Redistribution of black and brown carbon in aerosol particles undergoing liquid-liquid phase separation

Abstract:

Atmospheric black carbon (BC) and to a lesser degree brown carbon is a major anthropogenic greenhouse agent, yet substantial uncertainties obstruct understanding its radiative forcing. Particularly debated is the extent of the absorption enhancement by internally compared to externally mixed BC, which critically depends on the interior morphology of the BC-containing particles. Here we suggest that a currently unaccounted morphology, optically very different from the customary core shell and volume-mixing assumptions, likely occurs in aerosol particles undergoing liquid-liquid phase separation (LLPS). Using Raman spectroscopy on micrometer-sized droplets, we show that LLPS of an organic/inorganic model system drives redistribution of BC into the outer (organic) phase of the host particle. This results in an inverted core-shell structure, in which a transparent aqueous core is surrounded by a BC-containing absorbing shell.

We also study the redistribution of a model proxy for brown carbon, carminic acid, in single, levitated aqueous aerosol particles undergoing LLPS and compare the measured absorption efficiency with corresponding Mie calculations.