Sources of non-fossil-fuel emissions in carbonaceous aerosols during early winter in Chinese cities

China experiences frequent and severe haze outbreaks from the beginning of winter. Carbonaceous aerosols are regarded as an essential factor in controlling the formation and evolution of haze episodes. To elucidate the carbon sources of air pollution, source apportionment was conducted using radiocarbon (C-14) and unique molecular organic tracers. Daily 24 h PM2.5 samples were collected continuously from October 2013 to November 2013 in 10 Chinese cities. The C-14 results indicated that non-fossil-fuel (NF) emissions were predominant in total carbon (TC; average = 65 +/- 7 %). Approximately half of the EC was derived primarily from biomass burning (BB) (average = 46 +/- 11 %), while over half of the organic carbon (OC) fraction comprised NF (average = 68 +/- 7 %). On average, the largest contributor to TC was NF-derived secondary OC (SOCnf), which accounted for 46 +/- 7% of TC, followed by SOC derived from fossil fuels (FF) (SOCf; 16 +/- 3 %), BB-derived primary OC (POCbb; 13 +/- 5 %), POC derived from FF (POCf; 12 +/- 3 %), EC derived from FF (ECf; 7 +/- 2 %) and EC derived from BB (ECbb; 6 +/- 2 %). The regional background carbonaceous aerosol composition was characterized by NF sources; POCs played a major role in northern China, while SOCs contributed more in other regions. However, during haze episodes, there were no dramatic changes in the carbon source or composition in the cities under study, but the contribution of POC from both FF and NF increased significantly.