Comparison of modeled (black) and observed (red) optical depth cross-sections across (a), c), e)) and along (b), d), f)) the trail for the 2006 Spitzer observation of the 67P trail described in Agarwal et al. (2010) for three velocity models (a), b) Crifo & Rodionov 1997; c), d) Kelley et al. 2008; e), f) Agarwal et al. 2010). Panels in (a), c), e)) refer to the angular distance (x, as used by Agarwal et al. 2010) from the comet along the trail at which the cross-section cut is taken. For (b), d), f)) the positive direction is the trailing direction of the comet movement. For (a), c), e)) the cut is perpendicular to the projected comet orbit, with positive values in the direction of positive declination. The Spitzer observations discussed in Agarwal et al. (2010) are limited to within ~ 30′ of the comet.