| Abstract
| - Heterogeneous freezing of aqueous particles with solid inclusions of crystallized (NH4)2SO4, ice, and letovicitewere studied using optical microscopy and differential scanning calorimetry. For (NH4)2SO4−H2O particles,the heterogeneous freezing temperature was found to be dependent on the morphology of the (NH4)2SO4solid. If the crystallized solid was in the form of microcrystals, the heterogeneous ice-freezing temperaturewas 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 closeto the homogeneous freezing temperature. For particles with one or two large (NH4)2SO4 crystals in equilibriumwith (NH4)2SO4−H2O solution, we have estimated an upper limit of 1.5 × 10-5 s-1 μm-2 for Jhet (heterogeneousnucleation rate of ice, immersion freezing mode). Our results for NH4HSO4−H2O particles show that whenone or two large crystals of either ice or letovicite are present in the solution, the freezing temperature doesnot deviate significantly from the homogeneous freezing temperature, consistent with the (NH4)2SO4−H2Oexperiments. Our work shows that the surface area and surface microstructure of crystalline solids present inaqueous aerosols can significantly change the heterogeneous freezing temperature and critical ice saturationsand that heterogeneous ice nucleation induced by crystalline salts may be very important in the formation ofupper tropospheric clouds.
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