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First Chemical Characterization of Refractory Black Carbon Aerosols and Associated Coatings over the Tibetan Plateau (4730 m a.s.l)

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journal contribution
posted on 13.11.2017, 00:00 authored by Junfeng Wang, Qi Zhang, Mindong Chen, Sonya Collier, Shan Zhou, Xinlei Ge, Jianzhong Xu, Jinsen Shi, Conghui Xie, Jianlin Hu, Shun Ge, Yele Sun, Hugh Coe
Refractory black carbon (rBC) aerosol is an important climate forcer, and its impacts are greatly influenced by the species associated with rBC cores. However, relevant knowledge is particularly lacking at the Tibetan Plateau (TP). Here we report, for the first time, highly time-resolved measurement results of rBC and its coating species in central TP (4730 m a.s.l), using an Aerodyne soot particle aerosol mass spectrometer (SP-AMS), which selectively measured rBC-containing particles. We found that the rBC was overall thickly coated with an average mass ratio of coating to rBC (RBC) of ∼7.7, and the coating species were predominantly secondarily formed by photochemical reactions. Interestingly, the thickly coated rBC was less oxygenated than the thinly coated rBC, mainly due to influence of the transported biomass burning organic aerosol (BBOA). This BBOA was relatively fresh but formed very thick coating on rBC. We further estimated the “lensing effect” of coating semiquantitatively by comparing the measurement data from a multiangle absorption photometer and SP-AMS, and found it could lead to up to 40% light absorption enhancement at RBC > 10. Our findings highlight that BBOA can significantly affect the “lensing effect”, in addition to its relatively well-known role as light-absorbing “brown carbon.”