Many open questions exist about the importance of different sources of carbonaceous aerosol, which is a substantial contributor to the global aerosol budget and, therefore, to climate change and human mortality. In this work, 14C was determined in elemental carbon (EC) and different organic carbon (OC) fractions from ambient urban aerosols with aerodynamic diameter <10 ¿m collected in Zurich (Switzerland). This enabled a more detailed source attribution of the carbonaceous aerosol mass than is possible with other currently available methods. The three major sources, fossil fuel, wood combustion (both anthropogenic emissions), and biogenic emissions, were quantified, making specific regulatory air quality management measures possible. EC originates nearly exclusively from fossil fuel usage during summer, whereas biomass-burning emissions become substantial during winter with ~25%, even though this source contributes only marginally to the local energy consumption. For OC, biogenic sources are dominant in summer with ~60%, where secondary organic aerosol prevails. Wood combustion accounts for up to ~41% of OC in winter. Fossil fuels represent ~30% of OC throughout the year.