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Observations have revealed that the elemental abundances of carbon and oxygenin the warm molecular layers of some protoplanetary disks are depleted comparedto those is the interstellar medium by a factor of ~10-100. Meanwhile, littleis known about nitrogen. To investigate the time evolution of nitrogen, carbon,and oxygen elemental abundances in disks, we develop a one-dimensional modelthat incorporates dust settling, turbulent diffusion of dust and ices, as wellas gas-ice chemistry including the chemistry driven by stellar UV/X-rays andthe galactic cosmic rays. We find that gaseous CO in the warm molecular layeris converted to CO2 ice and locked up near the midplane via the combination ofturbulent mixing (i.e., the vertical cold finger effect) and ice chemistrydriven by stellar UV photons. On the other hand, gaseous N2, the main nitrogenreservoir in the warm molecular layer, is less processed by ice chemistry, andexists as it is. Then the nitrogen depletion occurs solely by the vertical coldfinger effect of N2. As the binding energy of N2 is lower than that of CO andCO2, the degree of nitrogen depletion is smaller than that of carbon and oxygendepletion, leading to higher elemental abundance of nitrogen than that ofcarbon and oxygen. This evolution occurs within 1 Myr and proceeds further,when the $\alpha$ parameter for the diffusion coefficient is ~0.001.Consequently, the N2H+/CO column density ratio increases with time. How thevertical transport affects the midplane ice composition is briefly discussed.

Different degrees of nitrogen and carbon depletion in the warm molecular layers of protoplanetary disks