Role of aerosol particles in formation of precipitation

Abstract

Variable concentrations of different‐sized water insoluble particles were found in single raindrops and bulk precipitation water and in single snowflakes and hailstones collected in different parts of the world. The experimental evidence shows that aerosol particles are of extreme importance in the formation of the liquid and solid phases of precipitation. A fraction of particles in the 0.2‐ to 2‐µm‐diameter size range acts as cloud condensation nuclei. Condensation of water vapor creates an aerosol cloud that now consists of a population whose size distribution differs from that of the aerosol in the surrounding air and that consists on a mass basis largely of water. Very large particles create conditions for aerodynamic capture of different‐sized cloud particles with subsequent growth of precipitation elements. Those particles are sea‐salt particles in maritime clouds, ice crystals and ice pellets in storms, and deka‐ and hecto‐µm soil particles in severe hailstorms. The ice phase in clouds is initiated on ice‐forming nuclei that are derived from the aerosol population. The ice phase is formed through three basic mechanisms. Contact nucleation takes place when a supercooled water droplet collides with an aerosol particle at the temperature at which that particle can nucleate ice. If the supercooled waterdrop is warmer than the ice nucleation temperature of the colliding aerosol particles, the latter may be captured and become a hydrosol particle. Upon further supercooling the hydrosol particle will act as a freezing nucleus and nucleate ice. The temperature of a freezing waterdrop (liquid ice system) is 0°C, and the released water vapor during freezing produces in the wake of the drop a region of water vapor supersaturation with respect to liquid water at the temperature of the environment. Some of the aerosol particles exposed to that supersaturation will nucleate ice through condensation followed by freezing. Ice‐forming nuclei in snow crystals vary from 0.1 to 13 µm. The majority of particles present in snow crystals are scavenged. Soil particles from arid regions play an important role as ice‐forming nuclei during the northwest monsoon season in Japan. The majority of particles in cirrus clouds are of terrestrial origin but particles of extraterrestrial origin were found in large numbers on certain occasions. Preferential transfer of magnetic particles from air into the ice phase of clouds seems to be evident. The heat and water vapor exchange during phase transitions affect the aerosol population present between cloud particles. Thermophoresis and diffusiophoresis are both active in the transfer of 0.02‐ to 2‐µm‐diameter aerosol particles into precipitation elements. Particles smaller than 0.02 µm are transferred by Brownian diffusion and those larger than 2 µm by impaction. Determination of a lifetime of an aerosol particle within a cloud, or in other words determination of the rate of an aerosol particle transfer into a liquid or solid phase of a cloud, depends on the sizes of interacting particles and their physical state at the time of interaction.