Charge modulation of scavenging in clouds: Extension of Monte Carlo simulations and initial parameterization

Abstract

Parameterization has been applied to extended Monte Carlo simulations of the electric charge modulation of aerosol scavenging by droplets, both for particle charges of the same sign as droplets and of opposite sign. Separate parameterizations are given for electro-antiscavenging for small aerosol particles with same-sign charges; electroscavenging for larger particles with same-sign charges; and electroscavenging for opposite-sign particles. A wide range of sizes and charges is treated, together with altitude and humidity variations. The simulations of particle trajectories take Brownian diffusion into account by Monte Carlo perturbations and include the full conducting-sphere electrical force, and phoretic and gravitational forces. For larger particles with larger droplets, the image force causes increases in collision rate coefficients irrespective of the sign of the particle charge, or the amount and sign of the droplet charge, or the degree to which space charge in the cloud is unipolar. Thus, even with little space charge in a cloud, the atmospheric ion production increases the scavenging of larger aerosol particles. With the presence of space charge, the scavenging rate for small particles with high mobility decreases or increases depending on the unipolarity of the space charge. The results are applicable to a wide range of electrical environments associated with clouds and stratified aerosol layers, and are relevant to precipitation and ice-nucleation processes and cloud albedo effects. Key Points Electric charge effects are comparble with diffusive and phoretic effects Electro-scavenging of larger CCN decrease their concentration Sign of effect on scavenging of smaller CCN depends on space charge unipolarity ©2013. American Geophysical Union. All Rights Reserved.