The formation and sedimentation of nitric acid-aerosols (NA-aerosols) in the winter polar stratosphere is investigated in the light of recent mass spectrometric in-situ measurements of gaseous nitric acid performed in the winter arctic stratosphere. We revisit the nitric acid measurements and extend their interpretation by carrying out isentropic trajectory calculations of the airmasses encountered in these experiments and by considering aerosol measurements, which became available recently. Our analysis shows that the mass spectrometer measurements provide strong evidence for nitric acid due to sedimentation of HNO3-containing aerosols. Both these processes promote halogen-catalysed destruction of ozone. Moreover, it is concluded that nitric acid nucleation is less efficient than expected from simple equilibrium considerations. Probably, this is due to rapid cooling and inefficient nucleation (inefficient activation of preexisting H2SO4/H2O-aerosols for HNO3/H2O-condensation) allowing only a small fraction of the preexisting H2SO4/H2O-aerosols to serve as active nuclei. ¿ American Geophysical Union 1992 ¿ American Geophysical Union 1992