Most chemicals in our study can absorb water vapor, increasing in volume by a factor of 1.6 -4.1 at 85% RH (Supplementary Table ). For most atmospheric particles this hygroscopic growth is partially suppressed by the positive curvature from the Kelvin effect, but in fractal soot the curvature is negative in the gaps between primary spheres, promoting capillary condensation of water, and resulting in a higher growth factor by volume, Gfv. To compute the equilibrium fraction of absorbed water in menisci, we used an iterative procedure (Supplementary section 7), similar to the previous work. The predicted dependence of the humidified filling angle 𝜃 2 on RH (Supplementary Fig. )