Secondary organic aerosol (SOA) formed from the oxidation of biogenic hydrocarbons is one of the largest sources of organic aerosol in Earth�??s atmosphere, and a significant component of total aerosol. While the carbon in biogenic SOA is derived from naturally emitted volatile organic compounds (VOCs), it has become clear that anthropogenic pollutants like NOx and SO2 play important roles in biogenic SOA formation, and thus affect both air quality and climate. While progress has been made elucidating mechanisms of biogenic SOA formation in the presence of anthropogenic pollutants, the net effect of anthropogenic emissions on biogenic SOA loadings in the atmosphere are still highly uncertain. In this proposed project, titled �??Quantitative Investigation of Mechanisms of Oxidation and SOA Formation from Terpenes for Different Oxidants and NOx Regimes�?�, we will conduct experimental and modeling studies in order to achieve an improved understanding and quantitative description of the mechanisms of atmospheric oxidation of monoterpenes, with an emphasis on the effect of oxidation regime and NOx on reaction pathways, gas- and particle-phase products, and SOA formation. Such information is necessary to understand the role of NOx in the evolution of SOA composition due to future changes in NOx and BVOC emissions and climate. Experiments will be conducted in the CU-Boulder chamber facility using a suite of state-of-the-art online and offline instruments and methods and will focus on reactions of OH and NO3 radicals, the major daytime and nighttime oxidants, respectively, with three of the most atmospherically abundant monoterpenes: ?�-pinene, ?�-pinene, and ??-3-carene. Results will be used to evaluate and improve monoterpene oxidation and SOA formation mechanisms in the explicit GECKO-A model and in simplified versions suitable for use in chemical transport models. The investigators on this proposal were major participants in the SENEX, SOAS, SAS, SEAC4RS and GoAmazon field campaigns, and because the chamber experiments will be conducted with some of the same instruments used in the field the results will be immediately useful to the interpretation of field data.