Line-to-continuum for ¹²CO₂ as function of abundance for different gas-to-dust ratios in solid lines. In the bottom plot the line-to-continuum for ¹³CO₂Q-branch is shown in dashed lines. A dotted black line shows a line-to-continuum of 0.01, lines with this line-to-continuum ratio can be observed if the signal-to-noise (S/N) on the continuum is more than 300. With a S/N on the continuum of 300, observations of the Q-branch should be able to probe down to 10^-9 in abundance for a gas-to-dust ratio of 1000. With similar gas-to-dust ratio and S/N, individual lines the 15 μm band will only be observable in disk with CO₂ abundances higher than 3 × 10^-8.