Question

Heterogeneous nucleation can only occur at humidities in excess of 100 percent. results in a water droplet that is 100 percent water. cannot occur in the presence of hygroscopic particles. requires condensation nuclei.

Answer 1

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In the atmosphere, cloud drops form on aerosols called condensation nuclei. The formation rate of the cloud droplets is determined by the number of cores. In general, aerosols can be classified, according to their affinity with water, into hygroscopic, neutral and hydrophobic. Nucleation on a neutral aerosol needs almost the same supersaturation as homogeneous nucleation; on a hydrophobic aerosol, nucleation is more difficult and more supersaturation is required. On the hygroscopic particles, which are soluble and have an affinity with water, the supersaturation that is needed for the formation of the droplets may be less than for homogeneous nucleation.

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