On the contrary, as predicted by MCCCM, capillary condensation on fractal soot begins already at 7 am (Fig. ), and by 9 am there is sufficient amount of condensate to make nearly 10% of soot particles CCN-active (Fig. ). The prompt condensation on fractal soot at low 𝑆 tot is a result of the vapor pressure depression from the negative meniscus curvature (Supplementary Fig. ), giving such fractal particles a significant advantage over spherical particles. The volume fraction of condensate closely follows 𝑆 tot and peaks at 0.34 around 7 pm, when nearly 80% of fractal soot particles become CCN. Although in later hours some of the condensate is lost by evaporation, a significant fraction of particles remain cloud active. Notably, when the filling angle reaches 45° at 4 pm (Fig. ), fractal aggregates are expected to begin restructuring (Fig. ), becoming significantly compacted by 7 pm when the filling angle reaches 80°. The partial evaporation of the condensate during late afternoon would convert these aggregates into highly compact globules. Although described as solid spheres in aerosol models, such compact globules possess appreciable void space that can be treated as pores and hold significant amount of condensate. If such atmospheric soot were sampled in the afternoon, it would be found to consist of soot particles that are compact, but lack visible coating, as has been reported earlier in field studies conducted at various locations. Notably, the morphological and CCN behavior similar as predicted by our MCCCM model has been observed in chamber experiments with fractal soot aged by OH-initiated photochemical oxidation of monoethanolamine. Intermediate volatility oxidation products of monoethanolamine underwent reversible capillary condensation, causing a partial collapse of soot aggregates at a condensate mass fraction of 17%. The CCN activity of such particles followed the amount of condensate, increasing initially and then decreasing when the condensate partially evaporated. Another remarkable feature observed in our simulation is that after midnight, despite the lower average amount of condensate (Fig. ) fractal soot is more CCN active than spherical soot (Fig. ). This is because for fractal particles the fraction of condensate does not vary with the size of aggregates, whereas for spherical particles it decreases with increasing particle size, making larger spherical particles relatively less hygroscopic. Since cloud activation depends both on particle size and amount of water-soluble material, fractal soot produces more CCN.