## 5 Conclusions  
Comprehensive literature studies allowed to summarize knowledge about the monitoring of air quality in Europe in terms of particulate matter, especially in relation to PM\({}_{2.5}\). The present work was addressed on quantitative comparison of the PM\({}_{2.5}\) chemical characteristics measured at different rural and urban background stations in Europe. The concentrations of PM\({}_{2.5}\), as well as PM\({}_{2.5}\)-related SIA and TC, registered both at rural and urban background stations varied within wide range of values. Considering data for the former type of stations, it can be observed that the concentrations of PM\({}_{2.5}\) from Zloty Potok were visibly higher compared to the values observed on most rural sites (north-western Europe, Mediterranean area) especially during the heating period. The average concentrations of the main components in PM\({}_{2.5}\) from Zloty Potok were also higher, which is particularly evident in the case of the carbonaceous aerosol, which was the predominant PM\({}_{2.5}\) component from mentioned station, as well as other rural stations located in different parts of Poland. Generally, PM\({}_{2.5}\) concentrations recorded at urban and urban background stations are higher than the PM\({}_{2.5}\) concentrations from the rural background stations. Some exceptions from this rule could be explained by the different research period and the specific location conditions in the measurement stations. This for example applies to the sites in Szczecin and Trzebinia, which generally differed significantly in terms of concentrations of PM\({}_{2.5}\) main constituents, with higher values for the latter station. Taking into consideration the European data, the highest concentration of carbonaceous aerosol was found in the central parts of Europe (southern Poland--Trzebinia,Katowice; Czech Republic--Prague) and in some southern European countries (Greece--Thessaloniki; Turkey--Istanbul). The concentrations of SIA were more diversified, in general, with the highest levels recorded in some French stations (Saint-Omer, Dunkerque), following by Menen (Belgium), Thessaloniki (Greece), Helsinki (Finland), Rovigo (Italy) and stations located in southern Poland (Trzebinia, Katowice, Zabrze). The relative content of SIA as well as carbonaceous aerosol demonstrated different spatial and seasonal variability. 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), which applies both to urban and urban background stations as well as the rural background stations. In many cases (e.g., Czech Republic, Germany, Greece) the largest proportion of SIA in the PM mass was identified at the rural background sites. However, research conducted as a part of this work has shown that those ratios differs from site to site without any trend--for example in Italy, Finland and Poland, with higher SIA shares in PM\({}_{2.5}\) from urban background sites. Taking into account the results recorded at the rural background station in Zloty Potok, the total share of secondary inorganic ions was generally lower or comparable to many European stations having similar characteristics, without any significant seasonal variations. Another difference is related to a strong dominance of TC over SIA in the case of Zloty Potok compared to the European data. A comparable share of TC in the fine particulate matter was recorded only at the regional background station in Ispra (Italy), however without any distinct seasonal variation as in the case of Zloty Potok. Urban and urban background stations were typically characterized by a higher share of TC in PM\({}_{2.5}\), compared to the rural background sites, which ranged from \(\sim\)14% (Dunkerque (France)) to \(\sim\)49% (Trzebinia (Poland)). Moreover, for the former type of stations, there is also no such visible difference in the share of SIA and TC in PM\({}_{2.5}\).