## 3 Results and Discussion
### Chemical Composition
#### 3.2.2 Carbonaceous Compounds  
Average OC and EC concentrations as well as the standard deviations, median values, and ranges are summarized in Table 2. Figure 5 shows the correlation between OC and EC concentrations, as the OC/EC ratio is commonly used to distinguish between different sources of carbonaceous material [33]. Previous studies reported lower OC/EC ratios (up to 2.3) to be associated with fossil fuel burning, while OC/EC ratios associated to emissions from biomass combustion are higher and exhibit a wider range (up to 16) [29; 33; 34]. The OC/EC ratios found in this study ranged from 2.4 to 11. The classification according to DB/BB PM\({}_{10}\) ratios characterizes differences in chemical composition and possible aerosol sources (Figure 5). Scenario 2 (DB/BB PM\({}_{10}\) ratios \(\leq\) 1.5) shows higher OC/EC ratios (5.2 to 11), characteristic for aerosols related to biomass burning, while scenario 1 (DB/BB PM\({}_{10}\) ratios \(>\) 2) yields samples with lower OC/EC ratios (3.2 to 4.6) indicating the influence of biomass burning as well as fossil fuel combustion and thereby a plausible, stronger urban impact. Figure 4: (**a**) Temperature dependence of mass concentrations and (**b**) relative contributions of major ions (nitrate, ammonium, sulfate) to PM\({}_{2.5}\) samples at the urban sampling site in Graz DB, 2 January–31 March 2017. Black highlighted squares mark time periods with DB/BB PM\({}_{10}\) ratios \(>\) 2 (A: 2 January–3 January 2017; B: 11 January–12 January 2017; C: 21 February–23 February 2017). Black highlighted triangles mark time periods with DB/BB PM\({}_{10}\) ratios \(\leq\) 1.5 (D: 22 January–23 January 2017; E: 31 January–2 February 2017 and F: 9 February–17 February 2017). Remaining data points are shown by grey squares. The highest OC/EC ratios (\(>\)5) occurred when ambient temperatures were rather low (until mid-February), pointing to a pronounced influence of biomass combustion. Yet, this was not accompanied with lower relative contributions of inorganic ions, as could be expected because of the increased source for carbonaceous aerosols. Figure 6 shows the correlation between OC/EC ratios and the contribution of ammonium, nitrate, and sulfate (summarized as SIA, see Section 2.4) to PM\({}_{2.5}\). Scenario 1 shows generally lower OC/EC ratios, and also lower contributions of SIA. The opposite can be observed during time periods of scenario 2 where the more regional character of the pollution event characterized by similar PM concentrations at the urban and the background site leads to higher OC/EC ratios and an above average contribution of SIA. The influence of biomass combustion indicated by the OC/EC ratios was further investigated using the molecular tracer levoglucosan. Emission studies linked levoglucosan (Lev), mannosan (Man), and galactosan (Gal) to biomass combustion processes [18; 35], whereas the ratios of Lev/Man and Lev/Gal provide information about the combustion material [13; 30]. Lev was found to be the most abundant saccharide during the sampling period with an average ambient concentration of 0.86 ug/m\({}^{3}\) (0.22 to 2.6 ug/m\({}^{3}\)), contributing 1% to 4% of total PM\({}_{2.5}\) mass. Atmospheric concentrations of Lev were higher during cold periods and decreased with time and increasing temperatures. Man concentrations were on average eight times lower than those of Lev, with concentrations ranging from 0.03 to 0.35 ug/m\({}^{3}\). Average Lev/Man ratios yielded 7.9 indicative for the use of softwood and hardwood as fuel for biomass combustion [15]. Gal concentrations were mainly below the limit of detection. Figure 7a shows a high correlation between Lev and OC concentrations (r = 0.93) for the whole data set, suggesting a major fraction of OC originating from biomass burning across all samples. The correlation between Lev concentrations and OC/EC ratios (Figure 7b) shows larger scatter (r = 0.74) and some difference between samples representing different DB/BB PM\({}_{10}\) ratios.