New particle formation happens frequently in the troposphere. It can directly affect the aerosol lifecycle and has the potential to change the cloud and precipitation properties indirectly. The newly formed particles are often very small, so including a nanometer-scale size bin or nucleation mode is critical to better represent these ultra-fine particles and their impact on aerosol lifecycle and radiative forcing in global aerosol models. In this work, we add a nucleation mode to the Modal Aerosol Module (MAM) used in the atmospheric component of the Energy Exascale Earth System Model (E3SM). The lower bound of geometric mean dry diameter of aerosols is extended from 8.7 nm in the Aitken mode of the default model to 1 nm in the new nucleation mode. The condensational growth, coagulation, and other important physical processes of ultra-fine particles are now explicitly simulated. Preliminary evaluations show that with the new model can well simulate the aerosol number and size distribution below 10 nm, which is largely underestimated in E3SM. Compared to the default model, the total aerosol number concentration increases almost everywhere (global total column-integrated number is 4 times higher), particularly in the upper troposphere where the binary H₂SO₄-H₂O nucleation is active. On the other hand, the aerosol number concentrations in the Aitken and accumulation modes are reduced significantly, mainly due to the strong nucleation and condensational sink of H₂SO₄ in the nucleation mode. Since the Aitken mode aerosol number has a significant impact on the homogeneous ice nucleation in E3SM, this results in a slightly weaker cloud radiative effect for both the longwave and shortwave components, especially over the tropics. The impact on the anthropogenic aerosol forcing will also be discussed.