## 1 Introduction
SIA influences, to a large extent, the concentrations and composition of fine PM, both in areas far from significant emission sources as well as in urban areas [46; 47; 48; 49]. In general, many studies have shown that the main part of SIA is associated with fine particles and the largest contribution of SIA to fine PM mass is usually identified at the rural background stations [9; 48; 50]. According to EEA report [41], SIA constitute about one third of the PM\({}_{10}\) and a half of the PM\({}_{2.5}\) concentrations, measured at regional background stations in Europe. In urban air, the concentration of SIA in PM mass is smaller, due to the increased proportion of primary particles, coming from local sources [41]. The chemical composition of PM is relatively different in various parts of Europe. In general, there is more carbon matter in PM\({}_{10}\) in Central Europe, more nitrates in North-Western Europe and more mineral dust in Southern Europe [34]. The share of carbonaceous matter in PM\({}_{10}\) varies from 10% to 40% at EMEP (_European Monitoring and Evaluation Programme_) sites and between 35 and 50% at sites located in the Mediterranean area. In addition, measurements of the PM chemical composition show that there is a clear decrease in the relative contribution of SO\({}_{4}\)[2]- and NO\({}_{3}\)[3]- ions in the PM\({}_{10}\) mass as the distance from rural sites towards urban and communication stations increases. On the contrary, the share of carbonaceous matter in the PM\({}_{10}\) mass increases in the direction from rural sites to traffic locations [34]. Although the last two decades have brought the improvement of ambient air quality, the problem of exceeding the standards for PM\({}_{10}\) and PM\({}_{2.5}\) still remains unresolved in many areas within the Europe [12; 17; 18; 32]. As indicated in the latest EEA report [12], the long-term WHO AQG for PM\({}_{10}\) (20 \(\upmu\)g\(\cdot\)m\({}^{-3}\)) and PM\({}_{2.5}\) (10 \(\upmu\)g\(\cdot\)m\({}^{-3}\)) was exceeded at 48% and 68% stations respectively and in all the reporting countries except Estonia (PM\({}_{2.5}\), PM\({}_{10}\)), Switzerland (PM\({}_{2.5}\), PM\({}_{10}\)), Finland (PM\({}_{2.5}\)), Hungary (PM\({}_{2.5}\)), Norway (PM\({}_{2.5}\)), Iceland (PM\({}_{10}\)) and Ireland (PM\({}_{10}\)). The research concerning the PM chemical composition and the identification of its sources have been undertaken by many authors, for example, [7; 17; 25; 32; 36; 51; 52; 53; 54; 55; 56]. Also, some review studies has been published, for example, [4; 12; 19; 34; 57; 58], which allowed to significantly improve knowledge about PM\({}_{2.5}\) air pollution in areas with different emission characteristics. However, there is still a lack of works that would present the above subject in quantitative terms by providing specific data on concentrations and percentage contributions of PM main chemical constituents. Such studies would greatly facilitate comparison of the results obtained by atmospheric pollution researchers, who are often forced to review a significant number of publications. Comparative analysis is highly difficult because the number of sites on which the aerosol was fully characterized is still limited, especially in the southern and eastern parts of Europe [4; 12; 17; 19; 31; 57]; this is particularly true for the fine PMfraction (PM\({}_{2.5}\), PM\({}_{1.0}\)). Another problem is the inconsistency of location criteria and often limited data sets (in many countries less than 5 years of the reporting time) [58]. In the context of the above-mentioned facts, the aim of the present study is to investigate the concentrations and chemical composition of PM\({}_{2.5}\) from selected urban and rural background stations in Europe through a systematic review. This is the first such comprehensive and extensive analysis in this issue, covering spatial and seasonal variability.