Test of depleting carbon and oxygen abundances in the PDS 70 disk using C¹⁸O emission. Each panel represents a different model, and their relevant parameters are noted. Panels (a) and (c) use the fiducial critical surface density as in the preferred model in the main text. In panels (b) and (d), the critical densities are enhanced by the same factor as the abundances of carbon and oxygen were depleted. A depleted abundance is clearly inconsistent with observations.