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. Sources of particulate organic carbon (OC) with differentvolatility have rarely been investigated, despite the significant importancefor better understanding of the atmospheric processes of organic aerosols.In this study we develop a radiocarbon-based ( 14C ) approach for sourceapportionment of more volatile OC (mvOC) and apply to ambient aerosolsamples collected in winter in six Chinese megacities. mvOC is isolated bydesorbing organic carbon from the filter samples in helium (He) at 200  ∘ C in a custom-made aerosol combustion system for 14C analysis. Evaluation of this new isolation method shows that the isolatedmvOC amount agrees very well with the OC1 fraction (also desorbed at 200  ∘ C in He) measured by a thermal–optical analyzer using theEUSAAR_2 protocol. The mvOC, OC and elemental carbon (EC) of13 combined PM 2.5 samples in six Chinese cities are analyzedfor 14C to investigate their sources and formation mechanisms. Therelative contribution of fossil sources to mvOC is 59±11  %,consistently larger than the contribution to OC ( 48±16  %) andsmaller than that to EC ( 73±9  %), despite large differences infossil contributions in different cities. The average difference in thefossil fractions between mvOC and OC is 13 % (range of 7 %–25 %),similar to that between mvOC and EC (13 %, with a range 4 %–25 %). Secondary OC(SOC) concentrations and sources are modeled based on the 14C -apportioned OC and EC and compared with concentrations and sourcesof mvOC. SOC concentrations ( 15.4±9.0 µ g m −3 ) areconsistently higher than those of mvOC ( 3.3±2.2 µ g m −3 ),indicating that only a fraction of SOC is accounted for by the more volatilecarbon fraction desorbed at 200  ∘ C. The fossil fraction in SOC is43 % (10 %–70 %), lower than that in mvOC (59 %, with a range of 45 %–78 %). Correlation between mvOC and SOC from nonfossil sources (mvOC nf vs. SOC nf ) and from fossil sources (mvOC fossil vs.SOC fossil ) is examined to further explore sources and formationprocesses of mvOC and SOC.

High contributions of fossil sources to more volatile organic aerosol