The synergy of morphology and mixing state on the absorption of coated black carbon soot

Abstract. Black carbon (BC) strongly absorbs light and contributes to climate warming. Coating formation on fractal BC-containing soot is a ubiquitous atmospheric process that enhances its absorption, but the extent of this absorption enhancement (E_abs) remains uncertain and difficult to represent in models. In this study, we use large simulation chamber experiments to investigate BC absorption and its modification by coating formation and ageing. Polydisperse fractal BC-containing soot was mixed with non-absorbing atmospherically-relevant secondary aerosols (sulfuric acid and α-pinene secondary organic aerosols). Varying reaction dynamics produced different coating rates and masses, mimicking atmospheric processing. The evolution of soot and coating properties was tracked for up to 20 hours. The results of this study suggest a general positive correlation between E_abs and coating amount on soot. However, they also experimentally demonstrate the strong effect of morphological modifications and mixing state heterogeneity on BC absorption, both depending on the reaction dynamics of coating formation and ageing of the coated soot. Combined variations in morphology and mixing state affect the magnitude of E_abs and its further evolution, leading, depending on processing, to either an increase or decrease in absorption. The range of measured E_abs resulting from soot processing cannot be reliably predicted using a fixed E_abs value or by a core-shell optical model, two approximations commonly used in climate models. These results support the need for new formalisms that would take into account both morphological and mixing state heterogeneity to improve model predictions of BC absorption and its role on the radiative budget.