Model of volatile loss from the disk atmosphere. Left-hand panel: a fraction Δ_X of volatile atoms are lost in the midplane during each of N_mix mixing cycles. Equation (3) is shown for a depletion X₀/X₁ = 100 (solid curve). A theoretical turbulent mixing timescale gives N_mix = 200 (black dashed line) and Δ_X = 2 × 10^-2 (dotted line), consistent with an upper limit of v_turb ≤ 40 m s^-1 on the speed of turbulent mixing in TW Hya, which gives N_mix ≤ 1300 (gray dashed line, Hughes et al. 2011). Right-hand panel: Δ_X from Eq. (4), taking a_max = 10 cm and a_min = 0.001, 0.005, or 0.05 μm (solid lines). The smallest grain size settled below the CO freeze-out height at r = 100 au in our TW Hya model is also shown (dashed line, Eq. (5)).