Electroscavenging of evaporation nuclei by cloud droplets and consequences for contact ice nucleation

Tinsley, Brian A.
August 2000
AIP Conference Proceedings;2000, Vol. 534 Issue 1, p459
Academic Journal
Droplets in clouds collect aerosol particles that are charged due to the (hitherto largely neglected) action of image charge forces between the conducting surface of the droplet and the particle. This process (electroscavenging) is in addition to phoretic scavenging. We have made new computations of scavenging to include electrical, phoretic, and gravitational forces in a self consistent way. We show that for only a few tens of elementary charges on particles of radii from 0.1 μm to more than 1.0 μm, and for droplets of radii more than 15 μm and for 98% relative humidity, the collision efficiency is dominated by electroscavenging. Charges on the droplets (in comparison to charges on the particles) have little effect on the collection efficiencies, even when the charges are one or two orders of magnitude larger and of the same or opposite sign as those on the particles. Electroscavenging provides a pathway for contact ice nucleation in supercooled clouds, when aerosol particles with charges of order 10² elementary charges result from evaporation of droplets, with typical droplet charges, that are subject to mixing processes at cloud tops. Contact ice nucleation of remaining unevaporated droplets is favored not only by collection rates enhanced by electroscavenging, but also because the sulfate and organic material that is adsorbed onto droplets before evaporation remains as coatings on the residual charged evaporation nuclei. The coatings are thought to have good ice nucleating properties. The retention of the coatings and the charges, although temporary, is long enough (with continued mixing) to greatly increase the contact ice nucleation rate compared to that from other aerosol particles that have not been just previously processed through droplets.


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