The effect of hygroscopicity on cloud droplet formation

The effects of particle hygroscopicity and the availability of condensable material (other than water) in the gas phase on cloud droplet formation and the radiative properties of clouds have been studied using an adiabatic air parcel model with detailed multicomponent condensation. The pre-existing log-normal particle distribution used is bimodal in size and bimodal in hygroscopicity. To simulate this, four log-normal distributions were used and in each mode particles were assumed to be internally mixed, i.e., they are composed partly of salt and partly of an insoluble substance. The mean diameters, standard deviations, total number of pre-existing particles, the mass fraction of the soluble salt and initial concentration of condensable vapour were varied in the simulations. There is a clear effect of hygroscopicity on the activated fraction of aerosol particles in our simulations. Thus hygroscopicity of pre-existing aerosol particles and concentrations of condensable gases can also influence the optical thickness and reflectance of clouds. The change in optical thickness varies as a function of the number concentration of pre-existing particles, having a maximum (Delta tau/tau = 0.2) near a concentration of 1000/cc.