## 3 Results and Discussion
### Chemical Composition
#### 3.2.1 Ionic Composition  
Mean concentrations of ionic species, as well as standard deviations, medians, and ranges are summarized in Table 2. Quantified anions contributed an average of 27% to the total PM\({}_{2.5}\) mass, with maximum contributions of 45% (27 January-30 January 2017) and minimum contributions of 18% (27 March-31 March 2017). Total cation mass contributions were 12% on average and ranged between 8% and 14% of the total PM\({}_{2.5}\) concentration. Overall equivalent concentrations of cations and anions are well balanced with an average value of 0.97 (Figure 3a), but show a distinct temporal evolution (Figure 3b). At the beginning of the sampling period a cation deficit emerges, which vanishes in early February with changing ambient conditions. This shift is primarily caused by decreasing concentrations of anions and ammonium, while the concentrations of the other cations remain rather constant over time, or in the case of magnesium and calcium, even show an increasing tendency. We link these changes to an increasing influence of resuspension of mineral dust, which will occur predominantly in the coarse fraction, but slightly affects PM\({}_{2.5}\) as well [4]. Regarding the anion deficit we assume the occurrence of different anions like carbonate, but also organic acids and phosphate, which were not quantified in this study. Elevated chloride concentrations were only found during January, concurrent with above average sodium concentrations. During January the molar ratio of chloride to sodium showed excess chloride with an average molar ratio of 1.4, while the opposite became true in February and March, where average molar ratios of 0.52 and 0.15, respectively, were observed. While the concurrent increase of sodium and chloride might indicate the use of de-icing salt applied to the urban environment, the influence of combustion sources can also contribute to the excess chloride [29; 30]. Nitrate was identified as the dominant anion over the whole sampling period, with concentrations ranging between 1.6 and 20.0 \(\upmu\)g/m\({}^{3}\), while concentrations of sulfate ranged between 0.73 and 12.9 \(\upmu\)g/m\({}^{3}\) and concentrations of ammonium, the most abundant cation, ranged between 0.12 and 8.0 \(\upmu\)g/m\({}^{3}\). Nitrate concentrations emerge as highly correlated with sulfate (r = 0.93) and ammonium (r = 0.97), indicating the formation of (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) and semi-volatile NH\({}_{4}\)NO\({}_{3}\)[31]. Neutralization ratios based on equivalent concentrations of ammonium and sulfate showed excess ammonium for all but two sample pools with rather low PM mass in March (1 March-10 March 2017; 27 March-31 March 2017). If nitrate is included, the neutralization ratios (NH\({}_{4}\)\({}^{+}\)/\(\sum\)(SO\({}_{4}\)\({}^{2-}\) + NO\({}_{3}\)\({}^{-}\))) show a maximum value of 0.83, indicating that a fraction of the nitrate present is associated with other ions such as potassium (r = 0.75). These conditions allow to estimate the concentrations of (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) and NH\({}_{4}\)NO\({}_{3}\) based on the molar concentrations of sulfate and excess ammonium (each multiplied with the respective molar mass of the salt). During time periods \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Mean (\(\upmu\)g/m\({}^{3}\))** & **Standard Deviation (\(\upmu\)g/m\({}^{3}\))** & **Median (\(\upmu\)g/m\({}^{3}\))** & **Range (\(\upmu\)g/m\({}^{3}\))** \\ \hline Cl\({}^{-}\) & 1.5 & 2.7 & 0.92 & 0.17–7.7 \\ NO\({}_{3}\)\({}^{-}\) & 6.4 & 6.0 & 5.3 & 1.6–20.0 \\ SO\({}_{4}\)\({}^{2-}\) & 3.5 & 3.7 & 2.7 & 0.73–12.9 \\ Na\({}^{+}\) & 1.3 & 0.66 & 1.2 & 0.26–2.5 \\ NH\({}_{4}\)\({}^{+}\) & 2.0 & 2.5 & 1.4 & 0.12–8.0 \\ K\({}^{+}\) & 0.81 & 0.73 & 0.53 & 0.14–2.2 \\ Mg\({}^{2+}\) & 0.06 & 0.02 & 0.06 & 0.04–0.09 \\ Ca\({}^{2+}\) & 0.29 & 0.17 & 0.28 & 0.02–0.62 \\ OC & 9.1 & 6.6 & 10.5 & 3.6–25.4 \\ EC & 2.0 & 0.95 & 2.1 & 1.0–4.4 \\ Lev & 0.86 & 0.84 & 0.91 & 0.22–2.6 \\ Man & 0.11 & 0.11 & 0.12 & 0.03–0.35 \\ \hline \hline \end{tabular}
\end{table}