Evolution of the spectral optical properties of black carbon soot due to coating and ageing: insights from simulation chamber experiments

Black carbon (BC) soot aerosol, produced during the incomplete combustion of fossil fuels, biofuels, and biomasses, is a major light-absorbing species and a key climate forcer. Despite its importance, BC remains challenging to represent in models due to persistent uncertainties in its spectral optical properties. In particular, the formation of non-absorbing coatings on fractal BC soot is a ubiquitous atmospheric process that enhances absorption, yet the magnitude of this enhancement (Eabs) remains highly uncertain and poorly represented in current models.In order to advance on this topic, a set of experiments were performed using the 4.2 m3 CESAM simulation chamber on BC-soot aerosol generated from a propane diffusion flame. Experiments were conceived to systematically investigate the impact of coating formation and further ageing on soot spectral optical properties. Two chemical systems inducing the formation of a coating by a second scattering aerosol phase produced via the photo-oxidation of SO2 and the ozonolysis of α-pinene were considered.The resulting dataset quantifies the magnitude and variability of Eabs under varying conditions, highlighting its dependence on soot morphology, soot–coating structure, and particle-to-particle heterogeneous mixing state. We show that the relative importance of these factors evolves with the dynamics of coating formation and ageing. Importantly, the Eabs cannot be reliably predicted using a fixed value or simple core–shell optical models, as commonly assumed in climate simulations.