## 2 Materials and Methods
### Source Apportionment  
The definition of PM sources follows the review of [PERSON] et al. [11] who lists six major source categories for PM in Europe (atmospheric formation of secondary inorganic aerosols, traffic, resuspension of crustal/mineral dust, biomass burning, (industrial) point sources, and sea/road salt). As the limited number of samples analyzed for Graz (14 sample pools covering 64 days) does not permit source apportionment based on a multivariate technique like positive matrix factorization, an exploratory method, a macro-tracer approach, was used to investigate the contributions of the main PM sources in Austria. These were identified during previous work [12; 13; 4]. This macro-tracer approach is based on concentrations of marker compounds and respective conversion factors optimized for the region of interest [14; 15; 16]. Secondary inorganic aerosols (SIA) include nitrate, sulfate, and ammonium generated from precursor gases NO\({}_{\mathrm{x}}\), SO\({}_{2}\) and NH\({}_{3}\). A conversion factor of 1.1 is applied to the sum of respective ions (\(\sum\)(NO\({}_{3}\)\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), NH\({}_{4}\)\({}^{+}\))) to account for water uptake due to humidity. SIA cannot be attributed to a specific source of PM but indicate an influence of regional-scale or long-range transboundary anthropogenic pollution [17; 11]. The contribution of wood burning aerosol (WS) to PM was determined based on the concentrations of the anhydro-sugar levoglucosan (Lev) as proposed by [PERSON] et al. [18] and since then widely deployed. As emissions depend on the type of wood and stove used, as well as combustion conditions, the conversion factor needs to be adjusted to local conditions: yielding 10.7 [15]. The related emissions of elemental carbon (i.e., EC\({}_{\mathrm{WS}}\)) were assumed to be 5% of WS, which is within the lower range of data reported in the literature [19], but matched to observed EC values. A proxy for PM emissions attributable to traffic was deduced from ECtr (ECtr = EC - ECWS) concentrations. Traffic primary' was calculated as the sum of traffic exhaust (ECtr multiplied with a conversion factor of 1.33, accounting mainly for organics) and the contribution of abrasion (which was estimated to be 30% of traffic exhaust). The respective conversion factors used are based on tunnel measurements in Austria [20; 21]. We highlight that this proxy, called 'traffic primary', does not include secondary inorganic aerosols related to traffic (e.g., particulate nitrate formed of precursor NOx), as those contributions are already included in SIA, which denote several types of regional-scale anthropogenic pollution. In Austria traffic emissions accounted for 51% of the total NOx emissions in 2017 [22], resulting in a pronounced share of traffic-related SIA. We note in passing that this approach neglects possible contributions of coal and oil to overall EC concentrations, which is justifiable according to the total emissions reported for Austria [23]. Furthermore, mineral dust (MD) is separately identified. MD is emitted by both anthropogenic and natural sources. MD is related to resuspension of split, erosion, construction work or agricultural activities, and is preferably found in PM10 samples [10]; thus, only a minor contribution is expected for our data set. MD was determined based on the analysis of calcium and aluminum and the respective conversion factors and geogenic ratios to account for silicates and carbonates [24]. We note in passing that a fraction of MD might overlap with traffic abrasion, but due to the overall minor contribution of MD to PM2.5 no further considerations were made. Regarding the carbonaceous fraction, WS has been identified as a major contributor to organic carbon during the cold season [13]. OC contributed by WS is also scaled via levoglucosan concentrations [13; 15]. Further contributions are formed by HULIS, which indicate the presence of secondary organic aerosols [9].