Satellite observations of denitrification and ice clouds in the Arctic lower stratosphere in February 1997 are used with Lagrangian microphysical box model calculations to evaluate nucleation mechanisms of solid polar stratospheric cloud (PSC) particles. The occurrences of ice clouds are not correlated in time and space with the locations of back trajectories of denitrified air masses, indicating that ice particle surfaces are not always a prerequisite for the formation of solid PSCs that lead to denitrification. In contrast, the model calculations incorporating a pseudo-heterogeneous freezing process occurring at the vapor-liquid interface can quantitatively explain most of the observed denitrification when the nucleation activation free energy for nitric acid dihydrate formation is raised by only ~10% relative to the current published values. Once nucleated, the conversion of nitric acid dihydrate to the stable trihydrate phase brings the computed levels of denitrification closer to the measurements.