High contributions of fossil sources to more volatile organic aerosol

Sources of particulate organic carbon (OC) with different volatility have rarely been investigated, despite the significant importance for better understanding of the atmospheric processes of organic aerosols. In this study we develop a radiocarbon-based (14C) approach for source apportionment of more volatile OC (mvOC) and apply to ambient aerosol samples collected in winter in six Chinese megacities. mvOC is isolated by desorbing 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 isolated mvOC amount agrees very well with the OC1 fraction (also desorbed at 200 °C in He) measured by a thermal-optical analyzer using the EUSAAR_2 protocol. The mvOC, OC and elemental carbon (EC) of 13 combined PM 2.5 samples in six Chinese cities are analyzed for 14C to investigate their sources and formation mechanisms. The relative contribution of fossil sources to mvOC is 59±11 %, consistently larger than the contribution to OC (48±16 %) and smaller than that to EC (73±9 %), despite large differences in fossil contributions in different cities. The average difference in the fossil 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 sources of mvOC. SOC concentrations (15.4±9.0 μg m-3) are consistently higher than those of mvOC (3.3±2.2 μg m-3), indicating that only a fraction of SOC is accounted for by the more volatile carbon fraction desorbed at 200 °C. The fossil fraction in SOC is 43 % (10 %-70 %), lower than that in mvOC (59 %, with a range of 45 %-78 %). Correlation between mvOC and SOC from nonfossil sources (mvOCnf vs. SOCnf) and from fossil sources (mvOCfossil vs. SOCfossil) is examined to further explore sources and formation processes of mvOC and SOC.