Abundance of CO₂, CO, and O₂ ice relative to water ice as a function of radius and time for the protoplanetary disk model with evolving physical conditions and using fully atomic initial abundances (left) and fully molecular initial abundances (middle). Note that the data for CO₂ are shown on a linear scale as opposed to logarithmic because of the low dynamic range in the chemical model results. The right-hand column shows the corresponding values measured for each species in cometary comae. The vertical dashed lines are included solely to guide the eye. Oort cloud comets and Jupiter family comets are represented by the squares and circles, respectively. For comets without a stated observed error (or range), we have assumed a conservative error of 50% of the observed ratio with respect to H₂O ice.