  An abundance decrease in carbon- and oxygen-bearing species relative to dust
has been frequently found in planet-forming disks, which can be attributed to
an overall reduction of gas mass. However, in the case of TW Hya, the only disk
with gas mass measured directly with HD rotational lines, the inferred gas mass
($\lesssim$0.005 solar mass) is significantly below the directly measured value
($\gtrsim$0.05 solar mass). We show that this apparent conflict can be resolved
if the elemental abundances of carbon and oxygen are reduced in the upper
layers of the outer disk but are normal elsewhere (except for a possible
enhancement of their abundances in the inner disk). The implication is that in
the outer disk, the main reservoir of the volatiles (CO, water, ...) resides
close to the midplane, locked up inside solid bodies that are too heavy to be
transported back to the atmosphere by turbulence. An enhancement in the carbon
and oxygen abundances in the inner disk can be caused by inward migration of
these solid bodies. This is consistent with estimates based on previous models
of dust grain dynamics. Indirect measurements of the disk gas mass and disk
structure from species such as CO will thus be intertwined with the evolution
of dust grains, and possibly also with the formation of planetesimals.
