## 4 Results and Discussion
### Chemical Composition of PM\({}_{2.5}\)--Contributions  
The relative content of SIA as well as carbonaceous aerosol and other chemical constituents bounded with particulate matter can demonstrate considerable seasonal and temporal variability [31; 34; 79]. Both short-term and long-term studies indicate that meteorological parameters, such as temperature, precipitation, wind direction and wind velocity, have great influence on atmospheric aerosol properties [67; 96; 107]. In addition, temporal and spatial variation of particulate matter characteristics is the function of the local emission sources as well as long-range transport of pollutants [96; 49]. The contribution of main chemical compounds in PM\({}_{2.5}\) mass (Figures 2 and 3) was calculated on the basis of data contained in Tables 1 and 2. In case of the rural background stations selected for comparisons, the SIA share in PM\({}_{2.5}\) were within the range from \(\sim\)12% (_Hp_) (Uto, Finland) to \(\sim\)62% (_Hp_) (Waldhof, Germany) (Figure 2). The contribution of SIA in PM\({}_{2.5}\) from the selected urban and urban background stations ranged from \(\sim\)12% (_Ss_) (Madrid, Spain) to \(\sim\)68% (Menen, Belgium) (Figure 3). The share of the secondary inorganic aerosol (SIA) was typically dominated by sulphates and less often by nitrates (North-Western Europe, some urban background stations in Italy) (Figures 2 and 3). The content of ammonium ions was quite lower. In general, higher SIA shares in PM\({}_{2.5}\) were found at measurement stations located in central (Switzerland) and western Europe (France, Germany) compared to the central and southern parts (Slovenia, Hungary, Italy, Spain). This applies to both urban and urban background stations (Figure 3) as well as the rural background stations (Figure 2). It is also worth to note that the contribution of SIA can change significantly within one country, as confirmed by the urban background stations in France (from \(\sim\)28% (Marsylia) to \(\sim\)50% (Saint-Omer)) and Spain (from \(\sim\)12% (_Ss_) (Madrid) to \(\sim\)36% (Elche) and rural background stations in Greece (from \(\sim\)29% (Penteli) to \(\sim\)50% (Finokalia)) and Germany (from \(\sim\)33% (_Ss_) (Melpitz) to \(\sim\)62% (_Hp_) (Waldhof)). Numerous studies have shown that the largest proportion of SIA in the PM mass is usually identified at the background sites [9; 48; 50]. This can be also observed when looking into the data presented in Figures 2 and 3, with the following stations as an example: Czech Republic (SIA: \(\sim\)42% (_Ep_) and \(\sim\)35% in Kosetice and Prague, respectively), Germany (SIA: \(\sim\)53% (_Ep_), \(\sim\)51% (_Ep_) and \(\sim\)39% in Waldhof, Neuglobosow and Duisburg, respectively) and Greece (SIA: \(\sim\)50%, \(\sim\)30% and \(\sim\)27% (_Ep_) in Penteli, Finokalia and Thessaloniki, respectively). However, the situation is often more complex--for example in Finland, PM\({}_{2.5}\) from Helsinki was characterized by a higher SIA content (\(\sim\)27%) compared to Uto (\(\sim\)13% (_Ep_)). Another example could be the stations in Italy where, in general, higher SIA shares were recorded in PM\({}_{2.5}\) from urban and urban background stations. An exception may be the rural background station in Monte Martano with a relatively high proportion of SIA in the fine particulate matter (\(\sim\)30%). In case of Poland, results from the studies conducted as part of the mentioned research project, have shown a significantly higher share of SIA in PM\({}_{2.5}\) from Szczecin (\(\sim\)31% (_Ep_)) and Trzebinia (\(\sim\)29% (_Ep_)) compared to Zloty Potok (\(\sim\)17% (_Ep_)). Nevertheless, the share of SIA in PM\({}_{2.5}\)from two considered urban background stations was quite lower with reference to the values recorded in Puszcza Borecka (\(\sim\)34% (_Ep_)).