Heterogeneous freezing of aqueous particles induced by crystallized (NH4)2SO4, ice, and letovicite

Heterogeneous freezing of aqueous particles with solid inclusions of crystallized (NH4)(2)SO4, ice, and letovicite were studied using optical microscopy and differential scanning calorimetry. For (NH4)(2)SO4-H2O particles, the heterogeneous freezing temperature was found to be dependent on the morphology of the (NH4)(2)SO4 solid. If the crystallized solid was in the form of microcrystals, the heterogeneous ice-freezing temperature was close to the eutectic temperature and the critical saturation with respect to ice was close to 1. However, if the solid was in the form of one or two large crystals, the heterogeneous freezing temperature was close to the homogeneous freezing temperature. For particles with one or two large (NH4)(2)SO4 crystals in equilibrium with (NH4)(2)SO4-H2O solution, we have estimated an upper limit of 1.5 x 10(-5) s(-1) mum(-2) for J(het) (heterogeneous nucleation rate of ice, immersion freezing mode). Our results for NH4HSO4-H2O particles show that when one or two large crystals of either ice or letovicite are present in the solution, the freezing temperature does not deviate significantly from the homogeneous freezing temperature, consistent with the (NH4)SO4-H2O experiments. Our work shows that the surface area and surface microstructure of crystalline solids present in aqueous aerosols can significantly change the heterogeneous freezing temperature and critical ice, saturations and that heterogeneous ice nucleation induced by crystalline salts may be very important in the formation of upper tropospheric clouds.