Anthropogenic release of aerosols into the upper troposphere has raised questions regarding a possible increase in efficiency of cirrus cloud formation via heterogeneous ice nucleation, which would have corresponding effects on the Earth's radiative budget. Previous studies have shown that ice nucleation depends on both the composition of the aerosol acting as an ice nucleus, as well as the mechanism of nucleation. Several laboratory studies have shown that contact freezing, where an ice nucleus is in contact with the surface of a pure water droplet, occurs at roughly 10-15 K higher than does homogeneous ice nucleation of pure water, and also roughly 2-3 K higher than immersion freezing. Other experiments have focused on deposition freezing, where ice forms directly onto a dry aerosol particle, and have concluded that the efficiency of this kind of nucleation is dependent upon the physical and chemical composition of the aerosol particle. Using our recently developed ice nucleation microscope apparatus, we can observe immersion, contact, and deposition freezing events. In this study, the aerosols tested for ice nucleation activity include soot, ash, and humic-like materials. Using an Olympus BX51M microscope equipped with a Linkam LTS-350 cold stage and digital imaging devices, we idenitify the freezing temperatures of ice nuclei as a function of nuclei composition and freezing mechanism. The results will be presented and atmospheric implications will be discussed.