We report the kinetically constrained phase diagram for nitric acid-water aerosols' over the range of nitric acid mole fractions from 0 to 0.5, measured using a temperature-programmable cryogenic flow tube. The freezing temperatures of the aerosol particles, which have radii on the order of I mum, are 30-90 K lower than those of the bulk, and the liquid-solid phase boundaries are modified by the differences in the activation energies for nucleation of the nitric acid hydrates. We infer from the shapes of the respective supersaturation curves that water ice nucleates in the liquid droplets at nitric acid mole fractions below about 0.15, and nitric acid trihydrate grows on these ice nuclei. At higher concentrations, nitric acid dihydrate crystals nucleate, and either ice or nitric acid trihydrate crystals grow on the nuclei. Using realistic estimates of the nucleation rate constants, we conclude that ice will nucleate homogeneously in upper tropospheric clouds having nitric acid mole fractions as high as 0.1 if the temperature, decreases to 2 10 K.