## 3 Results and Discussion
### Chemical Composition
#### 3.2.1 Ionic Composition

Table 2: Summary of ambient concentrations of major PM\({}_{2.5}\) constituents (\(\upmu\)g/m\({}^{3}\)) at the monitoring station located in Graz Don Bosco (DB), 2 January–31 March 2017.  
with average ambient temperature below 0 \({}^{\circ}\)C the contribution of NH\({}_{4}\)NO\({}_{3}\) to major ions (ammonium, nitrate, and sulfate) was on average 33% (up to a maximum of 47%). Conversely, during warmer periods (average temperature above 0 \({}^{\circ}\)C) the contribution of NH\({}_{4}\)NO\({}_{3}\) never exceeded 26% and appeared even as negligible during some periods. The contribution of (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) to major ions showed no variation with temperature (average contribution of 37% during time periods with temperatures below 0 \({}^{\circ}\)C and 33% during time periods above freezing, respectively). Figure 4 shows the temperature dependency of major ions presented as mass concentrations (Figure 4a) and relative contributions to PM\({}_{2.5}\) mass (Figure 4b). The sampling periods representing the two scenarios defined before are indicated with black squares (scenario 1) and black triangles (scenario 2). Summed ammonium, nitrate, and sulfate contributed between 17% and 51% of PM\({}_{2.5}\). A weak tendency is found towards higher contributions during periods with lower air temperatures (Figure 4b). Independently from a temperature dependence, pronounced differences in the contribution of major ions to the overall mass of PM\({}_{2.5}\) were observed, when classification according to the DB/BB PM\({}_{10}\) ratios is considered. During time periods with DB/BB PM\({}_{10}\) ratios \(\leq\) 1.5 (scenario 2) contributions were rather high (36-40%), compared to periods with DB/BB PM\({}_{10}\) ratios \(>\) 2 (scenario 1; 17-19%). Even during the coldest time span of the study period (11 January-12 January 2017, average T= \(-\)7.8 \({}^{\circ}\)C), the contribution of nitrate, sulfate, and ammonium yielded only 17%. The classifications according to the DB/BB PM\({}_{10}\) ratio indicate a more pronounced influence of secondary inorganic ions during time periods characterized by transport of air pollutants. The differentiation according to the DB/BB PM\({}_{10}\) ratios is also reflected by the neutralization ratios (NH\({}_{4}\)+/\(\sum\)(SO\({}_{4}\)\({}^{2-}\) + NO\({}_{3}\)\({}^{-}\)). These are higher during periods characterized by scenario 2 (average value of 0.72) than scenario 1 (average neutralization ratio of 0.49). Likewise, the contribution of NH\({}_{4}\)NO\({}_{3}\), calculated as above, to major ions differs. It was found to be higher during time periods of scenario 2 (30%) than during time periods of scenario 1 (16%), although the time periods representing scenario 1 were characterized by lower temperatures. Average concentrations of sodium, calcium, and chloride are slightly higher during time periods with DB/BB PM\({}_{10}\) ratios \(>\) 2 (i.e., when urban sources dominate). Figure 3: (**a**) Ion balance of pooled PM\({}_{2.5}\) samples at the monitoring station located in Graz Don Bosco (DB), 2 January–31 March 2017. (**b**) Temporal variations of ion balances for individual sampling pools. Overall ion concentrations observed during winter 2017 showed slightly lower contributions of nitrate and sulfate to PM\({}_{2.5}\) (15% and 8%, respectively) than previously observed [12]. During time periods with elevated PM\({}_{2.5}\) concentrations (\(>\)50 \(\upmu\)g/m\({}^{3}\)), the average contributions of nitrate to PM\({}_{2.5}\) mass were in a comparable range for the present and previous study (accounting to 18% in 2017 and \(<\)20% of PM\({}_{2.5}\) mass in 2000/2001). For sulfate, lower contributions to PM\({}_{2.5}\) (11%) were observed during time periods in 2017, while higher contributions (\(>\)20%) were reported in 2000/2001 [12]. This difference can be attributed to generally decreasing SO\({}_{2}\) emissions [32].