The goal of this project is to further develop the metastable atom bombardment (MAB) HR-ToF-AMS and to deploy the instrumentation to a field study, e.g., Front Range 2009 or California 2010 for the purposse of obtaining a more detailed characterization of secondary organic aerosols (SOA) formation and aging. By limiting molecular fragmentation, MAB ionization reduces redundancies in the mass spectra of organic aerosols and enables the determination of molecular composition. The soft nature and the high sensitivity of the MAB using an HR-ToF-AMS that is configured to allow interchange between electron impact (EI) and MAB ionization sources on a timescale of minutes has already been demonstrated. Combination of the quantitative data retrieved from EI mass spectra with the enriched chemical information obtained by MAB ionization will yield more detailed characterization of SOA formation and aging. The increased specificity in aerosol mass spectra will greatly expand the power of Positive Matrix Factorization (PMF) to isolate organic aerosol (OA) sources. As part of the campaign analysis, the enriched OA characterization will be used in a Cloud Condensation Nuclei (CCN) closure study and determine whether improved agreement is achieved. The proposed research develops and applies analytical capabilities not available at present, but which are needed for attacking major uncertainties in the sources, processes, and effects of organic aerosols. The unique data generated by these techniques will furnish critically needed information on organic aerosols and submicron aerosol climatic effects that can inform climate models and scientific assessments by, for example IPCC, and through them, policy decisions.