N₂H⁺J = 4−3 /J = 3−2 (left panels) and J = 4−3 /J = 1−0 line ratios (right panels) for three different CO and N₂ abundances, as indicated in the leftmost panels, in models with large grains settled to 20% (column one and three) or 80% of the small grain scale height (column two and four). Dashed lines show the snow surface only models, while solid lines represent the full models. The simulated emission is either convolved with a (black lines) or beam (red lines).