The goal of this project is to investigate the impacts of aerosols in California on radiative forcing and cloud formation. The investigation is motivated by the following set of questions: (1) How can ambient measurements be used to decipher the origins of the carbonaceous fraction of the aerosol? How do the properties of the secondary (SOA) component of carbonaceous aerosols compare with those in laboratory chamber experiments? (2) What are the chemical composition, hygroscopicity, CCN activity, and optical properties of California aerosols? How do these properties evolve as a function of atmospheric residence time? (3) What is the aerosol mixing state and how does it influence hygroscopicity, CCN activity, and optical properties? (4) What are black carbon (BC) levels in California aerosols? How do these affect hygroscopic and optical properties? (5) Based on data obtained during CalNex 2010, what are the direct and indirect radiative forcings attributable to California aerosols? A combination of AMS spectra, PILS water-soluble organic composition, and high-resolution mass spectrometry analysis of filter samples will be used to evaluate the extent of oxidation of the aerosol and to look for molecular markers for biogenic and anthropogenic sources. The aerosol mixing state will be assessed with the Aerosol Time of Flight Mass Spectrometer (ATOFMS). BC mass concentrations will be measured by the Single-Particle Soot Photometer (SP2) and aerosol absorption coefficients will be measured with a three-wavelength Photoacoustic Aerosol Spectrometer (PASS-3) and a three-wavelength Particle Soot Absorption Photometer (PSAP). Single particle size and refractive index will be measured by the Multiangle Light Scattering Spectrometer (MLS). In the closure study, theoretically calculated single-particle refractive index based on SP2 BC concentration and non-BC composition from the AMS will be compared with absorption coefficients measured by the PASS-3 and PSAP and with single particle refractive indexes measured by the MLS. Aerosol properties derived from the CalNex 2010 field program will be imbedded in General Circulation Model (GCM) vertical columns over California to assess the sensitivity of radiative forcing to these properties versus those that are currently used in climate assessments.