Measurement report: New insights into the mixing structures of black carbon on the eastern Tibetan Plateau – soot redistribution and fractal dimension enhancement by liquid–liquid phase separation

Abstract. Black carbon (BC; i.e. soot) absorbs radiation and contributes to glacier retreat over the Tibetan Plateau (TP). A lack of comprehensive understanding of the actual mixing state leads to large controversies in the climatic simulation of soot over the TP. In this study, ground-based sampling, electron microscopy analyses, and theoretical calculations were used to investigate the interactions among the liquid–liquid phase separation (LLPS), soot redistribution in secondary particles, and fractal dimension (Df) of soot particles on the eastern rim of the TP. We found that more than half of the total analysed particles were soot-containing particles. One-third of soot-containing particles showed a core shell structure that probably formed the LLPS phenomenon after long-range transport. Particle size and the ratio of organic coating thickness to soot size are two of the major possible factors that likely induce soot redistribution between organic matter and inorganic aerosols in individual particles. The Df sequence is ranked as externally mixed soot (1.79±0.09)< sulfate-coated soot (1.84±0.07)< organic-coated soot (1.95±0.06). We concluded that the soot redistribution process promoted the morphological compaction of soot particles. This study indicates that soot-containing particles experienced consistent ageing processes that induced a more compact morphology and soot redistribution in the LLPS particles on the remote eastern rim of the TP. Understanding the microscopic changes in aged soot particles could further improve the current climate models and evaluations of BC's radiative impacts on the eastern TP and similar remote air.