r ins r cir The sphericity distribution of the NMC parti-cle population has a median value of 0.67 (Fig. ), with a substantial fraction (45%) around 0.7-0.8. This makes the shape of the particles closer to the value of a cube (0.71) than a sphere. Their roundness R is defined as the ratio of the average radius of curvature of the corners of the object's silhouette to the radius of the maximum inscribed sphere :