The size effect of hematite and corundum inclusions on the efflorescence relative humidities of aqueous ammonium sulfate particles

Mineral dusts inside aqueous atmospheric particles provide surfaces that induce crystallization during episodes of decreasing relative humidity (RH). Submicron aqueous ammonium sulfate pal-tides containing hematite (alpha -Fe2O3) and corundum (alpha -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 10(10.4) sites cm(-2) and m < 0 for alpha -Al2O3 and 10(9) sites cm(-2) and m = 0.04 for alpha -Fe2O3 particles.