Grain lifetimes regarding sublimation: pure ice grains (solid line); three-layer grains with a fractional mass of the mantle component α_ice = 0.90 (dash-dotted-dotted line); two-layer (Ol) grains with a core of amorphous silicate and a mantle of water ice with α_ice = 0.98 (long-dashed line); two-layer (C) grains with a core of amorphous carbon and a mantle of water ice with α_ice = 0.98 (dash-dotted line). The dash-dotted-dotted line and the dash-dotted line are nearly identical. The grain residence time within the 9.25′′  ×  9.25′′ FOV is shown by the red line. We assumed v_ref = 230 m s^-1 and q_v = 0.4.