Nitric acid–containing particles with diameters of 10–20 μm were detected inside the Arctic polar vortex in the period January to March 2000. We present the results of a unique three-dimensional microphysical simulation of these large HNO3-containing particles covering the entire Arctic vortex. The model describes the simultaneous growth, evaporation, sedimentation, and advection of several thousand individual nitric acid hydrate particles over their complete lifetime. We compare modeled and observed particle size distributions as a test of different particle nucleation mechanisms. The model is able to produce particles with sizes typical of those observed and broadly reproduces the change in particle characteristics through the winter assuming nitric acid trihydrate (NAT) particle growth. The possibility that the observed large nitric acid–containing particles were composed of nitric acid dihydrate (NAD) cannot be excluded within the uncertainty of the HNO3 field above the aircraft. The formation of nitric acid hydrate particles on synoptic ice clouds may be a source of some of the observed large nitric acid–containing particles. However, a direct, but highly selective, nucleation of NAT or NAD particles over wide regions appears to be necessary to explain the observations.