Laboratory measurements are presented for the freezing kinetics of H2O/HNO3 aerosols over the temperature range of 188--204 K. For 2:1 H2O:HNO3 aerosols crystallizing to NAD we observed a maximum nucleation rate of J=9.3¿109 cm-3 s-1 at 194 K. This temperature is between the glass point of 161 K <Ji et al., 1993> and the melting point of 235.5 K <Ji et al., 1996>. This can be compared to a previous measurement of J=6.7¿109 cm-3 s-1 at 193 K <Disselkamp et al., 1996> and lower temperature measurements of J≈1010-1012 cm-3 s-1 at 178.8-175.8 K <Bertram and Sloan, 1998a>. Measured nucleation rates decrease as the aerosol becomes dilute, but NAD formation is still observable for 2.5:1 H2O:HNO3 at temperatures near 195 K. In contrast, freezing of 3:1 H2O:HNO3 aerosol was not observed for constant temperature experiments throughout this temperature range, yielding an upper limit of J<1.5¿109 cm-3 s-1. This is the lowest experimental value determined for 3:1 H2O:HNO3 freezing rates at these temperatures. From the measured freezing rates and knowledge of the free energy of diffusion the average interfacial free energy for NAD in a 2:1 H2O:HNO3 solution was determined to be &sgr;=25.2 ergs cm-2. A limit for the interfacial free energy was placed on 3:1 H2O:HNO3 particles, for which freezing was not observed. These data imply that if aerosols reach compositions more concentrated than 3:1 H2O:HNO3 in the atmosphere, NAD may play a role in polar stratospheric cloud formation. ¿ 1998 American Geophysical Union