Polar stratospheric clouds (PSCs) were simulated via a mixture of (1) liquid-ternary-solution particles consisting of H2SO4, HNO3, and H2O, and (2) solid nitric acid trihydrate (NAT) particles in a microphysical box model. The simulations indicated that the scattering ratio increases as the temperature decreases to near the frost point of ice without a concomitant increase in the depolarization. When the decrease in temperature was halted a few degrees above the frost point, the time-dependent depolarization was subsequently observed to increase. This response was found when the total number of solid particles was in the finite range, 0.001--0.01/cm3. This number range agrees well with the (large size) particle number concentration observed via an optical particle counter. ¿ 1999 American Geophysical Union