Vertical τ_z = 1 emission surfaces for the two transitions (J = 2 − 1, J = 3 − 2) of each of the three isotopologues (¹²CO,¹³CO,C¹⁸O) in the initial condition for our fiducial protoplanetary disk. The black solid lines indicate simulation boundaries (in this case, r_out = 100 au), while the dashed gray lines indicate wave-damping regions in the hydrodynamical simulations. The sharp transition in the ¹³COJ = 3 − 2 τ_z = 1 surface is caused by freeze-out of the immediately overlying material at large radii.