While several studies have investigated winter-time air pollution with a wide range of concentration levels, hardly any results are available for longer time periods covering several winter-smog episodes at various locations; e.g., often only a few weeks from a single winter are investigated. Here, we present source apportionment results of winter-smog episodes from 16 air pollution monitoring stations across Switzerland from five consecutive winters. Radiocarbon (<sup>14</sup>C) analyses of the elemental (EC) and organic (OC) carbon fractions, as well as levoglucosan, major water-soluble ionic species and gas-phase pollutant measurements were used to characterize the different sources of PM<sub>10</sub>. The most important contributions to PM<sub>10</sub> during winter-smog episodes in Switzerland were on average the secondary inorganic constituents (sum of nitrate, sulfate and ammonium = 41 ± 15%) followed by organic matter (OM) (34 ± 13%) and EC (5 ± 2%). The non-fossil fractions of OC (<i>f</i><sub>NF,OC</sub>) ranged on average from 69 to 85 and 80 to 95% for stations north and south of the Alps, respectively, showing that traffic contributes on average only up to ~ 30% to OC. The non-fossil fraction of EC (<i>f</i><sub>NF,EC</sub>), entirely attributable to primary wood burning, was on average 42 ± 13 and 49 ± 15% for north and south of the Alps, respectively. While a high correlation was observed between fossil EC and nitrogen oxides, both primarily emitted by traffic, these species did not significantly correlate with fossil OC (OC<sub>F</sub>), which seems to suggest that a considerable amount of OC<sub>F</sub> is secondary, from fossil precursors. Elevated <i>f</i><sub>NF,EC</sub> and <i>f</i><sub>NF,OC</sub> values and the high correlation of the latter with other wood burning markers, including levoglucosan and water soluble potassium (K<sup>+</sup>) indicate that residential wood burning is the major source of carbonaceous aerosols during winter-smog episodes in Switzerland. The inspection of the non-fossil OC and EC levels and the relation with levoglucosan and water-soluble K<sup>+</sup> shows different ratios for stations north and south of the Alps (most likely because of differences in burning technologies) for these two regions in Switzerland.