Ratio of CO gas surface density to total gas surface density after 1 Myr in the region of the ice line. Shown are the fiducial model with α = 10^-3 (solid green line), a low-viscosity model with α = 10^-4 (dashed blue), and a high-viscosity model with α = 10^-2 (dotted purple). The ratio of is plotted with a dashed black line. Also plotted is the case of a disk without viscous spreading (red dash-dotted line). The horizontal gray line is our input value for the CO-to-gas mass ratio.