## 3 Results and Discussion
### Source Apportionment with a Macro-Tracer Approach  
The concentrations of the PM source groups defined in Section 2.4 are shown in Figure 9. For all sample pools WS and SIA emerge as the dominant sources, although with alternating dominance. WS contributions to PM2.5 mass ranged between 10% and 43% (2.4-27.7 ug/m3) and contributions from SIA ranged between 19% and 57% (2.8-44.9 ug/m3). Contributions of 'traffic primary' ranged between 1.6% and 16% (1.4-6.0 ug/m3). Contributions of HULIS and MD showed rather low average contributions to PM2.5 mass of 2.3% (1.0 ug/m3) and 4.1% (2.2 ug/m3), respectively. Uncertainties in conversion factors lead to under- and over-estimations of PM sources in certain cases. However, during the study period the carbonaceous fraction was slightly overestimated (1.8 ug/m3) only between 2 January-3 January 2017 via this macro-tracer approach. As the discussions of the chemical composition revealed marked differences between the two scenarios, source apportionment via the macro-tracer approach is shown in more detail for these samples (Figure 10). Time periods of scenario 1 (DB/BB PM10 ratios \(>\) 2; Figure 10a) showed higher average contributions of 'traffic primary', contributing 7-11% to PM2.5 mass (average value of 10%), while contributions of SIA ranged from 19-21% (average value of 20%). Contributions of WS ranged between 28% and 43% (average  
Figure 8: PAH profiles according to their classification of molecular weights. Middle molecular weight (MMW) PAHs are shown in black and high molecular weight (HMW) PAHs in red. Black squares mark the PAHtotal/PM2.5 ratios. value of 35%). Furthermore, the mass of not identified PM was found quite high. During time periods of scenario 2 (DB/BB PM\({}_{10}\) ratios \(\leq\) 1.5; Figure 10b), SIA increased and ranged from 40-44%, even though these days were not the coldest during the study period. During this scenario the contribution of 'traffic primary' decreased to an average value of 4%. Contributions of WS ranged from 15-30% (average value of 20%), and thus are lower than during samples of scenario 1. The time period of 9 February-17 February 2017 shows elevated contributions of MD and also OMnd, which can be linked to an input of desert dust identified during this time period [28]. Generally speaking, MD did not show pronounced variations during either scenario considered (4% during scenario 1 and 5% during scenario 2). The same was found for HULIS, with average contributions of 2% and 3%, respectively. Contributions of OMnd were found to be higher during time periods of scenario 2 (9%) than during scenario 1 (2%). Thus, the simple classification based on the difference in DB/BB PM\({}_{10}\) ratios (local vs. regional) is reflected by chemical characterization and findings of source apportionment. Supporting Information from Meteorological Analyses  
The chemical analyses described above detail the PM source apportionment to individual subgroups/components of interest. To supplement the findings of the chemical analyses regarding contributions from local and regional emissions to PM burdens meteorological data is analyzed. Today it is well understood that ambient meteorological conditions strongly modulate PM burdens and determine particulate accumulation and decay [40; 41; 42; 43]. High PM burdens coincide normally with one or multiple of the following meteorological phenomena: stagnant weather conditions, calm winds and limited ventilation, cold ambient temperatures leading to a shallow boundary layer, and temperature inversions inhibiting vertical mixing. To characterize the meteorological conditions on a regional basis we used the WLKC733 weather classification [44], which provides information on dominant wind direction, cyclonicity at 925 hPa and 500 hPa, and dominance of wet or dry conditions. To characterize local meteorological conditions, we considered measurements of ambient air temperature and wind direction and wind speed. Of these covariates, temperature is most important as it is relatively variable over the study period.