Radial intensity and spectra of observed H₂CO 3₀₃–2₀₂ and C¹⁸O 2–1 versus the best-fit models. Left: radial intensity curves from azimuthally-averaged elliptical annuli projected to i = 44°, PA = 133°. HD 163296 data is show in black, best-fits for H₂CO and C¹⁸O are in gold. The vertical dashed lines indicate the CO snow line (blue dash) from Qi et al. (2015), the 5σ outer radius of the 1.3 mm grains (black dash), and the change-over radii, R_c, for the best-fit radial step-abundance models (gold dash). H₂CO profiles are taken from integrated intensity maps after applying a Keplerian mask. Right: disk-integrated spectra. HD 163296 data is shown in filled gray. H₂CO spectra are Hanning smoothed to 0.336 km s^-1 channels.