## 4 Summary and Conclusions  
The study investigates the applicability of the methodology suggested by the European Commission (EC) to assess the contribution of desert dust (DD), to PM\({}_{10}\) loads in the region of Graz in Southern Austria over a time period of six years from January 2013 to December 2018. The station Masenberg (MB) could be identified as a suitable regional background station and the BGL\({}_{\text{MB}}\) was calculated without an exclusion of DD affected days. Furthermore thresholds for the calculated BGL\({}_{\text{MB}}\) and the NDL\({}_{\text{MB}}\) of 12 ug/m\({}^{3}\) and 10 ug/m\({}^{3}\) were used to identify whether the NDL\({}_{\text{MB}}\) is really defined by DD and may be subtracted. We want to point out that this procedure is a more conservative approach than given in the EC guideline, representing the highest computable BGL, and thus the lowest NDL. A detailed investigation of all days exceeding the daily limit PM\({}_{10}\) concentration showed that the risk of undue reductions is rather small. Within 16% of days which were not correctly classified, concentrations were not reduced for 33 days (false negatives), while an undue reduction happened only during five days (false positives). The evaluation of these false negative days reveals the limitations of the method. Both calculation of NDLs and their restriction via thresholds and interpretation of models and trajectories might be biased. It is obvious that using the proposed approach the BGLMB will be overestimated as soon as the DD influence holds longer than one day, leading to a corresponding underestimation of the NDLMB-Still, the calculated NDLMB is higher than the crustal loads chemically measured on quartz fiber filters sampled during two DD events in 2016. Overall the influence of the subtraction of natural sources (i.e., desert dust) from PM\({}_{10}\) concentrations at two sites in Graz yielded a number of reductions; however, on an annual basis, changes of the amount of days exceeding the daily limit were found to be rather small. Within this study, we investigated stations in the region of Graz, representing a hot spot regarding PM\({}_{10}\) concentrations in Austria. Due to the limitation to days exceeding the daily limit, the impact on mean annual concentrations of the proposed approach was not assessed. Additionally, the applicability of the proposed approach to other stations needs further investigation and remains of great importance, but should be supported by a chemical analysis of the crustal loads. The following are available online at [[http://www.mdpi.com/2076-3417/9/11/2265/s1](http://www.mdpi.com/2076-3417/9/11/2265/s1)]([http://www.mdpi.com/2076-3417/9/11/2265/s1](http://www.mdpi.com/2076-3417/9/11/2265/s1)), Table S1: Detailed investigation of the days where the subtraction of the NDL MB reduced the PM\({}_{10}\) load at the urban-traffic station Don Bosco (DB) below the limit value of 50 \(\upmu\)g/m\({}^{3}\). Conceptualization of the study was done by [PERSON] and [PERSON]. XRF filter measurements were made available by [PERSON], who also helped to interpret the data. [PERSON] processed all the data. The manuscript was drafted by [PERSON] together with [PERSON]-[PERSON], with all co-authors commenting on the results and the manuscript content. This research was part of the project DUSTFALL funded by the Austrian Research Promotion Agency (FFG), grant number 848858. The APC was funded by TU Wien University Library through its Open Access Funding Program. The authors wish to acknowledge the provincial government of Styria, especially [PERSON] [PERSON], for the air quality monitoring data from their operational network. Thanks go to [PERSON] for drawing the maps in ArcGIS, [PERSON] for the XRF calibration and analysis of the filters, [PERSON] and [PERSON] for the operational WRF-Chem model forecasts, [PERSON] and [PERSON] for the operational computation of the back-trajectories as well as [PERSON] for the operational computation of the Saharan dust index at Sonmblick.