Relative contributions of individual phoretic effect in the below-cloud scavenging process

We systematically estimate the thermophoretic and diffusiophoretic effect to the below-cloud scavenging coefficient based on the below-cloud scavenging coefficient expression derived in Bae, Jung, and Kim [(2006). Development and evaluation of an expression for polydisperse particle scavenging coefficient for the below-cloud scavenging as a function of rain intensity using the moment method. J. Aerosol Sci. 37, 1507]. The temperature difference between the ambient air temperature (T) and raindrop surface temperature (T_s), denoted (T-T_s), is the most important factor to the thermophoretic effect. The scavenging coefficient by diffusiophoresis is mainly affected by T, (T-T_s), and relative humidity (RH). The diffusiophoresis increases with the decrease of (T-T_s). The driving force of diffusiophoresis increases with the increase of T. As RH becomes lower, diffusiophoresis is more effective. The relative contribution of thermophoresis and diffusiophoresis shows opposite trend to the changes of both T and (T-T_s) at all T except (T-T_s) being 1 °C. Thermophoresis and diffusiophoresis are important to the scavenging process when particle diameter is between 0.1 and 10 μm, especially around 1 μm.