The goal of the project is to investigate quantitatively the role of the nitrate radical in secondary organic aerosol formation and to test and improve the theories and parameterizations developed from laboratory studies and used in current models. The project starts with an analysis of measurements from BEACHON-RoMBAS, a field campaign at a mountain forested site. The proposed work will utilize a large suite of state-of-the art ambient data (e.g. HR-ToF-AMS, TD-LIF, HR-PTR-MS, and the recently developed MOVI-HRToF-CIMS: Chemical-Ionization HRToFMS coupled to a Micro-Orifice Volatilization Impactor) in combination with the Potential Aerosol Mass (PAM) flow reactor with the NO3 oxidant. To help interpret the field measurements, laboratory studies with the PAM flow tube and a new temperature-controlled environmental chamber, under controlled conditions representing those encountered at BEACHON-RoMBAS, will be conducted to directly link the atmospheric observations to the SOA yields and properties and NO3 reactivity observed in the same experimental system in the laboratory, and to compare with the literature and theoretical predictions. Additionally, a photochemical model of gas-phase chemistry and gas-particle partitioning will be constructed based on the Leeds Master Chemical Mechanism to model both the chemistry in the atmosphere and in the PAM reactor. A computational fluid dynamics model (CFD) will be used to understand the flow within the PAM reactor and thus better represent gas and aerosol losses and chemical evolution and inhomogeneities through coupling with the chemical modeling. A new flow tube reactor, optimized for ambient and laboratory NO3-VOC reactivity studies, will be designed and tested, guided by the CFD modeling and results from the first NO3-PAM field deployment at BEACHON-RoMBAS. Model development in support of data collected at BEACHON-RoMBAS will also find applications in analysis of planned nighttime aircraft measurements that will take place during NOAA SENEX, an aircraft (P-3) campaign in summer 2013 that will target forested regions in the Southeast U.S.