Source of nitrogen isotope anomaly in HCN in the atmosphere of Titan

The N-14/N-15 ratio for N-2 in the atmosphere of Titan was recently measured to be a factor of 2 higher than the corresponding ratio for HCN. Using a one-dimensional photochemical model with transport, we incorporate new isotopic photoabsorption and photodissociation cross sections of N-2, computed quantum-mechanically, and show that the difference in the ratio of N-14/N-15 between N-2 and HCN can be explained primarily by the photolytic fractionation of (NN)-N-14-N-14 and (NN)-N-14-N-15. The [(HCN)-N-14]/[(HCN)-N-15] ratio produced by N-2 photolysis alone is 23. This value, together with the observed ratio, constrains the flux of atomic nitrogen input from the top of the atmosphere to be in the range (1-2) x 10(9) atoms cm(-2) s(-1).