2D histograms of the projected radial particle accelerations, a_rad, as a function of particle radius (for the selected particles, see Fig. 4). The histogram values were determined analogously to the description in Fig. 12. Also shown are the predicted surface acceleration, a_s (dotted line, see also Fig. 11), and two models (dashed and solid curves) of the particle acceleration, as described by Eqs. (8) and (9). The parameters for both curves are based on our gas models (see Sect. 4.5), but for the solid curve, a local gas production rate was assumed that is about five times higher.