<p>To investigate the sources and formation mechanisms of carbonaceous aerosols, a major contributor to severe particulate air pollution, radiocarbon&#160;(<span><sup>14</sup>C</span>) measurements were conducted on aerosols sampled from November&#160;2015 to November&#160;2016 in Xi'an, China. Based on the&#160;<span><sup>14</sup>C</span>&#160;content in elemental carbon&#160;(EC), organic carbon&#160;(OC) and water-insoluble OC&#160;(WIOC), contributions of major sources to carbonaceous aerosols are estimated over a whole seasonal cycle: primary and secondary fossil sources, primary biomass burning, and other non-fossil carbon formed mainly from secondary processes. Primary fossil sources of&#160;EC were further sub-divided into coal and liquid fossil fuel combustion by complementing&#160;<span><sup>14</sup>C</span>&#160;data with stable carbon isotopic signatures.</p><p>The dominant EC&#160;source was liquid fossil fuel combustion (i.e., vehicle emissions), accounting for 64&#8201;% (median; 45&#8201;%&#8211;74&#8201;%, interquartile range) of&#160;EC in autumn, 60&#8201;% (41&#8201;%&#8211;72&#8201;%) in summer, 53&#8201;% (33&#8201;%&#8211;69&#8201;%) in spring and 46&#8201;% (29&#8201;%&#8211;59&#8201;%) in winter. An increased contribution from biomass burning to&#160;EC was observed in winter (<span>&#8764;28</span>&#8201;%) compared to other seasons (warm period;&#160;<span>&#8764;15</span>&#8201;%). In winter, coal combustion (<span>&#8764;25</span>&#8201;%) and biomass burning equally contributed to&#160;EC, whereas in the warm period, coal combustion accounted for a larger fraction of&#160;EC than biomass burning. The relative contribution of fossil sources to&#160;OC was consistently lower than that to&#160;EC, with an annual average of&#160;<span>47&#177;4</span>&#8201;%. Non-fossil OC&#160;of secondary origin was an important contributor to total&#160;OC (<span>35&#177;4</span>&#8201;%) and accounted for more than half of non-fossil&#160;OC (<span>67&#177;6</span>&#8201;%) throughout the year. Secondary fossil&#160;OC&#160;(SOC<span><sub>fossil</sub></span>) concentrations were higher than primary fossil&#160;OC&#160;(POC<span><sub>fossil</sub></span>) concentrations in winter but lower than POC<span><sub>fossil</sub></span>&#160;in the warm period.</p><p>Fossil WIOC and water-soluble&#160;OC&#160;(WSOC) have been widely used as proxies for POC<span><sub>fossil</sub></span>&#160;and SOC<span><sub>fossil</sub></span>, respectively. This assumption was evaluated by (1)&#160;comparing their mass concentrations with POC<span><sub>fossil</sub></span>&#160;and SOC<span><sub>fossil</sub></span>&#160;and (2)&#160;comparing ratios of fossil WIOC to fossil&#160;EC to typical primary&#160;OC-to-EC ratios from fossil sources including both coal combustion and vehicle emissions. The results suggest that fossil WIOC and fossil WSOC are probably a better approximation for primary and secondary fossil&#160;OC, respectively, than POC<span><sub>fossil</sub></span>&#160;and SOC<span><sub>fossil</sub></span>&#160;estimated using the EC&#160;tracer method.</p>