Aerosol and cloud condensation nuclei(cCn Cloud Condensation Nuclei( ccn) are particles that can become activated to grow to fog or cloud droplets in the presence of a supersaturation of water vapor. If the earth's atmosphere were totally devoid of particles, clouds could not formAerosol and cloud condensation nuclei (CCN) • Cloud Condensation Nuclei (CCN) are particles that can become activated to grow to fog or cloud droplets in the presence of a supersaturation of water vapor. • If the Earth’s atmosphere were totally devoid of particles, clouds could not form