Mineral dusts inside aqueous atmospheric particles provide surfaces that induce crystallization during episodes of decreasing relative humidity (RH). Submicron aqueous ammonium sulfate particles containing hematite (&agr;-Fe2O3) and corundum (&agr;-Al2O3) inclusions are investigated in an aerosol flow tube at 298 K. As compared to 35% RH where homogeneous nucleation is rapid, the heterogeneous nuclei regulate the RH from 35% up to 60% RH as the inclusion size varies from 50 to 450 nm. The strong size dependence can be rationalized by an active site model. Model optimization yields 1010.4 sites cm-2 and m<0 for &agr;-Al2O3 and 109 sites cm-2 and m=0.04 for &agr;-Fe2O3 particles. ¿ 2001 American Geophysical Union