Left, top: gas-to-dust ratio in the line-emitting region of each species, over time. The significant bump seen in the gas-to-dust ratio of OH is misleading. Our measurement of the line-emitting area is biased by the fact that there is a small amount of OH emission coming from warm upper layers at relatively large radii. These upper layers have a disproportionately high gas-to-dust ratio. Left, middle: the line-emitting area, in square astronomical units, of each species overtime. This area is calculated from a face-on perspective, assuming that the line emission comes from an annulus encompassed by the inner and outer radii of the line emission (x15 and x85). Much like the gas-to-dust ratio for less-evolved models, the line-emitting area calculation of OH is biased by small amounts of emission at larger radii. Left, bottom: the escape probability line flux of each species over time. The dashed horizontal line indicates the nominal 1σ Spitzer sensitivity of 8.5 × 10⁻¹⁹ W m⁻² achievable with a 512 second integration. Right, top: the density-averaged gas temperature in the line-emitting region of each species over time. Right, middle: the density-averaged vertical optical depth of the dust at 20 μm in the line-emitting region of each species over time.