Recent results obtained during the SUCCESS experiment show that aerosols containing ammonium ion are commonly found in the upper troposphere and therefore it is of great importance to investigate the ice freezing properties of such aerosols. For this purpose we present mathematical expressions, derived from recent model calculations and laboratory data, for calculating the deliquescence relative humidity of (NH4)2SO4, NH4HSO4 and NH4NO3 under upper tropospheric conditions. Using these functions we define the temperature and relative humidity requirements for the deliquescence of dry particles in the upper troposphere. Further using a thermodynamic model of the H2SO4/HNO3/NH3/H2O system, we calculate the variation in the supercooled aerosol composition and saturation ratios of various ionic salts in solution as a function of temperature and relative humidity. Using these values we show salt crystallization in the upper troposphere may occur in two distinct type of ammoniated aerosol solutions. First (NH4)3H(SO4)2 (letovicite) may crystallize in acidic solutions of (NH4)2SO4. Second double salts of (NH4)2SO4 and NH4NO3 may crystallize in nearly neutralized solutions of (NH4)2SO4 and NH4NO3. Finally we discuss how the presence of crystalline salt surfaces in aqueous solutions may alter the mode of cirrus cloud formation from homogeneous to heterogeneous freezing nucleation. ¿ 1998 American Geophysical Union