A modal aerosol treatment is implemented in the NCAR CCSM CAM model which predicts both aerosol mass and number, and internal mixing between aerosol components. Several new processes have been added to CAM: new particle formation, coagulation within and between aerosol modes, condensation of water vapor and trace gas on aerosols, aging of primary carbon to accumulation mode based on sulfate coating from condensation & coagulation, and aerosol activation. A number of other processes have been improved: in-cloud rainout based on activated (cloud phase) aerosol calculated, below-cloud impaction scavenging rates depending on particle sizes for mass and number of aerosols, optical properties of internally-mixed hydrated aerosol.

In this presentation indirect effect using the modal aerosol treatment will be compared with simulations using the current CAM3 aerosol treatment (the bulk approach). The much smaller indirect effect from the modal aerosol points out to the importance of different aerosol treatments: aerosol size simulated in modal versus a prescribed size in bulk and internal mixing of different aerosol components in modal versus external mixing in the bulk. The sensitivity of indirect effect to aerosol emissions will also be discussed.