Intercomparison of Global Aerosol Microphysics Models: Insights Into Cloud Condensation Nuclei Formation

A major goal of global aerosol modeling is the prediction of aerosol number concentrations, size distributions and composition that allows prediction of cloud condensation nuclei (CCN) concentrations and estimation of the aerosol indirect effects. To date, field campaigns provide the most detailed observations of aerosol microphysical behavior. Meaningful model-observation comparisons require assimilated meteorology so simulations reproduce the specific meteorological and pollution events of a given field campaign. In this work, the TwO-Moment Aerosol Sectional (TOMAS) microphysical model is integrated into the GEOS- CHEM model. GEOS-CHEM is a global three-dimensional model of tropospheric chemistry driven by assimilated meteorological observations from the Goddard Earth Observing System (GEOS). GEOS-CHEM, driven by assimilated meteorology, allows TOMAS predicted aerosol microphysics to be compared to field campaign observations. TOMAS in GEOS-CHEM improves the previous bulk aerosol model by simulating size- resolved aerosol microphysics. The TOMAS model conserves both number and mass concentrations of aerosol segregated into 30 size bins from 0.01 μm to 10 μm. The microphysical processes include coagulation, condensation, evaporation, nucleation, size-resolved wet deposition and size-resolved dry deposition. Sulfate and sea-salt aerosols are included. To test the aerosol model, the model predictions are compared with the observational data from a variety of field campaign data. Additionally, model intercomparisons are made between the following three models: (1) TOMAS in GEOS-CHEM; (2) TOMAS in GISS GCM II-prime; (3) GLOMAP in TOMCAT. All three models include two-moment sectional microphysics algorithms with high size resolution. The suite of comparisons allows us to examine the effects of different meteorological fields and chemical schemes (e.g. online versus offline oxidants) on model performance. Points of comparison include aerosol number concentrations, CCN concentrations, and size distributions. Special attention will be given to microphysics of the marine boundary layer as well as predicted nucleation in each model.