Cloud processing of internal mixed aerosol: A numerical study using a bin aerosol-microphysics scheme coupled with WRF

Atmospheric aerosol affects cloud macrophysical and microphysical properties, influences precipitation type and amount, and eventually impacts climate through the interaction between cloud and precipitation. On the other hand, aerosol particles are modulated by cloud and precipitation. Processed by clouds, aerosol particles are redistributed in the air through scavenging (sink) and evaporation (source). Their chemical and hygroscopic features are changed by clouds as well. These redistributed and modified aerosol particles affect cloud/precipitation and their subsequent development. A new detailed bin warm-phase aerosol-microphysics scheme has been developed to study how aerosol properties such as size, mass and solubility are modified by cloud processing. This scheme simulates the following cloud processes: haze activation by dry aerosol particles, diffusional growth of haze and cloud drops accounting for both solute and curvature effects, collision-coalescence of cloud drops, Brownian, gravitational, and phoretic collection of haze and dry aerosol particles by cloud drops, sedimentation of dry aerosol particles and rain drops, and wet deposition of aerosol particles by precipitation. By implementing this scheme into the Weather Research and Forecast model (WRF) version 3, several sensitivity simulations have been performed by simulating cloud formation over a two-dimensional bell-shaped mountain with periodic lateral boundary condition. The initial internal mixed aerosol population with solubility as a function of aerosol size is continuously processed by haze and cloud drops for 12 hours. The soluble and insoluble aerosol mass is tracked both inside and outside haze and cloud drops throughout the simulation. The main findings of this study are: 1) The bulk solubility of background aerosol increases after cloud processing if most insoluble mass is associated with large size particles while it decreases if insoluble mass is associated with small ones. 2) Larger aerosol particles are generated after cloud processing as a result of collision-coalescence. 3) Most in-drop aerosol mass is associated with haze and small cloud droplets. But the average in-drop aerosol size is proportional to the drop size. 4) Brownian and phoretic collection of haze and dry aerosol particles are crucial to modify aerosol bulk solubility under dry condition while collision-coalescence is more important in wet condition.