\(\sim\)18% and \(\sim\)32%). Also urban background stations in Szczecin and Trzebinnia (Poland) were characterized by a clear dominance of TC over SIA, with mean values averaged over entire period amounted to: \(\sim\)31% and \(\sim\)41% (Szczecin) and \(\sim\)29% and \(\sim\)43% (Trzebinia), respectively. The total share of secondary inorganic ions in PM\({}_{2.5}\) from Szczecin and Trzebinia was generally lower compared to the data registered at many stations located in the northern and central parts of Europe, for example, Menen (Belgium) (\(\sim\)68%), Birmingham (UK) (\(\sim\)51%), Saint-Omer (France) (\(\sim\)50%), Dunkerque (France) (\(\sim\)41% (\(H\!p\)) and 49% (\(NH\!p\))) and Basel (Switzerland) (\(\sim\)49%) (Figure 3). The share of SIA remained in a similar range of values as SIA contribution registered at stations in Marseille (France) (\(\sim\)28%), Bern (Switzerland) (\(\sim\)30%), Budapest (Hungary) (\(\sim\)28-30%), Padua (Italy) (\(\sim\)31%) or Thessaloniki (\(\sim\)28%, \(H\!p\)). Slightly lower shares of SIA were noted, among others, at stations in Helsinki (Finland) (\(\sim\)27%), Rotterdam (the Netherlands) (\(\sim\)23%), Leece and Conegliano (Italy) (\(\sim\)26%), L'Eliana and Barcelona (Spain) (\(\sim\)25%) and Istanbul (Turkey) (\(\sim\)18%). Both stations in Szczecin and Trzebinia were also characterized by a higher SIA share in PM\({}_{2.5}\) and less pronounced seasonal variation of this share compared to other urban background stations located in Poland (Figure 3).