We present data to show that at least some of the observed polar stratospheric cloud (PSC) particles sampled by the ER 2 on January 20, 1989, during the Airborne Arctic Stratospheric Expedition (AASE 1) were composed of a water-rich HNO3/H2O solid phase. The PSC water content derived from the ER 2 data on this day is larger than that of nitric acid trihydrate (NAT), nitric acid dihydrate (NAD) or an aqueous ternary solution of H2SO4/HNO3/H2O. Here, we suggest that these particles were composed of a water-rich metastable HNO3/H2O solid phase and refer to such clouds as Type 1c PSCs, which are different from Type 1a (crystalline NAT or NAD particles) or Type 1b (aqueous ternary solution droplets) PSCs. Type 1c PSCs could either be crystalline (a higher hydrate of HNO3 and H2O) or amorphous, and the data analysis presented here cannot distinguish between these different solid phases. The observed PSC on this day could have been a mixture of water-rich HNO3-containing solid particles with liquid ternary droplets and/or NAT (NAD) aerosols. However, a mixture of cloud particles composed of ternary solution droplets and NAT (NAD) aerosols is inconsistent with the data. A surface adsorption/reaction process occurring on frozen sulfate surfaces is suggested as a formation mechanism for Type 1c PSCs in the stratosphere. A possible mechanism for the formation of large HNO3-containing aerosols (NAT or NAD) starting with Type 1c PSCs is discussed. ¿ American Geophysical Union 1996