Biogenic secondary organic aerosol over the United States: 3 Comparison of climatological simulations with observations

6 [1] Understanding the effects of global climate change on regional air quality is central in 7 future air quality planning. We report here on the use of the Goddard Institute for Space 8 Studies (GISS) general circulation model (GCM) III to drive the GEOS-CHEM global 9 atmospheric chemical transport model to simulate climatological present-day aerosol 10 levels over the United States. Evaluation of model predictions using surface measurements 11 from the Interagency Monitoring of Protected Visual Environments (IMPROVE) 12 network indicates that the GISS GCM III/GEOS-CHEM model is a suitable tool for 13 simulating aerosols over the United States in the present climate. The model reproduces 14 fairly well the concentrations of sulfate (mean bias of 0.36 mg m , normalized mean 15 bias (NMB) of 25.9%), black carbon ( 0.004 mg m , 1.9%), organic carbon that 16 comprises primary and secondary components ( 0.56 mg m , 34.2%), and PM2.5 17 ( 0.87 mg m , 20.4%). Nitrate concentrations are overpredicted in the western United 18 States (west of 95 W) with a NMB of +75.6% and underestimated in the eastern 19 United States with a NMB of 54.4%. Special attention is paid to biogenic 20 secondary organic aerosol (SOA). The highest predicted seasonal mean SOA 21 concentrations of 1–2 mg m 3 and 0.5–1.5 mg m 3 are predicted over the northwestern 22 and southeastern United States, respectively, in the months of June–July–August. 23 Isoprene is predicted to contribute 49.5% of the biogenic SOA burden over the United 24 States, with the rest explained by the oxidation of terpenes. Predicted biogenic SOA 25 concentrations are in reasonable agreement with inferred SOA levels from IMPROVE 26 measurements. On an annual basis, SOA is predicted to contribute 10–20% of PM2.5 mass 27 in the southeastern United States, as high as 38% in the northwest and about 5–15% in 28 other regions, indicating the important role of SOA in understanding air quality 29 and visibility over the United States.