Modeled optical depth cross-sections along the trail for the 2004 (a), b)); and 2006 (c), d)) Spitzer observation of the 67P trail (Kelley et al. 2008; Agarwal et al. 2010) using the velocity model of Agarwal et al. (2010). (a), c)) show the contribution of particles of different masses to the total optical depth; (b), d)) show the contribution of particles created during different comet apparitions to the total optical depth. The positive direction is the trailing direction of the comet movement.