A new bulk cloud microphysics scheme that accounts for aerosol microphysical properties and size distribution is implemented into the single-column version of the ARCSyM. This scheme is distinguished from other bulk microphysics schemes by its prognostic determination of cloud particle number concentration and saturation ratio. The new scheme is compared to a simpler bulk microphysics scheme and observations taken during the FIRE Arctic Clouds Experiment in May 1998. Qualitatively, the two microphysics schemes are generally in agreement with the observed cloud formation and evolution. Comparison with aircraft measurements at 3 times shows that the new scheme better discriminates cloud phase and reproduces reasonably well the observed liquid and ice water content for two cases. The better performance of the new scheme is attributed to its more elaborated treatment of the freezing process which is made possible by the prognostic determination of cloud particle number concentration and the assumption of a bimodal lognormal cloud size distribution. Sensitivity studies are performed to assess four aerosol microphysical properties on the evolution of cloud microphysical processes. Results show that the IFN concentration, the aerosol number concentration, the slope of the aerosol size distribution, and the aerosol solubility may impact substantially on cloud phase and total water content. The liquid water path and ice water path can vary by as much as 100 g m-2 locally as a result of the variation of these parameters related to aerosols. ¿ 2001 American Geophysical Union