What determines aerosol number concentration for cloud droplet nucleation and radiative forcing?

Aerosol number concentrations, size distribution and chemical characteristics are all important for determining the cloud droplet concentration and hence the radiative forcing of anthropogenic aerosols. Aerosol number concentrations and CCN concentrations result from emissions of primary particles together with nucleation of new particles, their coagulation to accumulation mode aerosols, and the condensation of gas phase precursors onto pre-existing particles. Significant uncertainty in determining the forcing by anthropogenic aerosols has remained because the total number of new particles generated in the present-day atmosphere that grow to CCN sizes is unknown. Here, we examine the importance of binary homogenous nucleation of H2SO4(g) and a first- order nucleation scheme based on measured rates for determination of aerosol number, cloud droplet number, and indirect radiative forcing by anthropogenic aerosols.