Schematic representation of the CO₂ abundance near the iceline in the r–z plane. Panels B, C and D approximately represent the top half of the blue box in panel A. The straight green lines denote the τ = 1 surface for ¹²CO₂ (solid) and ¹³CO₂ (dashed). Panels B, C and D: the abundance structure under different mechanism to explain the difference in CO₂ abundance between the predictions of the viscous model and the IR observations. Panel B: the case for a constant gas-phase destruction rate due to some as yet unidentified process (Sect. 4.4). Panel C: the abundance structure for deep UV/X-ray penetration (Sects. 5.2.5 and 5.2.6). In more extreme cases, the ¹³ CO ₂ τ = 1 surface can also be in the UV/X-ray dominated region. Panel D: the abundance structure for negligible vertical mixing (Sect. 5.2.1). Image not to scale.