Gas-phase CO in protoplanetary disks: a challenge for turbulent mixing
  This is the first paper in a series where we study the influence of turbulent diffusion and advective transport on the chemical evolution of protoplanetary disks, using a 2D flared disk model and a 2D mixing gas-grain chemical code with surface reactions. A first interesting result concerns the abundance of gas-phase CO in the outer regions of protoplanetary disks. In this Letter we argue that the gas-phase CO concentration in the disk regions, where the temperature is lower than $\sim25$ K, can be significantly enhanced due to the combined effect of vertical and radial mixing. This finding has a potential implication for the current observational data on the DM~Tau disk chemistry. 
D. Semenov, D. Wiebe, Th. Henning