## 4 Summary and Conclusions  
The study presents results of the chemical characterization and source apportionment of atmospheric PM\({}_{2.5}\) at an urban monitoring station in Graz, Austria during January to March 2017. The chosen sampling station is a known hotspot for elevated PM\({}_{10}\) concentrations in Austria. Given the limited number of samples, source apportionment was performed via an exploratory method using specific marker compounds and conversion factors optimized for the region of interest. Building on this study, future work is suggested, which should include a detailed analysis of longer time periods to allow the application of more comprehensive receptor models such as positive matrix factorization. Based on the comparison of the PM ratios between an urban (DB) and background (BB) site (DB/BB PM\({}_{10}\) ratios), two scenarios were identified: (1) time periods with DB/BB PM\({}_{10}\) ratios \(>\) 2, indicating dominant urban impact; (2) time periods with DB/BB PM\({}_{10}\) ratios \(\leq\) 1.5, which characterize pollution episodes observed across a wider area. Pronounced differences in the chemical characterization of the respective samples and in source apportionment show the feasibility of this simple 'dual-site' approach. Time periods with DB/BB PM\({}_{10}\) ratios \(>\) 2: Several PM\({}_{2.5}\) associated components (e.g., EC, PAHs) were found at elevated fractions during these time periods indicating a dominant influence of urban sources. Higher EC contributions as well as ratios of individual PAHs/PM\({}_{2.5}\) related to traffic emissions and combustion pointed towards a pronounced local influence. According to source apportionment the influence of 'traffic primary' is markedly higher, but also WS is a dominant source. This is also indicated by OC/K\({}^{+}\) correlations and levoglucosan concentrations. Time periods with DB/BB PM\({}_{10}\) ratios \(\leq\) 1.5: Time periods have been identified as regional pollution episodes, indicated, for example, by a strong increase of the contribution of SIA. The largest ammonium and nitrate inputs were observed within these time periods, with elevated levels not only found during cold spells. Samples representing scenario 2 also showed differences in OC/K\({}^{+}\) correlation and the macro-tracer derived source apportionment showed a clear dominance of SIA. WS emerged as the second most important aerosol source. The comparison of the results of the chemical analyses with supporting meteorological information showed the presence of inversions during time periods with DB/BB PM\({}_{10}\) ratios \(>\) 2. These local inversions favored the accumulation of locally produced PM. On the other hand, the comparison of meteorological data across a wider area indicated that during other time periods high PM background concentrations occurred. During these episodes, local inversion, even if present, does not appear as the primary driver of elevated PM concentrations. The following are available online at [[http://www.mdpi.com/2076-3417/10/12/4222/s1](http://www.mdpi.com/2076-3417/10/12/4222/s1)]([http://www.mdpi.com/2076-3417/10/12/4222/s1](http://www.mdpi.com/2076-3417/10/12/4222/s1)), Table S1: List of conversion factors to calculate the source contribution; Table S2: Summary of ambient concentrations of quantified analytes related to PM\({}_{2.5}\) at Graz DB. Conceptualization of the study was done by B.K. and A.K. [PERSON]; chemical analyses were done by B.K., M.K., T.S., C.H., and A.L. ; meteorological data was interpreted and prepared by [PERSON] and [PERSON]; writing and original draft preparation was done by B.K. ; writing--review and editing was done by A.K.-G. and H.R. All authors have read and agree to the published version of the manuscript. This research was partly funded by the provincial government of Styria. Open Access Funding by TU Wien. **Acknowledgments:** The authors wish to acknowledge the provincial government of Styria for providing the filter material and air quality monitoring data and for partially funding the analysis.