Effects of Image Charges on the Scavenging of Aerosol Particles by Cloud Droplets and on Droplet Charging and Possible Ice Nucleation Processes

Abstract

Previous calculations of the rate at which falling droplets in clouds collide with aerosols have led to the conclusion that except in thunderclouds any electrical charges on the aerosols or droplets have little effect on the collision rate. However, it had been assumed that the aerosols would have only a few elementary charges on them, whereas it is now known that at the tops of nonthunderstorm clouds the evaporating droplets may have several hundred elementary charges on them and that much of this charge remains on the residual aerosol for 5 min or so after the evaporation. Also, most previous calculations neglected image charge forces that provide strong attraction at close range even when droplet and aerosol have charges of the same sign and of comparable magnitude.The authors present numerical calculations showing that electrical effects dominate collision rates for charged evaporation aerosols. The calculations are for the size range of 0.1-to 1.0-m radius with the collision efficiency compared to that for phoretic and Brownian effects being greater by up to a factor of 30 greater for droplets from 18.6-to 106-m radius with relative humidity in the range 95%-100% and only 50 elementary charges on the aerosol. The results imply that electrical effects can be important for the scavenging of evaporation aerosol particles in the size range of the Greenfield gap.The authors call this process ''electroscavenging.'' Electroscavenging of charged particles, when the particles are mostly of the same sign, is a previously unrecognized droplet charging process. Electroscavenging also provides a pathway for contact ice nucleation when charged aerosol particles from evaporated charged droplets collide with supercooled droplets. Ice nucleation can occur because aerosol particles from the evaporation of cloud droplets have been found to be more effective as ice forming nuclei than other aerosol particles that have not been processed through droplets.

Laboratory studies of collection efficiency of sub-micrometer aerosol particles by cloud droplets on a single-droplet basis

“…7, electro-scavenging have a significant impact on the curves. Previous work by Wang et al (1978), Byrne and Jennings (1993) and Tinsley et al (2000) shows the presence of charge on droplets and aerosol can increase the CE throughout the Greenfield gap. Moreover, as described by Tinsley et al (2001), the electrical effect is more important for smaller particle sizes (< 0.1 µm) than Brownian diffusion.…”

“…It is known that droplets carrying higher electric charge have higher CE (Barlow and Latham, 1983;Byrne and Jennings, 1993;Pranesha and Kamra, 1997a, b;Tinsley and Leddon, 2013;Tinsley et al, 2000;Tinsley, 2010), and this is consistent with our data in Fig. 9.…”

“…Droplet size can impact CE, where lower values correlate with larger droplets (Lai et al, 1978;Pranesha and Kamra, 1996). Higher charge also correlates with higher CE, indicative of greater electrical force (Beard, 1974;Wang and Pruppacher, 1977;Lai et al, 1978;Barlow and Latham, 1983;Pranesha and Kamra, 1997a, b;Tinsley et al, 2000). It should be noted that the number of elementary charges used in previous work was often motivated by atmospheric observations: a few tens to hundreds of elementary charges for altostratus and stratocumulus clouds (Phillips and Kinzer, 1958;Beard et al, 2004) and hundreds to thousands of elementary charges in cumulonimbus clouds (Thomson and Iribarne, 1978;Marshall and Winn, 1982).…”

“…Previous studies have theoretically determined the CE between droplets and aerosol particles (Slinn and Shen, 1970;Beard, 1974;Grover et al, 1977;Davenport and Peters, 1978;Wang et al, 1978;Park et al, 2005;Tinsley et al, 2000Tinsley et al, , 2006Chate, 2005;Andronache et al, 2006;Feng, 2007;Croft et al, 2009;Tinsley, 2010;Wang et al, 2010;Tinsley and Leddon, 2013). In order to understand our experimental data, we compare them to a theoretical treatment of CE.…”

Solar wind-atmospheric electricity-cloud microphysics connections to weather and climate

“…In the latter case, collisions of IFN with droplets below freezing temperatures result in contact ice nucleation, and collisions with warmer droplets provide immersion ice nuclei for freezing when droplets cool to below freezing temperatures. Trajectory simulations to determine scavenging rates (applicable to both incloud and below-cloud scavenging) have been made, without considering diffusion, by Tinsley et al (2000Tinsley et al ( , 2001, Tripathi and Harrison (2002) Trajectory simulations incorporating Brownian diffusion of the aerosol particles in the vicinity of the droplet, which dominates collisions for smaller particles interacting with droplets, have been made by Tinsley (2010). Below-cloud scavenging has been reviewed by Chate et al (2011).…”