Regional-scale modeling and CalNex field campaign data will be integrated to assess the impact of organic matter and black carbon on direct, semi-direct, and indirect radiative forcing over California. Particular attention will be given to quantifying how changes in primary particulate and particulate precursor emissions affect oxidant chemistry, PM2.5, and aerosol radiative forcing, since policy makers need to understand the impacts of potential emission mitigation strategies on both air quality and climate. Our analyses of the field data and model predictions will be guided by the following scientific questions: 1) Are current emission inventories of primary particulates, particularly black carbon and organic matter, and particulate precursors consistent with comparisons between observed and simulated aerosol mass and composition? 2) How well does the volatility basis set approach represent secondary organic aerosol evolution in California and what are the relative contribution of anthropogenic, biogenic, and biomass burning sources to organic aerosols in the South Coast Air Basin, the San Joaquin Valley, and the Sacramento Valley? 3) To what extent do aerosols impact aerosol direct and indirect radiative forcing over different geographical regions using a state-of-the-science regional model, and how does the mixing of organic matter components and black carbon with other species affect their optical properties? 4) What are the differences in aerosol radiative forcing resulting from local emissions in California compared with the long-range transport of aerosols? What are the differences in aerosol radiative forcing between anthropogenic and natural emission sources? We will use the chemistry version of the Weather Research and Forecasting (WRF) model that includes representations of the interactions of aerosols, radiation, clouds, and chemistry implemented by PNNL scientists. Recently, two new components have been added to WRF-Chem to help us address the science questions. First, the �??volatility basis set�?? approach has been coupled with the SAPRC 1999 gas-phase photochemical mechanism and the MOSAIC aerosol model in WRF to better represent secondary organic aerosols (SOA). In this approach the current static representation of primary organic aerosol (POA) has been replaced by a dynamic approach in which low volatility organic material evaporates, undergoes multi-generational chemistry and recondenses over varying time and spatial scales to form SOA. In addition, the VBS approach represents SOA formation due to multi-generational chemistry of a complex mixture of thousands of un-identified organic species that are missing in existing inventories. This highly improved representation of SOA is expected to have profound implications on the current understanding of organic aerosols, their mixing state with black carbon, and their relation to aerosol radiative forcing and climate change. Second, physics modules, including those for aerosol and aerosol radiative forcing, from the Community Atmospheric Model 5 (CAM5) global climate model have been ported to WRF. Using the CAM5 and standard WRF parameterizations, we will also be able to compare the strengths and weaknesses of parameterizations employed by global climate models and regional models, using the same modeling framework and emissions.