Black carbon aerosols absorb radiation and their absorptive strength is influenced by particle mixing structures and coating compositions. Liquid-liquid phase separation can move black carbon to organic particle coatings which affects absorptive capacity, but it is unclear which conditions favour this redistribution. Here we combine field observations, laboratory experiments, and transmission electron microscopy to demonstrate that liquid-liquid phase separation redistributes black carbon from inorganic particle cores to organic coatings under a wide range of relative humidity. We find that the ratio of organic coating thickness to black carbon size influences the redistribution. When the ratio is lower than 0.12, over 90% of black carbon is inside inorganic salt cores. However, when the ratio exceeds 0.24, most black carbon is redistributed to organic coatings, due to a change in its affinity for inorganic and organic phases. Using an optical calculation model, we estimate that black carbon redistribution reduces the absorption enhancement effect by 28–34%. We suggest that climate models assuming a core-shell particle structure probably overestimate radiative absorption of black carbon aerosols by approximately 18%.

Ambient atmospheric aerosol particles comprised of various inorganic and organic substances ubiquitously undergo phase transition, such as efflorescence, amorphization, and especially liquid-liquid phase separation (LLPS). Resultant changes of physicochemical properties in aerosols then deeply affect the climate system. However, finely detecting these processes occurring in single aerosol particles, especially under the acidic condition of real atmospheric environment, remains a challenge. In this work, we investigated the pH-dependent phase separation in single levitated microdroplets using a self-developed laser tweezers Raman spectroscopy (LTRS) system. The dynamic process of LLPS in laser-trapped droplets over the course of humidity cycles was detected with the time-resolved cavity-enhanced Raman spectra. These measurements provide the first comprehensive account of the pH-dependent LLPS in single levitated aerosol microdroplets and bring possible implications on phase separation in actual atmospheric particles.

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