Understanding the effects of global climate change on regional air quality is central in future air quality planning. We report here on the use of the Goddard Institute for Space Studies (GISS) general circulation model (GCM) III to drive the GEOS-CHEM global atmospheric chemical transport model to simulate climatological present-day aerosol levels over the United States. Evaluation of model predictions using surface measurements from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network indicates that the GISS GCM III/GEOS-CHEM model is a suitable tool for simulating aerosols over the United States in the present climate. The model reproduces fairly well the concentrations of sulfate (mean bias of -0.36 5g m-3, normalized mean bias (NMB) of -25.9%), black carbon (-0.004 5g m-3, -1.9%), organic carbon that comprises primary and secondary components (-0.56 5g m-3, -34.2%), and PM2.5 (-0.87 5g m-3, -20.4%). Nitrate concentrations are overpredicted in the western United States (west of 950W) with a NMB of +75.6% and underestimated in the eastern United States with a NMB of -54.4%. Special attention is paid to biogenic secondary organic aerosol (SOA). The highest predicted seasonal mean SOA concentrations of 1--2 5g m-3 and 0.5--1.5 5g m-3 are predicted over the northwestern and southeastern United States, respectively, in the months of June--July--August. Isoprene is predicted to contribute 49.5% of the biogenic SOA burden over the United States, with the rest explained by the oxidation of terpenes. Predicted biogenic SOA concentrations are in reasonable agreement with inferred SOA levels from IMPROVE measurements. On an annual basis, SOA is predicted to contribute 10--20% of PM2.5 mass in the southeastern United States, as high as 38% in the northwest and about 5--15% in other regions, indicating the important role of SOA in understanding air quality and visibility over the United States.