## 3 Results and Discussion
### Application of the Modified Methodology
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Year**} & **PM\({}_{\mathrm{MB}}\) DB** & **PM\({}_{\mathrm{MB}}\)-ND\({}_{\mathrm{MB}}\)** & **PM\({}_{\mathrm{MB}}\)-ND\({}_{\mathrm{MB}}\)** & **DD Days** & **PM\({}_{\mathrm{MB}}\) GS** & **PM\({}_{\mathrm{MB}}\)-ND\({}_{\mathrm{MB}}\)** & **PM\({}_{\mathrm{MB}}\) GS**-ND\({}_{\mathrm{MB}}\)** & **DD Days** \\  & \(>\) 50 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\) & \(>\) 50 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\) & \(\leq\) 50 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\) & **DB** & \(>\) 50 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\) & \(>\) 50 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\) & \(\leq\) 50 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\) & **GS** \\ \hline
2013 & 44 & 36 & 8 & 4 & 31 & 25 & 6 & 1 \\
2014 & 27 & 16 & 11 & 7 & 23 & 18 & 5 & 3 \\
2015 & 39 & 30 & 9 & 5 & 35 & 29 & 6 & 3 \\
2016 & 39 & 35 & 4 & 2 & 34 & 29 & 5 & 3 \\
2017 & 54 & 49 & 5 & 1 & **43** & 37 & 6 & 1 \\
2018 & 39 & 36 & 3 & 1 & 30 & 23 & 7 & 4 \\ \hline Sum & 242 & 202 & 40 & 20 & 196 & 161 & 35 & 15 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Number of days with daily PM\({}_{10}\)\(>\) 50 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\) at the urban-traffic station Graz Don Bosco (DB) and the urban-background station Graz Stud (GS), number of days which are exceeding or falling below the 50 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\) limit value after the subtraction of the NDL of the regional background station MB (NDL\({}_{\mathrm{MB}}\)) without any further evaluation as well as with the identification of desert dust (DD) days via model calculations. Amount of days exceeding the limit of 35 days/year with PM\({}_{10}\) concentrations \(>\) 50 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\) are marked in bold. values are considered. The resulting thresholds (\(\mathrm{BGL_{MB}}\): 12 \(\mathrm{\mu g/m^{3}}\) and \(\mathrm{NDL_{MB}}\): 10 \(\mathrm{\mu g/m^{3}}\)) are based on a descriptive approach, but give a better understanding of the several factors influencing \(\mathrm{NDL_{MB}}\). \(\mathrm{BGL_{MB}}\) concentrations, determined as a 30-day average, should not fall below 12 \(\mathrm{\mu g/m^{3}}\), a value close to the annual average concentration of \(\mathrm{PM_{10}}\) determined at Masenberg. We want to point out that monthly averaged \(\mathrm{PM_{10}}\) values of the whole period of 6 years vary between 6-8 \(\mathrm{\mu g/m^{3}}\) for November, December and January and between 10-14 \(\mathrm{\mu g/m^{3}}\) for all other months. Setting a threshold for \(\mathrm{BGL_{MB}}\) at 12 \(\mathrm{\mu g/m^{3}}\) will preclude the determination of a DD event valid for reduction, during the winter month and also during most of the year 2017. Still a lower threshold value would lead to the identification of events not associated with the influence of DD, but by other sources or general differences between the background site at 1180 m asl and the urban sites in Graz. Furthermore, the calculated \(\mathrm{NDL_{MB}}\) should be at least 10 \(\mathrm{\mu g/m^{3}}\) and thus account for a substantial part of the \(\mathrm{PM_{10}}\) concentration, at least at the background site. This can be regarded as a precondition as the method should identify DD events which influence the air quality in Austria markedly. Otherwise, low concentration values more affected by random variations could account for undue reductions of \(\mathrm{PM_{10}}\) levels. This is especially important during days when daily limit values at polluted sites are exceeded only slightly. The identified \(\mathrm{NDL_{MB}}\) threshold is close to the 95 th percentile of 10.9 \(\mathrm{\mu g/m^{3}}\). These thresholds were applied to the 242 days exceeding the 50 \(\mathrm{\mu g/m^{3}}\) daily \(\mathrm{PM_{10}}\) limit within the six-year period at DB only, since this is the more critical station regarding the 35 day/year limit.