Mass of solid carbon needed to accrete into the atmosphere to bring C/H up to ~ 4 × C/H_solar. Also shown is the mass fraction of carbon relative to the refractories (M_C∕M_ref) for the solidswithin the CO₂ ice line (r < 1.4 AU); between the CO₂ ice line and the point where CO is converted to CO₂ ice (at T_gas ≲ 40 K, 1.4 AU < r < 8 AU); from there to the CO ice line (8 AU < r < 32AU); and outward of the CO ice line (r > 32 AU).