## 3 Results and Discussion  
### PM\({}_{10}\) Levels at the Stations Graz Don Bosco (DB) and Graz Sid (GS)  
Daily PM\({}_{10}\) levels recorded at the stations DB (urban-traffic) and GS (urban-background) exceed the 50 \(\mu\)g/m\({}^{3}\) on 242 and 196 days, respectively, within the six-year period from January 2013 to December 2018, thereby exceeding the 35 day/year limit in six years and one year, respectively. The time series are plotted in Figure 2 showing that maximum PM\({}_{10}\) values, and hence, exceedance of limit values mostly occur during wintertime. This pattern of the seasonal variation is observed at both stations and is characteristic for stations in this region. The increase of PM concentrations during winter can on one hand be attributed to an enhanced influence of inorganic secondary aerosols due to lower air temperature and increased humidity and also to a stronger influence of wood combustion used for residential heating, as well as regional transport of PM [22; 24]. Furthermore, meteorological conditions with wintertime inversions favor an enrichment of PM concentrations in the boundary layer. During wintertime inversion conditions are very likely in the area of Graz due to the orographic situation of the region, allowing air pollutants to concentrate over several days until the inversion is cleared out and concentrations drop [17].  
On top of these seasonal variations the influence of long-range transported desert dust may occur. As an example, an intensive desert dust episode in April 2016 is marked by a red star in Figure 2. The identification, intensity and duration of this event, covering large parts of Austria, was already thoroughly described by [PERSON] et al. [12]. Still a number of less pronounced events took place. Such events, identified via the use of WRF-Chem model forecasts [12], FLEXTRA  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Station** & **LON** & **LAT** & **Elevation** & **Device** \\  & [\({}^{\circ}\)] & [\({}^{\circ}\)] & [\({}^{\circ}\)] & [m a.s.l.] & \\ \hline Graz Don Bosco (DB) & 15.41643 & 47.05702 & 358 & Digital HVS DHA80 \\ Graz São (GS) & 15.43306 & 47.04167 & 345 & Digital HVS DHA80 \\ Graz Lustbühel (LB) & 15.49369 & 47.06700 & 473 & MetOne BAM 1020 \\ Bockberg (BB) & 15.49583 & 46.87139 & 449 & MetOne BAM 1020 \\ Masenberg (MB) & 15.88222 & 47.34806 & 1180 & MetOne BAM 1020 \\ Graz Ost (GO) & 15.46638 & 47.05944 & 366 & Digital HVS DHA80 \\ Gratwein (GW) & 15.32361 & 47.13555 & 382 & Digital HVS DHA80 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Monitoring stations used in the present study. See locations in Figure 1.  
back-trajectories [25] as well as the dust ensemble forecasts and forecast comparisons provided by the WMO sand and dust storm warning advisory and assessment system ([[https://sds-was.aemet.es/forecast-products/dust-forecasts](https://sds-was.aemet.es/forecast-products/dust-forecasts)]([https://sds-was.aemet.es/forecast-products/dust-forecasts](https://sds-was.aemet.es/forecast-products/dust-forecasts))), are marked in orange in Figure 2. Additionally optical aerosol properties measured at the high mountain global GAW station Hoher Sonnblick in the Austrian Alps are used for the identification of desert dust (DD) occurrence [14]. The aim of this study was the quantification of DD contributions for days with PM\({}_{10}\) exceedances only. An additional influence of desert dust might have occurred during periods of lower PM\({}_{10}\) concentrations as well.