The Pre-Existing Particle Distribution and The Formation of Cloud Droplets: A Model Study

Abstract

The effects of the preexisting particle distribution and condensation of nitric acid on cloud droplet formation are studied by using an air parcel model describing the formation of a convective cloud. In the used model the simultaneous condensation of water and nitric acid vapors occurs on particles. The preexisting particle distribution used is a bimodal in size and the particles are assumed to be internally mixed, i.e., they are composed partly of ammonium nitrate salt and partly of some insoluble substance. The mean radii, standard deviations, the mass fraction of the soluble salt, and initial concentration of condensable nitric acid vapor are varied in the simulations. There are clear effects of hygroscopicity on the activated aerosol particles and condensing nitric acid on the activation of cloud droplets. Generally, the mode with the larger mean radius become activated more easily than the smaller mode. The effects of standard deviation on the cloud droplet formation are not always clear.