Schematic of the vertical CO₂ abundance (top panel) and upwards column (bottom panel) as function of disk height. Three different cases areshown in solid black, dashed dark blue and dotted blue lines. The black line shows the case for shallow UV or X-ray penetration, in this case an abundance similar to the midplane CO₂ abundance will be measured. The dashed dark blue line shows a slightly deeper penetration of radiation, in which case a lower CO₂ abundance will be inferred from the observations because the gas is optically thick and hiding the higher abundance deeper into the disk. The dotted blue line shows the case of deep penetration. In this case a low CO₂ abundance will be inferred due to the dust hiding the bulk of the CO₂. The vertical lines in the upper panel show where the disk becomes optically thick to CO₂ lines, the solid and dashed grey line, show where the gas becomes optically thick for the small and intermediate PDR or XDR respectively, the green dotted line shows where the dust becomes optically thick, which is above where the gas becomes optically thick for the large PDR or XDR.