Aerosol indirect effect using a fast and accurate global aerosol microphysics model
Yunha Lee 

Aerosols can enhance cloud albedo and modify precipitation efficiency by acting as cloud condensation 
nuclei (CCN).  These two aerosol indirect effects are the most uncertain climate forcings.  The large 
uncertainty in estimates of the aerosol indirect effect is due partly to uncertainties in CCN 
predictions and can be improved with appropriate simulation of aerosol number and size. The TwO-
Moment Aerosol Sectional (TOMAS) microphysics model incorporated in the Goddard Institute for 
Space Studies General Circulation Model II' (GISS GCM II') predicts accurately the evolution of 
aerosol number by aerosol microphysical processes such as condensation and coagulation. My 
doctoral research at Carnegie Mellon University focuses on two broad topic areas: a) the 
improvements of GISS-TOMAS model and b) the study of aerosol indirect effects using the 
improved GISS-TOMAS model. This seminar will cover the latter.

First, the impact of mitigation of black carbon (BC) particulate matter on aerosol indirect forcing is 
studied. Mitigation of BC has been suggested as a strategy complementary to reduction of 
greenhouse gases due to its positive top-of-the-atmosphere (TOA) direct radiative forcing. However, 
BC-containing emissions can contribute to the CCN population when those particles become 
internally mixed with hydrophilic aerosol components, and a decrease in BC mass result in a 
reduction in TOA cloud radiative forcing. This study shows that the reduction in aerosol indirect 
forcing by the BC mitigation partly offsets the reduction in TOA direct BC radiative forcing, which 
might indicate less climate benefits by the BC mitigation than what has been suggested. 

Second, we investigated the uncertainty of nucleation on cloud microphysical properties, such as 
cloud droplet number concentration (CDNC), cloud droplet effective radius (Reff), and aerosol 
indirect forcing using the GISS-TOMAS model. The predicted rates of new particle formation 
(nucleation) by different nucleation theories varies by several orders of magnitude, which may 
affect atmospheric CCN concentrations, clouds and indirect forcing. Among several nucleation 
theories available, two nucleation mechanisms are chosen for this study to represent the overall 
uncertainty in nucleation rates: the binary (H2O-H2SO4) nucleation parameterization based on 
Vehkamaki et al. [2002] is used as a lower bound on nucleation rates, and the ternary (H2O-H2SO4-
NH3) nucleation parameterization based on Napari et al. [2002] is used as the upper bound of 
nucleation rates. The global difference in nucleation rates (binary vs. ternary) is ~10^6, but for CN10 
is ~2 and CDNC is ~10%. The choice of nucleation scheme influences the increase of global-average 
of CDNC from the preindustrial to the present-day (87% for the binary nucleation and 62% for the 
ternary nucleation) and the decrease of global-average of cloud albedo forcing (-0.89 W m-2 for 
binary nucleation and -0.65 W m-2 for ternary nucleation).