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Ice Nucleation
The process of forming a snowflake involves two steps: an initial nucleation phase, followed by a subsequent growth phase. Ice nucleation can happen homogeneously or heterogeneously depending on the temperature and the presence of other particles within a cloud. Regardless of the type of nucleation, larger sized droplets nucleate into ice crystals easier than smaller sized droplets. Once the process of ice crystal formation has occurred, the growth of an ice crystal depends on factors such as temperature, relative humidity, and the difference in the saturation vapor pressure of water and ice (see Wegener-Bergeron-Findeisen process).

Homogenous Nucleation of Ice Crystals
Homogenous nucleation is the formation of an ice nucleus within a water droplet that does not contain an aerosol. While the microphysical and chemical details of ice nucleation are complex and an active area of research, atmospheric temperatures must be below -35oC for homogenous nucleation to occur. At these low temperatures, the liquid compressibility reaches a maximum, and fluctuations in density reach a maximum (allowing for density to approach that of ice in regions of a given droplet). These conditions allow for water molecules within a droplet to aggregate which can result in the formation of an ice embryo. After an ice embryo forms, the water droplet is able to completely freeze and form an ice crystal. Due to the low temperatures required, homogenous nucleation is more likely to happen in colder regions and higher altitude clouds compared to heterogenous nucleation. Despite the colder conditions required to homogeneously nucleate, this pathway of nucleation is still quite common in the atmosphere.

Heterogenous Nucleation of Ice Crystals
Heterogenous nucleation is the process of water molecules freezing directly onto an aerosol particle that can act as an ice nucleus. There are multiple theorized types of heterogenous nucleation including deposition nucleation (can occur in the vapor phase as water vapor is absorbed on the particle), immersion nucleation (an ice nucleus is initially present in the drop), and contact freezing (ice nucleus and supercooled liquid droplet collide and form an ice crystal). In the presence of an ice nucleus, heterogenous nucleation can occur at temperatures below 0oC. The occurrence of heterogenous nucleation is largely dependent on the physical makeup of the ice nucleus. Aerosol particles that possess physical characteristics similar to ice, including a similar molecular spacing (109.5o bond angles) and hexagonal lattice structure, tend to act as better ice nuclei. When an aerosol particle has these aforementioned physical characteristics, the energy barrier required for a droplet to freeze is lowered and water can freeze onto the aerosol particle at higher temperatures compared to homogenous nucleation. Beyond the physical shape, ice nuclei that are insoluble in water tend to make efficient ice nuclei.

One example of an effective ice nucleus is silver iodide (AgI), an inorganic aerosol that has been extensively used in heterogenous ice nucleation studies due to the hexagonal structure that mimics the crystalline structure of frozen water. Due to its high efficiency as an ice nucleus, silver iodide has also been used in artificial cloud seeding efforts. Some other examples of effective ice nuclei include lead iodide (PbI2), kaolinite, and metaldehyde. The ability for an aerosol to act as an ice nucleus can also differ by aerosol type, a concept which is detailed below.

Mineral Dust
Mineral dust is one type of effective ice nucleus that can create ice crystals at a wide range of temperatures as high as 0oC. Mineral dusts tend to be good ice nuclei due to their hexagonal structure, insolubility in water, and ability to act as a nucleus in both immersion and contact nucleation situations. Common ice nuclei that fall under the classification of mineral dust includes the following molecules and groups: kaolinite, montmorillonite, chlorites, and illite. Because different regions emit different types of mineral dust, the effect mineral dusts have on the heterogeneous ice nucleation process differs significantly by region.

Biological Aerosols
Biological aerosols (see bioaerosol) are a class of aerosols that include biological matter (pollen, viruses, mold) and have been proven to be effective ice nuclei in regions with significant sources of biological aerosols. The temperature of ice nucleation in the presence of biological aerosols has been observed as high as -4oC making biological aerosols another aerosol type that can trigger ice nucleation at relatively warm temperatures. Due to the high variety of biogenic species that can be found in biological aerosols, the specific impacts of these aerosols on ice nucleation are highly dependent on regional emissions characteristics. Biological material can be observed in other aerosol types making it difficult to differentiate between biological aerosols, and other aerosol types that contain biogenic matter.

Anthropogenic Aerosols
Anthropogenic aerosols (also referred to urban aerosol) are a class of aerosol emitted from anthropogenic activities. Types of anthropogenic aerosols include black carbon, soot, sulfate aerosols, and other aerosols emitted from human/industrial activity. These aerosols are theorized to contribute a significant portion of the ice nucleating particle budget. Preliminary work identifies that the specific physical effects of anthropogenic aerosols on ice nucleation depend on the location within a cloud where the radius decreases at the top of the cloud but increases in more central locations in the cloud. However, a different study (Chen et al., 2018) does not observe an effect on ice nucleating particles in the presence of increased anthropogenic aerosols.

Other Aerosols Types
Marine aerosol, which often contain salt (NaCl), have been a focus of ice nucleation in coastal and remote ocean regions. Water particles that contain salt can nucleate by a homogenous pathway at sufficiently cold temperatures over ocean regions. When a marine aerosol contains a diatom, ice nucleation proceeds efficiently by heterogenous nucleation at warmer temperatures (-13oC). Current studies on heterogenous nucleation by marine aerosol is focused on identifying the specific effects diatoms have on the nucleation process.

Aerosol from biomass burning is another type that has been studied has the ability to undergo heterogenous ice nucleation. Studies that include both controlled and field experiments show that -15oC is the upper limit of heterogenous nucleation with biomass burning aerosol. While both maritime and biomass aerosol do undergo heterogenous nucleation, the fact that both nuclei can contain organic material makes it debatable whether to interpret the respective particle as a biological aerosol.