## 3 Results and Discussion  
### PM2.5 and PM10 Concentrations  
Figure 1 shows the daily PM concentrations at the urban sampling station DB over the study period. The PM10 concentrations observed between 2 January-31 March 2017 ranged from 9.9 to 140 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\). On 1 January 2017 (following fireworks on New Year's Eve) a maximum concentration of 157 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) of PM10 mass was measured, but due to the influence of fireworks this day is excluded from further evaluations in this study. During the sampling period 34 days occurred with a PM10 concentration exceeding 50 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) and thus violating the EU short time PM10 limit value. Of these exceedance days, 21 already occurred in January 2017. Considering the whole year of 2017, 54 days violating the EU short time PM10 limit value occurred, indicating a substantial PM burden in contrast to the general decreasing trend of annually occurring days of exceedances [28]. PM2.5 concentrations at the urban sampling station DB followed the trend of PM10, with concentrations between 5.6 and 120 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\), giving an average PM2.5 to PM10 ratio of 0.7. As chemical analysis could not be performed for the whole data set, selected filters were pooled to represent characteristic time periods. The pools were formed on the basis of PM\({}_{10}\) concentrations, and time periods selected for chemical analysis are marked with grey shading in Figure 1. In total, 14 sampling pools were analyzed, comprising 64 (out of 89) days of the sampling period. Overall, the highest PM concentrations occurred during January and are represented by six sampling pools (comprising 19 days). February and March 2017 are represented by five and three sampling pools, covering 25 and 20 days, respectively. Figure 2 combines the daily PM concentrations at the urban sampling site DB and the background site BB. As expected, lower PM\({}_{10}\) concentrations, ranging between 5.1 and 123 \(\upmu\)g/m\({}^{3}\), were observed at the background site BB. Nevertheless, individual days with high PM\({}_{10}\) concentrations emerge in the BB record. These periods of relatively similar PM concentrations at DB and BB have been considered as indicative for pollution episodes affecting an overall wider area. The differences in PM concentrations at DB and BB are evaluated via DB/BB PM\({}_{10}\) ratios (Figure 2). During the whole sampling period DB/BB PM\({}_{10}\) ratios ranged between 1.0 and 4.3, and between 1.0 and 3.9 when only days with PM\({}_{10}\) concentrations above 50 \(\upmu\)g/m\({}^{3}\) are considered. Elevated PM concentrations are represented by the filter pools of the time periods of 2 January-3 January 2017, 11 January-12 January 2017, 22 January-23 January 2017, 31 January-2 February 2017, 9 February-17 February 2017, and 21 February-23 February 2017. Based on the DB/BB PM\({}_{10}\) ratios, which indicate differences or conformity between the PM concentrations determined within the city and the background site, we postulate a subdivision of these six periods into two scenarios: (1) DB/BB PM\({}_{10}\) ratios larger than 2 represent a pollution period with local character (i.e., a marked impact of urban emissions); (2) DB/BB PM\({}_{10}\) ratios smaller or equal to 1.5 represent comparable PM concentrations on both sampling sites and thus indicate larger scale pollution. In the following we will refer to these scenarios as'scenario 1' and'scenario 2', respectively. Table 1 provides detailed information about the six time periods of interest (i.e., average temperature, average DB PM\({}_{10}\) and DB PM\({}_{2.5}\) concentrations, and DB/BB PM\({}_{10}\) ratios). PM ratios  
Figure 1: Daily average PM\({}_{10}\) and PM\({}_{2.5}\) concentrations observed at the urban sampling site Graz Don Bosco (DB). Grey shading marks selected time periods. For convenient reference, the red line marks the 50 \(\upmu\)g/m\({}^{3}\) limit value set by the European Commission.