Midplane models of protoplanetary disks Ðnd that the cold temperatures in the outer parts of the disk ensure that virtually all molecules are accreted onto the grains. However, molecules in the gas are observed at these radii. One possible explanation is that the emission arises from above the midplane, possibly in a heated layer at the surface of a Ñared disk. Models which take into account the vertical chemical distribution of molecules and can calculate column densities are therefore

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... for comparison with observations. We present the results of a calculation of the time-dependent two-dimensional chemical structure of a Ñared protoplanetary disk which includes photoprocesses driven by both the stellar and interstellar radiation Ðelds. Three layers are found in the disk consistent with previous work. In the upper layer photodissociation produces large abundances of atoms and ions. Below this molecules are shielded and can avoid dissociation, although sufficient radiation is present to remove molecules from the grain surfaces by photodesorption. The majority of the observable species come from this layer. Closer to the midplane of the disk, freezeout removes molecules from the gas. We Ðnd that photodesorption can account for the observed column densities if the photodesorption yield is higher than 10~3 molecules per photon. These results indicate that many observed molecules trace the physical and chemical conditions in the surface regions rather than the midplane although the contribution of the heated surface layer to the column densities is minimal.