Recent laboratory data on ice freezing from aqueous H2SO4 solutions are used to update our homogeneous ice freezing nucleation algorithm. The effect of particle size on the ice freezing process is demonstrated by calculating and comparing ice freezing curves for different size particles ranging from 0.2 to 4 micron in radius. Using the ice nucleation model we show that liquid water saturation is required above -44 ¿C to activate submicron H2SO4 particles into cloud droplets. A thermodynamic model is used to show that the available laboratory data on ice freezing from ternary H2SO4/HNO3/H2O solutions are insufficient to adequately address the effect of HNO3 uptake on the homogeneous freezing of aqueous H2SO4 particles into ice. ¿ 2000 American Geophysical Union