Collision probability including 3D effects between a dust particle of radius s (x-axis, in centimeters) and a planetesimal of radius R_p (y-axis, in kilometers), at 1AU with the same conditions as Fig. 15. The values of range from 10^-18 (very inefficient accretion) to almost 1 (perfect accretion). The probabilities are much lower than in Fig. 15 primarily because of vertical (turbulent) diffusion of dust particles.