## 3 Results and Discussion
### Selection of a Suitable Regional Background Station  
According to Equation (1) the NDLs of the respective days, calculated as the difference between PM\({}_{10}\) and BGL, will vary around zero. Negative values represent days where the BGL is larger than the central PM\({}_{10}\) value and vice versa. In Figures 3 and 4 it can be seen that the time series of MB feature different characteristics than the one of BB and LB. PM\({}_{10}\) values of BB and LB tend to cover a wider range compared to MB and show a seasonality similar to DB and GS with increased values especially during winter. Correspondingly, BGLs of BB and LB (Figure 3) show this seasonality as well and NDLs (Figure 4) yield a much larger scatter during the wintertime, than during summer. Negative NDL outliers at BB and LB were found to occur mostly during winter and could be traced back to days at the end of an inversion weather situation where the PM\({}_{10}\) concentration suddenly drops due to an air mass exchange. Days with inversion weather conditions, meaning an inversed vertical temperature profile with lower values at lower altitudes compared to higher ones, were identified using the daily mean temperature at the TAWES stations at the airport of Graz (337 m a.s.l.) and on top of Schockel (1445 m a.s.l.). If the daily mean temperature at the airport of Graz was lower than the one at Schockel the day was classified as inversion day. Inversion days occur all years whereas their number varies from year to year. Thus, for single years and during short time measurement campaigns BB and LB might serve as suitable background stations for Graz, but not over a longer period of several years. Positive NDL outliers at BB and LB are of course identic with PM\({}_{10}\) outliers and occur mostly during winter, again reflecting the inversion weather situation where PM\({}_{10}\) concentrations are enhanced. Due to this seasonal dependence, BB and LB are ineligible as regional background stations to compute the NDL. The calculated NDL at these stations cannot be interpreted as such due to the substantial influence of inversion weather conditions. It rather reflects a \"pollution load\" pointing to a variety of influencing factors from anthropogenic and natural sources. NDLs of MB are similar between winter and summer and, as a matter of fact, for the whole data set. Negative NDL outliers were found to be associated to days with precipitation rather than inversion weather situations shortly before or after. During precipitation the PM\({}_{10}\) concentration suddenly drops and becomes lower than during the previous and subsequent days representing the BGL. Positive outliers of the NDL are again identic with outliers of PM\({}_{10}\), but were quite equally distributed between the different seasons. The single outlier for the BGL was observed on July 25, 2013 related to a Saharan dust episode a few days later. Due to averaging over 30 days, also other days are influenced by this Saharan dust episode. Their BGL values were found to be only slightly smaller than the upper whisker and are therefore not highlighted. Figure 3: Background load (BGL) of three potential background stations using either the mean (red), the median (blue) or the 40 th percentile (green) of the 30-day period for calculation. Results show that MB represents a suitable background station because it is largely independent of inversion weather situations and hence seasonality. Based on an urban climate analysis study of Graz [17] it can be assumed that the representativeness of the background condition observed on MB is applicable for the stations in Graz. Thus, we investigate the contribution of desert dust on PM\({}_{10}\) measurements using MB to compute the BGL based on the mean of a 30-day period without extracting desert dust affected periods. This approach represents a modified methodology compared to the one proposed by the EC (40 th percentile, exclusion of DD days) which was established for Spain, but the demand for its validation for other countries is clearly highlighted.