CO depletion factors as a function of extinction based mass surface density for IRDC C (left) and IRDC F (right). The black crosses show the depletion factors derived assuming no temperature offset between the ¹³CO and ¹²CO while the blue ×’s are derived assuming that ¹³CO traces gas 4 K colder than the ¹²CO. Triangles indicate that the depletion factor was lower than 0.1. The y-axes are not the same for the two panels, with IRDC C having larger derived depletion factors.