A simple analytical model of aerosol properties with account for hygroscopic growth 1. Equilibrium size spectra and cloud condensation nuclei activity spectra

A microphysical model of aerosol size spectra that includes hygroscopic gro wth is presented. In the presence of atmospheric humidity, an aerosol size spectrum of the Junge [1952] type transforms into a superposition of two eq uilibrium power law spectra, with indices determined by the original Junge index and the fraction of soluble material contained in the aerosol. The ae rosol size distributions ale presented as explicit functions of relative hu midity H for three different humidify regimes: (1) moderate humidity, (2) h igh humidity but subsaturation, and (3) interstitial cloud aerosol. On the basis of this model of aerosol hygroscopic growth, the Twomey [1959] cloud condensation nuclei (CCN) power law is then derived so that its coefficient s are related to the aerosol microstructure and humidity dependence under s ubsaturation. This simple aerosol model can be used in cloud and climate mo dels that do not treat explicitly CCN activation and for evaluation of aero sol optical properties. An application for aerosol optical properties is co nsidered by Khvorostyanov and Curry [this issue].